McGraw-Hill Series in Water Resources and Environmental Engineering Rolf Eliassen, Paul H. King, and Ray K. Linsley Consulting Editors
ENVIRONMENTAL ENGINEERING
Bailey and Ollis: Biochemical Engineering Fundamentals Bishop : Marine Pollution and Its Control Biswas: Modelsfor Water Quality Managemeni Bockrath : Environmental Law for Engineers. Scientisls, and Managers Bouwer : Gro!{ndwater Hydrology Canter: En vironmental Impact Assessment Chanlett: Environmental Protection Gaudy and Gaudy: Micr obiologyfor Environmental Scientists and Engineers Haimes: Hierarchical Analysis of Water Resources Systems: Modelling and Optimization of L arge-Scale Systems Hall and Dracup: Water Resources Systems Engineering Linsley and Franzini: Water Resources Engineering Linsley, Kohler , and P aulhus : Hydrology for Engineers Metcalf & Eddy, Inc .: Wastewater Engineering.' Collec tion and Pumping of Wast ewater Metcalf & Eddy, Inc.: Wastewater Engineering. Trealment, Disposal. Reuse Peavy, Rowe, and Tchobanoglous : Environmental Engineering Rich : Low-Maintenance, Mechanically-Simple Wastewater Treatment Systems Sawyer and McCarty : Chemistry for Environmental Engineering Steel and McGhee : Water Supply and Sewerage Tchobanoglous, Theisen, and Eliassen: Solid Wastes, Engineering Principles and Management Issues
Howard S. Peavy Professor of Civil Engineering Montana Scate University
Donald R. Rowe Professor of Civil Engineering King Saud UniverSity Saudi Arabia
George Tchobanoglous Professor of Civil Engineering Univers ity of California, Davis
McGraw-Hill Book Company New York
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CONTENTS ENGI~EERING INTERNATIONAL EDITION 1985
ENVIRONMENTAL
Exclusive rights by McGraw-Hili Book Co., Singapore for manufacture and export. This book cannot be re-exported from the country to which it is consigned by McGraw- HilI.
40 39 38 37 36 35 34 33 32 31 15 14 13 12 11 CTP BJE Copyright © 1985 by McGraw-Hili, Inc. All rights reserved. No part of this publication m ay be reproduced or distributed in any form o r by any means , or stored in a data base or a retrieval system, without th e prior written permissio n of the publisher. This book was set in Times Roman. The editors were Kiran Ve rma and David A. Damstra. The production supervi so r was Leroy A. Youn g.
Preface
XI/ I
Intrnclu Cli on I- I 1-2 1-3 1-4
1- 5
·T he Em'i ron lll c nt T he Im[lact 0 1' Il ulllam upo n th e Enviro nm e nt Th e Im[lact o r th e En\ironm e nt upo n Hum an s Impro \'c me nt of Envi ro nme nt al Qu a li ty T he Ro lc of th e Environ ment a l Eng in ee r ReI-e rcnces
2 4 6 7 ~
Part 1 Water ...Libra./},.()f.c;:()1"!9.ress .Ca~loging in Publication Data Peavy, Howard S. Environmental engin ee ring . (MC Graw-Hili se ries in water resources and environmental engineering) Includes bibliographi ca l references and indexes. 1. Environmental engineering . I. Rowe, Donald R. II. Tchobanoglou s, George. III. Title. IV. Seri es. TD145.P43 1985 628 84-3854 ISBN 0-07-0491 34-8
Water Qu a lit y : Definiti ons, Charaderistics; and Perspect ives
II
2-1
T he Hyd raul ic Cycle a nd Water Qual ity
12
PHY S IC A L W AT E R-QUA LITY PA RA M ETER S
14
2-2 2-3
Suspend ed S,1 Iid , T urbidit y Co lo r Ta ste and Odo l Tempera ture
15 17
2
2-4 2-5 2-6
CHE M ICAL \\ATE R-QUA LI T Y PA RA M ETER S When ordering this title use ISBN 0-07 -1 00231-6
Printed in Singapore
2- 7 2-8 2-9 2· 10 :2-1 1 2· 12
Chemi stn of S" luti o ns ' T o ta l Dissoh cd So lid , Alkali nit\ Hard ness Fl uo ri de Me tal s 2-1 ] Orga nH.:" 2- 14 Nutllc nh
18 20 22
23 23 28 31
3(,
37 38 -1 4
CONTENTS
vi
vii
CONTENTS
BIOLOGICAL WATER-QU-\UTY PARAMETERS
47
2-15 Pathogens 2-16
2-17 2-18
2-19
3 3-1
3-2 3-3 3-4 3-5
3-6
Pathogen Indicators
50
WATER QUALITY REQUIREMENTS
54
In-Stream Standards Potable Water Standards Wastewater Effluent Standards Discussion Topics and Problem s References .
54
Water Purification Processes in Natural Systems
63
PHYSICAL PROCESSES
64
Dilution Sedimentation and Resuspension Filtration Gas Transfer Heat Transfer
64
CHEMICAL PROCESSES
73
Chemical Conversions
73
BIOCHEMICAL PROCESSES
3-7 3-8
3-9 3-10 3-.11
46
55 56 57
65
66 66 71
4-1· 4-2
4-4 4-5 4 -(, 4-7
5-1
5-2 5- 3
5-4
5-5 5-6 5-7 5-8 5-9 5-10 5·11
79
RESPONSE OF STREM...lS TO BIODEGRADABLE. ORGANIC WASTE
83
5-12
. . . . . . . .... ?;l. _ ...... . .. . . . ..... . g5 94
5·13 5-14
Dissolved Solids Removal DiSCUSSIOn Topics and Problems References
~ .
PRIMARY TREATMENT
2Pl
217
Sludge Digestion
Solids Removal
5-22
Historical Overview of Water Treatment Water-Treatment Processes
~ S'\ - \G1
1.1 1 ~ 151
Wastewater Di sposal Discussion Topics and Problems References
6- 1 6-2 6-3
224 224
Growth and Food Utilization Suspended-Culture Systems Activated Sludge Ponds and Lagoons Attached-Culture Systems Secondary Clarification
5-23 Wastewater Reuse
In
221
229
105
\j) \ - \IJ)
220
230 234 234 248
Engineered Systems for Water Purification
Aeraiillil So lid s Se paration Settlin g Operatioll s C llagulati o ll So ft enin g
212
SECONDARY TREATMENT
WASTEWATER DISPOSAL AND REUSE
6
208
Screening Comminuting Grit Removal Flow Measurement Primary Sedimentation
104
110 11 3
207 211
5-20 Nutrient Removal
109
'104
Wastewater Characteristics Effluent Standards Terminology in Wastewater Treatment
5-16 Sludge Characteristics 5-17 Sludge Thickening
107
190
Engineered Systems for Wastewater Treatment and Disposal
SLUDGE TREATMENT AND DISPOSAL
5-21
182
190
ADVANCED WASTEWATER TREATMENT
Chemical Processes Biological Processes Di scu ss ion T o pics and Problem s Refere.nces
,. t ·
OTHER WATER-TREATMENT PROCESSES
. ~cl.~. P.i~iflf~c.t.iml.of Effiw:ots ......................................................
5-18
165
Filtration Disinfecti o n
5-19 Sludge Disposal
WA TER-TRFA 1M ENT PROCESSE S. THEORY AND .APPLICATION
4-3
5
75
3-12 Physical Processes
4
4-10
74
APPLICATION OF NATURAL .pROCESSES IN ENGINEERED SYSTEMS 3- i3 3-14
~
62
Metabolic Processes Microorganisms in Natural Wat er Syst em s
Dissolved-Oxygen Balance Dissolved-Oxygen Model Organic Discharge and Stream Ecology
· 4-8 ' 4-9
255
268
.
277
278 279
281 285 292 294 295:'
301 302
303 306 .314 322
Environmental Engineering Hydraulics Design
324
WATER DISTRIBUTION SYSTEMS
324 324
Method s of Distributing Water Di stribut io n Rese rvoirs Di stribution Sys tem s
326 331
.
CONTENTS
ix
v iii CONTENTS
333 335 337 338 346 347 348
Distribution System Components Capacity and Pressure Requirements Design of Distribution Systems Hydraulic Analysis o f Di stribution Systems 6~7 6-8 Cross-Connections in Di stribution System s Constructi o n of W a ter Di stribution Systems 6-9 6-10 Pumping R equired for Water Supply System s
6-4 6-5 <6-6
348
WASTEWATER CO LLECTION -:
6-11 < 6-12 6-13 6-14 6-15 6-16 6-17 6-18 6-19
349 349
Types of Co llection Systems Types of Sewers Collection System Appurten a nces Basic Consideration s in the Design o f Sewers Design of Sanitary Sewers Preparation of Contact Drawings and Specifica ti ons Construction of Sewe rs Maintenan ce of Sewers Design of Storm wa ter Sewers
~
8- 1 go:
360
i)-}
369
8-4 8-5 8-6
372 375 378 384 3')0 397 397
0<"
<--" , -- -. . -.. - - - - - - . . -
397 ,--399,
406 412
8-7 8-8 8-9 8-1 0 8-11
Part 2 . Air -""""'
'7
~
7-1
'""""' ~.
7-2 7-3 7-4
7-5 7-6 7-7 7-8 -----..,
9-1 9-2
AIR POLLUTION -PAST , PRESENT , AND FUTURE
41 8
9-4
418
9-5 '-)- (, 9- 7
Particul a tes H ydroca rbons Carbon Monoxide Oxides o f Sulfur
477 480
Meteoro logy and Natural Purificati o n Processes
483
E L EME N T AL PR OPERT IES Of THE ATMOSP H ERE
483
Sca les of i\ lotill n
484 486
Heat Pressure Wlncl MOisture Rcla tive H umidit y
La pse Rat es and DI '> persio n Press ure Sys te ms and Di spe rsio n Wind s and Di sper sio n M OiS ture and Di spe rsio n Mod el ing
8- 12 Change s o n the Mesoscale and Microscale __ . __~ : 13__ ~~b 'Hlge.s _0.11_the_ M aC.fQseal e Di sc uss io n T o pics a nd Proble ms References
417
CLASSIFICATION OF POLLUTA N TS
Di sc uss io n T o pi cs a nd Pro blems Refe rences
EFFECTS or A IR POLLUT IO N ON METEOROLOGICAL
Air Quality : Definitions, Characteristics, and Perspectives Historical Overvie\>.' Global Impli ca ti o ns of A ir P o lluti on Units of Measurem ent Sources of poJlutan ts
464
COND IT IO N S
9
Engineered Sys tem s for Air Pollution Control Atm osphe ric Clea nsing Pr ocesses Approa ches to C o ntaminant Cont ro l CON TR OL DEVICES rOR PARTICULATE CONTAM INA NTS
420 424 426
429
9-3
455 461 463
A IR- QUA LIT Y MA N AGEMENT C ONCEPTS
IN FLUE N CE OF i\ I LTEOROLOG IC AL PHENOMENA ON A IR QUALITY
372
6-20 Pumps 6-21 Pump Drive Unit s 6-22 Pump Application Terminology and Usage 6-23 Pump Operating C haracte ristics and C urves 6-24 Analysis of Pump System s 6-25 Pump Stations fo r Water and Wa stewater HYDRAULIC ANALYSIS OF WATER AN D WASTEWATER TREATMENT 6-26 Treatment Plant Desi gn 6-27 Preparation of Hydraulic FFOfiks< <" <<" < <." " <" Discussion Topics and Problems References
8
353 354
371 37 1 37 1
WATER AND WASTEWATER PUMPIN G
-
7-9 Ox ides of N itrogen 7-10 Phot oc hemi cal Oxidant s 7-1 1 Ind oo r Air Po lluti o n
Gra vitati o nal Sel1! ing Ch a mbe rs Ce ntrifu gal Co llec t () ,~S Wc t Colkctnr, Fa hr ic F ilt e r, ( Bag lHlllse Filt e rs) Elec tr os tat ic Prcc ipi Llt () fS (ESP) ' CONT IWL DEV ICES FOR GASEOUS CON T AM INANTS
') -8
431 44 2 44 5
Ad so rp ti(lll 9-9 ,\ OSor ptll)Jl ,-)· 10 C o nli cns"tloll 9- 11 ( 'omo u'>1lt)n
449
l) _ I ~
, \utt.lllIl1 t l\r t:!l lh~H ) !l ( '~)flt r ol
491 493 495 495 495 496 498 498
499· 499 507 508
509 510 5 12
514 5 14
516 5 18 520 523
528 53 3 536
540 540
545 557 559 56J
r CONTENTS
x
xi
CONTENTS
565 567
Discuss io n Topics and Pro blems Refe re nces
Solid Waste: Definitions, Characteristics, and Perspectives
573
TYPES OF SOLID WASTES
573
Municipal Wastes Indu strial Wastes Hazard o us Wastes
574 574 575
SOURCES OF SOLID WASTES
10-4 10-5
Sources o f Municipal Wa stes Sources o f Hazardou s Was tes
575 575 576
PROPERTIES OF SOLID WASTES
576
Ph ys ical C omposition Chemical Co mpositi o n Chan ges in Compositi o n
576 582 588
SOLID-WASTE MANAGEMENT
10-9 10-10 10-11 10-12 10-13 10-14 10-15
11 II-I
11-2 11-3 11-4 11 -5 11 -6 11 -7 11 -8 11 -9
607 615
TRANSFER AND TRANSPORT
618
PROCESSING TECHNIQUES
10-1 10-2 10-3
10-6 10- 7 10-8
II-II
11-12 Transfer Station s 11-13 Location of Tra nsfe r Stations 11-14 Transfe r Means and Method s
Part 3 Solid Waste 10
Collection Routes
11-10 Det ermination of Vehic le and Labor Requirements
A N O VERVIEW
Materials Flow in Societ y Reduction in Raw Mat e ri a ls Usage Reducti o n in Solid-Was te Qu a ntit ies Reuse of So lid-Waste Mat e rial s .. I'vlil.tel:iil.l.s..R.e~o.Y\TY .... En e rgy Recovery Day-to-Day Solid- Wa ste Managem ent Discuss io n Topics and Pro blems Refere nce s
Mechan ical Volume Reducti o n 11 -16 Thermal Volume Reducti o n 11.-17 Manual Co mpo nent Separa ti o n
II-IS
588 589 590 590 591 592 592 592 593
Engineered Systems for Solid-Waste Management
594
Functional Elements
594
SOLID WASTE GENERATION
594
T ypi ca l G e neration Rates Estimat io n o f Solid-Waste Qu a ntit ies Fact o rs Tha t Affect G e nera ti o n Rat cs
595 598 598
ON-SITE HANDLING. STORAG E. t\ N D PROC ESS IN G
598
On -S it e Handlin g On-S it e Sto rage On -S it e Pr ocess in g of So lid Was tes
599 599 601
COLLECTIO N OF SO LID \Vt\ ST ES
601
Co ll ec ti o n Services T ypes o f Co llec ti o ll Sys tcm,
1i05
601
620 622 622 626 627 627 627 628
11-18 11-19 11-20 11-21
Landfi ll ing with Sol id Waste s Design and Operation of La ndfill s Landfa rming Deep-Well Injection Discuss ion Topics and Pro blems References
628 638 646 647 648 652
. 12
Engineered Systems for Resource and Energy Recovery PRO C ESSING TECHNIQUES
12- i 12-2 12-3 12-4
Mechanical Size Alterati o n Mechanical Component Separation Ma gne tic and Electro mec ha nical Separa tio n Drying and Dewatering MA TERIALS-RECOVER Y SYSTEMS
12-5 12-6 )2-7
Materials Specifications Processing and Recovery Sy"stems System Design a~d Layo ut RECOVERY OF BIOLOGICAL CONVERSION P·RODUCTS
12-8 17.-9
Composting (Aerobic Conve rsion) Ana erobic Digestion RECOVERY OF THERMAL CONVERSION PRODUCTS
12-10 12-11 12-12 12- 13 12- 14
C o mbustion of Waste Mat erials Incin eration with Hea t Recove ry Use of Refuse-Deri ved Fuel s (RDF) Ga sification Pyro lys is RE COVERY OF ENE R G Y FROM CON VERSION PRODUCTS
12- I 5 En ergy -Reco ve ry Systems 12- 16 Efficiency Fact or s 12- 17 Deterrninati on of t .ner g} Output and EtJi cicn cy
~' ~
ULTIMATE DISPOSAL
588
~ ,
653 653 65.4 656 656-·- .. 657 657 657 657 659 659 660 .663 665 665 670 671 671 672 672 673 674 675
r
...t
r
r ~
xii CONTENTS MATERIALS- AND ENERGY-RE CO VERY SYSTEMS
675
Discussion Topics and Problems References
677
PREFACE
677
Appendixes A B C D·
Quantities and Units Conversion Factors Properties of Wat e r and Air Water Quality Standards
Indexes ---"'.
---.
---.
~'.
679 683 693
696 II
Name Index Subject Index '
Eng inee rs and scientists from a n umber of re la ted di sci plin es ha ve been in vo lved in the d evelopmc nt of an academic ba sis for th e und e rs tanding and management of the envi ronment. Th e ' management of water qu a lit y has been dealt with in mi crob io logy and s
xiv PREFAn
The breadth and depth of th e matenal in thi s book precludes complete coverage in a one-se m ster or o ne-qu a rte r course. H owever, the a rran gement of the material lends itself to seve ral d iffe rent course format s. I. For introdu c to r y e nginee ring courses at the sopho m ore or junior \i.:vel , C haps. 1.2,3 .7,8 . and 10 provide a n overv iew of the principles involved in e nviro nmenta l en gi neerin g sys tem s. These chapters assume a basic knowledge of c hem is try. biology, ph ys ics, and mathe matics . However , becau se m a n y eng in eeri ng cu rri cu la co ntain few chemi st ry an d even fewer bi o logy or microbiology co urses . th e chapters review th ese subj ec ts in detail. The introd ucto r y sec ti o ns of Chaps. 4. S. 9, II , and 12 may be utili zed to add relevance to th e th eo ret ica l di sc uss ion s. A course following thi s format w ill satisfy ABET requirements for e ngin ee ring scie nce. 2. If th e hrst a ppr oach is used fo r an introductory co urse. th e remainin g chapters (4.5 ,6.9, II , and 12) ca n be used as a follow-up co urse in env ironmen tal engineer in g design . Thi s co urse sho uld be res trict ed to e ng ineering s tud e nt s at th e juni or or se ni o r leve l who ha ve co mpl eted ba s ic fluid m ec hani cs. Such a co ur se would meet ABET' s e nginee rin g design criteria. 3. A m ore cla ss ical app roac h would be to use the first six c hapt ers as a text for a o ne-se meste r or o ne -quart er co urse in water and wastewater engineering. A second o ne-se mester/ qu a rt er course o n air-pollution control and so lid-was te management would use Chaps. 7 thro ugh 12. D es igned fo r junio r- or se ni o rleve l e nginee rin g st ud e nt s that have co mpl e ted basic tluid mechanics, these tw o ' co urses wi ll m ee t ABET criteria for e ngin eer in g design and scie nce, o r a n approximate o ne to o ne ratio . 4. C hapte rs 1,2.3,7.8, and 10 can a lso bc lI sed for a companion course in en vironmental sc ience for nonen ginee rin g s tudent s , prov ided allowance is made .. for·the iimited mar-hema ti ta lbackgrou n'd 6fthc' itlldenis: ' . Whateve r the a pproac h used , th e text sho uld leave stud e nt s with a c lear und erstanding o f th e prin c ipl es of all three o f th e maj o r areas o f envir omen tal e n ginee rin g. U se r co mm e nt s a nd sugges ti ons conce rnin g the effec ti veness of this approach would be g reatly appreciated . The a uthors w is h to ac kn ow ledge til e fac t th at devel op ment and publication of Environmental El7,1jin eeriny wo uld not ha ve been possibl e wi th o ut th e he lp and in spirati on o f o ur former pro fe sso rs, th e c ha lle nge and mot ivation o f our stud en ts , the assistance and e ncoura geme nt o f ou r co lleag ues, the patience a nd forebearance of o ur edit o rs, and th e SUpP Grt and und e rstandin g of ou r familie s.
HOI1'ord S. Peon: Donald R . R owe' C eorql' T chohal/oglous
ENVIRONMENTAL ENGINEERING
•
'.:
C HAPT ER
ONE INTRODUCTION
Environmental eng ineering ha s been defined as th e bran c h of eng ineering th a t is co nce rn ed with protecting the en viron me nt fr o m th e p o tentiall y d e leterious effec ts of human act ivit y, pro tecting human popu la tion s fr o m th e effects of a dverse env iro nm ental factors, a nd improv in g e n viron m ent a l .q u a lit y for human health a nd we ll -being. [1-2J As th e above definiti o n im p lies, hum a ns interact w ith th ei r enviro nm e nt so metimes adve rsely imp acting the environmen t and some times bei ng adve rse ly imp acted by p o llut ants in the env ironment. An unders tandin g o f the n a tu re o f the enviro nm ent and of hum a n interaction w ith it is a neces ' ary prerequis ite to unders tanding th e work o f the env iro nm enta l e ngi neer.
1-] THE ENV IRONM ENT Simply stated, the e n vironmen t can be defined as one's surro undings. In terms of the env iro nmental e ngineer's invo lvement , howeve r, a m ore sp ecific definition is needed. To th e envi ronmental engineer. the word environment ma y ta k.e o n g loba l ciimen s io ns. may refer to a very loca li zed area in which a sp ec ific problem must be ad dr essed, o r may, in th e case of co nt ai ned en viron ments, refer to a sm a ll vo lume o r liquid. gaseo ll s. or so lid materials w ithin a treatment plan t react o r. The g loba l env ironment co nsist s of the atmosph ere, th e hydTosphere, and the lithosph e re in w hi c h the life-s ustainin g resources of th e ea rth are co nt ained. The atIJlOspi1l'rc.;t mixture of gases exte ndin g o utward fr om t~e s urface of th e ea rt h. cvo lved from e lemeo ts o f th e ear th that were gas ified d urin g it s fo rmati o n a n d metamorphos is . Th e hydrosphere consists o f th e oceans, th e lakes and strea ms. a nd the s ha ll ow gro und wa ter bod ies that int erflo w wi th th e surface water. The lithosphere is th e so il mant Ie th at wraps th e co re of th e ea rth . The hiosphcre. a thin she ll that encaps ulates th e ea rth. is made lip o r the ;Itmosphc re a nd iltlwsphere adjacent to the surface o f the earth. toge th er \\it h th e
2
INTROD UCTION
INTRODuCTION
hydrosphere. It is wi th in the b iosphere that the life form s of earth. includi ng h umans. li ve. Li fe-sus taining ma teria ls In gaseo us, liquid , and so li d form s are cycled t hr ough th e bi os p here, provid in g s us tenance to a ll living orga ni sms. Li fe-s ustaining reso urces - air, food , and water - are withdrawn from the biosphere. It is a lso into the biosphere that was te products in gaseous, liquid. and so li d form s are discharged. From the beginning of time, th e biosphere has received and assimilated the wastes generated by pla nt a nd animal life. Natural sys tems have been ever active, dispersing s m oke from forest fires, dilutin g animal was tes washed int o st reams and rivers, and conve rtin g debris of pa st ge nerati o ns of p lant and a nim al life int o soi l ric h enough to s upp or t future pop ul a tions. F o r every na tur a l act of po ll ution, for eve ry und es irab le alteration in the ph ys ical. chem ica l, or bi o logica l charac ter is tics o f the enviro nm e nt , fo r everv in ci dent that eroded the qua lit y of the immed ia te, or loca l, en viro nm ent , the r~ were na tur a l acti o ns that res to red that qua lit y. Onl yi n rece nt years has it beco me apparent th a t th e sustaining and ass imil at ive capac it y of the bi os p here, th oug h treme ndou s, is not, after a ll , infinite. Th o ug h the sys tem has o pe rat ed fo r millio ns of years, it has begu n to show sig ns o f stress, primar il y beca use o f the impact of humans up o n th e environment.
3
from a campfire. Eve n when use o f fire became common , the relatively small amoun ts o f sm oke genera ted we re easil y and rapidly dispersed and ass imilated hy t he at m0sp here. Early civi li za ti o ns o ft en drank from th e same rivers in which they bathed and dep osited th e ir wastes, yet the impact o f sLi c h u se was relatively s li g ht. as natural cleansing mec hanism s easily res tored water quality. These early humans used caves a nd other na tura l s helters or else fashioned their homes from wood, dirt, o r a nim a l skin s. Often no madic, ea rl y popUlations left behind few items that were not read il y bro ken down and absorbed b y the atmosphere, hydrosphere, or litho sphere. And th ose items th a t were no t b ro ken down with time were so few in number and so innocuo us as to present n o significant solid-waste problems. O nl y as ea rl y peoples bega n to ga ther toget her in larger, more o r less s table gro upin gs did their impact upon their loca l environments begin to be significant. [n 6 1 A.D., cooki ng a nd heating fir es caused air- pollution problem s so severe th a t th e Ro man philosopher Seneca co mpl a ined o f " the stink o f the sm o k y chimneys." By the la te e igh teenth century, the waters o f the Rhin e and the Th a mes had beco me too polluted to support game fi sh. Fro m the Middle. Ages the areas where food a nd human waste were dumped harbored rats, flies, and o th e r pes ts.
Satisfying Acquired Needs 1-2 THE IMPACT OF HUMANS UPON THE ENVIRONMENT [n a natural sta te, earth's life fo rm s li ve in eq uilibrium wi th the ir e n vironme nt. T he numbers and ac ti v iti es o f each species a re gover ned by the reso urces ava ilable to them. Spec ies in te raction is commo n, with th e was te prod uct of one speci es o ft en fo rm in g the fo od s upply o f a n o th er. H umans alone ha ve the ab ilit y to ga t her . r.es.oy rce.s.fr()l11.tJI.':Y9.n<;i'(heir. im mediate surro undin gs and p rocess th ose reso urces in to diffe rent. m o re versa til e forms . These a bilities have made it poss ible for hum a n po pulati o n to thr ive and fl ourish beyo nd na t ura l co nst raints. But the natura l and m an ufac tured wastes ge nera ted a nd re leased into th e bi osphe re by these increased numbers o f human be ings have up se t the natural equilibrium. Anthropogenic, or human-induced , po llut an ts ha ve ove rl oaded th e sys te m. The over loading ca m e rel a tive ly la te in the course o f hum a n int e ract io n wi th the environment , perhaps because ea rly socie t ies we re primarily co nce rned wi th m ee tin g natural needs , needs hu m ans sha re in C0m ll1 0n w it h mos t o f the hi g he r m a mm a ls. These peoples had no t ye t beg un to be concerned wi th meeting the acquired needs assoc iated w ith m ore advanced Civilizations.
Satisfying Natural Needs Earl y hum,!ns used nat ura l resoui-ces to sa tisfy th e ir need s for air, wa ter. fo o d, anc! s he lt er. These natural. unpr ocessed reso urces we re readi lv a vai lab le in th e bi osphe re, and th e re s iclues genera ted by t he use of such reso l;rces were genera ll y compa tible wit h, o r readil y ass imilat ed by, th e eIlI· lron ment. Prllnitl ve humans a te plant and a ni mal food s wi th o ut eve n di; tllrbing the atmosphere with the s moke
But these ea rl y evid ences of pollution ove rl oad were merely the prelude to greater ove rl oads to come. With the d aw n o f the industrial revolution, humans were bet ter able th a n ever to sa t isfy th eir age-old needs o f a ir. wa ter, food , and shelter. In creasi n gly th ey turned th e ir a ttenti o n to o th er needs beyond th ose ass oc iated With survival. By the la te nin eteen th a nd early twentieth ce nttlrje ~>.~.l!t 9.mobiles ........... .
II"TRODl ' CTI ON 5
4
INTRODUCTION
disposal problems that arise when it is ti me to get rid of th e toaster become a further factor. As a rule, meeting the acqu ired needs of modern societ ies genera tes more residuals thar: meeting natural need s, and these residuals a re lik elY to be less compatible with the environment and less li kely to be readil y assimila ted into the biosphere. As societies ascend th e socioeconomic ladder, th e li st of acquired needs, or lu xuries, increases, as do the complex ity of th e production chain and the ma~s and complexity of the pollutants generated. Consequently, the impac t of modern human popula tions· upon the environment is of major concern to the enviro nmental engineer:
1-3 THE IMPACT OF THE ENVIRONMENT UPON HUMANS Tho ug h rivers become stagnant, skies smoke-shro uded , a nd dumping gro unds od o riferous and unsightl y, populations generally manage to igno re their impact o n the environ men t unt il they begin to become awa re of the ill efrect s that a po llut ed environment can have upo n thei r own hea lt h a nd we ll-being. Th o ugh stag nant rivers , smoggy skies . a nd un sightl y dumps were aes th eticall y di spleasin g to the citizens of ove rcrowded cit ies of earlier centuries, no att empt was made to re\·erse the nega tive impact humans had o n thei r environm ent unlil it becamc ev iden t th a t heavi ly polluted water, a ir, a nd so il cou ld exert a n eq ually negat ive impact on the health, the aes thet ic a nd cu ltu ra l pleasures, and th e econom ic op po rtunities of humans.
Health Concerns Elemen ts of the air, the wa ter. and the land may host ·ha rmful biolog ical and chem ica l agents that impac t the hea lth of hum ans. A wide range of comm un icab le diseases can be spread th ro ugh elemen ts of the environment by hum a.n and an imal waste product s. T his is most clea rl y evidenced. by th e plagues of th e M idclle Ages when disease spread t hroug h rats tha t fed ·on co ntamin ated so lid and human waste and disease carried by wa terbo rne para5ites a nd ba cteria ran rampa nt thro ugh the populatio n of Europe. It has only been in th e las t century that the co rrelati on be tween wate rborne b iological age nts and huma n diseases has been proved and effec tive preve nti ve measures have been taken. Thro ugh immuniza ti on a nd env ironm ental contro l programs, the majo r diseases transm itt ed via the environment ha\'e all but bee n eliminated in deve lo ped count ries. No country, however, is totall y immun e fr om o utbreaks of env ironmen tall y tra nsmitted disease. The tran smiss io n uf viru ses and proiozoa has proved particu la rl y difficu lt to co ntrol, .and lapses in go()d sanitary practice have res ulted in min o r epidemics of o ther wa terborne diseases. P o ll ution of th e atm osphere has also posed severe health pro blems Ih at are of great concern to environment al engi neers. Peop le in crowded citics have
lik ely suffered from the il l effect s of air po lluti o n fo r centuries, but it is o nl y in this century that II1 creas ing ly heavy po ll ution has ca used hea lt h prob lems so dramat ic as to be easil y a ttribut ed to air pollution . Several key incidents helped ca ll a tten ti o n to the poten tia lly dead ly effec t of air po ll ution. Severa l kille r smogs settled ove r London in the last quarter of the nineteenth century. but the tru e ex tent o f the air-po lluti on problem in th a t cit y did no t become apparen t until 4000 deaths and countless illnesses were a ttributed to the Lond o n smog of 1952. Th o ugh the 20 dea ths ca used by a smog ove r Dono ra, Penn sy lva nia. in 1948 .raised so me alarm . It was not until th e New York i~ ve rs i o n of 1963 clcJlmed seve ra l hundred li ves that thi s co untr y began to take the fi ght aga inst a ir polluti on ser io u sl\". M onit o rin g o f th e sulfur diox ide, lead , and carbon mon ox ide leve ls in area s s u ~ h as the smog-s hro ud ed Los An ge les bas in has revea led th at the hi gh leve ls of t hese and other co nt aminan ts pose direc t and indirec t threa ts to human hea lth . Th ese findings ha ve made air-po llut ion con tro l a top pri o rit y of the En viro nme nt a l Protecti on Agenc y and a majo r conce rn of enviro nment al en gineers, who are now ca lled upon to devi se man age men t programs designed to alter th e patt ern o f air po lluti on begun ce nturies ago a nd co ntinued until the prese nt tim e. O ther envi ronme nt all y relat ed hea lth pro blems also co nce rn th e en vIro nmenta l engi neer. Th e wid espread ·use of chemica ls in ag riculture an d in dus t ry has introduced many new co mpo und s into th e environment. Some of these co mpo und s ha ve been diffu sed in sma ll quantiti es th rougho ut the enviro nment. whi le others ha ve been conce nt ra ted at disposa l sites. Such che mi ca ls ma y be spread through air. watc r, and so il , as we ll as thro ugh th e food chain. and thu s pose a potential threa t to a ll human s. The pesticide DDT was used extensively d uring the mid-centu ry decades and has been instr umenta I. i.n the. eli.rpioalion .0J malari a· in· many p arts· of·rhe· i.\l ai rd·.·· ·· .. . In addi ti on, thi s· pesticide was used ex tens ive ly to control in sect pests on food and fiber pla nt s. Its benefi cial use to humans was widely acclaimed , and its promoter. Pa ul Mu ller. was awa rded a Nobel prize in 1958 fo·r his contribu ti o n to publi c hea lth. Subsequ ent resea rc h. howeve r. has shown th a t DDT is a cumul a ti ve tox in . tha t has adverse ly ;tfrccted many non target species. Traces of DDT can be found in alm os t all li vin g o rg;lni sms thro ugho ut th e world - including human s. Alth o ug h · th e use of DDT is no\\' b:ln necl in the Un it ed States and seve ral other co untri es. the chemical is st dl being manufa ctured. primari ly for use in seve ral develop in g co untries. par ticular ly in tro pi cal zo nes where its benefit s a re still cons id ered to o ut weigh It s lia bilities. A mo re rece nt example of chem ical tox ins th a t threaten health is the chemica l d ioxi n The format illil of this chemical. the scientific name o f wh ich is 2,3.7,Rtetrach loro-dibenzo paradioxin. is an unint entional by- product of a manufac turlll g process used with so me herhi cides anc! wood-preserving co mpounds. It is also fo rmed in t he prod uct io n of some d isinfectan ts ancl ind ustri a I clean in g co m pou nd s. Di ox in is an ex treme ly toxi c substance. and it s presence in excess of I ppb (part per billi o n) in the en\' il·o nmen t;d ele ment s becomes cause fo r concern . (On e pa rt per bill io n co rrespo nds to o ne drop of wa ter in a sw imming poolme:l slIring IS i"t wiele, .\ 0 ft lo ng. and 11 ft dee p.)
6
INTRODUCTION
C hem icals contai ni ng diox in residua ls have been used on a widespread basis during the las t few decades. and the level of th is chemica l in th e general environment is no t currently known. The discovery o f di o xin res idu a ls in waste-dis posa l s ites a nd in so il<; that were con tam ina ted t h roug h app lication of the paren t materia l has ca used g reat co ncern and ha s resu lt ed in ex pensive c lea n in g effor ts. Th e c rea tion of a "s uperfund" in the Environme n ta l Pro tection Agency. ini ti a ll y fund .e d at seve ra l bil lio n d o llars. is bu t a s ta rt in the effo rt s to mi t iga te th e ha za rd s of c hem ica ls in the enviro nm en t. Oth e r C onc ern s C lean ai r and wa terare an aesthe ti c de li g ht , yet ci ty dw e llers ha ve a ll bu t forgo tt e n the s mell o f clea n a ir. a nd clear. sparkling lakes. rivers. :lI1d st reams are becoming increasing ly rare. Litt ered streets and highw ays ' o tTe nd . rather than deli g ht. and unfe nced junk ya rds and u nco n tro lled d umps give fu rt her evidence o f th e aest het ical ly d ispleasing e ffec t o f improper so lid-waste di s posa l techniques. O u r c ult ura l as we ll a s our aes th etic heritage is a lso being los t to poJlut io n. The Parthen o n in Athens. t he Sta tue of L rberty in N ew Yo rk ha rbor. the s ta tues a nd fr escoes in Ve nice have withstood the o ns laug ht of the cleme nt s for centuries, ye t are in increas ing danger of being dest royed by t he constituen ts o f a p o llu ted atmosphere. And poll ution poses econom ic threats to hu man popu lations Lake E fl e o nce su p po rt ed a thrivi ng fis h ing ind ustry a nd a ll th e a tt endant process ing a nd s hi pping fac il ities assoc iat ed with th a t indu str y. ye t th e eco no mic po tenti a l o f the lak e was nearl y lost befo re se ri o us cle anup effo rts were begun . Th e s ilting in o f r ive rs. harbors. and rese rv oirs du e to unco ntro lled eros io n. often exacerbated by hum a n ac til'ities, threaten s to strengt he n so m e ind ustries'a nlle' expeiise·bfolhe"r"s.· Erll'ironmenta l e ng ineers are com mitt ed to pro tec tin g huma ns fr om th e t hrea ts a po lluted environment pose to human health. aesthetic :lnd cultural e njoy m ent, ;Ind eco no mi c we ll-bei ng.
1-4 IMPROV EM ENT OF ENVIRONM EN T AL Q UA LITY Vitall y co ncern ed with the improve ment o f environmental qua lit y, the el1\'iro nm e nt:." engineer p la ys a n impo rta nt" ro le in environmen tal mana ge m ent progra ms. Such programs might be sa id to inl'olve tllO distinct aspec ts -en vi ronme ntal s trat eg ies and e Jlvir o nm enta l ta c ti cs. [I-IJ LlIlim lllll e/1(o/ sr rl/r cqics are co mp l'ehensil'e pl ans tha t usua ll y address a va ri ety o f prob le ms that co nfront a sing le a rea . T y pica l'elll'iro nm e nl a l strate g ies might be a prugram to IInpn l\e the qualit y of Lak e Erie. to im p rove the ai r quality of the Los Ang e les basin. or to collect a nd p ro pe rly iJispose of tlie so li d W;lste fr o m th e c it y of Philad elphia, Enl'ironmental strategies are lI sua lly worked O UI in public and political ~ l r e n" s, Cl) nsid era ti o ll s mu s t inc lu d e economic. s(lcia l. and demograph ic fa c to rs. II ist<,ric;I1h. elll'ironJllental CIli!lnee rs ha\'e Iwt l'I:lycd ;1 highh visible ru le In
IN TRODUCTIqN
7
devising environmental st ra teg ies. Neve rthe less, the en viro n me nt a l eng inee r sho ul d be an imp o rtan t member of a ma nageme nt team th a t incl ud es perso n s drawn from a wide va riety of disc iplines. The input o f the en viro nment a l eng ineer, especia ll y in assessing the li ke ly respo nse of the en viro nment to vari o us levels o f co nt amin a nt load ing a nd in we ig hin g th e va ri o us tec hnica l soluti o ns that m a y be proposed, is a necessa r y co mpo nent o f a ny en viro nmenia l stra tegy. En viro n me nt a l engineers are usua lly m o re d irectl y associa ted w ith the imple.- . menta ti o n of the ellvironmenra/caclics tha t a re th e m ea ns fo r ach ievin g the g o a ls se t forth in a s pecific portion o f a given en vi ro n ment a l stra tegy. T he engineer 's par t in thi s imp leme nt a ti o n co ns is ts pr ima rily o f th e d esign, co ns tru cti o n , and o pe ra ti o n o f treat m e n t fac ilit ies fo r water, a ir, a nd so lid was te. F o r example, the env iro nme nt a l en g ineer wo ul d be involved d irec tly in the a d d itio n o f te r ti a r y processes to remove p hosp horus fro m t he effl uent o f a was tew a te r-trea tment facility emptying into Lake E t ie. th e insta ll a tion o f a hydroca rbo n rem oval system at a gaso line refine ry ~ys t em in Los Ange les. o r th e d esign o f a so lid -w aste process ing pla nt In Phi lade lp h ia .
1-5 THE ROL E OF THE ENVIRONMENTAL ENGINEER ;\S po lluta nts en ter a ir, wa te r, o r so il, na tural p rocesses such as diluti o n, biolo gica l co n vers io ns, a nd c hemica l reac tio ns convert was te m a teri a l to m o re acceptable fo rms a nd d isperse t he m thro ugh a large r vo lume. Yet th ose na tura l processes ca n no 1'.1 nger perfo rm the c leanup alo ne. T he trea tme nt fac ilities d es ig ned by the e n viro nmenta l engineer are ba sed o n the princip les of se lf-cleansing obser ved in . nattl re.'.~ lIt. t he enKi.n(:e.re.d pr 9<;:.e$$e~ a tnplify a nd op tim ize the o per a ti o ns o bser ved in na ture to hand le large r vo lu mes o f po lluta nts a nd to trea t th em m o re ra pidly. Engineers adapt the prin c ip les o f na tural mecha nis ms to eng in ee red sys tems fo r po ll u ti o n co nt ro l w hen th ey constru ct ta ll stack s to di sperse a nd di lute air po llutan ts, des ign biologica l treatmen t fac il ities for th e remova l of o rgani cs from was tewa te r. use che mica ls to ox id ize a nd precipita te ou t the iro n a nd manga nese in drinki ng-wa te r s u pplies, or bur y so lid wastes in co ntr o lled la ndfill o pera tions. Occasio na ll y, th e e n viro n men ta l enginee r m ust d esig n to reve rse o r co untera ct na tura l processes. For examp le, the containers used fo r d isposa l o f haza rdo us was tes such as to xic chemica ls and radioac ti ve m ate ri a ls mus t iso la te th ose m a terials from the en vi ro n men t in o rder to p reven t th e onse t of the n a tura l, b ut hi g hly u ndesirab le. processes of d ilut io n a nd d ispers io n. will be de m o nstr a ted thr o ugho ut this' tex t, a n und e rs ta nd ing o f na tural an q engineered. purifica ti o n processes. requ ires an understand ing of th e bio logical and chemica l react io ns invo lved in these processes. T hus, in addi tion to bem g know ledgeab le in the ma th em a tica l, physica l, a nd en gi nee rin g sCiences, . the envi ro nmenta l e nginee r mus t a lso be we ll grounded in th e subject areas o f c hemist r y a nd microbio logy. s ubject a reas no t usua ll y emp hasized in eng inee rl1l g c urncul a . Indeed . an und erstanding of bio logica l and c hem ica l pri nciples is as esse ntt a l to
As
8
.-...
.-.\
INTRODUCTION
the environmental engineer as the understanding of statics and strength of materials is to the structural engineer. The environmental engineer's unique role is to build a bridge between biology and technology by applying all the techniques made available by modern engineering technology to the job of cleaning up the debris left in the wake of an indiscriminate use of that technoiogy. The delicare balance of our biosphere has been disturbed. and the state in which we now find ourselves is a direct consequence of our having ignored the limits of the earth's ability to overcome heavy pollution loads, and of our having been ignorant of the. constraints imposed by the limits of the self-cleansing mechanisms of our biosphere. A keen awareness of these natural constraints plays an important role in the work of environmental engineers. For example, the laws of conservation of mass and energy prevent the destruction of pollutants. and the engineer is bound by these limits. The principles of waste treatment must therefore be to convert the objectionable material to other, less objectionable forms: to disperse the pollutants so that their concentrations are minimal: or to concentrate them for isolation from the environment. In all instances, the end products of the treatment of polluted water or air or of the disposal of solid wastes must be compatible with the existing ~ nviron ment.al resources and must not overtax the assimilative powers of hydrosphere. atmosphere, or lithosphere. In structural engineering. the engineer can simply specify a larger or stronger beam to carry a heavier load. The environmental engineer, on the other hand, must accept the carrying capacity of a stream, an airshed, or a landmass because these can seldom be changed. It is the purpose of this text to demonstrate how the environmental engineer, working within these constraints, uses all available technological tools to design efficient control and treatment devices that are molleled after the natural processes
...... '(hili hit'ye' so 'fbrig' p'r'eservec! olii ·b{cisphe~e·. 'Fo;'o~iy' 'by' b'r'i~gi~it'~~il~~l~gy';~t~ ' .'. harmony with the natural environment can the engineer hope to achieve t he goals of the profession-the; protection of the environment from the potentially deleterious effects of human activity, the protection of human populations from the elTects of adverse environmental factors. and the improvement of environmental quality for human health and well-being.
REFERENCES 1·1 Bella , D. A., and W. S. OvenoJl' "Environmental Planning and Ecological Possibilities," pre· sented at the annual national environmental engineering meetlf1g of ASCE, St. Louis. M o .• October 18- 22. 1971. 1-2 "Guidelines for Environmental Engineering Visitors on ECPD Accreditation Tean'!s," Engineers Council for Proli::ssional.oevelopment. United Enginee ring Cellt er, 34-5 East 47111 St.'-New York . October 1977.
PART
ONE WATER
CHAPTER
TWO WATER QUALITY: DEFINITIONS, CHARACTERISTICS, AND PERSPECTIVES
The availability" of a water supply adequate in terms of both quantity and quality is essential to human existence. Early people recognized the importance of water from a q uanti ty viewpoint. Civ il ization developed around water bodies that could support agriculture and transportation as well as provide drinking water. Recognition of the importance of water qua lity developed more slowly. Early humans cou ld judge water qua lity only through the physical senses of sight, taste, and smell. Not until the biological, chemical, and medical sciences developed were methods available to measure water quality and to determine its effects on human health and well-being. . ~ It was not until the mid-nineteentrrcentUTy-rhatthe relationship between'hmmrn ..... . .... ~ . waste, drinking water. and disease was documented. Several more years intervened before the facts concerning this relationship became widely accepted and remedial action was taken. In .1854,* Dr. John Snow, a public-health worker in 'London, noted a high correlation between cholera cases and consumption of water from a well on Broad Street. Noi only was cholera running rampant.in the nelgh~orhood around the well, but outbreaks of the disease in other parts of the city could be traced to indiv idua ls who had had occasion to drink from the Broad Street well.:: Although the proof was conclusive by modern epidemiology standards, the evidence was not accepted by Snow's contemporaries. It is alleged that he physically removed the pump handle to prevent LIse of the contaminated water. thus abating the epidemic. [2'-2IJ Advances in the germ theory of disease were made by Pasteur and others in the lafe nineteenth century, and by 190.0 the concept of waterborne disease was well accepted. The development of the science of water chemistry roughly paralleled that of ivater microhiology Many of the chemic~1s used in industrial processes
*-
Th i~.
da le i... li sted as 18"",9 ill
$u m~ publi c ati o ll ~
II
WATE R QUA LI TY : IJEF IN ITIONS . C HARA CTER ISTICS, AND PERSP ECT IVES
11
13
WATER
Atm os ph e re
and agriculture ha ve been identified in wat e r. H owever, th e effor t to id entify .(her chemical' compounds which may already be fo und in trace qu a ntities in many wa ter supplies and (6 delermirietheir effect b ti huma n hea lth was onl y ~ cently begun . It is likely tha t new analytica l techniques will b e de ve lo p ed th a t ~ill identify compounds not yet known to exist in water, and it is co ncei va ble th a t -lese materials will also be linked to human health . Thus, the science o f wa ter ' quality will rem a in a ch a llenge for engineers a nd scientists fo r yea rs ·to co me. Lik e all sciences, the science o f wa ter qu a lit y h as develo p ed it s ow n t ermin o logy and the means of quantifying these terms. The purpose of this cha pt er is to intro d.uce the read er to the modern co n cepts o f water qu a lity. The mea ns b y w hi c h th e .• ature and extent of contaminants in wate r a re me as ured a nd ex pressed a re' Qresented alon g with th e so urces of vari o us co nt a min a nt s that find th e ir way int o .: aler. An understanding of the m a terial in thi s cha pter will b e essenti a l in su b~equent chapters dealing with water-quality cha nges in b o th n a tur a l and eng inee red
Condensation
Earth 's surface
.}s tems.
_-1 THE HYDROLOGIC CYCLE AND WATER QUALITY -Water is o ne o f the m ost a bund a nt co mp o und s fo un d in na ture, cove rin g a pprox i,n a tely three-fourths o f the surface o f the ea rth . In spite o f thi s a pparent ab un d an ce, ...s;evera l fac tors serve t o limit th e a m o unt of wa ter a va ila ble fo r human use. As ~ J1own in T a ble 2-1 , over 97 pe rcent o f the to ta l wa te r suppl y is co nt a ined in th e ..Qcea ns and other saline b o di es of wa ter a nd is n o t readil y usa ble fo r m os t purp oses. )f the rem a ining 3 p e. cent, a little o ver 2 percent is ti ed up in ice ca ps a nd g lac ie rs ~nd , alon g with atrp o spheric and soil moisture, is inaccess ible. [ 2- 17J Thu s. fo r :, e ir ge neral li velih ood a nd the suppo rt o f th e ir va ried technical a nd ag ri c ultur a l activities, hum an s must depend up o n the rema ining 0.62 percent fo und in fr es h-"ater lak es, ri ve rs, and g ro undwa ter supplies.
-:-T able 2-1 World wate r distribution VoJume. 10 ' 2 m '
Locati o n La nd areas Fresh wa ter ta kes Sa line lakes a nd i nland seas Ri vers (ave rage insta nt aneo us vo lume) So il moist ure G roundwater (above dept h o r 4000 m ) Ice ca ps a nd glac iers Total land a rea (rounded) ~!\ tm os ph ere
(wa ter VO pM)
Ocea ns -To ta l all iL'ca tlon s trc'unded)
So urce : Ad a rt ed fr,)]l1 T lxld . 12- t 7J
125 '104
125 67 8,350 29,200 37,800
13 1. 320,000 1.360.000
0.009 0.008 0.000 1 0.005 0.6 t 2. 14 2.8 0.00 1 97.3
100
--
Aquifer<
;;=l
; u
o
~ 100/, 011
Fi gll rl' 2- 1 H ;.dro log1 c cycle
Wate r is ina COllst an t sta te of mot io n. as depic ted in t he h yd ro logic cyc le s h ow n in Fig. 2- 1. Atmosph e ric \Ia ter condenses and fall s to th e ea rt h a s ra in. snow. o r so me othe r form of rrec ipita tio n. On ce o n t he ear th 's surface. wate r flOW S into strea ms, lakes. and eV'e ntu a lI y th e oceans. or rer co lates th ro u gh t he so il and into aLJu ifer s t ha t eventua ll y disc harge int o s u rface wa te rs. T h ro ug h evaporat ion fr o m surfa ce waters or by evapotran sriration from p lants. wate r mo lecu les return to the a t mosrh e re tll repea l the cyc le . Although th e move me nt t hr o ug h so me p a rts of the cyc le may be re la ti ve ly rar id . comr lete recyc ling o f g rou ndwa ter mu st o fte n be measu red in gco logic ti me. Wat e r in n ature is most nedriy pure in its ev aporation stat e. Beca use the ve r y act of co nd e nsa tion usua ll y requires a surface. o r nucle i. wate r ma y aCLJuir~ imp uri ti es ~I t the VC I'Y mo ment of co nde nsa li o n. Ad dit io n a l im pu rit ies are'added as th e liqu id w; lt e r t ravels t hrou g h Ihe rema ind e r of t he hvdrolof!.ic cvcle and comes int o co n tac t w it h m ate ri a ls 'in the ~ti r a nd on o r henea til th e s~lrra~e of the ea rr 'h H uman ac ti vi til:s contr ib ute furthcr Impurittes In the fo rm of indu st rial a nd d omes ti c w; lstes. ag l'icu ltu ra l chemica ls. a nd
WA TER QUA LIT Y: DEFINITIONS, CHARACTERISTI CS, AND PERSPECTIVES 15
14 WATER
2-2 SUSPENDED SOLIDS Collo ida l
Di sso lved
Suspend ed or non fiil e r"ble
Size of particl e. 11m 10 - 2
10 - 1
10
I
I
!
100
I 10 - 1
iO - 8
As noted ea rli er, so lid s can be dispersed in water in both suspended and dissolved .. forms. Althou gh some dissolved so lids may be perceived by the physical senses, they fall more appropriately under the ca tegory of chemica l parameters and will be di scussed more fully in a later section.
Size of p"rtici e, mm
Sources Figure 2-2 Size cl.ssification of solids in
wa ler. (Fr oln
;V/e !calj & Eddy , In c. [l-R].)
in th e int ermed iate s tage which is of grea test co ncern becau se it is tIle qu a lit y at this stage th at will affec t human use of the water. The impurities acc umulated by wa ter throughout the hydro log ic cycle and as a result of human ac ti vities may be in both suspended and disso lved fe rm. Suspend ed mat erial co nsists of particles larger than molecular size th at are suppor ted by bu oya nl and visco us fo rces within the water. Disso lved material consists of mo lecu les o r io ns (see Sec. 2-7) that are he ld by the molecular stru cture of wa ter. Co ll oid s are vei- y sma ll particles that tec hnica ll y are suspend ed but oft en ex hibit man y of the cha rac ter isti cs of disso lved substa nces. Size ran ges of di sso lved, co llo idaL a nd suspended substances are shown in Fig. 2-2. Wal eI' pulluriol1 ma y be defined as the presence in wa ter of impurities in such quantity and of such nature as to impair the use of the wa ter for a stat ed purpose. Thu s the definition of water qua lit y is predicted on the int ended use of the water , and a gross det erminati o n o r the qu antit y of suspended and d isso lved impuriti es. whi le useful III some cases, is no t sufficient to co mpletely define water qua lit v. Man v pa rameters have evo lved that qualitativelv reflect the impact tha t v~ri6usi;n pti i-i ii'es .have onseIectec! ',y',iter .Ll ses.- An ~I I Y tic·,i i -p~oc~(i ~;~~~' h~ ;~" been deve loped th at quantitatively measure these para meters. Standard ;'v1 elhods /01' Ih e Examinaliol1 of WaleI' and Wa sl ew(J( er. [2- I SJ has been the auth orita ti ve stand ard for test procedures fo r many yea rs. For detailed cove rage of the subject . the interested reader is referred to thi s' publica tion and to an Enviro nmental . Protectio n Age ncy publica ti on that offers sim ila r informatio n. [2-9J A kn owledge of the pa rameters most commo nl y associated wi th wa ter- and wast ewa ter-treatment processes is esse nti a l to th e en vironmental engineer. The remai nd er of this chapter will be devoted to a disc ll ss ion of param eters used to assess th e ph ys ical. chemica l. and bio log ica l charac teristics of wa ter. T estin g procedures described for each parameter are based on those desc ribed in Stal/dard ,V /di wris [2- 15J
Physical Water-Quality· Parameters Ph ys ica I para meterS define t hDse cha racterist ics of \\'ater t ha t respond to t he senses of sig ht. t(luc ll. ta ste. or sme ll. Suspend ed so lids. lurbiclity. co lol-. taste an d odor. ;Ind temperatu re fall int o thi s ca tego ry.
So lid s suspended in water may consist of inorganic o r organic particles or of im misc ible liquids. In organic so lids such as clay, silt, and o ther soil constituents are co mm on in surface water. Organic material such as plant fibers and biological so lid s (a lga l ce lls, bac teria, etc.) are also common constituents of surface waters. These materials are often nat ural contaminants resulting from the erosive action of wa ter fl ow in g over surfaces. Because offne filtering capacity of the soil, suspended material is seld o m a co nstituent of groundwater. Other suspe nd ed material ma y result from hum an lise of the water. Domes tic wastewa ter usua lly contai ns large quantities of suspended so lids th at are mostly organic in nature. Industrial use of water may result in a wide variety of suspended impurities of either orga nic or inorganic nature. Imm'iscible liquids such as oils and greases are often constituents of wastewater.
Impa cts Suspended materi al may be objectionab le in water fo r severa l reasons. It · is aesthet ica ll y di spleasing and provides adsorpt ion sites for chemical and biological agent s. Suspended organic so li ds ma y be degraded biologica lly, resulting in . " .. .... ". ······obje·ci·i() nable by-products. Bio logically active (live) sllspended solids may include di sease-ca usin g orga ni sms as well as orga ni sms such as tox,in-producing strain s of a lgae. iVI easurement
Th ere are seve ral tests avai lable for measuring so lid s. Most are gravimetric 'tests in vo lving the ma ss of resid ues. The total solids t esc quantifies all the solids in the water. suspend ed and disso lved, orga nic and inorgan ic. Thi s parameter is measured by eva porating a samp le to dryness and weighing the residue. The 'total quantity of residue is exp ressed as milligrams per liter (mgj L) o n a dry-mass-of-solid s basis. A drying temperature sligh tl y above boiling (I04 c C) is sufficient to drive off t he liquid and the water ad so rbed to the sllrface of the particles, wh ile a temperature - of about 180°C is necessa ry to evaporate the occl uded wa ter. . Mos t suspended so lids can be remo ved from water by filtration . Thus, the suspended fraction of the so lid s in a water samp le can be app rox im ated by filt erill [! th e wa ler, drying the residue and filler to a constant wei ght at 104°C (± l C). ~ llId Lieterminillg the ma ss of th e residue retained on the filter. The result s of this O/l SIJ,,"i/ei/ soliris II:'SC are also expressed as dry mass per vo lume (mil ligramsper
16
WATER
WATER QUALITY: DEFINITIONS, CHARACTERISTICS, AND PERSPECTIVES
liter). The amount of dissolved solids passing through the filters, also expressed as milligrams per liter, is the difference between the total-solids and suspendedsolids content of a water sample. It should be emphasized that filtration of a water sample does not exactly divide the solids into suspended and dissolved fractions according to the definitions p~esented ~arlier. Some colloids may pass through the filter and be measured along With the dIssolved fraction while some of the dissolved solids adsorb to the filter material. The extent to which this occurs depends on the size and nature of the solids and on the pore size and surface characteristics of the filter material. For this reason, the termsfilterable residiles and Iloll{ilrerable residues are often used. Filterable residues pass through the filter along with the water and relate more · closely to dissolved solids, while nonfilterable residues are retained on the filter and relate more closely to suspended solids. "Filterable residues ,. and" nonfilterable residues" are terms more frequently used in laboratory analysis while the" dissolved solids" and "suspended solids" are terms more frequently used in water-quality-management practice. For most practical applications, the distinction between the two is not necessary. Once samples have been dried and measured, the organic content of bot h total and suspended solids can be determined by firing the residues at 600°C for J h. The organic fraction of the residues will be converted to carbon dioxide, water vapor, and other gases and will escape. The remaining material will represent the JJ1orgamc, or fixed. residue. When organic suspended solids are being measured , a filter made of glass fiber or some other material that will not decompose at the elevated temperature must be used. The following example illustrates the calculations involved in suspended solids analysis.
17
Use Suspended solids, where such material is likely to be organic and/or biological in nature, are an important parameter of wastewater. The suspended-solids parameter is used to measure the quality of the wastewater influent, to monitor several treatment processes, and to measure the quality of the effluent. EPA has set a maximum suspended-solids standard of 30 mg/L for most treated wastewater discharges.
2-3 TURBIDITY
A direct measurement of suspended solids is not usually performed on samples from natural bodies of water or on potable (drinkable) water supplies. The nature of the solids in these waters and the secondary effects they produce are more important than the actual quantity. For such waters a test for turbidity is commonly used. Turbidity is a measure of the extent to which light is either absorbed or scattered by suspended material in water. Because absorption and scattering are influenced by both size and surface ·characteristics of the suspended material, turbidity is not a direct quantitative measurement of suspended solids. For example, one small pebble in a glass of water would produce virtually no turbidity. If this pebble were crushed into thousands of particles of colloidal size, a measurable turbidity would result, even thoughthe mass of solids had not changed.
Sources .. ~ .... " ..... ~?,.a~p!~ .2: I .:. pt;t.er!l)!I,l!ng. ~~~. ~.Qn~.enlmtiQ/1 . .o( sllSpended . solids :. A filter.able resid ue
analysis is run on a sample or water as rollows. Prior to filtering, the crucible and filter pad are kept overnight in Ihe drying oven. cooled. and the dry mass (tare mass) or the pair determ1l1ed to be 54.352 g. Two hundred and fifty milliliters or the sample is drawII through a hlter pad contained in the porous-bottom crucible. The crucible and filter pad are then placed 111 a drY1l1g oven at 104°(, and dried untit a conSlant mass of 54.389 g is reached. Determ1l1e the suspended solids concentration of Ihe sample. SOLUTION
Most turbioity in surface waters results from the erosion of colloidal material such as clay, silt, rock fragments. and metal oxides from the soil. Vegetable fibers and microorganisms may also contribute to turbidity. Household and industrial wastewaters ma y contain a wide variet y of turbidity-producing material. Soaps, cietergents. and emulsifying agents produce stable colloids that result in turbidity. Although turbidity llle
I. Determine the mass of solids removed.
Tare mass + solids = 5-1.3S9 g - Tare mass = 54.352 g Mass of solids
= .=
0.037 g 37 mg
2. Determine the concentration of the solids. mg solids x 1000 mL!L = conc in mg ·L I11L of sample .
-~--.---- .-.-
37 x 1000 250
...- - - - . - =
148
l11 o :L e
Impacts When turbid water in a small. transparent container, such as a drinking glass. is helel up to the light. an aesthetically·displeasing opaqueness or "milky" coloration is J.pp:lrent. The col1oiLialm
18
WATER QUALITY: DEFINITIONS, CHARACTERISTICS, AND PERSPECTIVES 19
WATER
li g ht pe netra ti o n and photosy nthetic reacti o ns in strea ms a nd la kes. Accumulation o f turbidity-cau sing particles in po ro us streambeds res ult s in sediment depos its th at ca n ad ve rsel y affect the fl o ra and faun a of the s tream.
Measurement Turbidit y is measured pho to metrica ll y by determining th e percentage of light of a given intens it y that is either absorbed o r scattered . The o rig inal meas urin g a ppara tus, ca lled a J ackson ltIrbidim eler. was based on light abso rpti on and employed a long tube and standardi zed candle. The candle was placed beneath the g lass tube that was th en housed in a b lack metal s hea th so that th e light fr o m the candle cou ld only be see n from above the apparatus. The wa ter sa mple was th en ·p ou red s low ly into the tube until the lig ht ed candl e was no longe r vis ible, i.e., complete absorp ti on had occurred. Theg la ss tube was calibrated with readings fo r turbidity produced by s uspen s ions o f silica dioxid e (Si O l ), with o ne Jackson turbidity unit (JTU) being eq ual to th e turbidit y produ ced by I mg Si0 2 in I L o r dis tilled water. In recent years this awkward ap pa ratus has been re placed by a turbidit y meter in which a standardized electric bulb produces a light that is th en direc ted through a sma ll sample via l. In the absorption m ode, a photometer meas ures th e li gh t intens it y o n the s ide of the vial opposite from the lig ht so urce, w hile in the sca tterin g mode, a photometer measures th e li g ht inten s it y a t a 90 c an g le fro m th e lig ht source. Alth o ugh most turbidity me te rs in use today wo rk o n th e sca tt e rin g principl e, turbidit y cau sed by dark s ubsta nces th ilt absorb ra th e r than rerlect li gh t s ho uld be meas ured by the absorption technique, h)rmazin. a c hemical compound , provides m o re re produc ible' sta nd ards th a n S iO 2 a nd has rep laced it as a refe rence. Turbidity meter readin gs a re now expressed as /ormm ill lIIrhidit y lIliits. or FTU s. Th e te rm nephelometry IUrbiditr Llllits (NTU) is o ft e n used to indicate th a t the test was ru 'n accordi ng to Ih e sc alte rin g pr inci ple ,
Usc Turbidit y meas ur ement s are norma ll y mad e' o n "c lea n " waters as op posed to wastewaters. Na tur a l wa ters ma y ha ve tUI'bid ili es ran ging fro m a few FT Us to seve ral hundred . EPA drinking-waleI' s tand a rd s specify " m~p;i!llulll o f I FTU . while th e American W a ter W o rk s Assoc iati o n has SCi 0.1 FT L! as it s goal for drink ing water. [ 2- 1]
2-4 COLOR Pure wa te r is co lo rl ess. but water in nature is often eoki red by foreig n s ubsta nces. Water w hose co lo r is part ly du e to s uspe nd ed maltcr is sa id 10 have lI{J{'(/rcnt color Co lo r co ntributed by disso lved so lid s th ai rema lll aha 1"C lllll\';li o f suspended mailer is kn ow n as lrue color.
Sources After contact with orga ni c debris such as leaves, conifer needles, we~ds, or wood, water pick s up tannins. humic acid, and humatesand takes on yellowish-brown hues. Iron oxides cause reddish water, and manganese oxides cause brown or blackish water. Industrial wastes from textile and dyeing operations, pulp and paper production, food processing, chemical production, and mining, refining, and slaughterho use operations may add su bstantial coloration to water in receiving streams.
Impacts Colo red water is not aes thetica lly acceptable to the general public. In fact, given a c hoice consumers tend to choose clear. nonco lo red water of otherwise poorer qu a lit; -o~et trea ted potable water supp'lies with an objectionable color. Highly co lored water is unsuitable for laundering, dyeing, papermaking, beverage manufacturi ng, dairy prodlJcti o n and other food processing, and textile and plas li e pro duction. Thus, the color of water affects its marketability fo r both domesti c and indu strial use. While true co lor is not usually considered unsanitary or unsafe, the organic co mpounds cau sing true color may exert a chlorine demand and thereby seriously red uce the e ffe c tiveness of chlorine as a disinfectant. Perhaps more important are the produc ts fo rmed by the combination of chlorine with so me color-producing o rganics. Pheno lic compounds, common constituents of vegetative decay products, . prod uce ve ry o bjecliql)
. Measurement Although severa l method s of COIOf measurement are available, methods involvin~ co mparison with standardi zed co lored materials are most o ften used. Color: comparison lubes containing a se ries of sta ndards may be used for direct compariso n or water samples that have been filtered to remove appa rent color. Results are expressed in true color units (TCUs) where one unit is equivalent to the color produced by I mgj L of platinum in the form of chlorplatinate ions. For colors o ther Ihan ye ll ow is h-brow n hues. especially for colored waters originating from indu strial was te effi uents, special spec trophotom~ trjc tech niques are usually . emp loyed. In fieldwork . instruments employing colorcd glass disks that are calibrated to t he color slandards are o ften used . Because biological and physical changes uccllrri ng during storage may affec t color. samp les s hould be tested within 72 h of co ll ec t ion.
20
-,
WATER
\VATER QUALITY : DEFINITIONS, CHARACTERISTI CS, AND PERSPECTIVES
21
Use '
Measurement
Color is not a parameter usually included in wastewater analysis. In potable water analysis; the common practice is to measure only the true color produced by organic acid resulting from decaying vegetation in the water. The resulting value can be taken as an indirect measurement of humic substances in the water.
Direct measurement of materials that produce tastes and odors can be made if the cau sative agents are known. Several types of analysis are available for measuring taste-producing inorganics. Measurement of taste- and odor-causing organics can be made using gas or liquid chromatography. Because chromatographic analysis is time-consuming and requires expensive equipment, it is not routinely perfo rmed on wat er samples. but sho uld be done if problem organics are suspected. H o we ver. becau se o f the synergism noted earlier, quantifying the sources does not necessarily quantify the nature o r Intensity of taste and odor. Quantitative tests that employ the human senses of taste and smell can be used for this purpose. An example is the test for the threshold odor number (TON). Varying amounts of odorous water are poured into containers and diluted with enough odor-free distilled water to make a 200-mL mixture. An assembled panel of fi ve to ten " noses" is used to determine the mixture in which the odor is just barely detectable to t he sense of smell. The TON of that sample is then calculated, using the formula
2-5 TASTE AND ODOR The terms taste and odor are themselves ,definitive of this parameter. Because the sensations of taste and smell are closely related and often confused, a wide variety of tastes and odors may be attributed to water by consumers. Substances that produce an odor in water will almost invariably imparLa..taste as well. The converse is not true, as there are many mineral substances that produce taste but no odor.
TON
Sources
A + B A
(2-1 )
Many substances with which water comes into contact in nature or during human wh ere A is the volume of odorou s water (mL) and B is the volume of odor-free use may impart perceptible taste and odor. These include minerals, metals, and water required to produce a 200-m L mixture. Threshold odor numbers correspondsalts from the soil, end products from biological reactions, and constituents of in g to variolls sample volumes are shown in Table 2-2. A similar test can be used to wastewater. Inorganic substances are more likely to produce tastes unaccompanied quantify taste, or the panel can simply rate the water qualitatively on an "acceptby odor. Alkaline material imparts a bitter taste to water. while metallic salts abilit y " scale . may give a salty or bitter taste. Organic material, on the other hand, is likely to produce both taste.and.odor,.· .... · · .. .. ..... ..... . Table 2-2 Threshold odor A multitude of organic ch-emicals may cause taste and odor problems in water, numbers corresponding 10 with .petroleum-based products being prime offenders. Biological decomposition sample "olume dilul~d to of organics may also result in taste- and odor-producing liquids and gases in water. .200 mL Principal among these are the reduced products of sulfLlr that impart a " ro tten egg" taste and odor. Also. certain· species ~f algae secr~te 'an oily substance that Sa mpfe vo lume (A), may result in both taste and odor. The comb,ination.of two or more substances, mL TON neither of which would produce taste or odor by itself, may sometimes result in 200 1.0 taste and odor problems. This synergistic effect was noted earlier in t he case of 175 I I organics and chlorine.
Impacts Consumers find taste and odor aesthetically displeasing for obvious reasons, Because water is tliought of as tasteless and odorless, the consumer associates taste and odor with cont"amination and may prefer to' use a tasteless. odorless water that might actually pose more of a health threat. And odors produced by organic substances may pose more than a problem of simple aesthetics. since some of those substances may be carcinogenic.
150 125 100
U 1.6 2. 0
75
2.7
67
3.0
50 ,, 0
4 .D
25 10
8.0 20.0
2
5.0
100
200
---+--- - ---_..
22 WATER
Use Alth o ugh odo rs can be a problem with wastewater. the taste and odor parameter is on ly associated with potable water. EPA does not have a maximum standard for TON . A m ax imum TON of 3 has been recommend ed by the Public H ea lth Service a nd se rvesas a gu id e lin e rather th an a lega l standa rd. [2- lgJ
2-6 TEMPERATURE Te mpe rature is not used to e va lu a te direc tl y either potable wa te r or was tewa te r. It is_ however, o ne of the mos t importa nt p arame te rs in na tura l surface-wa ter systems. The temperature o f surface wate rs governs to a large extent the biological s pecies present a nd their ra tes of activ it y. T empe rature has a n effec t on most chemical reactions th a t occur in natural wa ter systems. Temperature a lso has a pronounced effect on the so lubilities of gases in wa ter.
Sources The temperature o f natural wa ter sys tems responds to many fac to rs. th e ambie nt temperature (temperature o f the surro unding atmosphere) being th e m os t un ive rsa l. Generally, shallow bodies of wa ter are more affected by ambie nt temp eratures than a re deeper bodies. The use of water fo r di ssipa ti on of was te heat in indu stry and the subsequ e nt discharge of th e heated water may result in dramatic. th oug h perhaps loca lized. temperature change~ m receiving streams. Removal of forest ca no pi es and irri gat io n return flows can also res ult in increased stream - ..... .... . . . . . .. . .. . ..... .. ... te mp'enitllte: ......... .
I rnpacts Coo ler wate rs lisu,tll y have a wide r d i\'ersity ofh io lllglc; iI s pecies. At lower temperatures. th e rate of biological ac ti vity. i.e .. utiliza t io n of fo od s upp li es. growth. reproduction. etc.. is s lower. If the temperature is inneased. biolog ica l act ivit y increases. An increase of loce is usuallv s ufficient to doub le the biological acti v it y, ifesse ntial nutrients are present. At elcv:lted tempcratllres anti increa sed metabolic rates. urga nisms that are more eflicicnt at rood IItilizatl n n and reproduction rlouflsh. while llthe r species decli'ne and ,Ire pC,rhaps e limin ,lteci a lt ogether. Accelera ted growth of a lgae often occurs in \\arm \\a;tcr ;llld can becoille a problem when ce l" cluster into algae Illat s. Natural secretl(lIl o f pils by ,i1 gJe 111 the mat s and the decay pr oducts o f dcad a lgae ce ll s Cdll n:sllit ill la sle and odor pl'oblem s, Higher-order species: s uch as fi sh. arc ,dkclCd dram;llica ll y by Icmper,i ture a nd by dissolved oxygen le ve ls. which a rc ;( , rll l~ ctl(ln o r tcmperatmc. Game fi s h ge nera ll y requ ire co o ler temperatures ;Incl 11I1,!hcr dl sso l\e Ll -\lxygell le vels. T e mpera ture c hanges a lTec t the rc,lctioll rates ami so lubilit y le ve ls of chemicals. a s ubj eci more fu ll y exp lored ill later s e c tlnn ~ (l r I hi ~, L'ilaptel·. Mus t c hemical
WATER QUA LITY : DEFINITIONS, C HARA CTER ISTICS, AND PERSPECTIVES 23
reacti o ns in vo lving dissolution of so lids are accelerated by increased temperatures. The so lubilit y of gases. o n the other hand , decreases at elevated temperatures. Because bio log ica l ox idatio n o f o rganics in streams and impoundments ' is de~ " pendent o n an adequate supply of dissolved oxygen, decrease in oxygen solubility is und esi rable. T he relationship between temperature a nd dissolved oxygen levels is s hown in Table C-3 of the appendix. Temperature also affect s other physical properties of water. The viscosity of water increases with decreasing temperature. The maximum density of water occ urs at 4°C, and den sity decreases on either side of that temperature, a unique phenomenon among liquids. Both temperature and density have a subtle effect on plankto nic microorga nisms in natural water systems. The relationship of temperature a nd density to st ratification of impoundments is discussed in Chap. 3.
Chemical Water-Quality Parameters Water has been called the universal so lvent , and chemical parameters are related to the so lvent capabilities of water. Total dissolved solids, alkalinity, hardness, fluorides, metals, organics. and nutrients are chemical p a rameters of concern in water-quality management. The following review of some basic chemistry related to solutions s ho uld be helpful in understanding subsequent discussions of chemical par ameters.
2-7 CHEMISTRY OF SOLUTIONS An atom is the sm a llest un it of each of the elements. Atoms are building blocks from w hi ch molecules of elements and compounds are constructed. For instance, tw o hydr oge n atoms combine to form a molecule of hydrogen gas:
H+ H
-->
H2
Adding one a tom of oxyge n to the hydroge n molecule results in one molecule of the compound wa ter: 1-1 2
+
0
-->
H 20
A relative mass has been assigned to a single atom of eac h element based on a mass of 12 for carbo n. The s um o f the atomic m ass of all th e atoms in a molecule IS the lIlu /cclila/' mass of that m o lec ule. The atomic mass of hydrogen is I arid the . atomic mass of oxyge n is 16. Thus, the molecula r mass o f the hydrogen molecule is 2 a nd the mo lec u lar mass o f water is 18. A mole of an e lement or compound is its molecular mass expressed in common mass units. usually grams. A mole of· hydrogen is 2 g, while a mole of wa ter is 1g g. One mole o f a substance dissolved in suffi cien t wa ter to m a ke o ne liter of soluti o n is called a one molar solution . Bonding of element s int o compounds is so metimes acco mplished by electrical forces resultin g from tran sferred electrons. When these compounds dissociate
'-
24
WATE R Q UA LI TY: DEFINITIONS. CH ARA("TERI STI CS , A N D PERSPECT I VES
WATER
In water, th ey prod uce species with oppos ite c harges. An exa mple is sodium chloride :
NaCI The charged species a re ca lJed ions. Positively charged io ns a re ca lled cal ions, and negatively charged ions a re called anions. The number o f p osi tive charges must equal the number of negative c harges to prese rve electrica l neutra lit y in a chemical compound. The number o f charges o n an io n is referred to as the va lence of th a t ion. Thus, the va lence o f sodium (Na +) is 1, w hile the va lence of calcium (Ca 2 +) is 2. So me compounds, ca lled radicals , als o p ossesscha rges. An exa mple o f a cationic radica l is a mm o nium (NH 4 +), wh ile carbo na te (CO) 2 - ) is a n a nioni c radic a l. Wh en io ns or radic a ls react w ith eac h o th er to fo rm new compo und s, t he reactions m ay not a lways proceed o n a o ne-t o -one bas is as was th e case fo r sodiu m ch loride . The y do, however, proceed o n a n equi va lence basi s that can be 'related to electroneutrality. Technically, th e equivalence o f an element o r radi ca l is defi ned as th e number o f h ydrogen atoms th at e lem ent or radic a l ca n hold in combinati o n or can replace in a reacti o n. In most cases, th e equivalen ce o f a n io n is the sa me as the abso lute va lue o f it s va lence. An eqllivalenl of a n e le ment o r rad ica l is it s g ram molecula r mass di vided by it s equiva lence. A m illieq ui valent is th e mo lec ular mass expressed in m illigrams d ivid ed b y the equ iva le nce a nd is o ften m o re useful in water c hemistry beca u se co nce ntr a ti o ns o f disso lved su bsta nces are mo re o ften in the milligrams per lit er range. Comp o und s are fo rmed by th e co mbin a ti on of e lemen ts o r ra dic a ls o n a o ne-t o-o ne eq ui va lent basis. The ca lcul a ti o n o f equi valents is illustrated in Exa mple 2-2. Example 2-2 : Calculating equivalents H ow many grams o f calci um will be required to . . . . . .... ... c o mbi'lie' w,ih'90'g'of carb
l. Ca rbo na te (CO / - ) is a radica l composed o f carbon and oxygen. In thi s particula r com bin a li o n. carbon has an a lomic m ass of 12 and a va lence of + 4. wh il e oxygen ha s a n atomic m ass of 16 a nd a va lence o f - 2. Therefo re. th e radica l has a to tal valen ce of - 2 a nd an equivalence o f 2. One eq uiva len l o f carbo na-I e is
Eq ui va lents a re very impo rt a nt in wa ter c hemistr y. In additi on to being useful in calc ul a tin g c he mi ca l qu a ntities for des ired reac ti o ns in wate r and was tewater treatment, equ iva lents a lso prov id e a means o f express in g var iou s co ns titu ents o f di sso lved so li d s in a co mm o n term. An eq ui va lent o f o ne substance is c hemi ca ll y equa l to a n equi va lent of a ny o th e r substance. Therefore, th e co nce ntra ti o n o f sub sta nce A ca n be expressed a s a n equiva lent co nce nt ra ti o n o f s ubstra te B by th e fo llow ing meth od . (g, L)A - - - -- x (g/eq ui v) B = (gj L)A (gjeq ui v)A
SOLUT ION
(iI ) I , One equiva le nt o r ca lCi um ca rbona te is 40 1- 12 + 3( I 0)
-- '--2- '
" = 50 g/eq uiv = 50.000 mg/ equl v = 50 mg/ mequlv
2. On e .equivalen t o f S(l(\ IlIl11 chloride is
23 + 35.5
= 58,5 g/equlv = 5 .5 mg/ mequi v
3, By Eq , (2-2)
11 7 mg!L
.
- _- - ' -,- x 50 mg/ mcqui v = 100 mg/ L o f NaCi as.CaCO , 58,) mg/ mequlv (il) I , One mo le o f a su bsl:ln cc di\ided by ils ;'a lence is o ne equ ivalent. 2 x 10 - .1 mo l/ L
..____ ..- - - - = 2 x 10
3
equi v/ L
I m o l/ equiv
+ 2 , Ihe refore. o ne equ iva len t
40.
2
(2-2)
Exa mpl e 2-3: Determinin g e'luiv a lenl co ncen lr a li ons What is the equi va le nl ca lcium car bo nale con ce ntratio n 0 1'( 0) I 17 mg/ L o fN aC I a nd (b) 2 x 10 - 3 m o l o fNa CJ ?
2
-
ex pressed as B
Hi sto ric a ll y, co nstitu e nts o f d issol ved so lid s have been reported in te rms o f equ iva leJll ca lc ium carbo na te co nce ntrations. The fo llowi ng exam ple illustrates thi s tec hniqu e.
12 + 3( 16) - - - - = 30 g/ eq ui v
2. The calcium Io n ha s an a to mic mass of 40 a nd a va lence o f o f calcium is
25
= 20 g/ equi v
3. The number of eq lli valent s o r ~a lcium mu st eq ual the number of eq ui va lent s o f ca rbo nale. th'ercfllre .
90 g . ..- ----= 3 equ lv or carbo n ate 30 g; equiv Thererore. 3 equi v x 20 g/ equiv = 60 g o f calc ium. a nd th a t amoun l w ill be required to reac t wio h 90 g o f c'lr nl> na te.
2, Th us, 2 x 10 - .1 equi\ · L x 50.GOO mg iequi v = 100 mg.' L
Man y so lid· su bs ta nces. particular ly th ose with crysta ll ine stru ctu re, io niz e re adi ly in wat er. Wa ter m ayor ma y no t be a chel1) ical reac ta n t in th e process. I n Eq, (2-3), wa ter is a reactant. w hil e in Eq. (2- 4) it is not. Ca O NaC I
+ H20
+ 11 /')
Wh en wat er is no t a reac t:lnl. it is c ust0m ar y to o mi t it fr om th e eq uati o n ,
(2-3) (2- 4)
26
WATER QUALITY: DEFINITIONS , CHARACTERISTICS, AND PERSPECTIVES
WATER
If x is the I1ll1nhe r of moles of Mg2 + resulting from the disi;ociation . then OH - is equal to 2x . Therefore.
The double arrows in Eq. (2-4) indicate a reversible conditi o n. That is, the solid form (NaCl) may be dissociating into its io nic co mponents (dissolution), or the ionic component s may be recombinin g int o the so lid fo rm (precipitatioll). When the solid material is first contacted with water, the net reaction will be toward the ionic form. If a sufficient mass of solid is present. a condition of dynami c equilibrium will be reached in which the rate of disso luti o n and the rate of precipitation will be exactly equal. At thi s pOint, the water is saturated with the dissolved species. Conditions of equilibrium can be expressed by the /"1I(/SS action equation. For ' the ge neralized reactio n
.\:A
A,Bv
X
1.3 x
10- 4 Illol/ L = Mg
2x = 2.6 x 10 - · m o ljL = OH
1.3 x 10 - 4 m o l/ L
3. - - ----- - .0..5 mol/equiv
x 50..0.0.0.. mg!equiv = 13.0. mg,'L of Mg as CaCO J
2.6 x 10 - 4 mol/ L 4. - - -- - -.- - x 50..0.0.0. mg/ equiv = 13.0. mg /L ofOH as CaC0 3 I mol/equlv '
(2-5)
K
=
In addition to solid substances, many gases also dissolve in water. Elements fr o m some of these gases may combine with water or with substances in the water to produce compounds or radicals that can be recovered in a solid form, thus becoming a part of the dissolved-solids load. An example is carbon dioxide.
the mass action equation is
[AYEB}' - - -[AxB\.]
[X][2XJ2 = 9 x 10 - 12 4x J = 9 X 10.- 12
+ vB
lonie component s
So lid compound
27
CO 2 + HzO
H ZC0 3
I-J '
+ HC0 3 -
(2-7)
a nd Tlie brackets around' t he io nic and so lid species ind icate molar co ncentrat ions. Th e K value is an equilibrium constant for a given substance in pure water at a given temperature. At equilibrium, the solid phase does not change concentrati o ns because dissolution and precipitation are equal. Thu s and
[AY[B}' = KK , = Ksp
Example 2-4: Determining equilibrium concentrations The sol u bili lY prodllct for the dissociation of Mg(OH), is shown in T a b.Ie 2-3 as 9 x I () - 12 De term ine Ih" co ncentra· t io n o f M g" a nd OH - a t eq uilibrium. ex pressed as milligra:ns pe r liler or (":1('0 3 ,
I. W rite the equa ti on for the reacti on
Mg(OH) ,
==
M g"
+ l O ll '
Both the bicarbonate (HC0 3 fo rm.
and carbonate (CO/-) are recoverable in solid
Table 2-3 Solubility products of selected ion pairs K ,p al Significance in . EqllilibTinm ·equatiun···· . . .. . . .. ... '25°(" ...... . 'envrronmental errgineering' .... .. . ..... ........ .
MgCO , ~ Mg' +
+
4 x 10 ' , 9 :x 10 - 12 5 x 10 - 9 8 x 10 - b 2 x 10 - , 2 x 10 - 19 3 x 10 - 17 2 x 10 - I b
C O,' -
MgtOH), ~ Mg " + 20W .. CaCO J ~ Ca" + CO J C'utO H), ~ Ca" + 20HCaSO., ~ Ca" + SO. Cu(O H), ~ Cu " + 20H Zn(OH ), ~ Zn' " + 2 0H NI(OH), ~ Ni ]+ + 20W ("r(O I'/) , ;:"" C'r" I- lO H AI( OI'l) J ~ AI' . + lO l r Fc(O H ), ~ Fe" + ,OW Fc(O H) , ~ Fe" + 201-1 1'v11l(O H ), ~ Mn" + ,OW MII(Ol-!) , ~ Mn " + 20W ·C : , (PO.) , ~ 3Ca " + 2 P0 4 CaHPO. ~ Ca" + HPO/ C a F , :.= Ca " I- 2 r -
,
,.
I\ ~ (
2, The so lubil ity producl equa t ion becomes
)
(2-6)
The quantity K ,p is known as the solubility product for the io n pair. If the co ncentration of either or both o f the ions is increased, the prod uct o f th e ioni c concentration will exceed the K sp and precipitation will occur to maJJ1t ain eq uilibrium co nditions. The solubility products for several substances common to natural water systems are given in Table 2-3. Use of the solubilit y product to calculate ionic concentra-· tions is illustrated in the following example.
SO l. UT IO N
(2-8)
'I
~
l~ aSU .,
Ag
.,. cr
:.= Bal ' + SO.,'
x 10 - 3J I x 10 - 32 (, x 10 - j(,
(,
5 x 10 - "
,-
I x 10 - Jb 10 " 1-> I x 10 " 27 x 10 - 7 x 10 - I I
~ X
.,
3 x 10 - 10 I x 10 - 10
Hardness remo va l, scaling. Hardness rem oval. scaling Hardness remo va l, scaling Hardness removal Flue gas liesulfurization Heavy me tal removal Heavy metal removal H eavy metal removal H eavy metal removal Coagulation Coagulation, iron re m ova l. corrosion Coagulation, If all rem()\·;!I, corrosion Manganese re mo val Mangane se removal Ph os phalc rem oval Phosphate rem ova l Flu o rid a li o n Chloride anal ys is Sulrale analysis
S o"r,l' : Adarleli from Sawyer and M cCa rt y. [2·12].
28
WATER QUAL rlY : DEFINITIONS. CHARACT ERISTICS, AND PER SPECTIVES
WATER
2-8 TOTAL DISSOLVED SOLIDS
specific cOl/dl/cra l1ce, is a fun c ti o n o f its ionic str ength. Specific conductance is
Dissolved material results from the solvent action of wa ter on so lid s. liquid s, a nd gases. Like suspended material. dissolved substances may be organic or inorganic in nature. Inorganic substances which may be dissolved in water include minerals, metals. and gases. Water may come in contact with these substances in the a tmosphere, on surfaces, and within the soil. Materials from the decay product s of vegetation. from organic chemicals, and from the organic gases are common organic dissolved constituents of water. The solvent capabi lit y of water makes it an ideal means by which waste products can be carried away from industrial sites and homes.
meas ured by a co nductivit y me ter employing the Wheatstone bridge prjnciple. The stand a rd procedure is to measure the conductivity in a cubic-centimeter field at 25 °C a nd express the res ul ts in millisiemens per meter (mS/ m). U nfort un a tely, specific cond uc ta nce and concentration of TDS are not re la led o n a o ne- Io-o ne bas is. Only io nized substances con tribute to spec ific conductance. Organic mo lcc ules and co mpo unds that dissolve witho ut ioniz ing are not measured. Additionally. t he magnitude of the specific conductance is influenced by th e va lence of Ihe ions in so lution , their mobility, and relative numbers. Th e te mperature a lso has an import a nt effect, with specific conductance increasi ng as I he water temp era l ur e increases. Conversion of units to milligrams per lit er o r milliequi valent s per lite r mus t be made by use of an appropriate constant. A multiplier ranging fr o m 0.055 to 0.09 is used to convert millisiemens to milligrams per liter. [ 2-1 5J T o use spec ific co nductance as a quantitati ve test, s ufficie nt anal ys is for filte ra ble residue must be run to determine the convers ion factor. For thi s reaso n. spec ific co ndu ctan ce is m os t often used in a qualitative se nse to mo nitor changes in TDS occ urring in natural streams or treatment processes.
Impacts
Use
The material remaining in the water after filtration for the suspended-so lid s ~~~I)'sisis considered to be dissolved. This material is left as a solid residue upon evaporation of the water and constitutes a p a rt of total so lids discussed in Sec . 2-2.
Sources
Many dissolved substances are undesirable in water. Di ssolved minerals, gases. and organic constituents may produce aesthetically displ eas ing co lor, tastes. and odors. Some chemicals may be toxic. and some of the dissolved organic constituents have been shown to be carcinogenic. Quite often , two or more dissolved substances-especially organic substances and members o f the halogen group will combine to form a conlpound whose characteristics a're' mO ore" obje
Measurement
~ .....
29
A direct measurement of total dissolved solids can be m ade by evaporat ing to dryness a sample of watet which has been filtered to remove the suspended solids. The remaining residue· is weighed and represent s th e IOwl dissolved solids (T DS) in the \vater. The TDS is expressed as milligrams per liter o n a dry-ma ss basis. The organic and inorga;1ic fractions can be determined by firing the residue at 600 ~ C as discussed in Sec. 2-2. An approximate analysis for TDSis often made by determining the electrical conductivity of the water . The ability of a water to conduct electricity. kno\\'n ;I S th e
.. -' ..
Beca use no distinct io n amung the co nstituents is made. the TDS parameter is in clud ed in the a nal ysis o f wa te r and wastewater only as a gross measurement of th e disso lved material. Whil e this is o ft en sufficient fo r was tewaters, it is frequently des irable to know more about th e co mp os ition of the solids in water that is intended for use in· p otabl e suppli es. agric ultur e. and some industrial processes. . When th is is·the·case: tests forseverat 'Oflhc'j'onic CO"m;(rtuents of TDS are made.
Ion Balance The ions us ually acc o unting fo r th e vas t maj o rity of TDS in natural waters arc listed in Tab le 2-4. Those Ii.<;ted uncler maj o r constituents a re often sufficient to Table 2-4 Common ions in natural waters Major con stituen ts, 1.0- 1000 mg! L
Secondary co nstituents.
O.O t· I O.n rng . L '
Sodium
fron
Catcium
StrollliUn1
M agnesium Bien rbona Ie
Potassiulll Car bonale
Sutfal c
Nltrale
Chloride
Flu or1(k BL)r On
S, i1 Cl
30
WATER QUALITY: DEFINITIONS, CHARACTERISTICS, AND PERSPECTIVES
WATER
characterize the dissolved-solids content of wa ter. These are called common ion s and are often measured individually and summ ed on an eq ui valent basis to represent the approximate TDS. As a check , th e sum of th e anions should equal th e sum of the cations beca use electroneutralit y must be preserved. A significan t imbalance suggests that additional cons tituents are present o r that a n erro r has been made in th e ana lys is of o ne or more of th e iOIlS . The fo ll owin g exam ple illu stra tes the io n balance p roced ure. Example 2-5: Testing for ion balance Tes ts for common ions are run o n a sam ple of wa ter and the results are show n be low. ·If a 10 perce nt e rror in the bala nce is acce pt ab le, should the analysis be co nside red complet e?
31
It is important to arrange the cations and anions in the order shown for convenience in determining types of hardness and the quantities of chemicals needed for so ftenin g, a subject m o re fully developed in· a later chapter of this text. Several of the constituents of dissolved solids have properties that necessitate specia l .attenti on. These constituents include alkalinity, hardness, fluoride, metals, organics, and nutrients.
2-9 ALKALINITY Alkalinity is defined as the quantity of ions in water that will react to neutralize hyd rogen io ns. Alkalinity is thus a measure of the ability of water to neutralize acid s.
Constituent s Ca' > = 55 mg/ L HCO , - = 250 mgl L Mg2+ = 18 mg/ L SO.' 60 mg/ L Na + = mg/ L C I = 89 m gi L
n
SOLUTtON
l. Conve rt the co ncen tra ti o ns of cations a nd anions fr o m milli grams per lit er to milli-
equ iva lents per liter and slim them .
- - - - . - - - - - - - - --- - - - - _. _ - - - - - - -- - - - Ca ti ons [on
Ca 2 , M g2+ Na+
Anion s
Cone , Equiv , mg/ L mg/ mequiv
Eq uiv co ne, meq / L
Ion
55 18 98
2.75 1.48
HCO , 2SO.
4.26
cr
40/ 2 243/ 2 23/ I
T o tal io ns
Cone , E4Uiv , mgi L mg/ m ~ quiv 250 60
6 1/1 96/ 2 89 ... 355 / I . . .. .
8.49
Equiv cone, m ~q ui v/ L
4.10 1.2 5 2.5 1
786
Sources Constituents of alkalinity in natural water systems include C0 32 - , HC0 3 - , OH - , HSi0 3 - , H 2 B0 3 - , HPO/ -, H 2 P0 4 -, HS -, and NH30 [2-3] These compound s result from the dissolution of mineral substances in the soil and a tm osphere_ Phosphates may also originate from detergents in wastewater discharges and from fertilizers and insecticides from ag ricultural land. Hydrogen sulfide and ammonia m ay be products of microbial decomposition of organic material. By far th e most co mmon constituents of alkalinity are bicarbonate (HC0 3 -), ca rb onate (CO/ - ), and hydr oxide (OH - ). In addition to their mineral origin, these su bstances can originate from carbon dioxide, a constituent of the atmosphere and a product of microbial decomposition of organic material. These reactions a re as fo llows:
2. Calcu la te percent of er ror.
+
CO 2
8.49 - 7.86
- - - - 100 = 8~"
H 2 C0 3 *
H2O
7.86
H2C0
Therefo re, accept ana lysis.
A co mm o n iOIl balance can be displayed conve ni en tl y in th e form of a bar diagram. A bar diagram for th e water in Example 2-5 ca n be drawn as shown be low. rn c(juiv / L
2.75
0
4 . )':\
(T
!-l eo ) m e'll/iv, L
0
4.10
5.3:'
! .Xh
3
*
HC0 3 -: CO/ -
+
H 2O
H+
+
H+
+
H C0 3 -
HC0 3 C0 3 2 -
+
OH -
(dissolved CO 2 and carbonic acid)
(2-9)
(bicarbonate)
(2-10) .
(carbonate)
(2-11 )
(hydroxide)
(2-12)
The reaction represented by Eq. (2-12) is a weak reaction chemically. Howev:er, ut ilization o f th e bicarbonate ion as a carbon so urce by algae can drive the reaction to th e ri ght and resu lt in su bstanti a l accum ulati o n of OH - .. W ater with heavy a lga l g rowths often has pH va lues as high as· 9 to 10. Because the reac ti ons represented by the above equations invo lve hydrogen or hyd roxide ions, the rela ti ve qu a ntities of the alkalinity s pecies are pH dependent. These relatio nships a re show n graphically in Fig. 2-3.
32
WATER QUALITY: DEFINITIONS, CHARACTERISTICS, AND PERSPECTI VES
WATER
r-- ~
100 90
....l
eo
14
1\\
80 70
\
,l -
60
u
U .50 c'" ~
,S -;;; -'"
:;;:
40 30
12
\
\ \
20 10
o
6,5
10
HC0:i
\
E
O~
/ r\
!
C0 3
\ "7
V B 2 C0 3 '
~
7.5
[7
8.S
8
V 9.5
10
J\
4
I
Figure 2-4 Alkalinity titration curye
II
Figure 2-3 Alkalinityspecies vs. pH. Values are calculated for water at 25°C containing a total alkalinity of 100 mg/L as CaC03. (From Sawyer and McCorry [2-12].)
Impacts In large quantit.ies, alkalinity imparls a bitter taste to water. The principal ob~ jection to alkaline water, however, is the reactions that can occur between alkalinitv and certain cations in the water. The resultant precipitate can foul pipes and othe-r water-systems appurtenances.
Measurement Alkalinity measurements are made by titrating the water with an acid and determining the hydr9gen equivalent. Alkalinity is then expressed as milligrams per Ii.ter ofCaCO J . If 0.02 N H 2 S04 i~ used in the titration, then 1 mL of the acid will neutralize 1· mg of alkalinity as CaC0 3 . Hydrogen ions from the acid react with the'alkalinity according to ihe following equations:
+
+
oL---------------------------------------------------Milliliters of titrant
pH
H+
I -------,-------I ~ CO/~ HCO:; I
2
\
10.5
olr +, CO/~ L
6
'L
V
8
c.
/ \ / \/ OW
9
1:
\/ \ /
\
33
If acid is added slowly to water and the pH is recorded for each add it ion, a titration curve similar to that shown in Fig, 2-4 is obtained, Of particular significance are the inflection points in the curve that occur at approximately pH 8,3 and pH 4.S, The conversion of carbonate to bicarbonate [Eq. (2~14)J is essentially complete at pH 8.3. However, because bicarbonate is also an alkalinity species, an equal amount of acid must be added to complete the neutralization. Thus, the neut r aliza~ tion of carbonate is only one~half complete at pH 8.3. Because the conversion of hydroxide to water is virtually complete at pH 8.3 (see Fig. 2~3)..all ()ft.h.e )ly~r9xide. and one~half of t.he carbonate have been measured at. pH 8.3. At pH 4.S all of the bicarbonate has been converted to carbonic acid [Eq. (2~ I S)], including t.he bicarbonat.e resulling from t.he reaction of t.he acid and carbonate [Eq. (2~14)]. Thus, t.he amount of acid required to titrate a sample to pH 4.S is equivalent to the total alkalinity of the water. This 'point is i1iustra'ted in t.he following example. Example 2-6: Determining total alk'alinity A 200~mL sample of water has an initial pH. of 10. Thirty milliliters of 0,02 N H 2 S0 4 is required to titrate the sample to pH 4.5. What is the total alkalinity of Ihe waler in milligrams per liter as CaCO,') SOI.ti nON
Because each rnilligr;llll of ().02 ;V H 2 SO'4 will neutralize I Illg of alkalinity. t h ere IS 30 rng of alkalinity in the 200-nlL sample. Therefore. the concentration of alkalinity expressed as milligrams per lite'r will he 30 mg
OH-
H 20
(2-13)
C0 3 2 -
+
H+
HC0 3 -
(2-14)
HC0 3 -
+
H+
H1CO,
(2-15)
200 Illl
x
1000 rn L
L
150 mgj L
If the volume of acid needed to reach the 8,3 endpoint is known. the spec ies of alkalinity can also be determined. Because all of the hydroxide and o n e~half
WATER QUALITY: Q~FINITIONS, CHARACTERISTICS, AND PERSPECTIVES
34 WATER
of the carbonate have been neutralized at pH 8.3, the acid required to lower the pH from 8.3 to 4.5 must measure the other one-half of the carbonate, plus all of the original bicarbonate. If Pis the amount of acid requ ired to reach pH 8.3 and M is the total qLiantity of acid required to reach 4.5, the following genera lizations concerning the forms of alkalinity can be made: if P = i'vI, all alkalinity is OH P = M / 2, all alkalinity is CO/P = 0 (i.e .. initial pH is below 8.3), all alkalinity is HC0 3 P < M ; 2, predominant species are CO/ - and HC0 3 P > M / 2, predominant species are OH - and CO/ In observing the pH dependency of the species in Fig. 2-3, it is noted that the quantity of OH - becomes significant at pH less than about 9.0. Without introducing significant error, it can be assumed that the OH - of samp les with pH less than 9.0 is insignificant. The CO/- would then be measured by 2P and the HC0 3 - would be measured by the rema inder (M - 2P) One method of calculating the quantities of ea~h species is illustrated in the following example.
35
Use Alkalinity measurements are often included in the analysis of natural waters to determine their buffering capac ity. It is also used frequent ly as a process control variahle in walt;r and wastewater treatment. Maximum levels of alkalinity have not been set by EPA for drinking water or for wastewater discharges.
2-10 HARDNESS Hardness is defined as the concentration of multivalent metallic cations in solution. At supersaturated conditions. the hardness cations will react with anions in the water to form a so lid prec ipitate. Hardness is classified as carbonate hardness and l1oncarbonate hardness, depending upon the anion with which it associates. The hardness that IS equivalent to the alka linity is termed carbonate hardness, with any rema ining hardness being called noncarbonate hardness. Carbonate hardness is sensitive to heat and precipitates readiiy at high temperatures.
Example 2-7: Determining alkalinity species Determine the species , al~lI Ihe quantity of each specie, of alkalinity in Example 2-6 if the 8.3 equivalence point is reached at II mL of acid.
+
Ca(HCOJ)z
~
CaC0 3
Mg(HCO J )2
~
Mg(OH)z
CO 2
+ HzO
(2-16)
+ 2CO z
(2-17)
Sources
SOLUTION
Becallse the initial pH is 10, the initial pOH of the water is4. A Jeterl1linatio ll of the OH . concentration can be made as follows. 10 - 4 mol OH-
[OW] = =
I equi\
50,000 mg CaCO, X -I cqulv-- -
--'---L -- x ;;;;1 OH5 mg/ L as CaCO,
2. Five milli!Jters o f acid would be required to measure the OH - in a I-L sample. H owever. this sample is only 200 mL so the necessary volume of acid is: 200 5 --- = 1.0 mL 1000
.
3. If I mL of acid measures the OH -. then 10 mL of acill measures one-half of the carbollate and 10 more will be required to measure the remaining one-half of the CO /-, leaving') mL to measure the HCO) - . (See Fig. 2-4.) Thus. the quantity of each species is as follows. OH - (calculated from pH) CO ,
2-
HCO J
=
20 mg
. 200-mL I)
-
x
200 mL
5 mg i L 11l(J mg , L
L
mg
.. _.
1000 mL
'=
1000 IIlL x
L
Total albllnity
·.15 mg i L 150 Il1g ' L
The multivalent metallic ions most abundant in natural waters are calcium and magnes ium. O t hers may include iron and manganese in their reduced states (Fez +, M n 2+), stront ium (Sr2+), and aluminum (AI3+). The latter are usually
. fo'uiioinriiuch .smaI1er' 'cjllaiiiiii'es' 'thaii' cafCiu'iri--ilno--mag'nesiurrCarld' ro"i--:ilJ practica l purposes. hardness may be represented by the sum of the calcium and magnesium 19ns. Impacts . Soap consumption by hard waters represents an economic loss to the water usee Sodium soaps react with multivalent metallic cations to form a precipitate, thereby losing t heir surfactant propert ies. A typical divalent cation reaction is:
2NaCO/_~ 1 7 H33 Soap
+ cation 2+
--->
cationz+(COzCI,H33)z Precipitate
+
2Na+ (2-.18)
Lathcring does not occur until all of the hardness ioils are ~recipitated, at which point the water has been" softened" by the soap. The precipitate formed \;ly har~ ness and soap adheres to surfaces of tubs, sinks, and dishwashers and may stam clothing, dishes, and other items. Residues of the hardness-soap precipitate may remain in the pores. so that skin may feel rough and uncomfortable. In recent years these problems have been largely alleviated by the development of soaps Glnd detergents that clo not I'cact with hardness.
....
36
\VA'I U( QUALITY' J)EFIN ITIONS, C HARACTER ISTICS, AND PERSPECT I VES
WATER
Boiler scale, the res ult of the carbonate ha rdness precipitate ma y ca use considerable economic loss throu gh fouling of water heaters and hot-wa ter pipes. Changes in pH in the wa ter distribution systems ma y also result in deposits of precipitates. Eicarbonates begin to convert to the less so lu ble carbonates at pH val ues above 9.0. Magnesium hardness, particularly assoc ia ted with the sulfa te ion. has a laxative effect o n perso ns un acc ustomed to it. Magnesium concentrati ons of less than 50 mgj L are desirable in pot a ble wa ters, alth o ugh ma ny public wa ter supplies exceed thi s amount. Ca lcium hardness p rese nts no public health .prob lem. In fac t: ha rd water is appa rentl y beneficial to the human ca rdiovascul ar system. [2-4J Measurement H ardness ca n be measured by using spect rop ho tome tric techniqu es or chemica l titration to determine th e quantity of calcium a nd magnesium io ns in a give n sample. Hard ness can be measu red directl y by titrati o n with eth ylenediam ine tetraace t ic acid (EDT A) using eri oc hrome bl ack T (EBT) as a!' ; ~ .. . .. , .. _ LD J reacts wi th th e divalent met a llic cat io ns. forming a comp lex th at is red in co lo r. The EDT A replaces the EBT in the co mpl ex, and when the replacement is complete, th e so luti on changes from red to blue. If O.O J M EDTA is used, 1.0 mL of the titra nt measures J.O mg of ha rdness as CaC0 3 .
37
o ther anima ls in large quantities, wh ile small concentra tion s can be beneficial. Concen trati o ns of approx imat ely 1.0 mg/ L in drinking wa ter help to prevent dental cavi t ies in ch ildren. During fo rm ation of permanent teeth , flu o ride co mbines chem ica lly with too th e namel. resulting in ha rd er. stro nger teeth that are more res istant to decay. Flu o rid e is often added to drink ing water suppl ies if sufficient quan titi es fo r good dental format ion are not natura lly present. Ex cess ive JJ1tak es of flu o rid e ca n res ult in discolo ra ti o n of teeth. Noticeable disco lora ti o n, ca ll ed mOl/lillY. is relat ive ly common when fluoride concentrations in drinking wa ter exceed 2.0 mg/ L but is rare whe n co ncentrat io ns are less th a n 1.5 mgj L Adult t ~e th are no t affect ed by flu oride. a lth ough both the ben efit s and li abili ties of fluorid e during too t h-for mati o n years ca rr yove r in to ad ulth ood . Excess ive dosages of flu o rid e can a lso result in bone flu o ros is and other skeleta l abno rma liti es. Co ncentration s of less than 5 mgj L in drinkin g wa te r are no t lik ely to ca use bon e flu oros is o r related pro blems, and so me wa ter supplies-a re known to ha ve so mew hat hi gher flu oride co ncentrat ion s with no disce rni ble pr051 em other than severe mottl;n g of leelh . On the assum pti o n tha t peop le drink mo re wate r in warmer clim a tes, EPA drinkin g-wale l' sta nd a rd s base upper li mi ts fo r fl uor id e o n ambien t temperatures. Th ese standard s a re d isc ussed mo re fu ll y in Sec. 2- 18.
2-J2 METALS Use Analysis for hardness is co mm o nly made on na tura l waters and o n wa ters intended fo r potable suppl ies and fo r certain indu stri al uses. Hardness ma y range fro m practically zero to severa-j. hundred; 'or 'even ' several ' thou sand; 'IYaYts per ' million . Although accep ta bility leve ls vary acco rding to a co nsumer's acclim at io n to hardness, a generally accepted class ifica ti on is as follows: So I'l Mod e ral e ly hard Hard. Very pard
. < 50 mgi L as C"CO, . 50-- 150 mg i L as CaCO, 150-300 mg/ L a s CaCO, > 300 mg; L as CaCO,
The Public Health Service 'standards recommend a maximum of 500 mg/ L of hardness in drinking water. [2-18J A ma xi mum limit is not set by the EPA standa rd s.
2-11 FLUORIDE Generally associated in na ture with a few types of sedim entary or igneou s rocks. fln or ide is seldom found in appreciable qu a ntities in surface wa ters and a ppears in gro undwate r in on ly a felY geograph ical regio ns. F lu or id e is toxic l\) humans and
All meta ls are so lu ble to some exten t in wa ter. While excess ive a m o unts of a ny meta l may present hea lth haza rds. o nl y th ose metal s tha t a re harmfu l in relati ve ly .... small amounts.are.co.r.nmon ly.labeled .t ox ic : oth er meta ls fa ll int o th e no ntox ic grou p. So urces of meta ls in natura l waters inc lude disso luti o n from na tural deposits a nd discharges of d omes tic. indu stria l. or ag r icultura l wastewaters. Meas urement of me ta ls in wat er is usua lly mad e by atom ic abso rpti o n spectroph otometry.
Nontoxic Metals In add iti o n to the hard ness ions. calc iulll and magn esi um. o th er no ntox ic meta ls co mm o nl y found in wa ter include sodium. iron. manga nese. a lu minum, copper. and zinc. Sodium. by rar the mos t cOlllmon no nt ox ic Illetal fo und in natural waters, IS ~Ib un e!ant in the ea rth 's crust ~I nd is highl y reac ti ve wi th oth er elements. Th e sa lt s o f sodium are ve ry so lub le in water. Excessive co ncentrat io ns ca use a bi tt er taste ."l wa ter a nd are" health halard to'cardJ< lc and kidne y pati ent s. Sodium is also co rros ive to Ill etal surfaces and . III large co nce ntrati o ns. is tox ic to p lant s. Ir on ;lJld ma nga nese quit e fr equentl y occur toge ther a ne! present no health h ~l Z~ \r(i s a t co ncentratiollS normilily f'nli lld ill natur;l l wa ters. As no ted in Sec. 2-4. iron an e! man ga nesc in vc ry small quantities Illay cau se co lo r pro blems. Iron C (ln C<: l1tr ~ ltl()n S of 0.3 Ill g/ L. and ma nganese cnnce ntr atioJls as low as 0.0 5 mg/ L
31) WAHR
can ca use co lor problem s. Additionall y, so me bacteria li se iron and man ga nese co mpounds for a n energy sou rce, and the resulting sli me grow th may prod uce taste a nd odo r problems. Wh en significant quant ities of iron are enco untered in natura l wa ter sys tems. it is usuall y assoc ia ted with chloride (FeCI2). bicarbo nate [Fe( HC0 3)2]' o r s ulfate [Fe(S04)] anions a nd exists In a reduced state. In the presence of oxygen, the ferrous (Fe 2 ' ) io n is ox idized to the ferric (Fe J +) io n a nd forms an insoluble co mpound wi th hydroxide [ Fe(O Hhl Thu s. signifi can t quantiti es of iron wi ll usua ll y be found o nl y in systems devoid of oxygen such as gro und wa ters or perhaps th e bo tt om layers of stra tified lakes. Simi la rl y. manganese ions (MnH and Mn..) ' ) associa ted wi th chlo ride, nitrates . a nd su lfat es are so luble, wh ile ox idized compou nd s (M n J , and M n 5 +) a re virtually insolu ble. It is poss ible. however, for o rganic acids de ri ved fr om decomposing vege tation to c he lat e iro n and ma nga nese and prevent their ox idat ion a nd subseq ueht p rec ipit a ti on in natu ra l wa ters. "The other no nt ox ic metal s are ge nerally found in ve ry sm all quantities in natura l wate r sys tem s. and most wou ld ca use ta ste probl ems lo ng befo re toxi c len.: ls were reached. Howeve r. copper a nd zin c are sy nergetic and when both a re present. even in small quant iti es. may be tox ic to Illany bio logica l spec ies. Toxic M etals As noted earlier. toxic meta ls are harmful to huma ns and other urgan isms in sm:t11 quant ities. TOX IC meta ls that may be disso lved in wa ter IIl clud e arsenic. barium. ca dmluill. chro mium , lead, mercury. and sil ver. C umulative toxin s such as arsenic. cadmluill. lead. a nd merc ur y a re particu la rly hazardous. Th ese metals are concentrated by the food cha in . thereby posing th e g rea tes t dan ge l' to orga ni sms . ' ric',ii" t he lOt>' (~ t' The' th,i j ii.
Fort una tely. toxic meta ls are present in o nl y minute quant it ies in most na tural water sys tems. Alth o ug h· natural sources of all th e tO XIC me ta ls exist, signifi ca nt c()nu.:nt ration In "ate r can usuall y be traGed to mini ng. industrial. or agricultural Sllurces.
2-13 ORGANICS Many organic materia ls are so lu ble in water. Organics in na tur'al water systems mCl)' come from natural sou rces or ma y resu lt fr om human activities. Mo st na tura l o rganics consis t of th e decay products of organi c s.o lid s, wh ile synth etic o rga ni cs are usual ly the resu lt of was tewa t.e r discharges or agricultura l practices Di sso lved o rg,l nics in \va ter are usuall y divided int o ·two broad categor ies': biodegrada ble and n()no io(/cg radable {refractory). ' Biodegradable Organics HICldegr'adahlc ll1ateri~ t1 consists of o rganics t hat call he utilized for food by l1~ltLII;lIh llcclirring mlul)() rganisms \\'ithin a reasonahle lengrh (If time. In
WA TER QUALITY: DEFINITIONS , CHARM'T ER ISTICS, AND PERSPECTIVES
39
dissolved form. these materials usually consist of starches, fa ts, proteins, alcohols, acids. a lde hydes. a nd esters. They may be the end product of the initial microbial decomposition of pla nt oranimaHissue, or they may result fr o m domestic or industrial wastewater discharges. Although some of these materialS can cause co lo r, tas te, a nd odor pro blems. the principal problem associated with biodegradable organics is a secondary effect resulting fr o m the acti on of microo rganisms o n these substances. Mi cro bial utilization of dissolved organics can be accompanied by ox'idation (addition of oxygen to, or the deletion of hydro gen from, elements of the orga'nic molecule) or by redu ct ion (addition of hydrogen to, or deletion of oxygen from, elements of the o rganic molecule). Although it is possible for the two processes to occur simultaneously, the ox id ation process is by far more efficient and is predominant when oxygen is available. In aerobic (oxygen-present) environments, the end products of microbia l decompositio n of organics a re stable and acceptable co mpou nds. Anaerobic (oxygen-absent) decomposition results in unstable and objectionab le end products. Should oxygen later become ava ilable, anaero bic end product s wi ll be ox idized to aerobic end products. The oxygen-demanding nature of biodegradable orga nics is of utm ost importance in na tural water systems. Wh en oxygen utilizat io n occurs more rapidly than oxygen can be replenished by transfer from the a tm osp here, anaerobic conditions that seve rely affect the ecology of the system will resull. This situation is cove red in more detail in the next chapter. The a mo unt of oxygen co nsumed during microbial utilization of o rganics is called the biochemica l oxygen demand (BOD). The BOD is measured by determining the oxygen co'nsumed from a sample placed in an air-tight container and kept in a con tro lled environment for a prese lected period of time. In the sta ndard test: a 300-mL BOD bottle is used and the sample is incubated at 20°C for 5 days. Light must be excluded from the incuba to r to prevent a lga l growth that may prod uce oxyge n in .the bo ttle. Because the saturati on concentration for oxygen in water at 20°C is approx imately 9 mg/ L. dilution of the sa mple with BODfree, o xygen-sa turated water is necessary to measure BOD va lues greater than just a fe w milligrams per .liter. . '.. . Th e BOD of ad iluted sam ple is calculated by BOD = 001 - DO F P
(2-19)
where 001 a nd DO F are the initial and final di ssolved-ox ygen concentrations (mg/ L) and P is the decimal fracti o n of the sa mple in the 300-mL bottle . . Ran ges of BOD covered by various dilution s are shown in Table 2-5. These val ues ass ume an initial dissolved-oxygen concentration of9 mg/ L in the mixture. with a minimum of 2 and a max imum of 7 mg/ L of O 2 being consumed . Calculati o ns of BOD s from this testing procedure a re mustrated in the foll ow ing example. Example 2-8: Determinin g BOD ) The BOD of a wastewa ter is suspected to range from 50 to 200 mgjL. Three dilutions are prepared to cover thIS range. The procedure is the
same in cach casc. First the sample is placed in the standa rd BOD bo ttle and is then
40
""".
WATER QUALITY: DEFINITIONS, CHARACTER ISTICS , AND PERSPECTIVES 41
WATER
Table 2-5 Ranges of BOD values covered by various dilutions By using percent mixtures J~
"'
"".
mixture
0.01 0.02 0.05 0.1 0.2 0.5 1.0 2.0 5.0 10.0 20.0 50.0 100.0
utilized is propo rtional to the amo ttnt available. Mathematically, this can be expressed as follows:
By direct pipetting into 300-mL bottles
Range of BOD
.mL
Range of BOD
20,000-70,000 10,000-35,000 4,000- 14,000 2,000-7 ,000 1,000-3,500 400-1 ,400 200-700 100-350 40- 140 20-70 10-35 4- 14 0-- 7
0.02 0.05 0.10 0.20 0.50 1.0 2.0 5.0 10.0 20.0 50.0 1000 300.0
30,000-105,000 12,000-42,000 6.000-21,000 3,0007 10,500 1,200-4,200 600-2,100 lOO-I,050 120- 420 60- 210 30- 105 12- 42 6- 21 0- 7
elL,
elL,
-
L,
i.
dL,
Lo
L,
f
= - k ell
L, In - - = - kl
Lo
L, = L OI.'.-k'
diluted to 300 mL with organic-free, oxygen-saturated water. The initial dissolved oxygen is determined and the bottles tightly stoppered and placed in the incubator at 20°C for 5 days, after which the dissolved oxygen is again determined. .
mL
DOl , mg/ L
DO" mg/ L
O 2 used, mg!L
p
BOD ,. mg/L
5 10 20
9.2 9.1 8.9
6.9 4.4 1.5
2.3 4.7 7.4
0.0167 0.033 0.067
138 142 110
~.
Most natural water and municipal wastewaters will have a population of microorganisms that will consume the organics. In sterile waters, microorganisms must be added qnd the BOD of the material containing the organisms must be determined and subtracted from the total BOD of the mixture, The presence of toxic materials in the water will invalidate the BOD results. The BODs only represents the oxygen consumed in 5 days. The total BOD . . or BOD for any other time period. can be determined provided additional information is known or obtained. The rate at which organics are utilized by microorganisms is assumed to be a first-order reaction: that is. the rate at which organics
(2-21 )
The term Lo in this equation represe nts the total oxygen equivalenl of the organics at time 0, while L, represents th e amount remaining at time r, and decays exponentially with time, as shown in Fie. 2-5. The oxygen equivalent remaining is not the parameter of primary importance. However, the amount of oxygen llsed in the consumption of the organics, the BOD" can be found from the L, value. If Lo is the oxyge n equivalent of the total mass of
=:::.:==-------
If the third value is disregarded (the final DO being less than 2.0 mg/ L), th e average BOD of the wastewater is 140 mg! L.
\'
(2-20)
where L, isthe oxygen equivalent of the organics at time 1, and k is a reaction Consta nt. The units of L, are milligrams per liter, and the units of k are d- 1. Equation (2~20) can be rear ranged and integrated as follows:
Source: From Sawyer and McCarty, [2-12J
... Wastewater ,
- kL,
r/r
Tim e days Fi:!lIrr 2-~
non
and oxygcn-t;: qul\a lc!l1 rdaliO n shir~ .
42
WATER
WATER QUALITY: DEFINIT IO NS , CHARACTERISTICS, AND PERSPECTIVES
orga nics. then the difference between th e va lue Lo a nd L, is iht: l)xygen equivalent cons um ed. o r th e BOD exe rted . Mathematicall y )"
~
Table 2-6 Typical values of k and Yu for various waters k. d -
L o - L, WJlc r Iype
h,
)"
Lo - L ot'
.\' ,
Lo(l - e - kl)
(2 -22)
wh ere )" represents th e BOD, of th e waler . Th e va lu e of v, approaches Lo asymptotica lly. indica ting that ' the total. or ultim ate, BOD (v..) is eqUid 10 th e initial oxygen equ iva lent o f th e water Lo. Th ese relationships are shown in Fig.
43
Tap water Surface waters Wea k municipal wastewater Strong municipal wil ~ te\Valer Trea led efll lleni
base
I
Y., mgL .
t'
<0. 1 0. 1- 0.23 0.35 0.40 0. 12.. 0.23
0- 1. 1-30 150 250 10-30
2-5. Equati o n (2-22) represen ts the BOD exe rted by the carbo n compo nent of th e o rgani c co mpo unds. Other co mpo nents of orga n ics, such as nitroge n and sulfu r, ma y also be ox idi zed by microorganisms. resulting in a n oxygen demand . Equatio ns sim ilar to Eq. (2-22) ca n be derived for these reactio ns. The va lu e of J.:. de termin es the speed of the BOD reaction wi thout in flu encing th e m,lgnitude of th e uftim ate BOD. This iss howll grap hi ca ll y ill Fig. 2-6. Numerica l values ~f J.:. ran ge fr o m abo ut 0.1 to 0.5 d - '1 depending o n the nature'of the orga ni c mo lec ul es. Simple compounds such as sugars and starches are easi ly utilized by the microorganisms a nd ha ve a high J.:. ra te, while co ~nplex mo lecu le's such as phenol s a re difficult to ass im il a te an d have low k n du es. S(,lme typica l va lu es of k are show n in Tab le 2-6. The \'a lu e of k for any given o rgani c compound is temperature-depend ent. Beca use microo rganisms a re mo re ac ti ve at highe r temperatures. th e va lu e of k increases wi th increasin g temperatures. The c han ge in k can he approxima ted by the \ an't H o fl~Arrhe niu s model: (2-23) i\ va lu e of 1.047 for 0 is o ft en ~I se d a lth o ugh
(I
The use of Eqs. (2- 22) and (2-23) is illustrated in the foll ow ing example. xamPJe 2-9: BOD conversions The BOD 5 of a wa stewa ter is determined to be J 50 mg/ L at 20°C. The k va lue is k fl o wn tll be 0.23 per da y. What wou ld the BODs be if the test were rUIl at I S" C'.' ( OLl'T ION
I. Determine the ultimate BOD.
r .5 r =I -_ e--
•
U
I__
1a
150
-e--~s
.. =. 220 .mgj L. .. . ..... ... ....... ........ . 2. Correct the k va lue for 15°C.
is known t'o \'ar: sll lllC\\l!at with
te mperature ra nges. [2-8J
k " = 0 23( 1.04r 5) = 0. 18
J. Calcu la te)'8
" ,l ',
= .1',.( I _ e- k ')
.1'" = 220 (I _ (' -
0 . 1 8· 8 )
168 mg !L
Nonhi odegradabl c Organics
J-'i g ll l"('
2-6 Ij O I)
l"\L' / (1\'11 ;1 ')
a lunction of reaction
\..'11I1 S (;\nl /.:
So me organ ic materials are resis tant to bio log ical degradation. Ta nnic and lignic acid s. ce llulose. and phen ols are o ft en fo und in natural wa ter systems. These co nstituen ts of woody plant s biodegrade so slow ly that they are usually consid ered refractor y. Mo lecules with excep tio nall y stro ng bonds (some of the polysacc harides) and ringed struc tu res (benzene) are essen ti a lly non bi odegradable.
44
-.
WATER
An example is the d etergen t compound a lkyl benzene su lfo na te (ABS) w hich, with its benzene ring, does no t biod eg rade. Being a surfactant, A BS ca uses frothin g and foaming in wastewater tre a tment pla nts and increases turbidit y b y stab iliz ing colloidal suspensions, Thi s problem was largely alleviated when detergent manu facturers switched to a linear a lk yl sulfonate (LAS) compound, whi ch is biodegradable. Many of the organics assoc iated with petro leum a nd with it s refining a nd processing also contain benzene a nd are essentially non bi o d eg rada ble. Some organics are n o nbi odegrad a ble because they are toxic to organisms. These include the organic pesticides, so me industrial chemicals, and hydrocarbon co mpounds that have co mbined with chlorine. Pesticides, including insecticides a nd herbicides, have found wide-spread use in modern society in both urba n a nd agricultural settin gs. P oo r application prac tices and subsequent washoff by rainfall and run o ff m ay result in co ntaminati o n.of-s.~rface streams. Orga nic insec ti cid es are usuall y chlorinated hydroca rb o ns (i .e., a ldrin, dieldrin. endrin, a nd lindane), while herbicid es are usua ll y c hl orophenoxys (e.g., 2A-dichlorophenoxyaceti c acid and 2.4,5-trichlorophenoxypropionic acid). Many of the pesticides are cumulative tox ins and cause severe problems at the higher'end o f th e food c hain . An example is th e nea r-extinction o f the brown pelican that fe ed s o n fi sh and o ther macr oaq uatic spec ies by the insec tic ide DDT, the 'u se o f which is now banned in the United States. Measurement of no nbi odeg rad able orga nics is usuall y by the chemical oxygen d emand (COD) test. Non bi o deg rad ab le o rga nics m ay a lso be estimated fr o m a to ta l organic carbo n (TOC) analys is. Both COD and TOC meas ure the bi odegradable fraction o f the o rga ni cs, so the BOD" must be subtracted fr o m the COD or TOC to quantify the non biodegradable o rganics. Specific o rga ni c compo und s can be identified a nd quantified thr o ugh analysis by gas c hro matog ra ph y ..... . .... .... . . . .. ... . . . . ... . . . . . . . . . . .. .... .... ... .... .... .. ... ,.- . .................... ... .
2-14 NUTRIENTS . Nutti!!nt s are eleme nts essential to the growtli aod repr od ucti on o f plaI}ts and animals, and aquatic species depend o n th e surro undin g water to provide their . nutrients. Although a wide va riet y of minerals and trace e leme nts can be classified as nutrients, tho;e requ ired in ~ost abundance by a qu a tic species a re carbon. nitrogen. and p'hosph o ru s. Carbon is readily available from man y so urces. Carbon dioxide from the a tmosphere, a lk a linit y. a nd decay pro du c ts o f orga nic m a tt er all suppl y carbon to the aquatic sys tem. In most cases, nitroge n an d phosphoru s are the nutrients that are the limiting fac to rs in aquatic plant grow th . A discussion of the consequences o f ove renri c hment with nitrogen a nd phosphorus is prese nted in Chapter 3.
Nitrogen Nitrogen gas (N 2 ) is the prim ary co mponent of the ea rth 's a tm osphere and is extremely stabl e. It wi ll react w ith oxygen under high-energy cond itio ns (elec tri ca l
WATER QUALITY: DEFINI TI ONS. C H ARACTERIST ICS, AND PERSPECTIVES 45
discharges o r tlame inc in era ti o n) to fo rm nitrogen ox ides. A lth o ugh a few biological spec ies are a ble to ox idize nitrogen gas. nitr oge n in th e aql:atic environment is derived prim a rily from so urces o th er than atmospheric nitrogen. Nitrogen is a consti tLi en t of proteins, ch lorophy ll, and many o th er biologica l co mp o und s. Upon the dea th of plants or anima ls, complex orga nic ma tter is broken d ow n to simple fo rm s by bacteria l decomposition. Pro teins, for ins tan ce, are conver ted to am in o ae id s and further red uced to ammo ni a (NH3)' If oxygen is prese nt, the ammon ia is oxid ized to nitrite (N0 2 - ) a nd th en to nitrate (N0 3 - ). Th e llItrate ca n then be reco ns titut ed in to li ving organic matter by photosynthetic plants. Other sources o f nitrogen in aquatic systems include anima l was tes, chem ical (part icu lar ly c hemica l fer tili ze rs), and wastewater discharges. Nitrogen from th ese sou rces may be disc ha rged di rect ly into strea m s or ~ay enter waterways through surface run off o r groundwa ter discharge. · N itrogen com pounds ca n be oxid ized to nitrat e by so il bac te ri a a nd ma y be carried int o th e gro und wa te r by perco latlllg wa ter. Once in th e aq ui fe r. nitrates move fr ee ly with th e g round wate r Aow. Grou~dwater co nt a min at ion by nitroge n from animal, feed lo ts and sep ti c tank d rain field s ha s heen record ed in numer o us in sta nces. [ 2- 10,2- 11, 2- 13J I n adchtlon to the overennchme nt p roblems a llu ded to earlier, nitrogen can h3ve o the r se riou s consequences. Ammon ia is a gas at tempera tures a nd pressures normally found In natural wa ter sys tems. The gas (N H )) exis ts in equilibriulTl With the aqueous ion ic fo rm called ammo nium (N H .. +). (2-24 ) The hydroxyl io n co nc entrati o n of the water. and thu s the pH. co ntro ls th e relati ve .... ab unc.I
WATER QUALITY: DEFINITIONS, CH ARACTER ISTICS, AND PERSPECTIVES
46 WATER
o n was tewa ter and other polluted waters, wh ile the test fo r nitr:lte is th e most common o n c lea n-wat er sa mpl es and tre a ted was tewa ters. Phosphorus Ph os ph or us appears ex clus ive ly a s phos phat e (P0 4 ) - ) in aquatic en vi ro nm ents. There are seve ral form s of phosphate, however. inc ludin g o rth op hos phate, conde nsed phosphates (pyro-, meta-, and po ly ph ospha tes). and orga nica ll y bou ll d ph osp hates. Th ese ma y be in so luble or particulate fo rm or ma y be cons titu ents of plant or a llJlnal ti ss ue. Like nitrogen. ph os phates pass through the cycles of decompos iti() n and phot osy nthesis. Ph os phate is a const itue nt o f soi ls and is used ex tens ive ly in fertili zer to rep lace and /o r s uppl e ment natu ra l quantities o n ag ri c ultural la nd s. Ph ospha te is a lso a constituent o f anima l was te and may beco me incor po rated into th e so il in grazing and fe edin g areas. Run o tT from ag ri cu ltu ra l areas is a m ajor co ntribut o r to ph osphate in s urfa ce waters. The tenden cy for phosphate to :ld so rb to so il particles limit s it s mO\'eme nt in so il mois tur e and gro und wa ter, but res ult s ill it s trans po rt Int o surfa ce wa te rs by ero sion. Muni cipa l wastewater is ano th er major so urce of phosphate in s urface water. Condensed ph os phat es are used extens ive ly as builders in detergents , and o rga nic ph osph at es ar e co ns tituent s o f body was te and food res idue. Other so urces inc lud e indu st rial waste in which ph os phate compounds are used fo r suc h purposes as boiler-wZi te r co ndit ioni ng. While p hos phates are n o t toxi c and do no t re prese nt a direct hea lth threat to human o r o ther o rgani sm s, the y d o rep rese nt a se riou s indirect threat to wa ter qualit y. As no ted earlier. phos pha te is o ft e n th e limitin g nut rie nt in s urface wa ters. ···-Wf1 e il·th e"iv·~1ibb l e·· s·Llppli ls ini.:re::i se d, rapid grow th o f aquatic plant s usually results. w ith seve re co nsequences. Ph os phat e can a lso interfere with wa te rt reatl11ent processes. Co ncen tr a tion s as low as 0.2 mg/ L interfere with th e c he mical ' coag ulati u n o f turbidit v. [2-20J Ph os phat es are mea s ured co lor imet ri call y. Orth o ph osphates can be meas ured dir ect ly. whi le co nd e nseu form s mu st be converted to o rth ophosphate by ac id h vdl"U lyza tiun and o rg anic phosphates must be con verted to orthophosphates by
Biological Water-Quality Parameters Water ilia: ser\'e as a m ed ium in \\ hich lit era ll y th ousa nd s o f biological sr ecies spend pa rt. ir not ;dL ,)1' their lire cvc les. Aquatic organisl11s 1'<1nge in s ize
47
ete rs, because their presence or absence may indicate in general terms the characterIstiCS of a given body o f water. As an example, the general quality of water in a trout stream would be .ex.pected lo exceed that o f a stream in which the pred o mmant species o f fish IS carp. SlInIia rl y, abundant algal populations a re associated With a water ri c h in nutrients. '. Biologists often use a species-diversity index (re lated to th e number of species and the relative abundance oforga.nisms in each species) as a qualitative parameter fo r streams and lakes. A body of water hostin g large numbers of species with wellbalanced numbers of individuals is considered to be a health y system. Based on their kn own tolerance fo r a given pollutant, certain organisms can be used as mdl . IS . . ca .tors C of the prese nce o f pollutants. A more det a iled coverage of this t OplC give n m hap. 3.
2-]5 PATHOGENS Fro m the perspectIve of human use and consump ti on~ the most impor ta nt biological organis ms III water are pathogens, those organisms capable of infecting, or o f transmlttmg dIseases to, humans. These orga nisms are not na tive to aquatic sys tems and usually require an anim a l host for growth and reproduction. They can, however, be tra nspo rted by natural water systems, thu s becoming a temporary me mber of the aqua tI c community. Many species of pathogens are ab le to survive III water and maintain their infectious capabilities fo r sign ifican t periods of time, These. waterbo rne pathogens include species o f bacteria, viruses, protozoa, and hetm1l1ths (parasitic worms). The characteristics o f the p~im ary waterborne path oge ns are listed in Table 2-7.
Bacteria The word bact eria co mes fr o m th e Greek word meaning " rod " or "staff," a s hape characteristic o f mos t bacteria. Bacter"ia a re sin'gle-ceU microorganisms, usually co lorless, and are the lowes t fo rm of life capable of synthesizing protopla sm fr o m the surrounding environment. In addition io the rod s hape (bacilli) mentioned above, bacteria may also be sphe rical (cocci) or spiral-shaped (spirilla). Gas tr 0 1l1test1l1 al disorders a re common symptoms o f most diseases trqnsmitted by wate rbo rne pat hogenic bacteria. C holera, the di sease that ravaged Europe during the eighteenth and ninetee nth centuries, is tran smitted by Vibrio comma. Among the most violent of the waterborne bacteri a l diseases, cholera causes vomiting and diarrhea that, without treatme nt. resu lt in dehydration and death. Symptoms of typhoid. a disease tran smitted by th e wa terbo rne pathoge n, Salmoll ella typhos{l, include gas troIIltestllla l di so rders, high fever. ulceration o f the intestines, and possible nerve damage. Although immuniza tion o f indi viduals a nd disinfection of water supplies ha ve el l mlnated cholera and typhoid in m os t parts o f the world, a reas of deve lopIng cu untrIes where overc rowding and poo r sa nit a ry cond iti on s preva il still
48
WATER
WATER QUALITY: DEFINlTIONS, CHARACTERISTICS, ANl) PERSPECTIVES
Table 2-7 Common waterborne pathogens Organism Bacteria Francisella lUlarensis , Leptospirae
Salmoneila paratyphi (A,R,C) Salmonella typhi Shigella (S.jfexneri, S. sonnei, S .. dysenteriae, S. boydi!) Vibrio comma (Vibri~ cholerae) Viruses Enteric cytopathogenic human orphan (ECHO) (ECHO) Poliomyelitis (3 types) Unknown viruses Protozoa Entamoeba histolytira
Giardia lamblia Helminths (parasitic worms) Dracunculus medinensis
Disease
Tularemia (deer fly fever) Leptos pirosis (Wei!'s disease, swineherd's disease, hemorrhagic jaundice) Para typhoid (enteric fever) Typhoid fever, enteric fever Shigellosis (bacillary dysentery) Cholera (Asiatic, Indian, El Tor) Aseptic meningitis, epidemic exanthem, {nfantile diarrhea Acute anterior poliomyelitis, infantile paralysis Infectious hepatitis Amebiasis (amebic dysentery, amebic enteritis, amebic colitis) Giardiasis (Giardia enteritis, lambliasis) Dracontiasis (dracunculiasis ; dracunculosis, medina; serpent, dragon , or guinea-worm
Echinococcus Schistosoma (S. mansoni, S.japonicum, S. haematobium)
. infection) Echinococcosis (hydatidosis; granulosus; dog tapeworm) Schistosomiasis (bilharziasis or .. Bill Harris " or .. blood fluke " disease)
experience occasional outbreaks of these two diseases. Temporary lapses in good sanitary practices sometimes result in outbrea~s of gastroenteriti? caused by some of the other bacterial pathogens listed in Table ?-7.
49
Immunization of individuals has reduced the incidence of polio to a few isolated cases each year in developed nations. Outbreaks of hepatitis are more common, with around 60,000 cases reported in the United States each year. Most of the hepatitis cases result from persons eating shellfish contaminated by viruses from polluted waters, [2-2J although an occasional outbreak will occur at campgrounds or other facilities where crowds gather and where water-supply protection and sanitary facilities are poor. Although standard disinfection practices are known to kill viruses, confirmationof effective viral disinfection is difficult, owing to the small size of the organism and the lack of quick and conclusive tests for viable virus organisms. The uI.1certainty of viral disinfection is a major obstacle to direct recycling of wastewater and is a cause of concern regarding the increasing practice of land application of wastewater.
Protozoa The lowest form of animal life, protozoa are unicellular organisms more complex in their functional activity than. bacteria or viruses. They are complete, self-contained organisms that can be free-li ving or parasitic, pathogenic or nonpathogenic, microscopic or macroscopic. Highly adaptable, protozoa are widely distributed in natural waters, although only a few aquatic protozoa are pathogenic. Protozoal infections are usually characterized by gastrointestinal disorders of a milder order than those associated with the bacterial infections discussed earlier. Protozoal infections can be serious nonetheless, as illustrated by an epidemic in Chicago in 1933 in which over 1400 people were affected and 98 deaths resulted when drinking water was contaminated bY ,sewage containing Entamoeba histolytica. [2-14]. Many .cas.es .of .giardiasis,. D[·backpaekers 'disease,' have' 'been . reported in recent years among persons that drank untreated water from surface streams. This infection is caused by Giardia lamblia, a protozoan that may be carried by wild animals living in o r near natural water systems. Under' adverse environmental circumstance, aquatic protozoa form cysts that are difficult to deactivate by disinfection. Usually complete treatment, including filtration , is necessary to remove protozoal cysts. .
Helminths Viruses Viruses are the smallest biological structures known to contain all the genetic information necessary for their own reproduction. So small that they can only be "seen" with the aid of an electron microscope, viruses are obligate parasites that require a host in which to live. Symptoms associated with waterborne viral infections usually involve disorders of the nervous system rather than of 'the gastrointestinal tract. Waterborne viral pathogens are known to cause poliomyelitis and infectious hepatitis, and several other viruses are known to be, or suspected of being, waterborne.
b._ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ____
The life cycles of helmil1ths. or parasitic worms, often involve two or more animal hosts, one of which can be human , ' and water contamination may result from human or animal waste that contains helminths. Contamination may also be via aquatic species of other hosts, such as snails or insects. While aquatic systems can be the ve hi cle fo r transm itting helminthal pathogens, modern water-treatment met hod s are very effective in destro ying these orga nisms. Th'us, helminths pose ha za rds primarily to th ose persons who come into direct con tact with untreated water. Sewage plant opera tors. swimmers in recreational lakes polluted by sewage o r stormwater runoff from ca ttl e feedlots. a nd farm laborers employed IJ1 agnCultural irriga ti on operat ions are at particular risk. [2-5J
A. Presump tive tes t
50
WA T ER
2-1 6 PATHOGEN INDICATORS An a lys is of wa ter fo r a ll the kn ow n pa th oge ns wo uld be a very time-consumin g a nd ex pensive pro positio n. Tests for specific path ogens a re usua ll y made o nl y whe n there is a reaso n to suspect t ha t th ose particula r o rga ni sms a re prese nt . At other ti mes. th e pu rity of wa ter is checked using ind icato r o rganisms . An indicator organ ism is one whose presence p res umes that co ntami nat ion has occur red a nd sugges ts the natu re and exte nt of t he co ntam ina nt(s). T he idea l path ogen indica to r wo uld ( J) be ap plica ble to a ll types of wa ter. (2) al ways be present 'w il en pa t hogens are present . (3) a lways be a bsent when pa thogens a re a bsent. (4) lend itse lf to ro utine qu a ntit a ti ve tes ting procedu res wit ho ut in te rference fr om o r co nfu sio n of res ults beca use of ex tra neo us o rgan isms. a nd (5) for the sa fety of la bora to ry perso nn el. no t be a pa thoge n it se lf. [2-6J Most of the wat erbo rn e pathogens are lilt rod ucecJ t hroug h fecal con tamina tio n of wa ter. Thu s. any o rga ni sm na ti ve to t he in tes tinal trac t o f hu malls and meet in g the above criteri a wou ld be a good indica tor organism. The (lI"ganisms mos t nea rl y mee ting these req ui re ments be lo ng to t he fecal co lifor m gl·o up. Composed . of severa l st rain s o f'bacteri a, p ri ncip a l of whic h is Escherichia coli. these orga ni sms are fou nd exc lu sive ly in t he int est ina l tract of warm -blooded anllll~ " s a nd arc exc reted in la rge n umbers wit h feces . Feca l co lifo rm o rgan isms arc nonpa t hogenic a nd a re belie ved to have a lo nger surv iva l time o utsid e th e a nim a l body tha n d l) mos t path ogens. Beca use the d ie-o rT rate of feca l co liform , is loga rithm iC. the number of sur viv ing organisms may be an ind icat io n of th e time la pse sin ce conta min a tion . Th ere are o ther co liform gro ups whi ch fl o urish o utside the intestina l tract of a nll11 al s. Th ese orga nisms a re nati ve to th e soi l anc! decayi ng vegeta tin n a nd are oft en fo un d in wa ter tha t was in rece rr t' co nt ac t' wi th' th ese· m,lwr"i !d"s".· Bc'c,i lI SC' the life cycles of so me pa thogens (pa rti c ul arly helminth s) may inc lu de periods in the soil, this gro up of co lifo rm o rga ni sms a lso se rves as a n Indica to r of pathoge ns. It is the usua l prac tice in .th e United Sta tes to use the to ta l co li form gro up (th ose of bot h feca l a nd no nfeca l o ri gin ) as indica tors nf the sa nitar y q ua lity of d rinking wa ter. whi le the indi ca to r of cho ice for wastewa ter emucn ts is the feca l co lifo rm gro up. Rela ti ve ly simp le tests have been dev ised to deter mi ne the p resence of co lifo rm bacter ia In wa ter a nd to enumera te the qu a nt ity. T he tes ts 1'0 1' to ta l co liform orga nisms employ sli ghtl y differen t cultu re medi a and lowe r Illcubat ion te mperat ures t ha n th ose used to iden tify feca l co li form organisms. T he membra ne-fi lter tec hnique. a tec hniq ue po pu lar with cnVlrl.lllmenta l engin ee rs. g ives a direc t co un t of co lifo rm bac teri a. In thi s tes t. a por tion of the sa mple is fi ltered thro ugh a memb ra ne. the po res of whi ch d o lI ut exceed 0.45 li m . Bacteria are re tamed on the hlter tha t is t hen placed on selecti ve rn ed ia to prul1lu te growth of co ii form bacteria wh ile in h ibiti ng growt h of o th er spec ies. The memb rane and medi a are incuba ted a t the ap propfl ate tem perat ure' fDr 24 h. a ll owing co li fo r m bacteria to grow intn vis ible co lonies th at are t hen cll un ted . The resul ts arc re pc)I"ted In n umbe l' o f o rga nisms per 100 m L of wat er
r
Inoc ula Ie laur yl tryp lose b ro lh ferme nta lion lubes and incubale 24 ± 2 h a l 35 C ± O.SoC
i
(2)
( I) Cas p roduced Posit ive presumptive
No gas, o r gas produ c tion doubt ful Incub a le additi ona l 24 h ( lo lal 4 8 ± 3 h)
[esl
(a ) Cas pro du ced . Positi ve tes t
(b) . N o gas produced Nega live test Colifor m group absent
B. Confirmed le st
l
Inoculate la ury! tryplose broth fe rme ntation tu bes and incubat e 24 ± 2.h at 35 C ± O.SoC
( I) Cas p roduced Transfer to confirma tory brilliant
(2 ) No gas, Or gas prod uc tion doubt fu l In cubate addi tional 24 h' ( to tal 4 8 ± 3 h)
green lactose bi le bro th
Incub"le 48 ! 3 It al 35 C ! O.SoC (a) Cas p roduced Coliform group
confirmed
( b) No gas produced Nega tive lest Colifo rm group absent
(a) Cas produced o r d oub l ful Confir m as in B(l)
(b) No .gas p rod uced Negative tes t Colifo rm gro up absent
C. Com pie led test
l
Inoc ulate la ury l tryp lose broth fe rmentation tubes and incuba te 24 ± 2 h at 35 C ± O.SoC
( I) Gas produce d. Transfer [Q confirmatory brill ia nt gree n lac tose bile Incubale 48 ± 3 h al .. .... 35.C.±.D.tC .. . .... . . .. .. .. (a) Gas prod uced Transfer 10 End o o r EMB plales Incub",e 24 ± 2 h at oS C ± D.5°C
(2) No gas, or gas p ro du c tion do u btful In cubal e addit io nal 24 h (I ot al 4 8 ± 3 h) . ... . .
.
(b) No gas pro duce d Negative test Coliform gro up absent
( I I) Typical OR alypical coliform colonies Transfe r to agar slant and laury l try ptose bro th fermentat ion tube Incubale aga r slanl 181024 hand lauryl tryplose brolh 24 ± 2 h 1048 ± 3 h at 35 C ± O.5°C
( a) Gas produced Gram-st ain portion of ::lgar sian I grow th
(a) Gas produ ced o r do ub t ful Conti n ue as in C( I)
(b) No gas prod u ced Negat ive te st Coli fo rm group abse nt
( 1.1 2) Spores or gram·positive rods and sp ores presen t Com ple led lest: coliform group abse n t
Fig ure 2-7 P roceuure J','r r u nnin g lotal coliform analysis by th e m uli iple.[ube fe r mentat ion met h od.
(From SlanJaul Melhods [2.15].)
T
(1.2) Negati ve colonies Coli fo rm group abse nl
(b 'j No gas produced Nega tive tes t Colfform group absen l
(II I ) Cram-nega tive rods present, 11 0 spores prese nt Completed tesl: coliform group presen t Gram ·posit ive and ·negative rods hoth presen[ Repeat pro(cdu re beginning at 1.1
T
r
r
r
52 WATER
W ATER Q UALI T Y : DEFI N IT IONS, C HARACT ERI ST ICS, AN D PERSPECTI VES S}
An a ltern a ti ve met hod often preferred by micr o bi o logists is th e mu lti p letube ferment a tio n test. Colifo rm o rgan ism s are kno w n to ferment lac tose, w it h o n e o f the end prod u cts bei n g a g as. A brot h containing lactose a nd othe r substa nces whic h inhi b it n o nco lifo rm org a n ism s is p laced in a series of test t u bes wh ich a re then inocul a ted wit h a d ecimal fract ion of I mL (100. 10,1.0, 0. 1. 0.0 1, etc.). These tubes are inc u bated a t the appropria te tempe ra ture a nd inspec ted for d ev~l opment of gas. This fi rst stage of the procedure is ca lled the pres umpt ive l est. a nd tu bes wi th gas deve lopmen t are pres umed to have colifo rm s present. A sim ilar tes t. called th e confirmed leSI, is t he n set up to c o nfirm the prese nce o f co li fo rm o rgan ism s. A schema t ic o f"th is process is sho wn in Fi g. 2-7. A sta tistica l me thod is used in co nju nc tio n wi th Table 2-8 to de termine th e m os t pro ba ble n u m ber
(M P N) o f colifo rm ba c te ri a in J 00 m L o f the wa ter sa mp le. Th is meth od is ill ustra ted in th e fo ll ow ing exam ple. Exam pl e 2- 10 : Determin ing th e most probable num ber of co lifo rms A stand a rd m u ltiple tube fe rm e nt a ti on test .is ru n o n a sa m p le o f wa te r fro m a surfa ce strea m. The resu lts of the an a ly sis fo r t he c'o n fi rmed tes t a re shown be lo w .
Size o f sa m ple. mL
95\ co nfi d e nce limit s
0-0-0 . 0-0-1 0-1 -0 0-2-0
.. ' " T-2-0· · .. .. .. 2-0-0 2-0- I 2- 1-0 2- 1- 1 2-2-0 2-3-0
---
Lowe r
<0 2 2 4
< 0 .5 < 0 .5 < 0 .5
2 4 4 6
1-0-0 1-0- 1 1- 1-0 1-1 - 1
C o mbi n a t io n
MP N index /1 00 mL
..... o·.. .
< < < <
0 .5 0 .5 0 .5 0 .5
.. . '<'0)" " < 0. 5 I
7
9 9
2 2 3
12
U ppe r
II
II II 15 '1 5 "
I, 17 17 21 21
28
3-0-0 3-0- I 3-1-0 3- 1- I 3-2-0 3-2-1
8
19
II It 14 14 17
25
4-0-0 4-0 -t 4 - 1-0 4- 1-1 4- 1-2
13 17 17 2I 26
4
4
25 34 34 46 31 46
46 ')
63 7K
of
p os it i\"c~
4-2-0 4 -2- 1 4 -3 -0 4 -3- 1 4-4-0
22 26 27 33 34
5-0-0 5-0- 1 5-0- 2 5- 1-0 5- 1- 1 5- 1· 2
23 31 43
5-2 -0 5- 2-1 5-2· 2 5-3· 0 5-3- I 5-3 -2 5-3 -3 5· 4-0 5- 4- 1 5- 4-2 5-4 -3 5-4-4 5- 5-0 5-5 · I 5-5-2
I 5-5 -3
L~-4 2' - ) - )
SOl/ r cf: Fro m Sm ilh .
MP N in dex . 100 mL
. . .3~. 46 63
49 70 94 79 11 0 140 I KO 130 1711 ~2 0
280
YiO 24() 350 <;40
no 1. 600
Lowe r
9
9 II 12 II 15 II 16 21
Upper
70 89 110
51)0 300 490 70n
oK 12(1 180
}OO (41)
.-
0
5
.. ... ... ... . .
120 150
44 35 43 57 90 120
4
O.DOI
.4
Se lec t a series whe re th ree tubes ea ch have pos iti ve res u lts (n o t necessary, b u t reco m me nd ed): use samp le size s 10. I . and O. I o En te r T ahl e 2-8 w ilh th e number o f pos iti ve tubes Ollt o f fi ve (4. 2, I) : T he cor res ponding M P N is 26 wit h ;, ra n ge o f 9- 78 o rgani sm s pe r 100 m L p oss ible a l a 95 pe rce nt co nfid e nce le vel.
93
130 170 220 190 25 0 340
I
S OL UT IO N
93
17 23 28 25 31 37
0. 1 0. 01
D ete rm ine th e m os t p roba bl e numbe r o f co lifo rm orga n ism s.
67 78 80
.. ...93 .
4
I
Table 2-8 MPN index and 95 % confidence limits for va ri ous combina ti ons of positive results when fiv e tubes a re used per dilution (10 mL, 1.0 mL, 0.1 mL)
C ombin a tion o f p osit ives
No . nega li ve
----10
95 ~-:; co nfid e nce l im its
N o. posiJl ve
ALTER NA TE S OL UT IO N
I Selec i samp le sizes 1.0 , O. f , an d U.O f. 2. h o m Ta ble 2-X, Ihe co rre s po ndin g M PN is 9 a nd Ih e 9 5 pe rce nl co nfi d e nce range is 2 thr o ugh 2 1. 3. Bec
Samp lin g lec h n iqu es and s uh se qu ent ha nd li ng o f the sa mpl es a re extrem ely 'I m[1 o rt :lIlt beca use sa mp les ca n ea sil y be co nt a min a tecl . D e ta iled p rocedures fo r samp li ng. sa'm p'le prepa rat io n, a nd ste rili za tio n o f labo ra to ry equ ipme nt fo r bo th th e me mbr an e fili er tec hni q ue a nd t he muli ip le- tu be tec hniqu e are p rese nt ed in
X"II I. ()O()· 7~ 1J
I .OOG 1.400 3.200 5,gOO
SWI/r!ord At !'1hoi/.I. [2- 15J
It sh uu ld be em[1ha sized aga in th a t pa thoge ns arc no t Id en tifi ed by the co lifo rm l e~ t. Th e p re se nce of co lifo rm orga ni sm s in wa te r d oes. ho we ve r. indica te tha i so m e por ti() n uf th e wa ler ha s recen tl y co n tac ted so il o r d eca ying veg eta ti o n t) r h:t, hlT ll l ilro lig h the Int es tinal tra c t ora w:lrm -hl oocl ed an im a l. T he a ss umpt ion Illlh l tilt'n ht: 11 l: I(k tha t pa th oge ns Illa ), have acco m pa n ied th e co lifo rm bac teria.
2 2,40(1
r2· 14]
l
5;1
WATER QUALITY: DEFI NIT IONS, C HARA CTERISTICS, AND P ERSPECTIVES
WATER
Water-Quality Requirements W ater-qua lit y requirements va ry acco rding to the pro posed use o f the water. Detailed desc ripti o ns of the qualit y criteria for agricultural use, fish and wild life propagat ion. s pecific indu stri a l a nd recrea ti o nal uses, and powe r ge nera ti o n are prese nt ed elsew here, [2- 7, 2-6, 2- 17]. and s uch desc ription s are beyond th e sco pe o f this text. Water unsuitable for o ne use m ay be quite sa tis fac to ry fo r another. and wa ter may be deemed acceptable for a pa rti c ular use if water o f be tt er qu a lity is no t ava ilable. . W a ter-q uality req uire m e nt s s hould no t be co nfused w ith wa te r-qualit y sta nd ard s. Set by the po tenti ,il use r. wa ter-qualit y req uirem ents re present a known or ass um ed need and a re based o n th e pri o r ex perience o f the water use r. Walerqualil.'r' standards a re set b y a gove rnmental agency and represent a sta tut o ry req uire ment. For exa mpl e. a farm e r ma y hlOW fr o m prior ex perie nce Ihat highly sa lin e wale r will damage th e crops. bUI th ere a re n o o ffi c ial wa ter- qualit y s ta nd ard s that sa y s uch wa te r ca nn o t be used fo r irri ga ti o n purp oses. In th e U n ited Sta tes. s tand ard s have bee n pro mul ga ted fo r strea ms a nd lak es. for public wate r s upplies. and for was tewa ter disc harges: Stand a rd s fOI surface wa ters. potabl e wa ter. a nd was tewa ters a re di sc ussed in th e fo llowi n g scc ti on:
2-18 POTABLE-WATER STANDARDS Sta nd a rd s for drinkin g wa ter have evo lved over the years as knowledge of the nature and elfec ts o f various contaminants has grown. Current ly, it is considered desirable tha t dr inking wat er be free o f suspended so lids and turbidity, that it be tasteless a nd odorless. th a t di sso lved in organ ic so lids be in moderate quantities, and that o rga nics. toxic substances, a nd pathogens be absent. As more is learned about the co nstituents of water. additional requirements will probably be added to thi s list. making drinkin g-w a ter requirements even m o re stringent. The World H eal th Organizat io n has established minimum criteria for drinking wate r th a t a ll na ti o ns a re urged to meet. These standards are listed in T a ble 2-9. Co untries with m o re advanced technol ogy generally have sta nd a rd s that exce~d t hi s qua lit y.
Table 2-9 Drinking-water standards of the World Health Organization Concentrations in milli g ram s per liter W H O Internali o nal (1958)
Chem ica l constit uen t
2-17 IN-STREAM STANDARDS
Per mi ssib le
Excessive
iimit
limit
Maximum lim it
,\lI1monia (N H,)
WHO Euro pean (1961) Reco mmended limit
T o le ran ce limit
0.5
A r st.:nic
F o r reaso ns o f aes t he t ics as we ll a s hea lth . it is ge ne rall y co ns id e red de s irable to --'m ;;;rii il;ii ' il:ili.ir~1·1 water sys te ms at as hi g h a quality leve l as poss ible . /\11 SO o f th e s t:l! es ha ve set minimum qual it y s tand ards for a ll s urfa ce wate rs lVilh in th eir boundar ies. These standard s o ft e n re Aec t th e be ne fi c ia l use lilad e o f til e stream. That is. mo re stringent sta nda rd s are app li ed to a strea m used as a so urce o f water for muni ci pal rurr oses th a n to streams lI sed fo r o th er purp oses. Man y fa c to rs affec t stream qualit y. W as tewa ter di scharges and o th e r human ac ti vities o ft e n ha ve s ign ifi ca nt impac t o n in-stream wa ter qualil Y· Th ese ac ti vities m ay lend th e mse lves to con tr o l by legis la ti o n and gove rnm en t reg ulati o ns. F o r examp le. co nsis tent with w ha t it be lieved to be publi c o rini o n. th e U nit ed States C o ngress passed the Water P o lluti o n Co ntr o l Ac t o f 1972 ( Publi c L~t w 92-500) a nd , w ith min o r modi fi ca ti o ns. renewed it in 1977. A sta ted goa l o f thi s legis la ti on is that a ll s urface wa ters in th e United Sta tes be maintained a t " fi sha ble, sw immab le " qualit y. Attaimnen t o f thi s goa l s ho uld re sult in a qualit y s uffi c ie nt fo r m os t wa ter uses and simpl ify trea tm ent processes for wate rs int end ed fo r po tabl e and indu stria l use. While leg is lati o n such as Public La w 92-500 ca n con tr o l so me as pects o f wa ter pol iutillil. th~ geo illgy o f a wmershcd. coupled with Ol her natura l ph cno m ena . is o ft en th e co n trolling factor in wate r CJua lit y. Thi s fact mu st be co ns id ered if in s trea m water-qu al it y s tand a rd s ar~ 10 be reali sti c.
55
0.2
Ca clnllulll Calci um 75 Ch lo ride ~OIJ C hr omi um (hexava lent) Cn prcr 1.0 Cyanide Fluo r id e I ron 0 . .1 Lea d Magnesi um ·50 Magnesium + sod iu m sulfate s 500 Manganese 0. 1 Ni trat e (as NO,) Oxygen. di sso lwd (milllmulll) Ph e nolic c<) mpo und s (a s phen o ls) 0.001 . Se le nium Sul fat e 200 T ot ,iI so lid s ' SOU Zinc 5.U
. .. . . . . . . . . . . 200 600
0.2 0.05
350 0.0 5
0.05 3. 0'
1.5 0.01
0.0 1 1.5
0. 1
1.0 0.1 150 1000 0.5
0. 1 125t 0. 1 50 5.0 0.001
0.002 0.05 400 1'00 15
0.05 250
5.0
• After 16 h co nta ct \\1 th "ell pipes. hilt "'ato r entering a disl r iblltion system s ho uld have less than D.I)) mg/ L o f co pper. t If there is 250 rn g; L o f sulf:lIc prl·soIH. Illa)!"csillrn sho uld not exceed
S()/I/'n': Ad"p ted fr o nt Todd .
[2- 17J
30 mg! L.
56
WATER
The Safe Drinking Water Act of 1974 (Public Law 93-523) mandated the U,S, Environmental Protection Agency to establish drinking-water standards for all public water systems serving 25 or more people orhavingJ5 or more connections, Pursuant to this mandate, EPA has established maximum contaminant levels for drinking water delivered through public water supply distribution systems, These standards were published in 1975 under Title 40, Subchapter D, Part 141 of the Code of Federal Regulations, [2-19J The maximum contaminant level (MCL) of inorganics, organic chemicals, turbidity, and microbiological contaminants are shown in Tables D-l through D-5 of the appendix, EPA has also issued proposed ~egulations to serve as guidelines to the states with regard to the 50called secondary drinking-water standards, [2-16J These appear in Table D-6 of the appendix,
2-19 WASTEWATER EFFLUENT STANDARDS The water Pollution Control Act of 1972 (Public Law 92-500) mandated the Environmental Protection Agency to establish standards for · wastewater discharges. Current standards require that as a minimum all municipal wastewater be treated to "secondary" standards shown in Table D-7 of the appendix. More stringent standards may be imposed where necessary, and in some cases less stringent standards may ' be permitted for small flows. Industrial disc hargers are required to treat their wastewater to the level obtainable by the "best available technology" for wastewater treatment in that particular type of industry. If industry discharges to a municipal wastewater collection system, the industrial waste must be pretreated 'so as to be compatible with the untreated municipal wastewater. The EPA regulations define receiving streams as either "effluent-limited" or "water-quality-limited." An eJjlue';I-limiled stream is a stream whi<;h will meet its in-stream standards if all discharges to that stream meet the secondarytreatment and best-available-technol ogy s tandards. Municipalities a.nd indusrries discharging to effluent-limited streams are as~igned discha rge permits under the National Pollution Discharg!,! Elimination System (NPDES) which reflects tlie secondary-treatment and best-available-technology s ta nd a rds. A wacer-qualit),-limiled-s£ream would 11 0 1 m~et its proposed in-stream standards even if all discharges met secondar y- treatment and best-available-technology criteria. Discharges to these streams may be required to meet effluent co ndition s more stringent than secondary-treatment and best-available-technology. These discharge limits are established on a case-by-case ba sis. . . Effluent standards. potable-water standards, al)d ill-stream standards are obviously interrelated. Enforcement llf eftluent standards. along with the COnlroi of nonpoint sources of p o llution, s ho uld result in the attainmcnt o r in-stream standards. The improvement of in-st ream quality s hould res ult in a better raw water for potable supplies. However. it is impractical to expect surb ce waters. even if in-stream standards are mel. to meet all of the ma ximum contaminant
WATER QUALITY: DEFiNITIONS, CHARACTER ISTICS, AN D PERSPECTIVES
57
levels imposed by the potable-water standard s, Treatment of surface water for potab le use will a lwa ys bc rcquired, the nature and level of treatmen t dependll1g o n the in -st ream qu a lit y o r th e water so urce.
DISCUSSION TOPICS AND PROBLEMS 2-1 Name th e phys ical wat er-qualit y parameters of concern to environmental engineers. 2-2 Discuss th e so urces a nd impacts of suspended so lid s.
2-3 How are suspended so lid s mea surerP 2-4 An analvsi s for suspe nded so lid s is run as follow s: ( I) A fibergla ss filter is dried to a constan t ma;s of 0.137 g; (2) 100 mL o f a sa mple is drawn through the fi lt er ; a nd (3) the filt er and res idue are placed. in a dr ying over at 104°C until a co nstant ma ss of 0. 183 g IS reached. Determine the suspended-so lid s concen tra ti on in milli grams per lit er. 2-5 One hundred milliliters of th e frltrate from a suspended-so lids ana lysis is placed in an ev aporation di sh whose tare Illass has been determined to be 327.48 5 g. The co ntents of th e di sh are evaporated to dryness. and th e lotal mass of the dish and so lid s is fou nd to he 327.5 17 g Determine the quantit y of filterable residue (in milli grams per lit er). 2-0 Th e cr ucib le. tilter pad . ;lIld so lid s of Example 2- J arc rlaced in" muftle furnace at 600"C for I h. Afte~ cooling. the mass is delermlned to be 54.367 g. Dete rmin e the coneent ration o f th e vo latile (orga nic suspend ed so lids) 2-7 The evaroration di s h and resid ue from Prob. 2-5 is placed In a Illuflle furn ace at 600°C for I h. Aftercooling.lts ma s, IS found to be 317.498 g. Determ in e the blt era ble vo latile(orga nl c) so lids of the sample in milli gra ms per Iil er. 2-S Discuss the so urces and imp ac ts of tastes and odors in \\ ater suppli es. 2-9 Ho\\' arc 1,1stes and odors mea sured 0
2-10 \Vil,;t',,'r'e't i;~~~;lr~~'s ,;f' ; ~ ;l~ pe~;I; ~lr~
____ ____ __
'i;l~;~;;S~~ il;'; ,Iter l)~'(i;~~:' WI;,;ta~~ t l;~ impacts
of eleva ted temperatures') 2-11 Namc th e chemical parameters of concc rn 111 water -qu,rlity man;rgcment. 2-12 The reaclion of so da ash (Na,CO J) ,~rth calc ium su lfat e in water is represented by the follolVing chemical stat ement :
Assumi ng that this react io n is comp lete and that there. is 1:;3 rng,'L nf CaSO" init ial ly present~ what is tire ma ss of soJa ash that mu st be added \!) (II) I L of the lI'ater a nd (b) 10.000 m ol' the wat er to comp lete th e reac ti o ns. 2- n How man y grams of CaO arc reqUired to he t he chemical eq uivalent of 246 g of Mg( HCO, );)
2-14 Express Ihe foll owin g co ncentrations of clem ents and compounds as milligrams per
lit er ofCaCO,.
X7 mg. L " 'II" •
IX9 mg :L Na HCO ,
58
W ATE R QUA LIT Y: DE FI NITI ONS, C H ARACTERISTI CS, AND P ERSPECTiVES
WATER
2-15 Express the fo ll ow ing mo la r co nce ntra ti o ns o f e lements a nd co m pou nd s as mi lli g rams pe r li ter of ('aCO). 10 -
2
mol; L /\1 3
J.5x 10 -
3
mol; L SO/ -
x 10 -
.I
lll ol i L
I
32
X
cr
'
2-20 Draw a milli equivalent-per- liter ba r dia gram fo r the wa ter with the following common ion concentrati o ns. 70 mg/ L
1.8 x 10 - -' mo l ' L CaSO.
165 mg/ L
2. 1 x 10 -' moli L Mg(CI) ,
3.5 x 10
J
mol i L NaO H
59
M g2
28 mg/ L
N a + = 124 mg/L
SO / - = 173 mg/ L
C I- = 202 mg/ L
'
=
De termine tlie erro r in th e io n bala nce. 2-2 1 An an a lysis of wat er fr o m a su rface strea m yields the fo llowing results.
2-16 Determ ine the conce nt ra ti o n of the fo ll owing ions in solution at eq u ilibrium witll the so lid al ~5 ' C . . ('a(O H ), MgC0 3 CaSO. 2-17 A was tewater con tai nin g Fe( HCO}h is discharged to a sur face pond. Ass u min): wmplete ox id at ion o r t he Fe" to Fe} ' a nd s uffic ie n t 0 H for t he fo ll owi ng react io n to OCCLI r
Ca 2 +
= 60 mg/ L
Mg2+
10 m ~\/ L
SO/ -
96 mg/ I...
Na +
7 mg/ L
NO ) -
10 mg/L
K + = 20 mg/ L
CI -
II mg/L
H C O ) - = 11 5 mg/L
If an e rro r o f 10 percent is acceptable, sho uld the an a lys is be considered co mplete ? 2-22 What are the so urces and impacts o f di sso lved so~id s in water supplies? 2-23 How are di ssolved s o lid s measured? How are TDS measurements expressed?
d eter mine the concentratio n (mg/ L as CaC'OJ) of the Fe) ' remain in g dissolved in the pond wat er. 2-18 A sa m ple or water from a s urfa ce s t rea m is a nal yzed fo r the co m mo n ions with the foll ow ing resu ls Ca'
98 mg/ L
>
C1 -
I-I Co., -
89 mgi L = 317
ru g,il
22 mg/ L Na "
7 1 mg ' L
S.04' (a)
125 mg: L
W hat is the percen t e rro r in the catio n- a ni on ba lance ')
(h) Draw a bar diag ram for th e wa te r.
2-19 ,.\ s~lInp l e of wate r was a na lyzed fo r com m o n io ns with the res ul t show l1 beIcHI·.
I-I Co., ·
-.
30n mgL
Na ' = liS mgL
SO"
,-
Mg "
-- ~4() mg i L
= 36.6 mg, L
2-24 A so lids a nal ys is is to be co ndu cted o n a sample o f wa stewater. The procedure is a s fo ll ows : I . i\ G och crucible a nd fi lte r pad are dried to a constant mass of 25.439 g. 2. Tw o hundred millilite rs o f a well-sha ken sample of the wastewater is p assed through the
filt er.
3. Th e crucible, filt e r. pad . and rem oved so lid s a re dried to a constant mass o f 25.645 g. 4. O ne hundred millilit ers o f the filtr a te [ wa ter pass ing through the filter in (2) above] is p laced in an evapora t io n dish that had been prewe ighed a t 275.419 g. .. S"Tlie 'sartip!e'i'Ii'(4) IS' eViiporared to dryness and the dish a nd residue are we ighed at 276.227 g. 6. Bo th the crucible fr o m (3) and the evap o ra tion dish from (5) are placed in a muffle furnace at 600"C for a n ho ur. Aft er coolin g. the mass of the c ru cible is 25.501 g and the mass of the d is h is 275.944 g. De te rmin e th e fo llowi ng: (a) (h) (e) (d) (e)
Th e The The Th e Th e
filt erab le so lid s (mg/ L) nonfi lterabl e solid s (m g/ L) to ta l so lids (mg/ L) o rga ni c fr ac ti o n of th e fi lt erab le so lid s (mg/ L) o rga nic frac ti o n o f th e no nfilt erab le so lids (mg/ L)
2-25 Wh at a re th e mos t co mmo n co nst itu e nt s of a lk a linit y, and what a re their sources and imp acts') 2-26 H ow is a lka hnit y meas ured :) 2-27 Determ ine the a lk a linit y o f the waters descri bed in Pro bs. 2-1 8 to 2-2 1.
CI'
= 71.D Ill):
L
Ca ' ' = 100 mg L Co ns t ruct a bar diagram in Illi ll lcquil a le n ts per liter ror this Will er.
2-28 A 100- mL sa mp le o f wa ter is ti trated w ith 0.02 N H ,S 04 ' The initia l p H is 9.5. a nd 6.2 mL o f ac id is req u ired to reac h t he p H 8.3 endpo int. An additional 9.8 mL is required to reac h th e 4.5 endpo int. De termine t he spec ies o f a lk a linit y present and the co ncentra ti o n of each species.
'1
60
WATER QUALITY: DEFINITIONS, C HAR ACTERISTI CS, AND PERSPECTIVES
WATER
2-29 A 200-mL sample of water with an initi a l pH o f 10.6 is titra ted with 0.02 N H 2 S0 4 , The sample reaches pH 8.3 after an addition o f 8.8 mL of th e acid . and an additional 5.5 mL is required to bring the sample to pH 4.5. Identify th e spec ies o f a lkalinity present a nd det ermin e the concentrations (mg/ L) of each.
:. 1 ..f.·
!
2-30 The initial pH of a water sample is 7.5. A 200-m L samp le is titrated wi th 0.0 1 N H 2 S0 4 , The pH 4.5 endpoint is reached after the addition of 15 mL of th e acid. Determine the spec ies of alkalinity present and the concentration (mg/ L) of eac h. 2-31 'Define "hardness" of water. note the two broad cla ssificatio ns o f hardness. and di scuss the sources and impacts of hardness . 2-3'2 Would hard water be acceptab le in most drinking -wa ter supplies? Wh y or why not? Would ha rd water be an acceptable coolant for an industrial plant') Wh y o r why not ')
2-44 A BOD ana lysis is begu n o n Monday. Thirty (30) milliliters o f was te with a DO o f zero is Illlxed with 270 mL of dilution wa ter with a DO o f iO mg/ L. Th e samp le is th en put in the incubator. Sin ce t he fifth clay falls on Sa turd ay a nd lab personnel do not wo rk o n Saturday, the final DO does no t get measu red until Mo nd ay. the seventh day. The fin a l DO is mea sured at 4.0 mg/ L. However. it is discove red that the incubator was set at 30°C. Assume a k, of 0.2 a t 20°C and kT = k 20 1.05 T - 20. Determine the S-d ay. 20°C BOD o f the sam ple. 2-45 Define nonbiodegradable orga nics. Give exa mples. discuss sources. and assess the impa ct of nonbiodegradabJe o rga ni cs in water. 2-46 Deline chemica l oxygen demand (COD) and tota l o rga nic ca rbon (TOC), a nd d iscuss how these a nd o ther tests a re used to quantify non biodegradable organics in water. 2-47 Name the nutrient s required in great es t abundance by aquati c spec ies.
2-33 How is hardness measured?
2-48 Disc lj sS the so urces a nd impacts of nitrogen and phosphorus in water bodies.
2-34 Determine the carbonate hardness. n o ncarbo nate hardness. and to tal hardness o f the water described in Probs. 2-19 through 2-21.
2-50 Define methemog lobinemia and discuss itas a water-rela ted illness.
2-35 Discuss the sources and impacts of flu o rides in drinking-water supplies. 2-36 Name the most common nonto xic meta ls fo und in wa ter supplies. iu entify their sources. and discuss their impacts. 2-37 Name toxic metals that may he dissolved in water. identify thei r principal so urces. a nu discuss their impacts. 2-38 Define biodegradable organics. Give exam ples. discuss so urces. and assess the impact of biodegradable organics in water. 2-39 Define biochemical oxygen demand (BOD) and outline the s teps in the standard 5-d ay BOD test. 2-40 The 5-day BOD of a wastewater is 190 mg/ L. Determine th e ultimate oxygen d erna llu . Assume k, = 0.25 r '. 2-4] In a BOD determination. 6 mL of wastewater co ntaining no disso lved oxygen is mixecl ~ .. . .. ......... w~h 2.94mL0f cilution waterllt·aining 8·. 6'n1 &II..; of disso lved oxygen: After'a-S--'d
Wastewa ter , rnL
I
20 10
2 4
2-49 . Ho w are nitruge n and ph os ph or us meas ured ') 2-51 Path oge ns arc not always bac teria. Name tw o pathogenic bacteria . two viruses. a nd o ne p rotozoa n somet imes found in wa ter s upplies. 2~52 With which wa terbo rn e pat hoge ns are the fo llowing di seases associa ted? (a)
(n) (c) (d) (1')
Cholera Sw ine heru's ui sc Jse Amebic dysentery Giardiasis Bacillary dysentery
8.9 9.1 9.2 9.2
2-54 D iscuss t he use o f tota l co liform and fecal co li form tests in th e measurement of pathogens. Disc uss the memb ran e· tiller technique and exp lain'h ow test results are reported wh en thi s technique is used . 2-55 Di sc uss the multiple-tube fermentation test. What is a presumpti ve test ') A co nfi r med test ') How are resu lt s expI'essec!? 2-56 A sample o f wastewater is a na lyzed fo r co li form o rga ni sms hy the multiple-tube fermen tati o ll method. The results of th e confi rmed test are as fo llows: Number of Sample s ize .
mL.
DO ,
000 1 (1.0001
1.5
000001 000000 I
2.5 5.8 7.5
(a) Determine the BOD s o f the wastewa ter.
(b) If yOU know that the oxygen utili z3 ti o n rate is 0.2 1 per day at 20"C, what will be the BOD) if the test is run at 30°C?
(f) T yph o id fever (y) Paratyphoid (il) Infa ntil e paralysis (i) Infectio us hepatitis.
2-53 What is an indicator orga ni sm ? Discuss the charac teri s tics of t he id ea l pathogen ind icato r and ind icate which organisms mos t near ly exhibit these characteristics.
POSili\'e results out of 5 tuhes
Number of ncgolive resulls Oll' of 5 lubes
o
DOt DO,
61
()
Determine the most prohable number anu range of coliform o rganisms per 100 mL at the 95 percent con fid ence le ve l. 2-57 Disc uss in-stream sta ndards. etn uent standa rds. and potable -wa ter standards. Who sets these standard s in the United Sta tes? Elsewhere') 2-58 What is an eflluent-limitcd stream 'J
\
62
WATER
REFER ENCES 2- 1
CHAPTER
THREE
A merlean Wat e r W or ks Associa tion: .. Quality Goals ror Publ ic Wat e r -- Statement or Policy." JAW' W A. 60 : 13 17 ( 1968)
2-2
Berg. Gerald: Transmissioll oj Viru ses by llle WOl er ROlile. Wiley . New Yo r k. 1%5.
2-3
Camp. T. R .: Waler alld lis Impurilies. R einhold, New Yor k , 1973 .
WATER PURIFICATION PROCESSES IN NATURAL SYSTEMS
u.s.
2-4
En vironm enlol Quality, the Eighth Annual Reporr of Ihe Coullcil of Ellvirollmel/lal QlIalily.
2-5
Gov. Pr inting Office , Was hing lOn, D.C. , December 19 77, Ge ldreich. Edwin E.: .. Water Bor ne P athogens," in Ralph Mitchell (cd .), Warer Polllliioll Microhiology, Wiley, New York, 19 72.
2-6
Hahn . R oy W .. Jr . : F,mdomen{(l/'A>pecls of Waler QualilY Malloy emenr, Tec hnomic , West port. Conn .. 1972 . .
2-7
M c Kee. J . E .. and H . W . W o lr: WOl er QualilY Criteria, publ. no.3-A, S tate Water R esou rces Control Board, Sacramento, Calif. , 1971 .
2-8
IV1etcalr & Eddy. Inc . : Woslewoler Ellgille<'rinq: Treatmenr, Disposal, Reuse, 2d cd., M cG raw-Hili , New Y ork . 1979 .
2-9
M elhods for Chenllcal Analysis of Waler and Waste, EPA 600/ 4-79-020. U.S. EPA, Cincinnati,
March 1979. 2- 10 Milkr, J. c.: Nilralt COlllaminalioll of lh e iVoler Table Aquifer of Delo "'are, Report or In vestigations no. 20. Delawa re Geologica l Survey. U ni versity or De laware , 1972. 2-11 - - -. P . S·. H ackenberry. and F. A. D e lucca : Groulldwoler Pollulion Problems in Ihe SOlllheastern Uniled Slares. E PA 600{3-77 -012. U.S. E P A, 1977.
Natural form s o f pollutants have a lways been present in s urface waters. L o n g befo re the 'd awn of civi li za ti on , many of the impurities discussed in the previ o us chapter we re washed fr om the ai r, eroded fr om land surfaces. o r leac hed from the soi l a nd ultim a tel y fo und th e ir way int o surface water. With few exceptions, natural purificati on processes we re able to remove o r ot herwise render these materials harmless. Indeed, wi thout these se lf-cleaning processes, the wate r-dependent life o n ea rth cou ld no t ha ve deve loped as it did. As civiliza ti on evo lved, human ac ti vit y increased the amount a nd changed the nature of polluiants entering watercourses. As settlements grew into villages, villages into towns, a nd to wns into cities. th,e quantity' Qf :-Y,"W~.p.r.od,ucts increased ... _. until the self-purificatio n cap~lcit'y ' o flo~~ l wa ter bodies was exceeded . Smaller streams were a ffected firs t. with la rge r s trea ms and lakes ultim a tely becoming polluted. Only in recent decades have po llution contro l prog ram s been initiated in a n attempt to reduce th e co ntam inants discharged to these :-vater bodies to the le ve l that th e na tural pur ifi cation processes can o nce aga in assimila te them. The self-purificatio n mechani sms of natural water sys te ms include physica l, chemical, and' biologica l p rocesses. The speed a nd comp leteness w ith which these c processes occur depend o n man y variables that a re system-specific. Hydraulic charac teristics s uch as vo lu me_ rate, and turbul ence of flow , physical characteristics of bott om a nd bank ma teria l. va riations in sunlight and temperature, as we ll as th e chemica l na ture o f th e natural wa ter, a re a ll sys tem va ri a bles th a t have an influence'o n the natur a l purification processes. In na tura l waters, these system var iab les a re se t by nature and can se ld om be a lt ered . T he same physica l. c hemica l. and bio log ical processes that serve to purify natura l water sy~ tems a lso work in engineered sys tems. In water- and wastewa tertrea tm ent plant s_ the rate a nd ex tent o f th ese processes a re m an aged by controlling the sys tem var iables. A th oro ugh kn ow ledge o f the na tura l purificati o n processes is thus esse ntial to the understanding of both th e ass imilative capacity of surface
2- 12 Sawyer, C. N .. and P . L. M cCarty' Chemistryfor EIII'irol/lllelllal Ellgil/eers, 3d ed .. McGraw-HilI. New Yo rk . 19 78 . 2- 13 Schmidt. K . D .. "Nitrate in Groundwat er o f the Fresn o-C lovis Me tro po litan Arca. Califo rnia. " Groundlo'{tier. 10 50 ( 1972). 2- 14 Smit h , Alicc' !'v!icrobiology and Palhologr. M osby, St. l Ollis. 1976.
2- 15 Srandard Melhods for Ihe Examin alion 0/ WOler and W aS!elo'{tler, 15th cd ., American Pu b li c Health Association. Wa s hington , D.C., 198 1 2- 16 Steele. E . W . a nd T . J. McGhee : Wal er Supply and Se ll'erage, 'St h ed., M cGraw- H ili. New' Yo rk . 1979 . 2- 17 T od d, D . K . Th e Waler Encyclopedia, Wate r In ro rmati on Cc nt er, POri W as hin g ton. New York . 1970 2- 18 U.S . Departmenl or H calth, Educati on. and Wel rare ' Drinking Waler Srnndards. P HS bulletin no. 956, Publi c H ea lth Service. 1962 . . 2- 19 U.S. Environmental Protection I\gency: "Na tio nal Interim Pr imar y Drinking Wat er Regulation s. " Federal Re91.Her , pt. I V. D ecemhcr 24. 1975 . 2-20 Walk er . R odge r . Waler SlIpply. TreGtmn/l , and Dislribul ion, Prentice- H all. EnglewoOd CliO-s . N . J , 19 78 . 2-2 1 Vesil lnd, P. Ai.lrne Ennronmenta/ Pollution lJnd Control. Ann Arh or Science. AJln Arbor. Mich .. 1975 .
63
1
64
WATER PURIFI CA TION PROCESSES IN NATU RAL SYSTEMS
WATER
waters and the operation of engineered systems. The self-purification of natural water systems is discussed in this chapter, while wa ter purifica ti on in engineered systems is covereciinGhaps, 4 and 5..
6S
Example 3-1: Measuring dilution in streams A Irea ted wastewate r enters a s tream as shown in th e accom panyi ng fig ure. The con cen tra tion of sod ium in the s tream a t poin t A is 10 mg/ L. and Ihe flow ra te is 20 m 3 /s . The concentra ti on of sod ium in Ihe waste st ream is 250 m g/ L, and the flow rate is 1. 5 m) /s. D elermine the concentralion of sodium at ,-.o inl B
Physical Processes Stream
The major physical processes invo lved in self-purification of watercourses are dilution, sedimentation and resuspension, filtration , gas transfer, and heat transfer. These processes are not only importan t in and of theITiselves,. but ani also of significance in their relation to certain chemica l and biochemical self-purification processes.
B
~
" 1.)
S ()j $
::;
'-..J
d
~
3:
3-1 DILUTION Through the first decades of the present centur y. wastewater disposa l practices were based on the premise that "the solution to pollution is dilution. " Dilution was considered the most economical means of wastewater di sposal and as such was considered good engineering practice. [3-5, 3-25J Early workers in the fi eld devised mixing-zone concepts based on the lateral, vertica l, and longitudin a l dispersion cha racteristics of the receivi ng wate rs. Formulas predict ing space and time requirements for diluting cert ain pollutants to preselected concentrations were developed. Highly polluted water in the immediate vicinity of the discharge was tolerated as.inevitable, and little th o ught was given to the low levels of material transported downstream. Although dilution is a powerful adj unct to self-cleaning mechanisms of surface waters, its success depends upon discharging relatively small qu an tities of waste ' into large bodies of water. Growth in population and industrial activity, with attendant increases in water dema nd and wastewater quantities, precludes the use of many streams for dilution of raw or poo rly' treated wastewaters. In the United States, legal constraints further limit use of water bodies' for wastewater dilutiqn. Urider present regulation s, maximu~1 allowable loads are set ' independently of ~ilution capacity. Only when the standard maximum' load s result in violation of in-stream water-quality standards is the dilution capacity considered, and then only to determine the increment of treatment necessary. The dilution capacity 'of a stream can be calculated using the principles of mass balance. If the volumetric flow rate a nd the concentration of a given material are known in both the stream and waste discharge, the concentration after mixing can be calculated as follows. (3-1 )
where C represents the concentration (mass/ vol ume) of the selected material, Q is the volumeric flow rate (volume/ time). and the subscripts s, IV, and m designate stream, waste, and mixture conditions . T he following example illustra tes the use of this formula.
Sm. L'TIO N 1. Writ e a mass ba la nce between poin ts A and B
Mass in = Ma ss Ollt
C"u Q,.n Sin ce
=
c,. " Q,.A + CwQw
Q,.H is the sum of the other two fl ows
" , ..0 '
2. In sert numeri ca l val ues a nd so lve for C,.
/I
10 x 20 + 250 x 1.5 C D = - -- -- - - _ . _ _ . ,. 20 + L5
C,.u
= 26.7 mg/ L
3-2 SEDIMENT AnON AND RESUSPENSION Sou rces of suspended soli ds. one of th e most co mmon water pollutant s. include domestic a nd ind ustria l wastewater and runoff from agric ultura l. urban . or sil vicultural activities. As discussed in Sec. 2-2, th ese so lids may be'inorga'llic or organic materials and/ o r li ve orga ni sms. and they may vary in size from la rge organic prticies to tin y, almost in visibl e, co llo id s. In suspen sion, so li ds increase turbidi t y (see Sec. 2-3). a nd th e redu ced ligh t penetrati o n ma y restri ct th e ph otosynthet ic activity of plant s, inhib it the vision of aquatic anima ls. interfere with feeding o f aquatic anima ls that obta in food by filtratinn. and be abra sive to respira;o r y structures sll ch as gills of fi sh. [3-27J
WATER PURIFICATION PROCESSES IN NATURAL SYSTEMS 67
Sett lin g o ut. or sedimen tati o n. is nature's method of rem ov in g suspend ed partic les from a wa tercourse. a nd most large so lid s will se ttle o ut readil y in quiescent wa ter. P a rticles in th e co llo id al s ize range C~ 1I1 sta y in suspen sio n fo r long periods of time. though eventua ll y most of these wi ll a lso settle o ut. Thi s natu ra l sed im enta tion process is not wi th o ut it s dra wbacks. Anae robic co nditi o ns are likel y to develop in sed iment deposits. a nd a ny o rga ni cs trapped in th em will dec o mpose, releasing so luble compo un ds into the stream above. Sed iment deposits can a lso alter the strea mbed by fillin g up the pore space and creating un suit a ble cond iti o ns fo r the rep roducti o n of man y aquatic o rga nisms. [ 3- IX] Th e deve lopment o f bank s cl f sil r a nd mud along th e bott om o r strea ms ca n a lt e r its co urse or haJl1rer nav iga ti o n activi ties. Sediment accumu lati o ns red uce reservo ir storage capacities and silt in harbors. a nd increase flooding du e to channe l fill-in . Resuspension of so li ds is co rnm an in tim es or fl ooding or heavy runorf. In such cases. increa sed turbulence may res uspend so lid s fo rmerl y deposi ted along no rm a ll y quiescen t are;JS of a strea m and carry th em fo r cons iderab le di stances dow nstrea m. Eve ntu a ll y they wi ll agai n se ttk: o ul. but not before th eir presence has in creased the turbidit y of th e wa ters into wh ich th ey ha ve beeJl introduced.
Gas c
.2 0.
~ -<
8--0'"
. ~
.D
Lic;uid
Figure 3-1 Gas-l iqu id contact with gas transfer between the pha ses.
equilibrium is reached . At thi s po in t, the number of molecules leaving the liq uid is equa l to the number of molecules entering it agai n, and the liquid is said to be saru rat ed wi th the gas. Equilibrium in this case implies a dynamic steady sta te, not a static state in which a ll movement of gas molecules wou ld stop once saturat io n occ urred. Two characteristics of the a bove process tha t are importa nt in wa ter are (J) solubililY. o r th e ex tent to which the gas is so luble in the water (i.e., the concentrati o n of gas in th e wa ter at equilibrium), a nd (2) lransfer rale , or the ra te a t which disso luti on o r release occurs.
3-3 FILTRA TJO N As la rge bil S of d ebriS w~ l s h al o ng a streambed. they uften lodge OJl I·eeds o r sto nes where th ey remain ca ught until high waters wash th cm in to th e main st rcam agai n. Sma ll bits of o rganic matt er o r Ill o rganic cla ys and oth er sediment s may be filt ered o ut by pebb les o r rocks along th e sll'ea mbed. f\ S \\·,lI er perco lates frum the surface downward Into g roundwater aq ui fers. filtrati o n of (J mu c h more soph isticated type . ·OCCO l'S: ;:intl.if the ·soil1:iye( s· 'a re' ckcp .eli 6 i.igh· ~i Ii'd' Tin'e' 'e'li ollgll: ienl o \;ill'of ·s li s·_· pended material is essentiall y co mplete by th e tilll e wate r ente rs the aqu ifer. Man y stream s Interc hange freel y w ith the alluvia l aquifers und ernea th them . so th e filt ered wa ter may reent er the strea m at so me poi nt down stream .
c::
2
V
. ... "
~
. .. . .
Solubility The so lubility o f a gas in equi librium with a liquid is quantified by Henry's law and is expressed ma th ematically by
3-4 GAS TRANSFER
p X= -
H
The transfer of gases int o and o ut of wa ter is a n impo rt a nt part of the natural purifi ca tion process. The repleni shment of oxygen los t to bacteria l degradati o n o r o rga nic was te IS Jcco mrli shed by th e transfer of oxygen from the ~1Ir Into th e wa ter Converse ly. gases ero l\·ed in the wa ter hy c hemica l and bi ologica l processes ma y be t ransferred fr o m the wa ter t (1 t he a t mos phere. A k now ledge o r the princi pi es. of gas transfer is esse ntial to understa nd ing these natural processes. Cons id er the simple sys tem s hpw n in Fig . 3- 1 ill whic h it co nta lll er of li quid is sea led wi th a gas a bove it. If th e liquid is initiall y pure Wi th n;spect to the gas. mo lecules o f gas wi ll migrat e across Ih e gas-liquid int er face an d beco me disso lved in the liquid . Alth ough some mo lecu les o f gas will beg in leaving th e liquid
(3-2) .'
in wh ich x is the equilibrium mole fract ion of the dissolved gas at 1 aim or moles of gas (ng) mo les of gas (n g) + moles of liquid (n l )
X= -_·_-
II is tbe ' coeffic ient of absorpt io n (H enry's coefflciene , which is unique for each gas-liquid system), and P is th e pressure of the gas above the liquid. Other factors that affeci x are temperat ure (the so lubility increases as temperature decreases) and the concentra ti on of oth er disso lved gases and so lid s (the solubility decreases as ot her di sso lved material in Ihe liquid in creases).
.L
.--
r r
r ~
L .. ;
68
WATER PUR IFI CATION PROCESSES IN NATURAL SYSTEMS
3. The saturatio n concent ration is
If the space above the liqu id is occupied by a mi xture of gases, each gas wi ll ha ve its own equilibrium mole fracti on. According to Dalto n's law, each gas in a mixture exerts a parti al pressure In propor ti on to its percentage by vo lum e in the mixture; th at is.
~[ , ii
PV = (PI
+ P2 +
P3
+ ... + fln)V
or
P =
C,
1.287
= :0
X
10 - 3 g·m o ljl x 28.9 glg·mol x 10 3 mg/g
37.2 mg/ l
The so lubility of a ir can also be found by usin g its components and Dalton's law. Th e compo nents of air by vo lume are approxima tely as follows:
LP;
Substituting into Henry's law, we see that x for the ith gas in a mixture is N,
p;
Xi
, !
\_- -
69
WATER
(3-3)
= -
H;
~
79 %
0 , ~2 i %
in which x;, H;. and p; are, respecti ve ly. th e equilibrium mole fraction; abso rptio n coefficient , and partial pressure of"the ith gas. Absorption coefficients for seve ral gases co mmonl y fou nd in natural wa ters are given in T a ble C- 2 of the appendix. The coefficients are seen to vary substantially with temperature. Although th e tota l disso lv ed mater ial a lso affec ts the so lubility, the effect is insignificant in the range of dissolved material us uall y found in fresh water. To be precisely acc urate, the partial pressure of water vapor mu st be accounted for in Eq s. (3-2) and (3-3). Co nve rsion of the eq uilibrium mole fraction x to an equilibrium concentration Cs is illustrated in the fo llowi ng example.
4. Th e molecular mass of nitr oge n is 28 g/ mol a nd H from Table C-2 in the appendi x is 5. 29 x 10" XN
,
=
0.79 5.29 X 10'
=
1.49 x 10
lIN , - 1.49 x 10 - ' nN , =
e. = =
Example 3-2: Calculating the solubility of air in water Calc ulat e the so lubilit y of air in water at O°C a nd I atm press ure. Assume o th er d isso lved ma teria l is neg li gi bl e.
8.3 x 10
4
=
_5
1.49 x· 10 -
5
x 55.6
mo l/ L
8.3 x 10 - gnlOl/l x 28 g mol x 10 3 mg/ g 4
23.25 mg/ l
5. The equi li brium concentrations for 0 , and CO , can be found similarly and are 16.65 and 0.02 mg/ L. respec tively.
SOLUTION
I. From Table
The equ il ibr ium ·concentratio n of air is
C-2 in the appendix. Henry's cons tant for air at O°C is
........ . .. .... .. ' .. ... ....... .. f!. .7.. 'U2. x. 10."..alm/mE>1 fra.:! ion at I atm pressure. The mo le fr ac ti on o f a ir in wa ter is found by Eq. (3-2).
= -. - _._--132 x =
23.25
,L ,
,..~.
+
16.65
Th e discrepancy is acco unted fbr by the rounding off of the percentage of N, . 0, _ and CO, in air. .
Th e rate of gas transfer is an important parameter in aera ti on. The rate of transfer is governed by severa l factors and is mathematically expressed as
2.3 I x 10 " mo l fracti on
dC/dt
2. One liter of water contains
1000 gi l - - - = '''6 g llhl l. L 18 g/ mol "' and = __ 11 9
.r~~.• _ .. + 55.6
n. - (2.31 x 10 - ' n.) = 2.31 x to - ' x 55.6 II.
=
1.287 x 10 -
3
g . Il1ol.' l
0.02 = 39.92 mg/ l
Transfer Rate
1.0 atm I04 ~11;~;I~lOli~,;~'iio-n
2.3 1. x fo - .5
+
~.
t' ~
.~
,L
=
(C - C)k"
where dC/dl is the instantaneous.rate of change of the concentrati on of gas in t he liquid. C and C are the saturation co nce ntrati on and the ac tu al con centrati o n, respectively. and k" is a constan t related to give n physical co nditi ons. lt should be noted th at desorption of the gas occurs when C is greater th an C· The magnit u de of k" is known to depend upon the 'temperature of the system. the interfacial ar ea avai labl e for gas transfer. and resistance to movement from one ph ase to th e oth e r. While the effect of tempera ture ca n be predicted by the van't Hoff-Arrhen i us ru le. the other va riables are system-spec ific. Th e int erfacia l area a vai lab le for gas
70 WATER
WATER PURIFICATION PROCESSES IN NATURAL SYSTEMS
transfer is measured by the total contact surface between the gas and liquid. Larger interfacial a rea per given volume will result in greater opportunity for gas transfer. The resistance to movement between the phases is most often explained by the two-film theory of mass transfer initially postulated by Lewis and Whitma n in 1924. According to this theory, the interface is composed of two distinct films , one on the gas side and one on the liquid side, that serve as a barrier between the bulk phases. This system is shown graphically in Fig. 3-2(1. In order for a molecule of gas so mewhere in the interior of the gas phase to be transferr:ed into the interior of the liquid phase. it must move through the bulk gas to the interface, across the gas film , across the liquid film, and, finally, away from the interface and into the bulk liquid. In systems where the liquid is :supersaturated with respec t to the gas, movement of the gas molecule will be in the reverse direction (Fig. 3-2b).
.,
Bulk g as
u
~ tI - - - ~
~
O~
!
The driving force causing mass transfer is the concentration gradient, C s - C. Resistance to mass transfer must be overcome for the process to occur, and each one o f the steps lis ted above is likely to exhibit a different level of resistance. The step which offers the most resistance to the movement of gas molecules becomes the rate- limiting step. In stagnant situations (i.e .. no internal movement of the bulk phases). movement of gas molecules to and away from the interface depends to tally upon diffusion , and the process is verysfow. However, if internal movement of the bulk phases occurs, molecules of gas are transferred to and away from the interface by turbulence and eddy diffusion. and the rate of mass transfer is most likely to be governed by one or both of the films. In most natural water. sufficient agitation of the bulk phases exists, and the film s become the limiting factors. In general, gases that are highly soluble in water, such as ammonia, encounter more resistance in passing through the gas film and the process is sa id to be gas~jilm-conlrolled. Conversely, slightly soluble gases suc h as oxygen and nitrogen encounter more resistance in the liquid film , and the system is liquid-film-controlled with respect to these gases. Gases of intermediate so lubi lity. such as hydrogen su lfide. encounter approximately equal resistance through the two 'films and the system is said to be mixed-film-controlled.
-7~
_ Gas film
E
Li",," "'m Bulk liquid r
C" >C .Concentration (al
~E ro
-
Bulk
v
u
t ~----ll.:---_ ps __ "' . Gasfilill ~------------~~------------
., !
2 0
.~ I-----"'~ Cl
Liquid film
"'"
Bulk liquid
C,
-
-
3-5 HEAT TRANSFER
______________~------------
/
7]
Figure 3-2 Two·film model of the interfa ce between gas and liquid: (Ill absorption m ode and (h) desorption mode . ;"
Bodies of water lose and gain heat much more slowly than do land or air masses, . and under most c ircumstances. water temperature is fairly constant and changes grad ually with the seasons. Consequently. aquatic plants and animals have not dcveloped s u fllcien t ..ad.apta bi l.ilY 1.0 ..dei\l. \Vil.~. aI:>ru pI . ch.ange~ . in .. t~n1p.er?t.tJ r~., ............ . a nd only 'ti~~' mo st hardy species survive such changes. Thus, heat increases tend to decrease the number of species of aquatic plants and animals. [3-17J Furthermore, increases in water temperature affect ionic strength, conductivity. dissociation constants, so lubility, and corrosion. potential, all factors associated with . water quality. Given constant meteorologic conditions. water theoretically requires an infinite time of exposure to attain equilibrium after a heat load. Furthermore. an " tnfinite surface area would be required to cool warm water introduced into a . river or basin to the equilibrium temperature. However. because temperature decline is nearly loga rithmic , equilibrium can be closely approached within practical limitations of time and su rface area. Many meteorological variables-plus o ther factors suc h as channel characteristics (depth, width. surface area), channel volume, etc. - affect the rate of heat transfer in bodies of water. For streams heated by solar radiation over several miles of ht;at-Ioad area. cooling begins only in shaded areas or at night and may proceed much more slowly than cooling in streams which receive their heat load in one discharge. In temperate zones. heat transfer in reservoirs and lakes where the influence of turbulence and current is negligible is controlled by a phenomenon known as {iIennal sWlijic(I{ion. Fresh waters reach their maximum density at 4°C (39°F),
I
i
72
WATER PUR IFI CATION PROCESSES IN NATURAL SYSTEMS
73
WATER
with density declining as water moves toward the freezing point or grows warmer. (See Fig. 3-3.) Thus, during warm seasons and in impoundment s of suffic ient depth, water divides into an upper layer of warm , circulating water known as the epilimnion and a lower layer of cool, relatively undisturbed water known as the hypolimnion: These two layers are sepa rated by the th ermoclin e, or meralimnion , a region of sharp thermal gradient. This stratification is shown in Fig. 3-4a. Stratification is usually interrupted in autumn (Fig. 3-4b) as surface waters cool and begin to sink. Wind action can then cause circulation throughout th e entire body of water so that turnover in the lake's strata occurs, stratification disappears, and the body of water reverts to a uniform temperature throughout its depth. In cold regions, surface waters freeze over as winter sets in. Waters at 2 °e (36°F), being denser than the colder waters above. form a layer a long the bottom , a layer in which the aquatic ecosystem survives as long as s ufficient oxygen is a va ilable, despite the freezing of the lake's surface (see Fig. 3-4c). In spring, the process is reversed as ice melts and turnover occurs (Fig. 3-4d), and summer stratification begins as surface waters are warmed by increased solar radiation. [ 3-27J
9°C
Epilimnion
DJr k. sUgnan t , cooler Hypolimnion
(h
O°C l ee
4°C
(h
Vertica l circu la rion
Ver ti ca l cireu la t ion
t!
tl
water
A ugust
9°C
~oC
November
January
(b)
(el
(0)
Figure 3-4 T emperalure profi les of a deep lake. show ing (a) thermal strat ification, (b) autumnal circu lalion, (e) winler stagnalio n. and (d) spr in g ove rturn. (Adapted/rom H ammer [3-8].)
Water
I Water
1.000 1.00
I Ice I
,-r--~
The nature and extent o f stratification varies. depending upon the size, depth, configuration, and terrain of the body of water. area-vo lume-stage relations, orientation of prevailing wind s, and hydrologic (or induced) inAo w and outAow characteristics. as well as with seaso nal variations in temperature. [3-25J
0.9995
Chemical Processes
0.95
i'7
.~
0.9990
"
Cl
0.9 ci L -L..L-L---'-----'-_ - 10 0 10 20 30
Na tur a l wa terco urses co nt ai n man y di sso lved minerals and gases th a t in teract . c hemi ca ll y with o ne another in complex and varied ways. Oxidation-reduction, dissolution-precipitation. cwe! o th er chemical conversions ma y alternately aid o r obs tru ct natural purification processes o f natural water sys tems .
Temperature.oC
1,---
(b) 0.9985
II\~ Ij"
t4
6
8 10 12 14 16 18 20
2~
Temperature.oC
(al Figure 3-3 C hanges in the den si ty of (a) wa ler and (bl ice wilh changes in lemperalure. (From Warr ell
[3-27].)
,"'-----
L
3-6 CHEMICAL CONVERSIONS Strict ly speaki ng. mos t of the oxidat ion-red uction conversions that playa part in se lf-pLmtica tioll of Ilat erClllll'SeS are biochemically mediated and w ill th e refore be discussed in subsequent sect io ns of this c hap ter Be\
74
WATER PURIFICATION PROCESSES IN NATURAL SYSTEMS
WATER
Nitrogen and phosphorus are usually considered the most essential nutrients found in watercourses. Other materials me equally important to growth of microorganisms and plankton. though they are needed ill lesser amoullts. Iron. manganese. copper. zinc, molybdenum. and cobalt are micronutrients lIsually present in water Natural chemical conversions th:11 may take place ill water call change these materials into a form thai is soluble and therefore usable by various aquatic organisms, ~ Chemical conversions that occur in reservoirs and deep 1:lkes play an imponant role in the accessibility of ph'lsphorus. Phosphorus may enter the body of ,vater attached to particles and settle to the bottom with these particles. Phosphorus may also enter the water as soluble orthophosphate and become incorpoTated in biomass that eventually settles to the bottom. When ferric iron IS also present the fo1l0wmg reaction occurs. (3-4)
Fe."" + PO .. "
The insoluble ferric phosphate IS precipitated and settles to the bottom. There. in the relative absence of oxygen. the iron is reduced tpthe ferrous form and the lOllS go into solution. During spring or fall turnover. the phosphorus IS lllixed throughout the entire depth of the lake. With some of it being used by plant life and some of It recombining with ferric Iron and reforming the IIlsoluble ferriC phosphate compound. with that precipitate again settling to the bott()m to await reduction.
[3-19J Chemical con\ersions th:lt take place in streams :llld lakes Gin heir tll stabilize the pH of those bodiesofwater. For example.lilllestune and other furms otcalciulll carbonate (CaCO}) dissolve readily in water containing CO 2 , [3 -2oJ
which living organisms assimilate and use food for subsistence, growth, and reproduction is called metabolism. The metabolic processes and the organisms involved are a vital part of the self-purification process· of.naturalwater systems.
3~7
METABOLIC PROCESSES
The biochemical reactions involved in metabolism are extremely complicated and are not yet completely understood. It is 'known, however, that two types of processes, each involving many steps, must occur simultaneously. One process, called catabolism, provides the energy for the synthesis of new cells, as well as for the maintenance of other cell functions. The other process, called anabolism, provides the material necessary for cell growth. When an external food source is interrupted, the organisms will use stored food for maintenance energy in a process called endogenous catabolism. Each type of microorganism has its own metabolic pathway, from specific reactants to specific end products. A generalized concept of metabolic pathways of importance in natural water systems is shown in Fig. 3-5_ Enzymes playa major .role in biochemical reactions. Enzymes may be considered as organic catalysts that influence reactions without becoming a reattant themselves. In biochemical processes, enzymes lower the activation energy necessary to initiate reactions. The enzyme th'en reverts to its original form for reuse. A model of enzyme-substrate (food) reactions is shown in Fig. 3-6. Enzymes are complex protein compounds and are very specific in terms of the reactions that they support. A microorganism thus needs specific enzymes for each reaction in its
'H';6'f'C(5~" ~'~" 1-1 2 C0 3 *
H 2 CO)*
==
H+
+
HCO:;
The hydrogen ions thus formed react with slightly soluble calcium carbonate to yield highly soluble calcium and more bicarbonate ions. CaC0 3
+ H'
==
Organics Ca2+
+
I-ICO.J
Endogenous catabolism
+ mil'roorganisms
The bicarbonate acts as a bufTer to protect a stream from pH fluctuations that can be harmful to aquat ic systems. +
Biochemical Processes Many of the 'chemical reactions II1volvecl Il1 the self-purit\catIOIi process must be biolo-gicallv mediated. These chemical reactions are not spontaneous but reqUIre an ex~ernai suurce uf energy for initiation. In the case of biodegradable organics and other nutrients. this activation energy can be supplied by microurganisms that utilize these materials for f(loci and energy. The sum total of the processes by
75
Waste heat Figure 3-5 Generalized metabolic pathway.
Organic residue
\VATER PUR I FICATION PR OCESSES I N NATl'RAL
76
SYSTEMS 77
WATER
Enzyme
+
IlIlrogl:110llS
s,,',,"," ~
L' ~ l rbon~l ct'ou~
/
•
/Sl''' '
f
OI
-
t
Enzy me
+
I
protein, }
Prociu ct
I"t s
t
carbohyd rat es Figure 3-6 Enzyme reacti o n mod e l.
metabolic pathway, The fact that enzymes are not used up in the metabolic processes is indeed fortunate, as this frees the microorganism to devote its energies and resources to the building of new cel1ular material rather' than to the constant rebuilding of em,ymes, Microorganis ms are equipped with enzy mes that a re especia l1 y wel1 suited' to the use o f partic ular types of organic m a tter. When these e nzymes are a norm a l part of a particu la r microorganism. th ey are ca lled co ns tiltlti vl:, Ce lls produce special enzy mes. ca ll ed adoptive enzymes. when they are ex posed to unu suaL even tox ic. substances, This accl imati o n occ urs natura ll y. th o ugh at a relatively slow rate. In m a ny cases. th e continued presence of a tox ic Subst:lIl ce w il1 lead to th e gradual development of a specific bacteria capab le of decom p os ing and utili zing th a t toxic compound, For example. phenol-splittin g bacter ia are ofte n fo und in "" ' '' ''''s"t"ream'sHi:iCliave' j:eceived 'di sdiiiiges bf phen o li c wa ters, [ 3-1IJ Energy is transferred from th e cat
...- 0 , - C0 2
PLANT LIFE \
I I
,
LI V ING PLANTS
{Tam mol~ . H) CO 2 1,12 S
t
'
I
--
' {nitrr tes NO~ INTERM ED I A T E . CO __ PRODUCTS 2
c
~~ - u
~g6
~SU lfurs
~;::~ ~ ",t,-alc, NOj
"'-CO, - - - - - stl ll:ll ,"
SO.;-
(a)
1l11rn~l' 1l 0u:,
l" drboIlJ Cl'OLJ S
-----.......,.
J)~, CO MPO S I TIO N
\
o rg;Jl1ll' Jell.!::-
INI TI.,\L PRODUCTS
{
CO ,
~
H )S
~
t INTt RMEDI '\TE l' Jillill on;" N H ; ___ PRODUCTS CO ) . 'i'ulfllk, ~"
~. ( iI)
Fi gll'" .I-X NIII')~C " . "ar hnn , " " d su llur c\'cles
'- .
ADP + I'
DECOM POSITION
I NITI A L PRODUCT S .
ClO
AN IMAL LI FE
Z ~ <3
w cr:
"'I
...- C0 2
.
/
",<-;
--.........
tJ
I I
pro le lns } LI V I NG \) f"IS AN I MALS
Enzymes ubs trate (o mpkx
slill'urolls
/\~
WASTES
Figure 3--7 Energy transfe r mod e l.
(II)
aerobic and (h ) ""aerobiC , (Fro m Vesrl/lld I 3-~6J-) ,
WATER PURifi CATION PROCESSES IN NATURAL SYSTEMS
78 WA TER
Like matter, energy can be neither created nor destroyed. Energy released in th e catabolic process is transferred to the cellular material synthesized in the anabo lic process, s tored in the waste products o f catabolism, o r released as Ilea t or mec hanical energy. The relative quantities dispersed in these ways depend up o n the na ture of the reaction . as depicted in Fig. 3-9. The end products o f aerobic catabo li sm are low-energy, s table compounds, with most of the energy being sto red in the cellu lar materi a l. By co ntrast. m os t of the energy released in a naerobic cataboli sm remains in the waste product s.
a s there are man y s teps in th e process. eac h be ing m ediated by its own se t of enzy mes. The read e r is referred e lsewhere fo r a mo re co mplete coverage o f the s ubjec t. [ 3-14. 3-20, 3-7J Ca tabo lic processes in vo lve e ithe r the oxidati o n or the redu c ti o n of mat e ria l in th eslibst rCll e (food su pp ly). Iffree mo lecula r oxyge n is a va ilable, it will be added to th e s ubstr a te anq t he waste produ c ts will be ox id ized co mpo und s. I n the absence o f fr ee oxyge n. bo u nd oxyge n may be rem oved from oxygen-bea rin g com po un d s a nd hydroge n added to e lement s o f the substrat e. Th e result is was te produc ts co mposed of red uced co mp o und s. Oxida ti o n reac t io ns are m ore efficie nt because th ey release g rea te r a m ou nts o f energy . Con seq ue ntl y. ae ro bic metabo li s m predo min a tes when oxyge n is ava il a b le. Th is is fo rtunate becau se the ox idi zecl p roduc ts o f ae ro bic processes are less objec tion a ble in na tur a l water sys tem s than the redu ced produ cts o f ana e ro bic processes. H owever. anaerobic metabo lism d oes play a n impo rtant role in was te ass imilati o n in oxyge n-d epleted wat ers and sed Iment. Severa l interm ediat e ste ps ma y be in vo lved in th e metabo lis m o f orga ni c ma ter ia l. Eac h int e rm edia te step ha s its ow n end pr od ucts, some o f whi c h may beco me s u bs tr a te in s ubsequ en t reac ti o ns. Thi s is illl1 strat ed by th e nitr oge n. ca rb o n. a nd sulfu r cyc les s how n in Fig. 3-8.
3-8 MICROORGANISMS IN NATURAL WATER SYSTEMS C lassical no mencla ture divides living organisms into two major subdivisions or kin gdom s, plants and a nimals. Th e term protista is often used to classify organis ms in which there is no cell speciali za tion ; th a t is, each cell is capable of carrying o ut a ll of th e functi o ns of that organ ism. Members of the protista group are caUed protists and may belo ng to either the pla nt or anima l kingdom under the classical no mencla ture. Most of th e o rga ni sms of significance in natural purification processes ·.....: bac teria. a lgae. a nd protozoa - are protists.
B1 0lllJ SS
Aerobic p rocesses
Availabl e ellt..' rgy
Bacteria Bacteria are the primary decomposers of orga nic material. Bacteria are singlece ll protists th a t utili ze soluble food. Although bacteria may link together into chai ns or clusters, each cell is an independent ..orga.f)j~m .capable.of ca.Hying· out· ... .. .... .. . all · t he 'necess a ry 1ifHlt"rii::ti6 iiS.T3-·(4J· 'the' structures of bacterial cells typical of na tural wa ter syste ms are illu strated in Fig. 3-IOa. A listing of the relative abunda llce o f the element s comprisi.n g the ce ll is presented in Table 3-1. The chemical formula fo r bacterial cells is assumed to be C 5 H 7 O 2 N. [3-1 SJ Energy fo r bac teri a l grow th and reproduction may be derived from the biochemical ox idation of inorganic or organ ic compounds. or from the reduction o f these co mp o unds. A few bacteria are a ble to utilize ultraviolet energy from sunli ght. M a terial so urces can be derived from either organic or inorganic compounds. Bac teria are often classified accordi ng to the energy and material sources that th ey require. Organisms that derive both energy and material from inorganic so urces are called al.ltotrophs. while bacteria th at obtain both energy and material from o rga nic comp ou nd s are called li eterotrophs. Pliotorroph s. bacteria wh ich utili ze sunlight fo r an energy so urce and in o rganic substances for a material so urce. pla y an ins ignificant role in the natural water purification processes. He ter o tr o phic bacteria are 'the mos t important species in th e degr adation of o rgan ic material. Aerobic hel erotroplis require oxygen in their met a bolic processes whil e eIl/uerobic liet emtroplis utilize o rgan ics in the absence of oxyge n . A third g ro up. ca ll edjacu/tative het erotroplis. functi o n as aerobes when oxygen is present but sw it c h to a na ero hi c processes when oxyge n becomes unavailable. A maj o r
W aSlc' hC'al
Wa s.! e prod uc t s
Biomass
AJ1~le r obir
processes
Available ene rgy
Vv:I Slc pr o c!u . . . l\
Fi gure 3-9 Energy balance
III
mel~b,)li,m
79
U/ler Slrele (lnd ,1/ ciiiJel' [3·221 )
i'
~
.l
80
WATER PURIF ICATION PROCESSES IN NATURAL SYSTEMS
WATER
Ce ll wa ll gives shape to ce ll and preven ts destruction by . shear forces: may be 10-50% o f ce ll weight.
Cytoplasmic membrane regu la tes transport of food int o and was te produ cts out of ce ll.
o .
0 '" .'0 '0.' 0'
Table 3-1 Bacterial cell composition
Cy toplasm , con taining ribonucleic acid ( RNA ). contro ls anabo li sm, manufa c tures and recycles enzymes, stores food.
Elemenl Carbon Oxygen Nitrogen H ydrogen Ph osphoru s Sulfur
'.0
..0' '.'0 : '.
-'0· .
00'
. 0
0
., o·
, 0 ,.".0
81
Dry weig ht . ~~
SO 20 14
8
3
Pota ssi um 0 ",
Sodium Calcium Ma gnesi um Chlorine
Nucleus, a si ngle stra nd of deoxyribonucleic acid (DNA), co nt ains ge n e tic code: w ith RNA regulates metabo li sm .
S lim e layer of o rgan ic poly m e rs va ries in thi ck ness with "age" of t he ce ll and o th er env ironm e nt a l co nditi o n s: sto res food aQd bi nd s food and o ther bac te ri a int o floes.
1
OS
I ron
0.5 0.5 0.2
Allolhers
0.1
,
[ 1·71
funct ion of auto t rophic bacter ia is t he oxida ti on of nitrogen and sulfur compou nd s to stab le end prod ucts.
(a)
Protozoa
..
;
~. L~
~
I.
, f
r ~, f.
•
i" I
Like bacteria, protozoa are Single-ce il orga nisms that rep rod uce by binary fiss ion. Unlike bacteria, protozoa ingest sol id orga nics for food. Si nce protozoa are one to two orders of rTla!;llit LI CIc: .largertI1Cll1.pac;te.ri a..lhe prot ozoa d iet often includes 'b~ctc'l:iai' ce(l s' ,is we ll as co ll o idal organics. There are many aquat ic species of protozoa. most of which are strict aerobes. Lik e heterot rophi c bacteria , they obta in both energy a nd material for growth and reproduction fr om th e same orga nic food so urce. The mo st Important pro tozoa l gro up in natural wate r systems is th e ciliata. Th ese orga ni sms are charact erized by hairlike appendages called ,ilia and may be either free-sw imming or stalked (attached to a so lid particle). as illu strated in Fig :\-1 1. The free-swimming protozoa use
(b)
Figure 3- 10 (a) Generalized struclure of a bacterial cell; (h) photomicrograph of fre,hwatn bacteria attached 10 a su rface. Threadlike materials a re ex tracellu lar polymers that hind Ihe organisms togelher ;\Ild 10 the surface (photo courlesy"j W . C . Chararklis) .
Algae arc auto tro ph ic. photosy nth etic organ isms anu . eve n th o ug h they uo not utili7e orga nic compounus directly. playa significant role in th e natllral purification prc>cess. In the presence of sunlight. :J!gae metabo li ze th e waste pruduct s of heterotrorhll' hacteri:1 (C0 2 , 0 , . PO.,-'- . etc .) wh il e obt:lining energy from sunl ight.
82 WATER
WATER PURIFICATION PROCESSES IN NATURAL SYSTEMS
83
Response of Streams To Biodegradable Organic Waste Th e self-purification of naturalw a(er systemsisac()mplex process that ()ften in vo lves physical , chemical , and biological processes working sim ultaneously. C hemical and biochemical re ac ti o ns a re conversion processes rather than remo va l processes. The nature, a nd perhaps phase, of the waste may be changed, . but t he produ cts remain in th e water until phys ica l processes remove them from suspension by sedim entation or b y transfer to the a tmosphere. This is·illustrated by the reaction in Eq. (3-4). Here chemical processes combine iron and phosphate into solid form. and the physical process of sedimentation removes it from suspen sion. Another example is the metabolism of organics by microorganisms. Bi ochemical reactions convert the organics to biological solids and other end products that may be recycled several times (Fig. 3-8) before ultimately being incorporated into bo tt o m sed iments or released as gases to the atmosphere, both by physical processes. The self-purification processes can be modeled , provided the waste characteris ti cs and th e system var iables of the water body are known. The modeling process is co mplicated in lakes and estuaries by dilution and dispersion characteristics that are va riabl e with time. A complete discussion of water-quality modeling is be yo nd the scope of this text. Ho wever. some examples will be used to illustrate th e interaction of the physical, chemical, and biochemical processes described ear lier. The examples chosen relate to the assimilation of organic material by streams and the res ulting effects.on the oxygen balance and the ecosystem . Alth o ugh these topics are the o nes most frequently covered in the literature on natural purification processes. the reader should be aware that other self-purification processes, with respect to other contaminants, may be of equal importance.
Figure 3-11 Ph otomicrograph of a stalked protozoan: n o te haIrlike cli la used in the food-gathering process.
. " . . . .. . . On.e 9U.be.w'Is\e.PJ(xlnctsoflhis.reaclionisox.ygen.When s unligh t is no t a va ilable. the algae catabolize stored food for energy a nd use oxygen tn the process. This diurnal nature o f alg a l ca tab o li sm is an important factor in the oxygen balance o f natural waters that a re nutrient-rich . Th ere are lite ra ll y thousands of species o f algae of va ri o us sizes. s hapes, and co lors. Algal ce ll s ma y be fo und in clusters. in lo ng filaments a ttach ed to ba nk s or bottom materi::li. or may rem a in as single ce ll s suspended in wa ter. So me spec ies of algae can ha ve negati ve effect s on water qua lit y becau se th ey produce o il y s ubst ances th a t cause t::tste anet Ddor pro blem s.
3-9 DISSOLVED-OXYGEN BALANCE Disso lved oxygen is o ne of the m os t important constituents ·of natur~1 water sys tems. Fi sh and o th er aquatic animal·species require oxygen, and a stream mu st have a minimum of abo ut 2 mg/ L of dissolved oxygen to maintain higher. life for ms. At least 4 mg/ L of disso lved oxygen is required for game fish and some s pecies lllay require m ore. In addition to this life-sustaining aspect, oxygen is impo rtant beca use the end products of chemica l and biochemical reactions in anae ro bic sys tems often pro duce aes theti ca lly displea s ing co lo rs, tastes. and odors. in water. When biodegradabl e o rganics are discharged to a stream con tai ning.dissolved oxyge n. mi croorganisms beg in the metabulic processes that convert the organics, along with the disso lved oxyge n, int o new cells a nd' ox idized w:\ste product s. Th e q uantit y o f oxyge n required fo r thi s conversion is the biochemical oxygen demand discussed in Sec. 2- 13. The rate ;It w hi c h the dissolved oxygen is used will d epend on the quantit y of tli e organics. the ea se with which they are bicdegraded .. and th e d iluti on ca pac it y of the stre am .
Other Organisms Other microorganis ms may a lso play important ro les in the natural. pur ification process. R o {ij l'l"S anet uu.\"(uc·ca are 'Iower -o reie r a nim a ls th a t. prey o n bacter ia. prot oz()i!. Li nd al gJe. The\' flelp to maintain a baLtnce In th e p(ipulations ofprlln;t ry producers ;lI1d sc n e as an impo rt ant link in th e chai n by which o rgani c m ate rials a re passed o n to hi g her-o rd e r animals. SI1I£lCj(' worms s uc h as tublfex a ncl bl oodworlllS. a s we ll as u th c l' helminths and in sect la rvae. feed o n sl ud ge depos its and help to brea k down and s() lubilize th e part icu late organics.
l
",
--
84
WATER
WATER PUR IFICATION PROCESSES IN NATURA L SYSTEMS
The di ssolved oxygen that is used from the stre a m must be replaced o r a n aero bic co nditi o ns will devel o p. Two mech a nism s a re kn o wn to co ntribu te oxyge n to surface wa ters; (I) di ssolu\io n o f o xyge n fro m th e a tm osphe re, o ft en ca lled rea era ti o n, and (2) producti o n o f oxygen by a lga l ph o tosy nth esis.
85
bac teria l metabo lism. major a lgal ac t ivit y usua ll y occurs downs t ream fro m . rather tha n with in, t he area o f grea tes t bac teria l ac ti vit y w here t he oxyge n is needed th e mos t. Also, in th e abse nce o f li ght. a lgae o bta in ene rgy fr om e ndoge n o us catabo lism represented by th e fo llowing reactio n.
(3-8)
Reaeration
T h is reac ti o n contributes to the oxygen de m a nd ra th er th a n to th e oxygen sup p ly of th e stream.
Th e princ iples bf equilibrium b et ween wa te r an d gas in cont ac t wi th eac h o t he r a re desc ribed in Sec. 3-4. Equilibrium co ncent rati o n s o f oxygen in wa ter a t vario us tempe ra tures a nd salinit y va lu es are g ive n in T ab le C-3 o f th e a p pe nd ix. Whe n co ncentration s of disso lved oxygen drop below th e equili b rium va lue. th e n et m o vement of o xyge n will be . fr o m th e atm osph e'r e into th e wa ter. The diffe rence between the equilibrium co ncentra ti o n a nd th e ac tu a l co ncent ra ti o n is ca lled th e oxygen deficit a nd is represented m a them a ti call y b y
D = Cs
~
e
The difference in a lga l catabo li sm d urin g li g ht a nd da r k p e ri o d s res ults in diurnal variations in the dissolved oxyge n in strea ms wi t h heavy alga l g row ths. T he dissolved-oxygen concentration often peak s arou nd 2 to 4 P.M., wit h t he lowes t levels occu r ring j ust before sunrise. U nfo rtu nate ly, th e excess oxygen gen era ted du rin g t he d ay can no t be sto red fo r use du ring th e ni ght , as it is ex pe lled to the at mosphe re to m a in tai n equil ib riu m. In _cases w here th e a lga l g row th is heavy, the endogenous catabo lism may de pl ete t he di sso lved oxygen to t he point w here fish ki ll s occur. Because of tli c variability o f photosy nth eti call y p ro du ced oxyge n, reae ra tion is co ns idered th e Ill os t dependab le source o f di sso lve d oxyge n. "t may be necessary however. to Inc lu de p ho tosyn t hetic oxyge n in a d isso lved -oxygen mode l for wa ters where algal growths are heavy.
(3-5)
e
whe re D is th e di ss o lved o xygen d efi c it a nd C, and a re th e equilibrium co nce ntra ti o n a nd ac tu a l o xygen c oncentrati o n , respec ti ve ly. T he units o f a ll th e terms a re milli g ra ms pe r liter o f oxyge n. F or co n sta nt eq u ili b riu m co ndi ti o ns. i.e .. C d oes no t cha nge. th e ra te o f c h a nge in th e d e fi c it is
dD ci l
de dt
(3-6)
3-10 DISS OLVED-OXYG EN MODEL
T he d eficit thu s incre ases a t th e same ra te th a t th e o xyge n is used up. The di ssolved oxyge n d efic it is th e dr iving fo rce for reae ra ti o n. Th e g rea ter t he de fi c it , th e g rea ter the ra te o f reae ra ti o n. It fo ll ows, th en, fro m Eq. (3-6) that the rate o f reaerati o n in cre ases as th e co n ce ntra ti on o f di sso lved oxyge n d ecreases.
Most a ll of the di sso lved-oxyge n mo d e ls in c u rre nt use rela te in some way to the model de\e loped hy S tree ter
Algal Photosynthesis Rate of Ox ygen Removal
In t he presence o f s unli g ht , a lga e me tabo li ze inorgan ic com po u nd s, wi t h one of t he was te product s bein g oxygen. The fo ll ow in g fo r m ul a is a sim p li fi ed represe n tation o f thi s reac ti o n . .
The rate at \Vhich disso lved oxygen disappears from the stream coincides with the rate of BO D exert ion. Therefo re
(3-7) ~' ~C\\
algal cells
de
III
dl
(3-9 )
Substituting into Fl] . (1-6)
til' III
T he oxygen thu s re lea sed is immed ia te ly ava il ab le to reple n is h t he di sso l\ ed oxyg(': n in th e wa ter. In th e presence of excessive nu t rien ts and hr ight su nli ght. a lgal me tabo li sm m ay produ ce S(l muc h oxygen t ha t th e water hecomes s u persa tura ted . Th a t is, > C, a nd th e defic it has a nega tive v:t1u c. Ad\ erse fac to rs associa ted wit h excessi\'e a Iga I gn)w t hs (li'ten (lut we igh the be nefi ts o f the oxyge n th ey prod uce. Because algae use the waste produc ts from
dD cil
(3- 10)
l'onlirlllil1g th~lt :111 increase In tlie r:lte of BOD exertion results III an incre:lse in th e ,:lle "f ch: ll1 gc ()f (}.\:gcn delicit . III Sec. 2-1). It was s hown tha t
e
I'
i
.
dy
L
= L. o - L,
86
WATER WATER PURIFICATION PROCESSES IN NAl1JRAL SYSTEMS
87
Because Lo is the ultimate BOD a nd therefore a fixed va lue.
Table 3-2 Reaeration constants
dy
dL,
tit
dl
dL,
= -kL,
(3-11 )
Ranges of k, at 20'C. base e
Water body
Recalling Eq. (2-20)
dl
Small ponds and backwalers Sluggish st rcom s and large lakes Large s!reams of low velocily Large streams of normal ,·e loci ty Swift streams . Rapids and waterfalls
a nd making appropriate substi tuti o ns in Eqs. (3-11) and (3-10). the following relationship is obtained
ill
'
(3-12)
which states tha t the rate of change in the dissolved oxygen deficit at time I due to the BOD is a first-order reaction proportional to the oxygen equivalent of the remaining o rga nics. A more convenient form of Eq. (3-12) is rlJ
Grealer than I 15
Source: Afler Melcatf& Eddy, Inc. [3 - 15)
dD
-- = kL
0. 1- 0.23 0.23 - 0.35 0.35- 0.46 0.46 - 0.69 0.69- 1.15
opposi te effects on the deficit. This is shown gr;tphically in Fig. 3-12. The rate of cha nge in the deficit is the sLIm of the two reactions
= kiL,.
(3-13) where r D replaces the diffe·rential form as the rate of change in the oxygen deficit due to oxygen utilization. The reaction rate constant k, is th e same parameter described in Sec. 2-13 and is derived from laboratory tests on the wastewater. The rate constant is adjus ted for temperature changes. but is not usually adjusted for other effects of dilution wit h the st ream water.
(3-15) The actual oxygen concentration (e, - D,) has a c haracteristic dip as shown in Fig. 3-12. resulting in the term lixYif£'1/ sag ClIIT£', commonly used to describe the process.
Rate of Oxygen Addition As no ted in Sec. 3-9. the rate of reaerati on is a first-order reaction.withTespecl. to the magn ituci e uf the oxyge n deficit This is expressed mathematically by (3-14) where r R is the rate at which oxygen becomes disso lvell fr o m the atmosphere. {) is the oxygen deficit defined by Eq. (3-5), and k2 is a re;ler~ition rate constant that is sys tem-specific. The nega t ive sign reflects t he fac t.t hat an increase in t he oxygen s uppl y dlle to reaeration reduces the oxygen deticit. Factors atlec tin g kl illclude stream turbulence (a fun ction of velocity and channel characteristics), surface area. water depth. and temperature. Temperature corrections are mad e by Eq. (2-23) with a value of 1.016 fo r () being most common. Several models are available for determining numerical values for k 2 • [3-16. 3:4], the development of which is beyond the scope of this text. A range of\a lues typically found aprlicable to various flow regimes is given in Table 3-2.
The Oxygen Sag Curve The oxygen deficit in a stream is a function of both oxygen utili z;lIion and re;ler;lti on In spectlun of b.Js (.3-13) and 13- 14) s hows th"l these two processes ha\.c
. .~~ . Cl'"
J.
Equilibnuln .::onl·cnlr"~~~ _ _ _ _ _ _ _ _
I'
_J"
~'
...=1.
>,
6 'tJ :~
f"
-
C;
,
C
c;
--+,I I
"-
I
I
I I
I
Tiull'. day~ Figure .1-12 CharacteriSlics u f the o'ygen sag cuneo
88
WATER PUR IFI CATION PROCESSES IN NATURAL SYSTEMS WATER
89
The fina l so luti on becomes
The oxygen deficit, and therefore the oxygen concentration, at any point in time after the discharge can be determined by integrating Eq. (3-15). This is not, however, a straightforwa rd operation. Recalling from Eq. (2-21) that
Dekll =
L, = Lo e- kll
kILo ( e1k'- k,)r k2-kl
+
Do -
~) k2-k\
or
and rearranging Eq. (3-15), the following equation is .obtained dD
Tt + k2D
.. ' = kILoe - k"
(3-1 6)
and in final form
which is a first-order differential equation of the forr~
dy -d
+ Py
x ·
D
=
k L . \ 0 (e - k" _ e-k, ,) k2 - ki . .
where P and Qare functions of x, [3-1] The use of the integrat ing factor ex peS P dx) necessary for the solution of this type equation. For Eq. (3-16). the integrating factor is
X
where x is the distan ce along the stream and II is the stream velocit y. The units of 1 must always be days. Substituting va lues for t, or x/u. into Eq. (3-20), will result in a va lue of D for that point in the stream. The mos t important point on the oxygen sag curve is often the poin t of lowest concentration because this point represents the maximum impact on the dissolved oxygen due to wastewater discharge. This point is called the cricical deficit Dc> and the time of travel to this point is termed the critical cime tc' Recognizing that the rate of change of the deficit is zero at the maximum deficit. an expression for Dc can be ..fou l1 cl f.r ornEq. U,\ 6) ...
Multiplying both sides .of Eq . (3- 16) by the right side of Eq. (3-17) yields
+
k Dek" = k L e(k, - k,)1 2
I
0
(3-18)
The left side of this equation can be factored as follows dD ek" _ + k Dek" . . .. ..... dt ..... 2 . .. .. ·
d
= _ Dek" ··(/t .... ·· ..... . ...... .
..
Separating variables and integrat ing JdD ek"
=
or kiLo
J
e(k,-k,}! dt
and
The. integration of which yields
(3-22) (3-19)
The solution of thi s equa tion depends on a numerical va lue for cc' which is somewhat more ditfic ult to obtain. First, Eq. (3-20) is differentiated and set equal to zero. again because Df is a maximum at cc:
The constant of integrati on C can be determined from known boundary condit ions, that IS, D = Do at ( = O. Therefore ~.
,t
o=
.
k L
.
k-=--~ ( - kie - k",. + k 2e - k" c ) 2
Dividing throu gh by e- k" and
(3-20)
(3-21 )
t =u
(3-17)
--.\
Doe-k"
In thi s equat ion. t represents the time of travel in the stream from the point of discharge and is the on ly independent variable in the equation. The time of travel from the point of discharge to any given downstream 'point is:
= Q
IS
ek" _dD dc
+
\ c
-
k c k 2 Doe- "
90
WATER PURIFICATION PROCESSES IN NATURAL SYSTEMS
WATER
91
b. BOD [Eq. (3-1)]:
rea rra nging
y,Q, + y",Qw Q, + Q",
= .. .
'y'mix.
+ 40
3.0 x 0.5
and
x 0.17
0.67 "
_
dividing through
b~
k k - k kI La
~ _ 2 _ _1
f)
2
II
l2.4mgj L Convert to ultimate BOD. (Assume kl = 0.23 for mixture')
k I ami taking the logarithm of both sides
= .In (k- 2 -
- k )1
(1,. 2
I,
k, k2 -- kI) D ) -- - - - 'q
"I
y.
y
;=
Lo = 1 - e
Lo
Do /" - - -k
"I L o
I)J
)(5
= 18.2 mgj L
(3-23 ) c. Disso lved oxygen:
Equations (3-22) ane! U-23) can be used to determine the critical oxygen le vel in the stream and the position ~It \\ hich it OCClIl·S. Th e procedure is illu strated in the following examp le. Exa mple 3-3: Appl yi ng the BOD sag curve .- \ municipal wastewater-treatmcnt plant dischargcs secon dary cfJ-lucnt to a surl"ace stream . Th e wo rst co nditi ons arc known to occur in the summer month, whell stream tlow is low alld water tempera ture is hi gh. Unda the se conditIOns. mea surcmClll S arc made in the laborat o ry and in Ihe tield to ·deterrTllne- 'rhe- 'cha ra cleri'sl it~;· (if' ihl'" \\':l~ ie\\' :!kr' and , Irea III fl ows. The waslewalcr is I"ound 10 have a maximum flow rai l' \)1" 15.000 rn J;uay. a DOD , 01" 40 mgiL , a dissolved oxygen concenl r,lIion of2 mg L. and a Ic mpcra lu re o f2 5 C Thc stream (upslreal11 from Ihc puinl of wa
. Skclch Ih e disso h'cd O\ygcl1 IH o liie a IOO· klll reach of Ille , Ircam bel o w Ihe dls-
DO m ;,
=
8.0 x 0.5 + 2,0 x 0.17 0.67
= 6.5
mgj L
d_.Temperature:
22 x 0.5 + 25 x 0. 17 Tmix
0.67
=
2. Correct reaclion constants for temperature_ a. BOD reaction rate [Eq . (2-23)J:
k" .8
=
k 20 ( 1.047 12 .8 -
10)
= 0.23 x 114
/J. Stream reaeration rate
eha rge.
k 22R =
SOLl' IION
k 20 (I.01612.8
- 10)
= 0.4 x 1.05
I. Determine'c haracrenstics ( I.
If
12.4 _e- O. 23
or in more convent ional form I,
k
l)f
Q:, -
\\'aste\\"~'ler-Slrealll mixl;,re.· IS.noo m' d
I d X
2<1 h
I X
);"22" =
(,() min
0.42 d . I
=
k2
I min
Il X
---60 s
3. Determine initia l oxyge n deficit Do· (J. At T = 22.8. the eq uilibriul11 concen tration of oxygen in fresh water is 8.7; therefore
= 11 . 17 Ill J S
Qrnlx
= 11.17
+
n .) =
n.b7
J
Ill , S
Do = 8.7 - 6.5
=
2.2 mg/ L
92
WATER PURIFICATION PR OCESSES IN NATURAL SYSTEMS
WATER
These point s are connected by a smooth curve as show n in the accompanying figure to yield the desired oxygen profile of the stream.
4. Determine the critical deficit and its location.
_I_In[k2 (I _
Dok, -
I, =
a.
k,-k,
k,
k')] ·
(3-23)
k,Lo
10
I In [0.42 ( 1 - 20.42 2 -- -0.26)J -0.42 - 0.26 0.26 . 0.26 x 18.2
=
- - -- - - -
8
2.5 d
1, '=
-' tIi
k, Loe-k'tc k,
C
Dc = -
b.
93
c v
}
c,
----------------
. . Do
6
en A
x
0.26 18.2e - 0' = _ .6 X '.5
o
0.42
=
.~
5.9 mg/ L
c. This condition will ~ccur at a distance of x = 0.2 m/s x 86.400 sid x 2.5 d
{c :::.
X/II
= 43.2 km downstream from point of discharge
75
o
5. Determine the deficit at points 20, 75. and 100 km from the point of discharge.
100
125
Di st3n cc downstream, km
xkm
1= - - -
(I.
u km/d
Ikm u = 0.2 m/s x - - - x - - - = 17.3 km/d 1000 m d 86.400 s
1'0 = 175
1'00
20/17.3
=
Ll6d
= 75/17.3 = 4.3 d
=
100/ 17.3
= 5.8 d
b. The deficits at these times are: k L D = _ _ '_0_ (e - '" - e-"') k, - k,
D
-0 ,6x - 0.26 x 18.2 (e' 0.42 - 0.26
+
(3-20)
Doe -'"
II 6 _
e-O .42Xllb)·+2.2e - 0.42XI.'6
.
Bot h th e position and magnitude 6r the critical deficit ar e related to t he sys tem variabl es (/'1' " 2, L o, Do. and u). The time o f travel to the c riti ca l defi c it (rJ is innuenced more s trongl y by th e va lues of /.:.1 and /.:." . while the magnitude of the deficit is m os t affec ted by th e L o va lu e. N o t o nl y do heavier loads resu lt in greater defi c it s. but th ey extend th e inAu e nce of th e waste farther down s tream H eavy loads of organics may result in th e' developmei1t 'of'a'I1ae-rubic condiri·ooS·.Und'er·· t hese conditions, o xygen is tran s ferred in a t a high rate [Eq. (3-14)J but is used up by facultative o rga ni s m s that may also be utilizing the organic material produced by a naerobic m e tabolism. In a deep s tream , true anaerobic organisms may Aourish near the·bottom. Only after the strength of the waste has been sufficiently reduced . will aerobic condition s be re sto red. Since anaerobic metabolism is a siow. process, recovery of an overloaded st rea m will be slow and the oxygen sag will extend far downstream. - "
20 -
= 5.1 mg/ L D 75
Limitations of the Oxygen Sag Curve
= 5.2 mg/ L
D,oo = 4.1 mg/ L
.6. The dissolved-oxygen concentrations at each po int are found C 20
= 8.8
- 5.1
C n2 = 2.8 mg/ L
C 75 = 3.5 mg/ L
C'oo
= 4.1 mg/ L
=
3.4 Illg!L
to
be:
Th e limit a ti o n s o f the oxyge n sag c ur ve s hould be at o nc e appare nt Th e rate o f d eoxy gen ~llion and th e ra te o f reaeration a re each affected by many va ri ab les fo r which the mod el mak es no a ll owance . . BOD variables Th e eq uati on is ba sed on the a ss umption that th ere IS one source of BOD w hen there m ay actually be seve ra l diffe re nt point or n onpoin t so urce s of BOD. Additional di sc harges can be tak e n into cons ideration by s ubdi vidin g a ri ve r into short reache s, each feci by a single p o int so urce. If tributaries empty int o
94
W ATER
WATER PURIFICATION PROCESSES IN NATURAL SYSTEMS
the m ains tream , any di scharge they may have recei ved mu st al so be tak e n Illto co nside ra ti on, a s we ll as th e in crease in flow o f th e rece ivin g stream. E ven when care is gi ven to co ns id er all o rga ni c lo ael s intro du ced at di sc har ge POlll ts, the bi o chemica l ox ygen demand ora str eam ma y be affected by o th er fac to rs no t app ro ximated by t he k I co n stan t. A lga l re spira t ion in th e absence of s unlight, nitrificati o n processes that in c rea se ox ygen d e m a nd , and the presencc o f slud ge d e pos it s in pool area s can al l in c rea se a str eam's BOD. [3- 5, 3-8J In s hall o w s treams, masses of m icrobial growth attach ed to th e strea mbed lIlay be more e tncient at utili z ing o rgani cs, and con sequ entl y co nsum e more di ss ol ve d o xyge n th a n the s us p ended micro o rgani sm s used in th e laborat o ry BOD tes t. [3 -2 IJ This fac t, though recognized and va lu ed in engineered tr eatment sys tems, is o ften ig nor ed in self-purifica ti o n stu die s. Reaeration variables R ep lacement of o xygen is al so affected by man y fact o rs
not taken int o considera ti on by the _form ula s used t o derive oxyge n sag curves , notably the reaeration co ntribut ion of algae p h ot os ynthesi s . F u rt he r, th e math ematics assumes stead y-sta te condition s all along a ri ver c ha nn e L Becau se such steady-stat e condi tions wo uld indeed be rare , m o st streams must be s ubdiv ided and a k c va lue assigned to each reach. Even with subd·ivisio n into reach es, det e rmination o f the kl constant is probab ly th e one area most prone to error in oxygen -sag-curve wo rk, becau se n o theoretical as sumption of flow characteristi cs - channe l formation, obstacl es, pool s , effect s of impo undments , and othe r such var iables - is like ly to fit anyone particular stream perfectly. Addi t ion s ha ve been made to the bas ic Streeter - Ph e lps model that incorporate the diurnal effec t o f alga l ph o tosy nthesis, th e nitrifi cati o n process, and th e sedlm entation-resuspension of organic material. Th ese model s a re present ed e lse wher e In the lit erature [3-22J and require a much more sophi st icilted.llat a ba sefo r use: . . .
,. 1·
"r '
Confirmation of the Oxygen Sag Curve The di ssolved- o xygen pro file o bta in ed fr o m ma t hemalica l mod e ls s ho ul d be con firm ed by ac tu a l field mea s u rements. Id eall y, th e re s ho uld be a co m p reh e ns ive samp lin g und e r condi ti on s of kn ow n waste load s and rivt:r hydr o logy. A peri o d o f warm weat her a nd lo w flow s i·s desirable, and dail y samp lin g fo r I m o nth fo r all param eter s is prefe rred.· Once the DO d e ficit a nd th e time tll th e c ri t ica l () 2 concen tr a ti on have been verified by a deLl il eci \ vat e r-qllality sline y. oxygen sag c ur ves ca n be used to fo reca st stream co nditi o n s th a t ca n he ex pec ted fo r g i\T n Ila s te lo ad s and s tr eam fl o ws.
3-1 I ORGANIC DISCHARGE AND STREA iVI ECOLOGY In :l·ddltl o n to \ :Hia t i;) ns In th e ·o .\v ecn cn ncc n tr at i, )n s. lll ;lll .\ ,' II ,e l piIysiul. che mi cal. and bi o log ica l c han ges o:c~l r ill streams ;I fl n tiJ e disciI:t r!,!e "r hi,, d eg rad a h le o rgani c maleri :Ii . T oge liJ n ", ilh Ihe \' .\\gC ll "I lppl\ . Ih es<: I' r
. i
95
and their products grea tl y influence th e ecology (th e relationship between living organis m s a nd th eir envir o nm en t) o f th e strea m. Lik e the oxyge n b a la nce the ec; logical balance of a s t ream rece ivi ng a biodegradable organic discharge c;n be mod e led. M os t of the model s a ss ume t ha t t he organic waste is com p osed primaril y of municipal wastewa ter and d o es n o t conta in s ign ifica nt quantities of mat.eri a ls that wo uld be toxic to t he flora a nd fauna of th e stream. Ecologica l modeling usually involves divid in g th e strea m int o reaches, o r zo nes, in w hi ch certain species or certam processes predominate. The model mos t common ly used in the United States is the o ne devised by Whipple, Fair, and Whipp le. Thi s model d ivid es the stream into four zones labeled the zones o f degradation, active decom position, ,:eeaver)', and clean water. A summ a ry of th e pllyslcal, c hemica l, a nd biological c haracte ri stics o f each zo ne is presented in Table 3-3. M a n y o f the ph ysica l c haracteristics described in Table 3-3 may be n o ted b y the casual o bserve r, but the chemica l c ha rac te ri st ics (w ith the exception of th e presence of highly od o rou s H l S) ca n be determi n ed o nly thr oug h samp ling and laborator y test in g. Biol o gical s pecies and numbers are mark edly d ifferent from zone to zo ne, and spec ies d ive rsity is a prim ary means of establishing zone boundari es. The change in species and number s of o rganisms in each s pecies is illustrated in Fi g. 3-1 1 Th e food supply is a primary fac tor in determining the type o f o rga ni sm s that predom inate. Nea r the po int o f discha rge, bacter ia, protozoa, and molds predominate. Bac teria fi nd an abu nd a nt food supply in the form of carbo hydrates, pro tein s, and fats. As th ese microor gani sms decompose organic was tes, they co nvert them into nutri ent material s suc h as nitrates, p h osphates, a nd carbo n dioxide. The bac teria l populati o ns flouri s h until di ssolved o xygen and lor th e fo od su pply is . ex ha us ted.· Bemuse· bacteri·a··provrde··food··for . prOHizCia: ci liat es, ro tifers, and cr ustacean s, th ese higher fo rm s of life diminish as bac ter ia die o ff. Th e abundan t sup ply o f nutri ent materials made avai lab le by th e bacteria! decom position of o rganic m a tter brings abo ut still furth er changes. About midwa y through the zo ne of ac ti ve decomposition, w here miner a l nutri en ts (notably n it rat es ) abou nd, a lgae beg in a ra pid inc rea se. Blue-gree n (Phorl11idillm, Lyngbya, a nd Oscil/atoria) and g reen alga e (Spirogyru and St ig t'oc/uniul11), a n d diatoms :, (GulIlrhon ema and Nit zschiu) may be present in thi s zo ne. [ 3-3J .. In the zo ne of rec over y. a lgae g ro wth peak s, then declines, w ith a lga l p op ulat iu ns III th e clean zone beginnin g to appr ox imat e th ose found in the predischarge purtions of the stream. Blue-gree n (f\llicrocys Lis and Anabaena), pigmented Ila gellates (Euy lenu and Pando rina ), g reen al gae (Cladorlrol'u and Ankistrodesmus), and di a tom s (l'vl f' ridio/'l andCrc/o tella) are s pecies fo und in the zone·of recovery. [ 3-3J . . As nutrient lo ad s dec lin e, BOD d·ecrea ses, an d DO levels r eturn to th ei r p redi sc har ge le vels, alga e and ba c teria po pulation s return to their clea n- water statll S, and clean-wa ter invertebrate a nd \ertebrate fauna again popu late th e str eam. At thi s puint, the s tream 's natural se lf-purifi ca ti o n pro cess ha s esse nti a ll y bee n co mp leted, but u nl y Ill so far as biod eg radabl e o rg:ll1ic wa stes a re co ncerned.
:lIi'
96
WATER WATER PURIFI CATION PROCF.ssES IN NATURAL SYSTEMS
ill
--
Table 3-3 Whipple, Fair, and Whipple model for zones of stream self-purification Zone Degrad a ti on (Zone 2 in Fig . 3-13)
Physical characteri s tics
Chemica l characte ri s tics
Biologica l characteri sti cs
The water is turbid ; there are sl ud ge deposits a nd Roating debri s
Oxygen is redu ced to about 40 ~~ o f sat urat ion.
Fish and gree n algae a re declining ; litt o ral forms o f gree n and blue-green a lgae are trailin g from frequent ly wetted stones. These includ e
Table 3-3 (continued)
Zo ne
Ph ysica l cha ra cl eri st ics
Chemical characteristics
Bi o logica l characteri s tics
Recovery
\V aler IS clearer.
Di sso lved oxygen co nte nt move s upward fr o m 40 :%.
Pro tozoa, rotifers, and crustaceans ap pea r . Fun g i a re prese nt to a lim ited degree . A lgae ap pear 111 the .. fo ll owing o rel er:
(Zone 4 in Fig 3- 13)
or sa tur at ion; · nitrates are prese n t.
Cyanoph ycaea, Chlo rophycaea. and
SIi.qeoc/onium, Oscil/Moria, and Ulolhrix. Bo tt o m forms in slud ge include reddish worms (Tubilic idae) si m ila r 10 ea rthworms. s uc h a s Tuhifex and Limnodrilus. Wat e r fungi a re typical ly whit e. o live green, putt y g ra y, r usty brown . Sphacro/i/us norOIlS, Lep lOm ilus, a nd A chlya appear , as do ciliated protozoa or ciliata such as larchesium. £PI.'ily/is. and
VOrticel/a. Active decompos ition (Zone 3 in Fig. 3-13)
Wat e r is grayis h and darker t ha n in degradati o n zone; SC um may form. septic condition s may have set in .
Oxygen leve l moves be tween 40 ~,~ o f
dialoms. Large plants (sponges, bryozoans) ap pear. Bottom o r ga n isms include Tuhife x. mu sse ls, snails, and insect larvae.
Ca rp, s uc kers. a nd m o re resistant forms o f fish occur. Clean wafer
'!atLJrirI ')Irl'am
(Zo nes I
cl)l1dlll l )nS {{ I t'
and 5 in Fig . 3-13)
rt';..iored.
.\Ollrc~' : r\c.i:Jplc..'d rrnm
Ma yflies (Ephemerupleria). s to ne fl ies (Plecoplera). caddis fli es (Trichoptera). and gamefish arc fou nd
D isso lved oxyge n I S close to sa turatIOn .
r1 ahbltl. ["-2]
Bacter ia flora fl our ish;
a naerobes di splace aerohes , whic h eappear toward the then as active lower e nd of t he zone. decomposi t ion Pro tozoa follo\\; course of . .di n:inis hes" ,?xy.&e.n.. .... ae r.Qbic .baCle ria , fir",· .. co nl ent ri ses. diminishing and then Methane, hydrogen. reappeanng. Fungi fOllow a a nd s ulfide a re simil a r course, di sappearing given off. unde r true septic co ndition s a nd then reappearing . Organisms are thre adlik e a nd develop pink.. cream, and grayish tints . Algae are present to a ve ry slight exten t a t the lower end o f the zo ne. TUbifex are prese nt on ly at th e uppe r
/
---L-
---, /
I
Popu latIOn o f indiVIdu a ls In eac h s pecIe
"'"
/--
\
!"
_ _ _ - J,
",
---
/
""-- Di sso lved oxygen
Distance downs tream _ 4
mosquito larvae (Clilex) are found . There is no fi s h li fe.
(COl/lil/lIcd)
en
Wa s tcwa te r dis c ha rge Figure 1-13 Changes In POplIl
(hulII ""l11l1l1'r . )
98 WATER
Shou ld an;lerob ic condi ti ons de ve lo p in th e zone o f ac ti ve deco mpos ition, a drastic change in both materials and fl ora a nd faulla wo uld be observed. Reduced co mp o und s, rath e r th an oxi dized e nd prod uct s, wo uld appear, and aerobic spec ies would g ive way to a naer o bic and facultative o rga ni sms that , wi th o ut co mpe titi o n fmm th e a e robes. wou ld flouri s h In g reat numbe rs.
WATER PURIFICATION PROCESSES IN NATURAL SYSTEMS
99
th e liquid . Removal o f gases that are in low co ncen trations in the atmosphere'is enhanced by m axim iz in g contact between th e water and air, a n operation often used to strip undesirab le gases from water intended for potable use. Oxygen, a major co nstituent of the a tm os phere, may be ad d ed to wastewaters by much the sa m e principle. The addi ti o n o f gases such as carbo n dioxide and chlorine to meet s pecific treatmen t objec ti ves (recarbomi ti on a nd di sinfection, respectively) is us ually accomplished in cl osed pressurized sys tem s.
Application of Natural Processes in Engineered Systems Many o f the ph ysica l, c hemical, and bio logica l processes that fun c ti on in n atu ral wa ter sys te ms have been in co rp o rated into eng in eered sys tem s fo r wa ter a nd was tewafer treatment. By carefu ll y contro lli ng th e syste m va riabl es, the rate at w hi ch th e processes occ ur is maximized a nd th e tim e required for pur ification is minimi zed. Rea c,tions may thu s be ca rri ed to co mpletion in engi neered sys tem s in a fraction of the tim e and space required for similar e ffi ciencies in natur a l wate r sys tem s. The fo ll ow in g secti o n gives an ove rvi ew of the applicat io n o f natural processes in en gi neered sys tem s whilc Chaps. 4 and 5 prov ide a more co m p lete coverage o f th e eng in eered sys tems.
3-12 PHYSICAL PROCESSES T he physica l processes fr eq uently lI sed in e n g in ee red systems inclu de sed im en tation, filtration, and ga s tran sfer. Th ese are the basic re mo val processes and m ay be used to remol'e mat eria ls in raw water or was tewa ter o r m ay be used to re m ove the products of c hemi ca l or biological processes. ...5ed i.n1entatio n.is.used to re mo ve particles and co lloid s fr o m b o th wa ter and wastewater. This term is often used sy n o n ymous ly with c larifica tion , a lth o ug h there are su btle diffe rences in their meaning. In wa ter- and was tewate r'treatment systems . sed ime ntati on is ca rri ed o ut in la rge basin s o r tank s in wh ich the fl ow IS di spersed unifo rmly to minimize turbulence th a t often keep s particles suspe nd ed in n a tur a l water sys tem s. When particles arc too small to se ttl e in a reaso nable len g th of tim e. che mic a ls may be add ed to coagulate them int o larger ma sses that Ivi II ~e tlle m o re qu ic k Iy. The se t t led so Iid s. or slut/qt'. is rn echa n ica Il y rem oved fr om the bo ttom o f the tank to prevent accumulation . Like sed ime nt a ti on. filtr atio n is used as a su i id s-remova l ope rati on in wa ter and . less co mm o nl y . wa~ t ewate r tr eatme nt. Th e filt er mat erial mo st co millonl y useci is a gra nul ar medium s imilar to th e sa nd a nd g r<1 ve l enco unt e red in man y str eams and aquifer s. The mat el: ial. is sized to o ptillli ze filtrati o n rat es and partic le remo va l. and m ec hani s m s ar e pro vi ded for periodica ll y removing th e impulities t rapped by thdi lt e r. In mod e rn practice. filtrati o n is o ft en a po lis hin g step fo ll ()\IIll g se ttlin g o p e ra ti o n s that removc th e bu lk o f th e so li ds. Gas-transfer operat io n s ma y be lI sed in buth \Vate r a nd Ilastcwate r tr e:ltm ent. Depending on th e tr ea tment ob lcctll"CS. ~ases may be removed fn llll o r added tn
3-13 CHEMICAL PROCESSES C hem ica ls a re used in m a n y wa ter- and wastewaier-treatment processes. Chemicals Ill ay be added to a lter equilibrium conditions and cause precipitation of undesir- . a ble spec ies. An examp le is the addit io n o f lime to precipitate hardness in potable water treatm ent a nd to precipitate phosphate in wastewater treatment. Often the c hem ical adjus tm ent of pH is necessary to effect the desired precipitation. Ox idi z ing agents may be used if reduced compounds are to be removed. For exa mple, p o ta ssi um permanganate may be' added to oxidize so luble forms of iron and m a nga nese to forms that precipitate. Chlorine is sometimes used as an ox idi z in g agent as well as a disinfectant in both water and wastewater treatment. C hemi ca l coagulation, ofte n used as an adj unct to sedimentation or filtration, co nditions sma ll particles and co lloids so that they form large, sett leable flocs. In a dditi o n to the above, many other c hemicals may be used for special purp oses in water and was tewater trea tment. ~gain , it should be kept in mind that c hemical processes a re conversion processes and that actual removal is accompl is hed by physica ll y sepa ratin g the solid, liquid, or gaseous products of the chemica l rea c ti o n s. . .............. . . . .
3-14 BIOLOGICAL PROCESSES Bi o logical processes have fou'ndlittle use in the treatment of potable water supplies becau se of the low levels o f biodegradable orga ni cs in the raw water. However,:) bio logical processes arc used ex ten sively in was tewa ter treatment to convert :'biodegradab le organics and o th er nutrients into a more managea ble form. Biolog ica l pi'ocesses for m th e ba s is fo r seco ndar y treatment in which dissolved a nd co ll(lidal organics a rc co n verted into biomass that is subsequentl y separated from (he liquid stream. Secondary treatment sys te ni s are designed to op timize contact be t wee n m icroorga ni sm s and o rganics uncl er t he most favo rab le environmental co ndltiull s. . Ollce sep;i rated,. the b iomass becomes a concentra ted was te stream that must he dealt wi th promptly. l3iological ·treatmellt o f thi s and ot her orga ni c wastewater s ludg(;~, ca lled silldy e digestion. is one o f th e most impor ta nt, and m os t difficult, rll"ocesses in wastewate r treatm ent.
WATER PURIFICATION PROCESSES IN NATURAL SYSTEMS 101
100 WATER
3-5 An indu strial wastewater is discharged into a municipal wastewater sewer. The character-
DISCUSSION TOPICS AND PROBLEMS
istics of the two wastes are as follows:
3-1 Name and briefly describe the majorphysicalprocessesinvolvediil self:purification of watercourses. 3-2 Two streams converge as shown in the sketch below. Determine the flow. temperature. . and dissolved oxygen in the merged streams at point C.
Flow =0' Temp =')
.
(DO
Flow = 3.7 m}/s / Temp=::loC
C
Indu strial.
Muni cipa l
Fl ow = 3500 m' /d BOD , = 1200 mg/L PO~ - = 140 mg/L
Flow = 17,400m' /d BOD , = 210 mg/ L pol - = 2.3 mg/ L
Determine the characteristics of the mixture. 3-6 Di sc uss thermal stratification and its importance in temperature o f streams and lakes.
=')
3-7 What are three major chemical processes that may a ltern ately aid or obstruct na tural purificati o n processes of water systems?
Stream C
3-8 Calculate the solubi lity of hydrogen sulfide in water at 20
DO = '4 .5 mg/ L
c
e.
3-9 Determine the solubility o f the components of air in water at 20°C and 1.5 atm pressure.
3-10 Wh at is the so lubility of me than e in water at 20°C?
B
3-11 Define (a) metabolism ; (b) catabolism: (e) anabolism; and (d) endogenous catabolism. \
Flow = 2.5 m}/s Temp = 17° C DO
= 7.5
3-12 What are adaptive enzymes? What rol,e do they play in natural purification processes of bodies of water? 3-13 Define (a) autotrophs. (b) heterotrophs. (e) phototrophs. (d) aerobic heterotrophs. (e) a na erobic heterotrophs. a nd facultative heterotrophs
mg/ L
en
3-3 Effluent from a wastewater-treatment plant is discharged to a surface stream. The characteristics of the effluent and stream are as follows: I
I:
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Effluent
Stream
Flow = 8640 m'id BOD, = 25 mg/ L Ammonia = 7 mg/ L Nitrate = 10 mg/ L Chloride = 15 mg/ L
Flow = 1.2 m'/s BOD, = 2.1 mg/L Ammonia = 0 mg/ L Nitrate = 3.0 mg/L Chloride = 5.0 mg/ L
3-14 Exp lain the role of rot ifers. c ru stacea. and s lu dge worms in natural purification processes of bodies of water.
3-15 What a re the two mechanisms known to contribute oxygen to surface waters? 3-16 What is the oxygen deficit o fa st ream and how is this deficit represented mathematic a lly ? 3-17 Write a simplified 'formula' fonhe -photosynthetic' process by which algae' popula r iDns ' ma y repleni sh oxygen in a body of water.
3-18 Write a formula for the endogenous catabolism by which algae popUlations may contribute t.o oxygen demand. . ~19. A wastewater-tr,9tment plant disposes of its effluent in.a surface stream. Characteristics
of the stream and effluent are shown below.
Determine the stream characteristics after mixing with the waste has occurred.
3-4 Cooling tower blowdown from a power plant is discharged to characteristics of each are given as: Stream
Coo ling water
Flow = 10 m'; s Temperature = 15' C TDS = 125 mgjL Chromate = 0
Flow = 40 m' / min Temperature = 28"C TDS = 2520 mg/ L Chrom
Determine the characteristics of the stream after mixing.
it
surface stream. The
Wastewa te r
Flow. m'/s Di sso lved oxygen. mg/ L Temperature, OC BOD , at 20°C, mg/L K, at 20°C, d - 1 K , at 20°C, d - 1
Stream
0.2
5.0
1.0
8.0
15
20.2 2.0 mg/ L
100 mg/ L
.
0.2 0.3
(a) Wh a t will be the dissolved oxygen co ncentration in the stream after 2.0 d ') (b) What will be the lowes t dissolved oxygen concentration as a result of the waste discharge?
102
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WATER
,.
3-20 A municipal was te wa ter-treatm ent plant discharges 18.925 mJ / d o f treated wastewater to a s tream . The wastewater has a BOD s o f 30 mg/ L with a k I o f 0.23 d - '. The temperature o f e
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REFERENCES
;
3-1 Amirlh"r~iah . A.: by personal commun ication, 1982. 1-' Ba bbill, H . E. and E. R. Baumann' Sewerage and Se,rage Trealmelll , Blh ed., Wi ley, New York. 1952 .
fl ows a t a ve loci t y 0. 5 m /s a nd the reae ra ti o n co n s ta nt is 0.45 d - ' . (a) What is th e oxyge n le ve l of the st ream after 2 d ') (b) What is th e c ritical oxyge n leve l in the s t ream and how far downstream w ill it occur')
3· 3 8 arlsc h. A. F .. ami W. M. In gram : Biology of Wal eI' Po I/lil ian, U.S. Dept. o f Int e ri o r W a te r Po lluli on ('on!ro l Adrninislra lion, 1967. ' 3-4 Churchi ll. M . A., H . L. Elm o re , and ·R . A. Bucking ham: "The Prediclion.of Stream R eae1:ation Rales," WaleI' Polllliion Research , vo l. I , Perga m o n, Londo n, 1964. .1-5 Clark. John W .. Warrcn Viessman, Jr. , and Mark J. Hammer: WaleI' Supply and Pol/ulion Conlrol 3d eel .. Ha rper & Row, New York, 1977. '
3-2 1 A wastewater-treatment plant discharges to a sma ll s trea m . The characteristics o f the
"
\vastewate r and th echaracteri s tics o f s tream are give n be low.
3-6 ~oin. The"dore Siream
Waste
~'
'
Flow = 0.4 m' /s BOD = 2.0 mg/ L DO = 90 % sat uration e Temperature = 24 C
Flow = 10,000 m' Jd DO = 0 ms/ L Temperature = 21 °C k, =O.23d-'
:
:i I
McGraw-HilI. New Y o rk. 1980. .1-k Hammer. Mark J .' Waler and Was le- WaleI' Te chnology, Wiley , New York, 1975. 3-9 Hvnes H B N : Tlw Biololj)' of' Pol/uled WaleI' , Li verpoo l University Press . Liverpoo.l , 1960. 3- 10 Kemmer . Frank N.: The NA LCO Waler Handbook, M cGraw- Hili , New York, '1979. 3- 11 K lei n. LOlns: IIiI'''' POlllllion II . Causes alld E(fecls, Bullerwo rlh , Lp ndon , 1962. 3· 12 Lewis. W K and W . G . Whilman : .. Pr inciples o r Gas Abso r plion. "Ind. Eng. Chem., 16 ' 12 15 ( 1924)
'"' = 0.45 d - ,
'I
oxygen mu st b e maintained in t he s trea m .
3-22 A milk-produc ts industry discharges a wastewate r t o a s tream . Characteristics of the
.1- 13 Lind sley. R . K . and J . ll. Franzini: Waler Resources Engineering. 3d cd .. McGraw-Hili , New York. 1979. 3-14 McKinney. R . E.' Microhioloq),/or Sanilary Enqinee r.,·. McGraw-HilI. New York, 1962. 3- 15 Melcalr 8.: Eddy. Inc . W{fSlelJ'fI/er Engin eerinq.- Tr ealmenl and Disposal, 2d ed .. McG r aw- Hili . New York. 1979 .
:1, ,! ,
wastewater and the s tream a re s hown below. (a) If no treatment a t all is given to t he wastewater. what will be the lowest oxygen level
3- 16 O·Connor. D . J .. and W. E. Dobbins: "The Mechani sms or Rcaeralio n in Nat ural S i rea ms" J San Enq /)ir. AS.C.E.. 82 :S A6 (1956). '
1
.11!
C.. J r .. [(olo.qical Srslems and Ihe Enrironmenl. H oughlon Mifflin, Bosl.on. 1976 .
.,-7 (laudy. A. 1- .. Jr .. "nd E. T. Gaudy: Microhidlo.lJy./or EIIl'ironll1 l'lJ lal Scien li.H.I and Engineers.
t '"
103
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the wastewater is 27 and the d issolved oxygen is 2.0 m g/L The s t ream just above the po int o f wastewat er di scharge flows at 0.65 mJ! s. has a BOD s of 5. 0 mg/ L, and is 90 percent satu rated wi th oxygen . The temperature of the s tream is 23 e After mixin g, the stream and wastewater
WATER PUR ifi CATION PROCESSES IN NATURAL SYSTEMS
I! !
Determ ine the m axim um BOD s (20 "C) tha t can be discharged if a minimum of 4.0 mg/ L o f
in the s tream as a result of the discharge"
[, Parameter Flow BOD , a ' 20' C DO Temperalure, ' C k,a I 20°C I., a120°C
\\'a stewatcr
1000 m ' /d 1250 mg/ L mg/ L . 50 0:35 d - '
o
Stream 19.000 mJ id 2.0 mgj L 10 .0 mgj L 10
t,
t
0.55 d '"
(b) If the s trea m is a trou t fi s hery and the s tream s tan da rd s require a minimum D O o f 5.0 mg/ L what is the maximum B OD, (20 C e) that can be discha rged by t he indus tr y?
3-23 Write a comp uter progra m to model th e Streeter - Ph e lps equati on. Repeat P robs. 3- 19. 3-20. a nd 3-2 1 u si ng the computer.
3-24 What a r e the fo ur zo nes in th e W hipple. Fair. and Whipple model '! Define the zo nes by exp la in in g wha t happens in each.
3-25 Mos t oft he nat ura l purin ca t ion rrocesses discussed III thi s char ter have t heir co unt e rpart s in ~ng in eered processes for the treatme nt of potable water supp lies or th e t rea tment of was te waters. Di scuss bri e n y the ways in which the follow in g na t ura l processes a re utili zed in engineerin g systems: (a) scdimenta l ion. (h) filtration. (e) gas tra ns fe r. (d) precipitation. and (~) microbial ac t io n .
··l
(
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f ~
i
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I
L
.1- 17 Park er. F. L .. an d P. A. Krenkel Thermal Polllliion .- SWillS vf lite AN, Dep\. Environmenlal and Reso urces Engineering. Vanderbill Univcrsily. Nashvi lle, December 1969. ·3-18 Palrick , R.: .. EITecl or Suspe nded So lids, Organic Mall er and T oxic Malerials o n Aquatic Life ... in. R,iver~, .... .I:Valer. a!"1 ,~~IJ:ngf. W91:k~,.f.el? ru ary 1968. p . 90. 3- 19 RU liner. Franz. FllIlllanll'lIlals o/ Limnology, 3d ed" D . G . Frey and F. E. J. Fry (!rans.) , Uni ve rsily of Tornnlo Press. Toronlo. 1963. 3-20 Sawyer. C. N .. a nd P. L. McCariy' Chem islry'/or EnvirollmenlOl Engineers 3d ed ., McGra w- Hili . New York , 1978. 3-2 1 Srinanlhakumar. S., and A. Amirlharajah: "Organic Carbon Decay in a Slream w ith Biofilm K inelics." 1. In t'. Eng .. ASCE, 109( 1): 102 (February 1983). :1-22 Slee!. E. W .. and T. J . McGhee' WaleI' Supply and Sewerage. 91h ed. , McGraw-Hili, Ne\'-, York , 1979. 3-23 S!recler, H . W .. and E. B. Phelps: U.S. Pub. Health bullctin no. 146, 1925. .1 -24 Tsi vogloll. E. hacer Mea suremenl of Slream Reaerulion. u.s. Dept. or Inl er ior , Water
c.:
PoliUlion Con lro l Adntinislr
c.:
T
T
T
CHAPTER
FOUR ENGINEERED SYSTEMS FOR WATER PURIFICATION
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An adequate supply o f pure water is absolutely esse ntial to human ex iste nce. Th e con'sequences of a contaminated water s upply can be illustr a ted by co nditi o ns
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prevalent during the industrial revolution in E u ro pe when large numbers of peasant s were attracted to the cities where th ey crowded together with little or : no sanitary facilities. Human waste. or " night so il" as it was ca lled, was tossed int o the streets or emptied into pits in common courtyards, often near t he s ha llow wells that served as the neighborhood water supply. Seepage into these wells and runoff ~ into nearby streams provided a direct link in the infection cycle. and o nce an out~ " ........ ... .br.eak.:oLdis.ease .occmred.it. usually .spread .rapid.ly . lhwu&h.. L!:J.e. community,. The ...... .. resultmg loss of lIfe and suffering left scarcely a famil y unt o uched during several centuries prior to the 1900s. . The development of effective water-treatment method s has virtually elimin a ted major waterborne epidemics in developed countries. Thi s is no t to sugges t. however, that the problem of waterborne diseases has bee n e liminated . Developing natIOns. where treated water is not available to a ll the p o pUl a ti o n. still experience occasIOnal epidemi cs of cholera and typhoid . as we ll as manv o utbr ea ks of less severe disease. Even highl y developed countries. including 'th e United States. where public wat er s upplies are almost universall y tl·eated. are no t to tall v immun e from an occasional o utbreak of gastrointestinal illnesses tra cea ble to bi~logically contammated water s upplies. Chemical contamination of water s upplies has become a co ncern in more recent times. Industrial facilities in developed countries produce and use literally thousands of chemica.l compounds. Along with a n ab undant arrav or household' a ndagricultural chemicals. these materi a ls often find th eir way int o 'water suppli es. WhIle some of these chemical compounds are known tl)xicants. mut age nt s. or carcmogens. the health effects o f many others are no t prese ntl y kn mv n . Sufllcicnt 104
ENG INEERED SYSTEMS FOR WATER PURIFICATION
105
data are no t presently ava ilab le , to predict the consequences of in ges tin g smal l qu an titiesof chemicals ove r long periods of time. It is ironic that the high stand a rd of living that allows ind ustriali zed nation s to provide bio logically pure water to the majority of th eir popUlations a lso res ults in the discharge o f chemical waste th a t may eventually have more deleterious effects on human health than th e d omest ic waste .th a t helped sp read t he plagues of past centuries.
4-1 HISTORICAL OVERVIEW OF WATER TREATMENT The treatment of water int e nd ed for human consumption is a very old practice . Baker [4-6J report s references in Sa nsk rit literature dating back to 2000 B.C. to s llch practices as the boiling and filtering o f drinking water. Wick siphons tha t tr a nsferred water from one vesse l to a no ther. filtering out the suspended impurities in the process. we re pictured in Egyptian drawi ngs of the thirteenth century B. C . and were refe rred to in ear ly Greek and Ro m a n litera ture. The fact th at these practices were recorded in the medi ca l documents of the times indicates th a t the connection betw een water a nd health had been observed. In fact, Hippocrates (460- 354 B.C.). co nsidered to be the father o f mo dern medicine, wrote that " ... whosoever wishes to investiga te medicin e properly should - consider the water tha t the inhabitants use - for water co ntributes much to health ." [4-6J These ea rl y wate r-trea tm ent devices were used in indi vidu a l hou seholds: th ere is no indication o f co mmunit y wa ter supplies bt;ing treated until around the first ce ntury. Some of th e R o m a n aq ueduct s had settling basins a t the head wo rk s a nd incorpora ted" pebble ca tch ers" in the aqueduct channeL These aqueducts suppli ed a few priva te ta ps and prov ided fou ntains'o r reservo irs for us~ by th e ge ne ral public. The city of Venice. situated o n isla nds with no fr es hwate r reso urce. channeled ra inw a ter fr om roofs a nd courtyards into elaborate cisterns thr o ugh sa nd filters surro undin g the reservoir. Tl~e fir st of these cisterns was built arou~d the fifth century A.D . and provided private and public water suppli es for abou t 13 ce nturies. [4-6J Water-treatment practice appare ntly lagged during the Middle Ages, with a renewed interes t emerg in g in the e ighteenth cen tur y. Seve~a l patents were issued for filtering d evices. primarily in France and England. As in ancient tim es. however, th ese devices we re for use in pri \a te households. institutio ns. ships. etc. It was n ot until the beg inning of th e nineteenth century that th e treatment of public water supp li es was a tt emp ted on a large sca le. The c it y of Paisley. Scot la nd . is gene rall y credi ted with be in g th e fir>! c it y w ith a treated wa ter sup p ly. That sy'ste m consis ted bf.sett ling opera tion s followed by fi ltra tion and was put in se rvice in 1804. [4-6J ThiS practIce slow ly spread throu g h Europe and by the end of th e ce ntur y. most major municipal wate r s upplies \vere filtered. These filters were th e "slow sa nd" type described in Sec. 4-8 . . The development o f wa te r tr ea tm ent in America lagged behind the European 1JI'Clctice. The first a tt empt a t filtration was made a t Richmond. Virginia. in 1932. Thi S project was a failur e. :ll1d seve ra l ye ars interve ned befo re anot her significan t
ENGINEERED SYSTEMS FOR WATER PURIFICATION 107
106 WAT ER
e O'o n \vas ma d e. [ 4-6J After th e C ivil War. o th e r a tt e mpts we re m a d e to fo ll ow the sa nd filtrati o n pra ctice o f Euro pe. few o f whi c h were successful. Apparentl y the nature o f the s lls pe nded so lid s in America n str ea m s was s ignifi ca ntl y difTerent fro m t hat o f th e so lid s in European streams. and t he slow sand process was no t as e ffec ti ve. The d eve lo pment of th e hydraulica ll y c leaned rapid sa nd filt er durin g the latte r par t o f th e nin eteenth centur y p rov id ed a m o re wo rkabl e p rocess. and by th e e nd o f the ce ntur y its use was widespread. Dur ing th e fir st two- thirds o f th e nineteenth ce ntury. filtr a ti o n was prac ti ced to improve th e aes th e ti c qu a li ty o f th e drinkin g wa te r. An unkn ow n be nefit was the re m ova l o f mi c roo rgani s ms. includin g p a th oge ns. w hi c h mad e th e wa ter m o re w ho l.eso me a s we ll. Th e acce ptance o f thi s fa ct in th e last quart e r o f the centur y spurred th e co nstru c ti o n o f the filt er plant s thr o ug ho ut E uro pe a nd Ameri ca . At th e tu rn o f th e century. filtrati o n was the prim a ry defe nse aga in st wa ter bo rn e d isease. Acce ptance o f th e ge rm th eo ry o f di sease trans mittal led to the di s infecti o n l) f p ubli c wat e r s uppl ies. First used o n a temp o rar y basis. di sinfec ti o n w ith bleac h powd ers and hyp o chl o rites was used in iso lated cases in th e e ig ht ee n-nineti es. T he fir st pe rm a ne nt in stall a ti o n for c hl o rin a tin g wa ter was m ad e in Belg ium in 1902. Th e p rod uc t io n o f liqu id chlo rin e began in 1909 a nd was fi rs t used fo r wat er d is infec tio n in Philadelphia in 19 13. [4-6J Ot her m ea ns o f di sinfec t io n. no ta bl y ozo nat io n , we re d eve lo ped s imu ltan eo us ly but did no t find wid es pread use. Th e dr a sti c reducti o n in d ea ths du e to wa terbo rn e di sea ses as a res ult o f di s infec ti o n led to the w ides pread c hl o rin a ti o n o f publi c wa te r s uppli es. Othe r wa ter-trea tment pro cesses d eve lo ped m o re s low ly and less dr a matica ll y. Coag ul a ti o n a s a n adjunct to se ttlin g was d eve lo ped a lo ng with th e' ra pid sand filt e r in Am e ri ca. So ftening o f ha rd wa te rs w as de m o nstrated in E u rope durin g .. ... ..... . .the. nineteenth centur y but did no t find w id es pread li se in public wa ter suppli es until we ll int o th e twe nti e th ce ntur y. The capa c it y o f c harco al to re move di sso lved o rga n ics was obse rveu by early e xpe riment e rs in fi ltra:ti o n but d id no t find ap pli ca ti o n in p ubli c wa ter s uppli es. Th e improve me nt o f thi s m a terial int o" ac ti \',lI ed ca rbo n " a nd it s use in wa ter-t rea tm ent p la nt s is a recen t occ tlfrence. as is th e use o f sy nth e ti c m e mbran es fo r hype rfiltr a ti o n to re m O\'e di sso lved in o rga ni c m a teria l. . M o re prog ress ha s bee n mad e in wa te r purific ati o n in th e la st ce ntury th a n III a ll o f th e p revio us ly re co rd ed hi sto ry. With few exce pti o ns. trea tmen t p rocesses deye loped 111 th e abse nce o f sc ientifi c kn ow led ge cO llce rnin g the bas ic p rinc ip les UpO Il w hi c h th ey o pe ra te. and o ft en w ith little mea ns to quantitat ive ly assess the ir effec ti ve ness . Onl y within the last 30 to 40 years has the bod y o r sc ientifi c know -· led ge ca ug ht up w ith the practic e o r wa ter puri fic ati o n. It is int eres tin g to no te tha t the d e \:e l o pn~e nt o f a th eo ry base has resu lt ed in few c han ges in the basic processes 'o f wa ter purifi cati o n. U nde rsta nd in g o f sc ientifi c p rinc iples has. howe\·er. led to refi ne m e nt s o f processes. d eve lo pm e nt o f be tte r eq u ip ment , and a ll oye ra ll in c rease in o perating e ffici e nc ies in wa ter treatment. Th e fo llow ing sec ti o n !l i\'es a ll ove r view o f m o d e rn wa te r-t re atm e nt p mcesses. w hil e th e remainin g ~ections o f th e c h a pt e r co ntain a d eta iled d escr ip t il) 1l o f th e ind ivid ua l processes.
4-2 WATER-TREATMENT PROCESSES Past practices in America have often been to obtain the purest possible source, even at the expense of transpor ting water over long distances, and to deliver it to the consumer with litt le or no trea tment. Some ci ties sti ll own large tracts of land near the headwaters of stream and restrict activi ties on t hese watersheds to minimize contamination. Although the benefits of source protection a re recog-' nized as a " first line of defense" in preserving water quality. a ll na t ural waters will require so me degree of treatment in order to meet modern drinking-water " standards. The nature and exten t of treatment w ill, of course, depend upon the na t ure and ex ten t of impurities . .
Process
Waste stream
Ch emica ts added Raw wat er
Aeralia n . Rem oves u nd esir abl e gases and /o r ox id a tion of iron a nd m anga ne se.
Lim e So/ref/ing: Removes ca lcillm a nd /o r m ag nesiu m
ha rd ness; ma y be d o ne in o ne o r two s tages.
Sod a ash
2
S ludge removed and disposed of ; possible recovery and reuse o f lime. ."
F il lrario n : Re moves re.sidual CaCO J c ry s tal s
an d Mg(OH) 2 Ooc le ft a ve r fro m so ft e ni ng; disi nfec tant may be add ed to preve nt . . biological grow th o n rilt er medium .
D isin/eCiia n . Des t ro ys pathoge ns; en o ugh
Chl o rine
3
Chlorin e
added to prov ide a resi dual in th e dis tri buti o n sys tem .
Sro rage: Provid es con tac t time for d is infec tio n and s tores wat e r for peak de ma nds.
to dis tribution system Figure 4- I Typ ica l plan t trea ting ha rd groundwa ter.
.. . .. . . . .. . . . .. .. . . . . .... .
Bac kwas h wa teT d ecanted ; sludge combined with sludge from 2 above .
ENG I NEERED SYSTEMS FOR WATER PU RI FICATION
109
108 WATER
Wast e streams
Chemica ls added
Process
Raw wa te r
Presidimentation: May be necessary if water comes from fast-flowing streams. Removes larger suspended solid s. Chem icals may be ad ded to oxidize organics o r to arrest their biologica l oxida t·ion.
Sludge removed periodically and disposed of by spreading on land :
Ammonia
Alum
Mixing. flocculation. se ttling: Removes tur bidity by coagu lating coll oids and se ttling them o ut ; may also remove colo r caused by large organ ic molecules.
Polymers
Filtration: Polishes to remove re maini ng tur bid ity; d isinfec tant may be added to p reven t biologi ca l gro w th o n filter medium .
2
Sludge removed conti nuou sly ; disposa l by land fil ling or o ther sui table means aft er ' dewatering .
Ba ckwash wa le r deca nt ed, and dewatered sludge dispose d of wi th tha t from 2 above.
C hlorin e
Th e processes se lected I'or th e trea t ment of potable wa ter depend on the qua lit y of the r:lw water suppl y. Most grou nd wa ters are clear a nd pat hogen-free a nd do no t co ntain significa nt am o unt s of o rga nic materi a ls. Such wa ters may often be used in potab le systems with a minima l dose of chlorine to prevent contamination in the distribution sys tem. Other grou ndwaters ma y con tain la rge qu a ntiti es of disso lved so lid s or gases. Wh en these include excess ive a mount s of iron , ma nga nese, or ha rdn ess. chem ica l a nd ph ys ica l trea tme nt processes may be requ ired . Treatment systems co mm o nl y used to prepa re po table water fr o m gro undwater are show n in Fi g. 4 ,1 . . Surface wa ters o rten cont ilin awide r var iety of co ntaminant s th a n grou nd water. and trea tment processes may be mo re complex. Most surface waters conta in turbidit y in excess of drinking-wa ter stand ard s. Although fast-moving streams may carry larger material in suspension , most of the so lids will be co ll oidal in size and will requIre chemi ca l coagulat ion fo r rem ova l. Dependin g on the geo logy of the watershed. hardness mayo r ma y not be a problem in surface waters. If low levels of color and o th er orga nic mat eri a l are present, adso rptio n ont o surfaceac tive m::Jte rial, a process not s ignifi cant in natu ra l water systems, ma y be necessa ry. A wide vari ety of microo rga ni sms. some of which ma y be pathoge nic, are a lso co mmon co nstitu en ts of surface wa lers. Treatm ent systems co mmonly useci in treatin g surface W;lIers al'e shown in Fi g. 4-2.
Water-Treatment Processes: Theory and Application Adsorption : May be necessary if wa ter contains dissolved organics; may co nsist o f ac tiva ted ca rbo n co lu mns o r ac t-ivated ca rbon may be added in powdered form in opera tion sim il ar to 2 ·above.
S tea m fro m cleaning cycle co nd e nsed and disposed of.
Disinfection: Destroy s pathogen s; enough added to provide residua l in the dis t ribution sys tem. Storage : Provides contac t time ro r d isin fec ti on and stores water for peak demand .
to
diqributi o n sys tem Figure 4-2 Typica l pl an l Ireatin g lurbid
~urfac~
waler wilh organ ics.
It is genera ll y co n\'en ient to grou p human use of water int o two broad ca tego ries depending upo n th e locatio!l of th e use re lative to the so urce. In -place use of water . in cl.ud~~ .\l.iWig'lt ion•. rCl:rCllt ion , .wi ldlife .propa.ga t io n . .and. the . d~lutio n , .ass imi.J.a-· .. · li on, and transportatIon ll f was tew ater. Alt ho ugh hydroe lectric powe r ge neratio n requi res brief dive rsion of waler thro ugh turbine pen stocks, this use is a lso co nsid ered an in-place use . Quantilati\·e ly. in-place use is a nonconsumpti ve use and wil l no t be cove red III this tex!. For irriga ti o n and Indu strial ·u se. and fo r indi vid ual and public domesti c supp li es, wa ter mu st be \\ ithdra wn frc)m streams, lakes. o r aqu ifers in Ih e natural hyd ro log ic cyc le. Th e po llutant s mos t de leterio us to cro ps (inorga nic sa lt s and metals) are diflic ult and ex pensive to remove. The vas t quantit y of irrigation wa ter . used a nd the low marg.in of proli l assoc ialed with fa rmin g virt ually prec lude a ny tl'ea tmenl of thi s waler. Wat er not su ited for irrigati o n is simpl y a band o ned . and avadab le ca pit a l is used in ste:ld to secure an alternate so urce of acce pt a ble qu a lit y. Man y industries wilh nccds 1'01' sm;t11 ::tmou nt s o f essentially potab le wa le r o btaill th eir s upplies fr o m puh lil: sys tem s. SL)me Ill d usiri a l water su pp li es. such as bCl iler, ked \V;lter. tlla y require a c hcm ica l purity an o rd er nf magnitud e g rea teT than po tab le wa ter En gi ll eer ing dcsi!!n fo r treatm ent of o ther types of in dustria l \\',ll e r supplies Illa y al s\ ) hc neceSS; 11 y. ( '\Jo ll ng wa ter. parti cu lar ly th at used on ly o nce ;Intl d isc har!,cd h;lc k III II
ENGINEERED SYSTEMS FOR WATER PURIFICATION
lJO
111
WATER
individual ho mes or farmstead s. Such systems are seldom engineered but are installed and operated by the home owners, perhaps with the advice of the welldriller and the distributor of ho me water-treatment unit s. Public water supplies, while only a fraction of th e to tal wa tel' us e, require by far the larges t amount of efTort expended by environmental engineers in the water-treatment field. The remainder of this chapter will be devot ed to the principles of water purification for potable' supplies. The processes involved are discussed first from a theoretical standpoint and then from an applications standpoint.
4-3 AERATION Aeration is a process sometimes used in preparing potable water. It may be used to remove undes irable gases disso lved in water (drgasijicalion) or to add oxygen to water to convert undesirable substances to a more manageable form (oxidCl1ion). Aeration is more often used to treat groundwater, as most surfa ce waters have been in contact with the atmosphere for a sufficient period of time for gas tran sfer to occur naturally. Groundwater may contain appreciable quantities of gases slich as carbon dioxide (C0 2 ) and hydrogen sulfide (H 2 S). Thcse gases arc biological waste products from bac terial decomposition of organic matter in the so il or by-pr6ducts of reduction of sulfur from mineral deposits. Excessive carbon dioxide concentration results in a corrosive water. High carbon dioxide levels may also interfere with other treatment processes. Hydrogen s ulfide imparts an unpleasant taste and odor to water, even in small concentrations. Altlwugh these gases are only slightly soluble .at .atrn()~p.be.r.ic.c.QndjtioJJs,.groundw'Lter.may cont·ain considerably· higher concentrations under pressures commonly fou nd in deep aquifers. Aeration of water supersaturated with these gases serves to speed the release toward equilibrium conditions. Although volatile liquids such as humic; acids and phenols ca n be removed . from water by aeration , the removal rates are too'slow for the process to be practical except in extreme cases where excessiw quantities must be reduced to more manageable levels. Iron and manganese are common elements widely distributed in nature. In the absence o f oxidizing agents, both of these clements are soluble in w:tter. Forming compounds with other so luble ions. hoth iron and man ganese are 2 so luble in significant quantities o nly in the + :2 oxidation sta te, i.e" Fe' , and Mn +. U pon contact with oxygen, or any other ox idizin g agents. both ferrou s iron and manganese are ox idized to higher valances, forillin g new io nic compkxes that are not soluble to any appreciable extent. Thu s. the jron ancl manganese may be . removed as a precipitate after aeratiun. Chemically. these reacti ons may be wr itten as fo llows. 4Fe 2
'
2Mn2 ~
+
O~
+ IOH 2 0
+ O2 + 2 H1 0
41·e(OH).,
1 I-
:2 1\'1nO 2 ~ ~
.:j
8 1-1 >
H
0
(4-1 ) (4-2)
In Eq. (4- I), Fe goes from the + 2 .to the + 3 oxidation state and in Eq. (4-2) + 2 to the + 4 OXidation state. In both equations the free oxygen (0 J IS reduced, and the anion ongmally :tssociated with the ferrous and manganOtiS Ions recombin es With other cations in the solution. In both cases, the pH of the solution IS lowered by the production of hydrogen ions. Iron and manganese are found in appreciable amounts only in groundwater and in water from the hypolimnion of strailfied lakes where anaerobic conditions ' exist. Aeration of this water provides the oxygen necessary to convert both elements to t he insoluble form. Chemical oxidants, such as potassium permanganate, can also be used for this purpose. They are sometimes used in connection with aeration to speed up the process. When aeration is lIsed to precipitate iron and manganese, additional treatment will be required to remove the precipitated solids. Both degasification and oxidation are governed by the principles of gas transfer that were presented in Sec. 3-4. Subtle differences in liquid-gas contact systems can have a pronounced effect on the overall gas-transfer process. An understanding of gas-transfer principles is essential in aerator design, and the student is encouraged to reread Sec. 3-4 before proceeding into the following discussion.
M n goes from the
Liquid-Gas Contact Systems Liquid-gas contact systems are designed to drive the water-gas mixture toward equilibrium as quickly as possible for degasification purposes and to provide' supersaturation of oxygen for oxidation purposes. These goals may be accomplished by either dispersing the water into the air or by dispersing the air into the water. When water is dispersed into the air, as depicted in Fig. 4-3, the interfacial "a re;Lper..yo.lume .of..wate.r. is maximized by minimizing the drop size. This will inc rea se the desorption rate for supersaturated solutions (Fig. 4-3a) or increase the absorption rate for undersaturated solutions (Fig. 4-3b). In general, this approach works better for desorbing gases than for absorbing oxygen, although the latter can be accomplished for undersaturated waters.
( ;as
fo"n
~
Bulk gos
Lo q uod !'tlm
ra)
Figure 4-3 Water dispersed in air: (a) desorplion and (h) absorplion.
rb)
112
WATER
In water purification plants, water-in-air systems may consist of fountains, cascade towers. o r tray towers. F otln tains consist of a piping grid suspended over a catch basin. Nozzles located at the int ersection of the pipes are fixed to direct the flow of water upward. Once its kinetic energy is dissipated, the wa ter falls bac k into the catch basin where it is recovered. portions of the fl ow perhaps being recycled . The height of the spray, and therefore the water-air co ntact time. is determined by the pressure in the pipes, whi le the d ispers ion pattern is determined by the nozzle characteristics. Nozz le size may vary from 2 to 4 cm in diameter. While smaller nozzles result in finer sprays. which yie ld greater surface-to-volu me .ratios, freq uent clogging of small nozz~e s ca n resu lt in hi gh maintenance cost. . Design parameters for spra y ae rators includ e system pressure, nozzle spacing. and flow rates per nozzle. Pressures of aro und 70kPa ( 10 Ibj in 2 ) are commo n and prod uce fl ow rates offrom 5 to 10 Ljs through each nozzle. Grid spac in g may vary from 0.6 to 3.5 m depend in g on the distance necessary to prevent extensive overlap of nozz le discharges. A typical design may consist of 2.54-cm nozzles on 1.25-m centers operating at 70-k Pa pressure. resulting in an -area requirement of approximately 10 m 2 /(50 Ljs) o f water treated (or abou t 100 ft 2 j(Mgal/d». Cascade lowers consist of a series of wa terfalls th at drop int o smal l pools. In this case the water is no t dispersed as droplets bu t is exposed to the atmosphere in thin sheets as it cascades down each step. Each step in a cascade tower is usually abo ut 0.3 m in height. and as many as 10 steps may be emp loyed. The number of steps det erm ines t he con tac t time between the wa ter and t he air. Head loss th rough th e system is simpl y th e height of the topmost step. The cascades may be arranged longi tudinall y like stair steps or may be arranged in a ci rcle, wi th the steps extend ing concen trica ll y outward from top to bot tom. Area requirem ent for cascade aera tors ranges from 4 to 9 m 2/(50 Lis) (40-90 ft 2/(Mgal/d», depending upon t he number of _ steps used. [4-50J . .. . . .. .. ... " . .. Tra J Tt1IVi-i r.n't(e 'si'ril'i laY 'iii 'ifit ll're' i 0 'c'a~;cai:le' t'o\\':e'rs"iii "t'I;,it i i1e"\~a ic'r is lift ed and allowed to fall to a lowe r eleva ti o n. Instead of being intercep teu in poo ls, tray towers int ercept the flow with solid surfaces over which the water mus t pass in its downward journey. The so lid surfaces may be a series of redwood slat tra ys which break the fl ow of the wa ter or a ser ies of porous-boltom tra ys containing stones, ceram ic sp heres. or oth er porous packing. In any case. tray material prov ides large sur face areas ove r which th e fl ow is spread in thin fi lm s. Po rosit y of the system must be sufficie nt to ensure circulati on of air around the surfaces. Tray towers are most oft en used for oxida tion of iron and manganese. Usua ll y the tray packing wi ll be large chunks of coke wh ich have been precoa ted with a st rong oxidant such as potassium permanganate to help initiate th e ox idation process. Films of iron and manganese solid s are deposit ed on the' surface o f the medium. and th ese film s se rve to ca ta lyze the prec ipit at io n. reacti on. Manganese ' prec ipit ates very slow ly be low a pH of about 9. and it may be necessary. t6 raise the pH to thi s le vel in orde r to speed the react ion. In addition to the above operations, there are lIlany proprietary devices on th e marke t wh ich make use of' one or more of the basic princ iples just discmsed. Informatio n on these dev ices may be obtain ed fr om curren t lit erature or from th e manufac turers o r distribu tors.
ENG I NEERED SYSTEMS FOR WATER PURIFICATION
Bulk liquid
C,
Liquid film
11 3
Bulk liquid
C,
>C,
Liquid fi lm
(a)
(b)
Figure 4-4 Air dispersed in waler' (a) deso rplion and (b) absorption.
Another meth od or ae rating water is to disperse th e air into the water. Again. bo th absorp ti on and desorption are enhanced by maximizing the interfacia l area , in this case by minimizing th e size of the air bubble. Figure 4-4a can be used to illu strate the situati o n for a supersa'turated water (desorption). and the process for an undersaturated wa ter (absorptio n) is shown in Fig. 4-4b. In genera l. thi s approach work s better for absorption th an for deso rp ti on. Air-in-water systems mo st o ften consist of tanks fr om 2.5 to 5.0 m deep thro ugh which th e water /l ows. Air is then injected th(Ough a porous bo tt om or through spa rgers near th e bottom. Since the energy-for this system is expended on the air, not the water, smaller. less complicated equipment is requ ired. Blowe r capacity need on ly be sufficient to deliver the requ ired air vo lu me at the pressure determined by head loss through the distribu tin g mechanism, plu s the depth of th e wa ter. Thi s type of aerat ion dev ice has fo und -grea ter 'use in was tewa ter treatm ent th an in potable water treatment. Severa l varia ti ons of this process may be employed. Ca rr yi ng ou t the process in an enclosed tank with a positive pressure in the atmosphe re above the liquid wi ll speed th e absorption ra te. altho·ugh.i t will also decrease the desorption rate. An impeller pla ced Ju st above the poi n't of air injec ti on will break the a ir flow in to slIla ller bubb les and enh ance mixing patterns. As in th e case of water-in- air sys tems, there are several proprietary dev ices which make unique app li ca tion of the basic princip les discllssed here. All aera ti o n operati ons must be we ll ventila ted to prevent the buildup of gases which ma y be tox icant s or asph ixa nt s.
4-4 SOLIDS SE PAR AnO N The terms sedilllelllolioll and ('fliriliclil ion are co mm only used interchangea;l ly with regard to prcp;II';ilI OIl llf j)'ltable \\,;ltCI' ..\lthough there ,II'C some suhtl e t1i1lcrcllces in the Cllll ll otatlllllS t'f the two word s. th ey hoth co nvey the id e~t of
114 WATER
WATER PURIFICATION PROCESSES IN NATURAL SYSTEMS
physically separating solid material from water. Separation may occur by flotation if the water is denser than the solid matter. In the preparation of potable water, virtually all of the solids requiring removal are heavier than water; therefore, sedimentation with gravity as the driving force is the most common separation technique. Sedimentation may be classified into various types depending upon the characteristics and concentrations of suspended materials. Particles whose size, shape; and specific gravity do not change with time are referred to as discrete particles. Particles whose surface properties are such that they aggregate, or coalesce, with other particles upon contact, thus changing size, shape, and perhaps specific gravity with each contact, are called flocculating particles. Suspensions in which the concentration of particles is not sufficient to cause significant displacement of water as they settle or in which particles will not be close enough for velocity field interference to occur are termed dilute suspensions. Suspensions in which the concentration of particles is too great to meet these conditions are termed concentrated suspensions. These differences result in significantly diflerent settling patterns and require separate analysis. Settling in dilute suspensions is discussed below. Since concentrated suspensions are most often encountered in wastewater treatment , that discussion is presented in Chap. 5.
where CD is the coefficient of drag, Ap is the cross-sectional area of the particle perpendicular to the direction of movement, and v is the velocity of the particle. Because the drag force acts in the opposite direction to the driving force and increases as the square of the velocity, acceleration occurs at a decreasing rate until a steady velocity is reached at a point where the drag force equals the driving force:
(4-3) For spherical particles,
Substituting into Eq. (4-3)
(4-4) Expressions for CD change with characteristics of different flow regimes. For laminar, rransitional. and turbulent'flow, the values of CD are:
24
.
(4-5)
CD = - (Iamll1ar) Re
Type-l Settling Discrete particles in dilute suspension, type-I settling, is the easiest situation to analyze. If a particle is suspended in water. it initially has two forces act ing upon it: (I) the force of gravity
= -24 Re =
.. 1) + - 3- 2 + 0.34 ( transItlOna ReI !
fnel =
(p p
-
P,Jg Vp
This net force becomes the driving force for acceleration. Once motion has been initiated, a third force is created due This force, called the drag force, is quantified by
to
vi scous friction.
............. ............................. .
¢VrPw d
in which Pp is the density of the particle, g is the gravitational constant, and Vp is the volume of the particle; and (2) the buoyant force quantified by Archimedes as
Re = -
(4-6)
(4-7)
0.4 (turbulent)
where Re is the Reynolds nUITI.~~r
where Pw is the density of the water. Since these forces are in opposite directions. there will be no net force when Pp = p".. and no ·acceleration of the particle in . Tchtion to the water will occur. If, however. the density of the particle differs from that of the water, a net force is exerted and the particle is accelerated ~n the direction of the force:
115
- J1
.. . . . . . . . . . . . . . . . . . (4-8)
Reynolds numb'ers less than 1.0 indicate laminar flow, whil« values greater than 10· indic;lte'turblIlcnt flow. Intermediate values indicate transitional flow. The shape factor ¢ is added to correct for lack of spherosity. For perfect spheres, the value of ¢ is 1.0. For laminar flow, substitution of Eq. (4-5) into Eq. (4-4) yields:
g(pp - p,Jd 2 18 J1
= - - - --
V r
(4-9)
which is known as the Stokes equation. Terminal settling velocities for the transitiunal flow involve simultaneous solutions ofEqs. (4-6) and (4-4). Use of the above equations in determining the terminal settling velocities of discrete particles in dilute suspensions is illustrated in 'the following example. Example 4-1: Finding the terminal settling velocit)' of a sphere in water find the terminal
sell ling \'e!Ocil y or a spherical particle with diameter 0.5 mm and specific gravity of 2.65 , cliling Ihrou gh WOll er al 20Ve
ENGINEERED SYSTEMS FOR WATER PURIFICATION
117
116 WATER Wat er lev el
SOLUTION
I. Assume laminar flow; from Eq. (4-8) with p ..
= 998.2 kg/m 3 and J1 = 1.002
x 10 -
3
N ·s/m 2 at 20°C Vt
18 x 1.002 x 10':3 N· s/m2
=
(Recall that the units of N are kg . m/s2)
v, = 0.22 mls 2. Check Reynolds number: 0 .22 m ls x 5 x 10 - 4 m x '998.2 kg/ m 3 Re = - - - - ' - - - - - - , - - - - . , - - - = 1.002 x 10 - 3 N . s/m2 = 112, which indicates tran si tio nal flow
24
CD =
. 3.
= V,2
4.
ill +
3 112 1/2
+ 0.34
-
0.84
4
= - x 9.81 x 3
(2650 - 998.2) 5 10 - 4 x 0.84 x 998.2
~
S"mpllng [lort
Figure 4-5 Setll in g column for analyzing lype- I sus pen sion .
v, = 0.11 ml s
Observing that the tim e of travel is equal for the two particles. it follows that
5. With v, = 0.11. repeat steps 2. 3, and 4.
Zo
Re = 55
10 = -
t'o
CD = 1.18
v,. = 0.1 0 mls '" 0.11 m/s (see step 4)
distance traveled = - time of tr avel
Zo 10
Suppose also that another particle is initially suspended at a distance Zp above the sampling port a nd that its te~minal se ttlin g velocity. less than Vo , is suc h th a t it arrives at the port at the same time as the previous par(.icle. (ts se ttlin g velocity can be calculated as di stance traveled Zp v = - - -- -- - = -p
time o f travel
and
L'p
Some ge nera li zed statements can be made based o n th e above equation .
. Direct application of Eqs. (4-4) through (4-9) is se ld om possible in water treatment because the size of particles must be known and a correction fac to r · to account for departure from sphericity has to be determined. An indirect method of measuring settling velocities of discrete particles in dilute suspensions. and of determining settling characteristics of a suspension, was devised by. Camp. [4-11 ] A settling column is constructed as shown in Fig. 4-5. The suspension to be tested is placed in the column and is mixed comp.letely to ensure uniform di stributi on of the particles. The s\lspension is then allowed to se ttle. quiescently . S.uppose that a particle is just at the surface at time t;:qual zero and its set tlin g velocity is such that it arrives at the sampling port at a later time, say I = 10 , Now. the averaging settling velocity of this particle can be calculated as Vo
Zp
=-
10
equa iio~~'g~~~t~;; i1~;1do,s~~h 'that their se ttling velocities equal or exceed l 'n . will a rri ve at or pass the samp ling port in time co· 2. A particle with diameter ell' < do will have a terminal set tlin g velocity vI' < Vo and will arrive at or pass the sampling port in time 1o . provided its o ri gi nal position was at , or be low, a point Zp . 3. If the suspensio n is .mixed uniformly (i.e., all particle sizes are randomly distributed from top to bottom of the co lumn), th en the fracti o n o f particles of size el p with settling velocity 1'1' which will arrive at o r pass the sampling port in time 10 will be Z p/Zo = l' / l'O' Thus. the removal efficiency o f any size particle from suspension is the rati o of t he sell ling ve locity o f tha t particle to the settling ve locity Vo defined by Zo/Io·
.J. ' All'particles With dianiet'els
These principles can be used to determine th e settleabi lit y o f any given s us~ pension . An apparatus simi lar to th ;\t shown in Fig. ~- 5 is fi lled with the'suspen sion to be tested. Theoreticall y. the depth of the water column IS not a factor ' in the analysis. but practical considerations dictate a depth of about:2 m. The s uspens io n IS mixed complete ly to ensure an initially uniform di stribution of particles. A suspended-so lid s tes t is rliF on a sam ple of th e comp lete ly mixed suspension. a nd an initial concentratIOn CO IS determined . After the sus pensi o n is a ll owed to se ttle for a tim e I, a second sample IS then drawn o O'a nd an o ther concentration C t is
ENGINEERED SYSTEMS FOR WAlft PURIFICATION 119
Il8 WATER
determined. All particles comprising C j must have settling velocities less than Z 01II l' Thus , the mass fraction of particles with V I < Zo/t I is
C
j
x = --
Co
The process is repeated several times with Xi a lways being the ma ss fraction o f particles with Vi < ZO/Ii' When these values are plott ed o n a graph, as shown III Fig. 4-6, the fraction of particles co rresp o ndin g to a ny settlll1 g ve loc it y can be o bt a ined. For a given detention time [0, an overall percent removal can be o btained . All particles with settling velocities grea ter than lio = Zol lo wi ll be 100 .percent removed. Thus, 1 - xo fraction of particles Will be removed co mpletely Il1 time 1 , The re maining particles will be remo ved accord ing to the rati o vJvo, corre0 sponding to the shaded area in Fig. 4-6. If the equation relatlllg V and X IS kn ow n, th e a rea ca n be found by integrat io n :
x
xo [' .
= J -
Xo
+
I
-~
Jx
(4- J0)
Example 4-2 : Settling column analysis of type-I suspension A settling analysis is run on a type- I suspension. The column is 1.8 m deep, and data are shown below. Time, min
o
Cone, mg/ L
300
420 27
What will be the th eo re tical rem ova l efficiency in a settling basin with a loading rate of 25 m 3 / m 2 ·d (25 m id)? .. . SOUJTION
l. Calculate mass fraction remaining and corresponding settling rates. Time , min
80
100
130
200
240
420
'0.63
0.60
0.56
0.52
0.37
0.26
0.09
3.0
2.5
2.0
.I .55
1.0
0.83
0.48
60
mass fraction remaini ng
u, x 10 2 , m/min
o Vo
whe re X is the total mass fracti o n rem oved by sedimentation. In most cases, it is simpler to int egrate by finit e intervals as demonstrated in th e following ex -
2. Pl o t mass fr act ion remaining vs. settling velocity. 1.0r-----------------~------,
amp le.
0.9
1.0
0.8
0. 7 '0"
OJ)
0.
c c.
VI
"E
0.6
t:
t::
S 0.5
U
t. x = 0.06
'"
<.-
~ ,;
c
0.4
E
E
t.x = O. I
N
I
0.3
'J
0
t.x=O.1
'-'"
6.x = 0.06 Seltling ve IUCIII"s." = I
I,
Fi gure 4-6 Co ll ection efficie ncy as a funeli ol1 of se ttli ng velocity .
~~.JL-
6.x = 0.06 1£....l--..l_..L...L...L-..L........:L-....L---.JL--------.JL-----.......; 4.0 3.0 S; 2.0 ~ ~ ~ 1.0 ~
o
00
.
Ve locity, m/ min X 10- 2
3. Determine Vo = 25 m J / m 2 • d = 1.74 X 10 4. Determine Xo = 54 percent. 5. Determine ~x . v, by graphical integration.
.~
'.
D.x
v,
llx . fl
0.06 0.06 0.1 0. 1 0.1 0.06 0.06
1.50 1.22 1.00 0.85 0.70 0.48 0.16
0.09 0.07 0.10 0.09 0.07 0.03 0.01
2
I
m/ min .
ENGINEERED SYSTEMS FOR WATER PU RIFI CATION 121
I~
II
E
c E 0
u
. ~
LD.X '", = 0.46
6. Determine overall rem ova l efficiency. X = I -
Xo
+
" tJ.x . v,
L -110
0.46
= 0.54
+ --
= 0.46
+ 0.26
Figure 4·7 Isorernova lli!leS rrorn sell ling anal ysis.
1.74
given isoremova l line is the instantaneous veloci ty o f the fracti o n of particles represented by that line . . It should be noted that the velocity becomes greater (I.e., the slope of the isoremova l lines beco mes steeper) at grea ter depth. Thi s common characterIstIc of flocculating sllspensioI1s reflects the increase in particle size a nd subsequent increase in settling veloc ity because of con tinued collision a nd aggregatIon WIth
=72%
.- .
Type-2 Settling
Type-2 settling involves flocculating particles. in dilute suspension. Flocculating suspensions cannot be generalized in the same manner as discrete particle suspensions. Th'e Stokes equation cannot b~ u~~q.!:?~"l!-u~e flocculating particles are . ...-.. ........... continually 'changing 'ifniie: sh'
x 100
.--.,
(4-11 )
where xij is the mas~ fraction in percent tha t is removed at the ith depth at the jth time interval. These values are graphed as shown in Fig. 4-7, and a family of isoremovallines is drawn similar to a contour map. The slope at any point o n any
other particles. For any predetermined de tenti on time. the overa ll percentage removed can be obta ined as illustrated in the example below. Example 4-3: Settling column analysis of flocculaling particles A co lumn .ana lysis of. a . . .., I ., 1 ' 1 be low . The inilial so lids concen · flocculallng SLl spen slo n IS run In tIC appdf3 tu s s lOWI . tration is 250 mg/L. The resulting maHix is sh o wn be low What Wi ll be the .overa ll Femoval efficiency of a settling ba sin whi c h is 3 In deep with a detention time of I h and
45 min?
If ~
~
I ~
Time of sampling. min
Dep lh. III
30
60
0.5 1.0
1:1 3· 180 203 2 13
220
83 125 150 168 I ()
225
1 8~
U 2. 0
2.5 3.0
90 50 9} II
135 145 155
120
150
180
38
30 55 70 90 10J 11 3
23 43
65 93 110 Ii } I ,}
5~
70
80 95
• Result s or sll spen ded so lid s les t on sample C,' mg L
ENG INEERED SYSTEMS FOR WATER PURIFICATION 123
122 WATER SOLUTION
I. Determine the rem ova l rate at each depth and time . X;j
= (I - C;)Co) x 100
Tim e of sampling. min Depth. m
30
60
90
120
150
180
0.5 1.0 1.5 2.0 2.5 3.0
47 28 19 15 12 10
67 50 40 33 28 25
80 63 53 46 42 38
85 74 63 56 51 47
88
91
78 72 64
83
59 55
68 62
::J
cC> OJ)
.S
Z,
IV·Z;
0.Q7 0. 1 0.1 0.1 0.1 0.1
2.6 1.8 1.2 0.8 0.45 0. 15
0. 18 0. 18 0. 12 0.08 0.04 0.0 1
LIVZ; = 0.6 1
5. Determine the removal efficiency, R = '0
77 72
2. Plot isoconcentration lines a:; s hown in th e accompanying figure. 3. Construct vertica l lin e at 10 = 105 min. 4 . From the figure. approximately 43 percent 01' the so lids will reach the 3-m depth 111 10 , The y wlil pe 100 percent removed. Some percentage of the remaining particles will be removed. Working upward along t he 10 line, tktermine increments of rem oval and depths to the midpo int of these increme nt s.
"E
IV
=
0.43
=
63%
tJ.,·z
+ r --'
zo
0.6\
+-
3.0
4-5 SETTLING OPERATIONS The sed imentation process has ma ny applications in the preparation o f potable wa ter. Materials that may be remo ved by sedi menta ti on include suspended solids or igina lly present in the water or dissolved so lids which have been precipitated in the course of other treatment processes. S uspensions in water-treatment plants are ass umed to be dilute, a lthough some zone settling may occur near the bottom of settling basins. Cr iteria for desi gn o f settling basins have evolved as much from practice as. fro m theo ry .. S.ettling .. l:.~~i~s ..employed. J()r . s.o li~~ . rernoyal,in . \V.~t~r.-.t.re~t.!penL ........... . plants are classified as either long-rectangular, circular. or so lid s-contact clarifiers. Although these are all continuous-flow systems, the settling theory for batch analysis discussed in the previous section can be applied_
LO'ng-Rectangular Basins E
Long-recta ngu la r basins are commonly used in treatment plants processing large fl ows. This type of basin is hyd rau lica ll y more stab le, and flo w contro l thro u g h large volumes is easier with this configuration. A typical long-rectangular tank is shown in Fig. 4-8. Typical designs consist o f basins whose length ranges from 2 to 4 times their width a nd fr o m JO to 20 times their depth. The bottom is slightly sloped to fac ilitat e slud ge scrap ing. A slow-moving mechanical sludge scraper, usua ll y redw ood slats o n a ch a in drive. continuously pulls the settled material int o a sludge hoppe r whe re it is pumped o ut periodically. A long-rectangular settling tank can be divided' into four different functional
.c
0.
c"
"
Vl
Tim e.
IllIIl
zo nes: I. The inlet zone in which baffles intercep t th e incoming water and spread the flow ulllfo rmly both horizontally a nd vert icall y ac ross the tank
124 WATER
J"NGINEERED SYSTEMS FOR WATER PURIFICATION
~~~~--~~~~ Settling zone
\ Sludge scrapeJ
-
125
'* I I
I I I I
I
I I I
I I
...•..
"
I I
I
o Sludge out
(u)
I,
Figure 4-9 Di sc rete pdrl icle removal in Ihc se tlling zone o f a I,lng-rectangular setlling basin.
particles fallm g Ihrou gh the settling co lumn wtll n(lW have two co mp o nents of velocity. the vertical co mponent.
and the ho ri zont
\;,het:e:4 ;stli ec ; oss~sect;;';li ,il ~i;,e;1.~;;
(/J)
Figure 4-8, Lo~g-rectangular settling basin: (a)diagrammatic sketch and (b) view or interi o r sh ow ing sludge scraper arrangemenl. (PholO courtesy oj Em.irex Inc., 0 Rexnord Companv,)
2, The outlet zo ne in which water flows upw3rd and over the ou tlet weir 3. The sludge zone. wh ich extends from t·he bottom of the tank to ju st 3bove the scraper mechanIsm 4. The se:tlin g zone. which occupies the rcmaining vo lum e of the. tank
ii~ e\\; i(Jiiitlm es·thedepih.the sl.liTi of these velocities is th e absolute ve loc it y of th e particles. No\\' cO ll sid er th e panicle In the b 1:0 wil l be removed from suspens ion at some poi nt along the settlin g zone. Now consider the particle with settling vc loclty < l'o. lft helnitialdepthnfthisparticlewassuchthatZ p !u, = I". thiS particle wtll ,liso be removed. as sho\\n in the b~ltch anal ysis . Therefore. the rem ova l of suspe nd ed particlcs pa sslllg thr u ugh the settling zone wil l be in proporti o n to the ratio of the Incli\idual settling \'eloet ties to the settl ing ve locir-y t·
u
. Although a'II four zones musi function properly for efficient sol id s removal. pmr:ary a ttentIon here will be focused on the settling zone. Assume that the settllflg colu mn Ifl F'Ig. 4 - 5 IS . sllspended 111 . the flow of the settling zone as shown in . Ftg. 4-9. The column travels with th e flow across the settli;lg lone. Discrete
Anot her poi nt ca n bc mad e by this analysis. The lim e retention time in the seltltn g zone. In
=
I'
Q
LZ n IV Q
10
corres ponds to the
rn
i' ENGINEERED SYSTEMS FOR WATER PURIFICATION
127
126 WATER
A lso (0
therefore
LZ oW
Q
and
.
!·o
Q
=I w
or
Q "0=As
(4-12)
w here As is th e surface a rea o f the set tling basin . Thus, the depth of the basin is not a factor In determining th e size par ticle th at can be removed co mpl etely in the settlIng zon e. The determining factor is the quantity Q/ As' which has th e units o f ve loc It y and IS refe rred to as th e ove rfl ow rate qo· This ove rflow ra te, expressed as cub IC meters per squ a re-meter hour (or ga llo n s per sq ua re-fo ot day), is the design fac tor fo r settlIng baSInS a nd corres p o nd s to the terminal sett lin g ve loc it y of the partIcle that IS 100 percen t removed. If a similar co mparis on of flocculating particles is made be tween batch se ttll!l g and continuous se ttlin g in lo ng-rec tan gul a r ta nk s, th e path of the falling partIcles wIll not be a straIght lIne . As determined in the batch ana lysis, average velOCItIes of flo cc ulatIng partIcles increase w ith d ep th. Since the paths of particles tend to curve d ow nw a rd as illustr a ted in Fig. 4- 7, d epth is a factor in fl occ ul a nt se ttlin g. Therefore, the batch analysis mu st be p erfor med in a col umn of the same depth as th e basi n which it is to model. . Settling basins designed for di sc rete particles are usuall y from 2.5 to 3 m d ee p, wh Ile those for flocculating p art ic les are usuall y 3 to 4 m deep. [4-44J From a practIcal standpoint. w idth s in excess of about 12 m crea te problems wi th s lu dge removal equipment ; thu s length s a re usually kept to less than 48 m. Multiple un~s III paralle l are used to o btain the vo lume and retention tim es need ed for large flows. In fact , It IS always good practice to have at least two unit s so one can co ntinue fun c tio ning while the ot her is down for repairs or routine maintenance. F o r dilute suspen sion s, overflow rates for di scre te particle set t liu g usuall y range from 1.0 to 2.5 m / h (0.4 to 1.0 ga l/ ft 2 . min), wh ile ove rfl ow ra tes for fl occ ul a ting suspensIons range from 0.6 to 1.0 mi ll (0.25 to 0.4 gal /ft 2 . min). Detention times range from 2 to 4 h for discrete particles :1I1c1 from 4 to 6 h for flu cc ulating suspen s ions . [4-44J Alth ough se lec ti on of the overflow rat e and the detention time determine the size o f the basin, o th e r parameters also have to be co ns·idered. These include th e horizontal ve loc ity Vh and th e weir over f~ow rate (/"'. The motion of the s lud ge scraper may momentarily res uspe nd li g ht e r particles a nd fl ocs a few ce nt ime ters above the sc raper blades. Since excess ive horizo nt,d ve loc ities wou ld m ove this material progress ive ly toward the ~lIt lct zo ne where It wou ld be los t in the overflow, horizontal flow veloc! ty s hould not exceed 9.0 I1l/ Il (0.5 ft/ mlll) for hght fl occulent sll spensions or about 36 mi ll (2 ft / min) for hea vier. discrete-partIcle suspen s io ns. [4-44J
Figure 4-10 In board weir arrangement to increase weir length.
Large weir overflow rates result in excessive velocities at the outlet These ve locit ies extend backward into the settling zone, causing particles and floes which would o therwise be remo ved as sludge to be drawn into the outlet Overflow 3 rates ranging from 6 m 3 j h per meter of weir for light flocs to about 14 m / h per meter of weir for heavier discrete-particle suspensions are commonly used . [4-44J It may be necessar y to provide special inboard weir designs such as the one in Fig. 4-10 to acco mm o date over'flow' ra·tes.······ . .. . . ' ................... , .... . . .
the '!owe'r we{r
The design of long-rectangular settling b as ins i"S illustrated in the following examp le.
Example'4-4~ Designing a long-rectangular settling basin for type.- 2 settling A city mu s t treat about 15,000 m) /d (4 Mgal /d) of water. Flocculating particl~s are prod uced 'by coagulation, and a colum n analys is indicates that an ove rflow rate of20 mid will produce satisfactory remova l at a depth of 3.5 m. Determine the size of the required settling tank . SOLUTION 3
I. Compute surface area (provide two tank s at 7500 m /d each).
Q = qoAs .7500 m 3 /d = A , x 20 mi d A, = 7500/20 = 375 m
2
2. Se lecting a length -t o-width ratio of 3/ 1. calcu late surface dimensions.
w x 3w = 375 m'
Width = 11.18. say 11 m Length = 33.54, say 34 m
I
128
ENGtNEERED SYSTEMS FOR WATER PURIFI CATION 129
WATER
It is essential that the weir plates be precisely level. since a ve ry sli gh t difference in elevation will result in considerable short circuiting (dIrect channe lltl g from influ ent to effluent). Uneven distribution and wltld currents can also cause short circuiting. These fa cto rs make flow con trol more difficult Itl cIrcular bas ll1s than in lon g-;ectangu lar ones . Because fl ow-con trol problems become m o~e difficult to co ntro l as tank size increases. it is usually adVIsable to lImIt cIrcu lar . tank diameters to 30 1Tl or lcss. Design of circu lar sett lin g basins is based on overflow rates and detentIon times. Th e limit s presented for long-rectangular tanks are app lIcable to cIrcul ar tank s. For obvious reaso ns. neith er horizontal ve lOCIty. nor weI r overflow rates are a consideration in th e design of circlllnr settlIng bas1l1s. " . The following exa mpl e illu s trat es the ciesign of cIrcular se ttltn g bas1l1s.
3. Check retention time. volume
(= - - - =
flow rate
II m x 34 m x 3.5 m
7500 mid x -
Id
24 h
= 4.19 h
4. Check horizontal velocity.
Q t',
,
7500 m
Id x
d
-24- h
= - = - - - - - - - := 8. 1 mi h A, II m x 3.5 m
5. Check weir overflow rate. If simple weir is placed across end of tank, overflow length will be II m and overtlow rate will be
Example 4-5: Designing 3 circular settling basin Using the data in Ex a mplc-4-4. determine the diameter required fo r circu lar se ttling baslIl s.
m' I dIm' 7500 -- x ~- x - - = 211 A - d 24 h II m h .m
SOLUT tON
Five times this length will be needed . Design weir as shown in Fig. 4-10. 6. Add inlet and outlet zones equal to depth of tank. and sludge zones as shown in the accompanying figure. 34.0
/. Inlet zone
3.5E
.... . .... . ....... . ..................
,4 , = 375 m'
2. The tk"lIn eter is ca lculat ed by
./
Outlet zone
=:J
Slope Iii 00
. .
Aga in pro vid in g two ta nks. the surface area is cal cu lated as befo re.
. ............. AlI.dime-nsions in meters
rrtl' , -I = 375 m ' 35 .
tI = 2 1.85. say ~2 m
Dep th of se ttling zo ne plus 0.5 fre eboard plus 0.5 for slud ge Zone
. 3. Inlet. outlet. and sl udge 70nes
Circular Basins
.-....
Circular settling basins have the same functional zones as the long-rectangular basin, but the flow regime is quite different. When the flow enters at the center and is baffled to flow radially toward the perimeter. the horizontal velocity of the water is continually decreasing as the distance from the center increases. Thu s. a discrete particle with a settling velocity Vo is continually undergoing a change in its absolute velocity due to the decrease in ho rizontal velocity. Thus. the particle path in a circular basin is a parabola as opposed to th e straight particle path line in the lo ngrectangular tank. Circular t.anks have certain advantages. Sludge' removal mechanisms are simpler and require less maintenance. [4-57]. Excessive weir overfl ow shou ld never be a problem because the en'tire circumference is used for overfl ow. In fact. to prevent extremely thin sheets of water from being drawn off. overflow weirs on circular tanks usually consist of V-notched met a l plates which reduce the effective overflow area. These strips are bolted onto the collection trough and can be adjusted to correct for differential se ttling of the basi n after construction.
1-''''''''"' '
e. pro\ .1d ed as,'110"-1' 1 in the accompanyin eu fl!!Ur ~
Scrtllllg 101lt.;
110
110
Oulicl zone
afC
Inkl
4.0
-
OUlkt'
71 lilt'
---
4 .0 ) - - - - -
4.0
zone·
L
f-J .,./ ~
I
-
--=-Solids-Contact Basins . I 1'I0'cr Cll nstl .' ICI'.dII either CIrcular or Illng." !th ough sll lid s-cLl ntaCl b:t~tns ) : fl"')ll1 I . ~. , . t Ile\ S·tll rectan gular h:lsins with respec t 1(\ ll oll' regll11es. 1 nnk , c use of th e rrlnctples
-,
130
WATER
ENGINEERED SYSTEMS FOR WATER PURIFICATION
of sedimentation previously discussed . The upward veloci ty of the flow in solidscontact basins works contrary to the settling velocity of the suspended particles. Referring to Fig. 4-11 the vector sum of velocities for any given particle is its terminal settling velocity minus the upward velocity of the flow at the level of the particle. At high upward velocities (near the bottom o f the cone), vir tually all particles and floes are swept upward with the flow. As the cross-sectional area of the cone increases, the upward velocity of the water decreases, and the vector sum of the velocities reaches zero; the particle stops and is suspended at that height in the cone. At this point, a particle is considered" removed" from the water. Obviously, larger particles with greater sett ling velocities become suspended nearer the bottom of the cone where upflow velocities are higher. Particles whose settling ve locity is exceeded by the upflow velocity at the top of the cone are swept on upw ard and into the overflow of the tank. Particles collect at positions dictated by their settling velocities until a so lids blanket is formed . creating a concentrated suspension. even if the original suspension was dilute. The creation of the solids blanket is an importan t part of upflow clarification. Very small particles. which would normally be swe pt out of the clarifier. mu st first pass through the concentrated zone. In flocculating suspensions, the chances are excellent that enmes hment in the sludge blanket will occur so th~t even very small particles or floes win be removed. Thu s. the blanket acts similarly to a filter for solids removal. The elevation of the sludge blanket in the cone is determined by the concentration of the solids in the blanket. higher concentration resulting in a greater rise. The concentration, and thus the elevation, of the solids blanket is controlled by drawing off excess sludge once the desired concentration and height have 'been reached ..
b·b,
At section the upflow velocity is equ a l to the se ttling velocity of the particle . Th e net velo.ci ty is zero, 3nd all particles with ihis settling velocity. remain suspended in the flow at Ihis point-.
Cylindrical section
b-l-~l-Upflow velocity
Settling ve loci ty of particle
-b
a-
At sect ion a·a, the upflow velocity exceeds the sett lin g velocity of the particle. The net velocity of the particle is in the directio'l of flow. and the particle is swept upward at the net velocity.
.L,,,,,,, velocity
Figure 4-11 Principles of upAow clarification.
Uptlow velocity
Settling velocity 01' p3.ticl e
As a factor of safety, the unit is usually sized so that the upflow velocity at a point 1.5 m below the top of the cone is one-half of the settling velocity of the particle that is to be removed . [4-1 J Generally speaking, upflow velocities of about 1 m/h at the liquid-so lids interface for flocculent sludges and about 2 mlh for nonflocculated slurries are adequate. Solids concentration of approximately 3 percent by weight for flocculated sludge blankets and approximately 5 percent by weight for nonflocculated slurries s hould be maintained in the solids blanket. Since horizon tal velocities are to be avoided in this process, overflow weirs should be arranged to ensure essentially vertical flow. Horizontal flow distances should not exceed the depth of the clarified zone by more than a factor of 3. [4-44J
4-6 COAGULA nON Virtually all surface water sources contain perceptible turbidity. Some particle sizes common to most surface waters are listed in Table 4-1, along with their terminal settling velocity (assuming quiescent conditions and specific gravity of 2.65). From these values it is obvious that plain sedimentation will not be very efficient for the smaller suspended particles. Under conditions normally encountered in setiling basins, efficient removal of particles less than 50 pm in diameter cannot be expected. Agglomeration of particles into groups, increasing the effective size and therefore the settling velocities, is possible in some instances. Particles in the colloidal size range, however. possess certain properties that prevent agglomeration. Surface waters with turbidity resulting from colloidal particles cannot be clarified without special treatment. A knowledge of the nature of colloidal suspensions is essential . XO .tbis removaL.. ......................................................... . ............................. ..
Colloidal Stability Colloidal suspensions that do not agglomerate naturally are called slable. The most important factor contributing to the stability of colloidal suspensions is the excessively large surface-to-volume ratio resulting from their very small size,
Table 4-1 Settling velocities of various size particles*
Coni ca l section
Particle diameter
Size typical
mm
01
10
Pebble Coarse sand Fine sand Silt Large collnid Small colloid
Ntt
vdocity
131
01 001
O.UOOI OOOOUO I
Settling velocity
0.73 In iS Inf' 1.0 x' 10 - ' m!s (0.6 m / min) 1.0 x 10 4 m /s (R.6 m /d) 10 x 10 - ' Ill !, (0.3 In/ yr)
n.n
1.0 x 10
\J
Ill is (3 m / million yr)
------- --------- -. . Srhcrcs
With
srecific gravi ty or 2.65 in water cit 20"('
132
ENG INEERED SYSTEMS FOR WATER PURIFICATION 133
WATER
Surface phenomen a predominate over mass phenomena. The most impo rtant surface phenomenon is the accumulation of electrica l charges at the particle surface. Molecul a r a rrangement within crys tals. loss of atoms due to abrasion of the surfaces, or o ther fac to rs may result in th e s urfaces being charged. In most surface waters. colloidal surfaces are negatively charged. Ions contained in the wa ter near the colloid will be affected by the charged surface. A negatively charged colloid with a possible configuration of ions around it is shown in Fig. 4-12.The first layer of cations attracted to the negatively charged surface is " bound " to the colloid and will travel with it. shou ld displacement of the colloid relative to the water occur. Other ions in the vicinity of the colloid arrange themselves. as shown. with greater concentrations of positive. o r co unter. io ns being closer to th e co lloidal surface. This arrangement produces a net cha rge that is strongest at the bo und layer and decreases exponentially with distan ce fr o m the colloid .
+
'- + " \ +
\
Wh en two coll oids come in close proximity there a re two forces acting o n them. The electrosta ti c potential created by the " ha lo" of cou nter ions surrounding eac h colloid react s to repe l th e pa rti cles. thu s preventing contact. The second force. an attracti on forc e ca lled th e mn der Wau/s[orce, supports co nt act. This force is in verse ly proportional to the six th power of th e distance between the p a rticles and also decays expone nti a ll y with distance. It decreases more rapidly than th e electrostatic potential. but is a str o nger force at close distances. The sum of th e two fo rces as th ey relate to o ne coll oid in close proximity to ano ther is illustra ted in Fi g. 4- 13. As noted in the figure. th e net force is repulsive at grea ter distances and becomes attractive on ly after passing thr o ugh a maximum net repulsive force. ca ll ed th e eYler9.1' harrier. at some distance between co llo·id s. Once the force becomes attrac ti ve. con tact betwcen t il e particles tak es pl ace. /\. means of overco min g th e ene rgy barrier mu st be ava ilable before agglomeration of particles can occur. Brown ian movem ent. th e ra nd o m movement of sma ller co ll oids becau se of molec ular bombardment. may produce enough momentum for particles to nvercome the ene rgy barrier and thu s co llid e. Mechanical agitation of the water ma y impart eno ugh moment lim to la rge r particles to move them across the energy barrier. Th ese processes are too slow . howeve r. to be efficient
+ \ + \
\
+ \ +
/:
I
+
I
B:;/ +
+
+
_/
+
+ ......... ....... . . ......... . .
I ~ ~ "'1)
' 0"
Nco.
/
/
Distance o ~----~~------~=---~~------------ be t ween
co ll o ids
Net rorce (Lcta - va" dcc Waal s)
- - - - - - - - -~::a---r-
Diffused layer Di stance from particle Figure 4-12 Charge sys tem in a co ll oida l suspen sion.
L.
J Figure 4-1 3 Force fields b,,,ween (plio Ids of like charge.
134
WATER
ENGINEERED SYSTEMS FOR WATER PURIFICATION
135
in water purification. and neither results in collisions of medium-sized colloids. Thus. other means of agglomeration must be used. In water purification this is generally accomplished by chemically coagulating the colloids into clusters. or flocs. wh ich are large enough to be rem oved by gra vit y sett ling.
t
Coagulation Theory
~
:::
In water-treatment plants. chemical coagulation is usually accomplished by the addition of tri va lent metallic salts such as AL 2 (S04).1 (aluminum s ulfate) or FeCI ] (ferric chloride). Although the exact method by which coagulation is accomplished cannot be determined, four mechanisms are thought to occur. These include ionic layer compression, adsorption and charge nel tralization , entrapment in a flocculent mass, and adsorption and interparticle bridging. Ionic layer compression The quantity of ions in th e water surrounding a colloid has an effect on the decay function of the electrostatic potential. As illustrated in Fig. 4-14 a high ionic concentration compresses the layers composed predominantly of counter ion s toward the surface of the colloid. If this layer is sufficiently compressed, then the van del' Waals force will be pred o minant across the entire area of influ ence, so that the net force will be attractive a nd no energy barriers will exist. An example of ionic layer compression occurs in nature when a turbid stream flow s into the ocean. There the ion content of the water increases drastically and coagulation and settling occur. Eventually, deposits (deltas) are formed from material which was originally so small that It could not have se ttled without coa~ulation. Although coagulants such as aluminum and ferric salts used in water . treatmenl 'iDn ize ; 'at ·t he 'conce'ntrarion 'co ttjmtiiilY''tfsed''fhey \vQuld not increase the ionic concentration sufficiently to affect ' ion layer compression.
.2 '-
o
" E
.D
:;
Z
Diffused layer Distance from particle (a)
t
~
:::
Adsorption and charge neutralization The nature , rather than the quantity, of the
ions is of prime importance in the theory of adsorption and charge neutralization. Although aluminum sulfate (alum) is used, as in the example below, ferric chloride behaves similarly. The ionization of aluminum sulfate in water produces sulfate anion s (SO/ - ) and aluminum cations (A1 3 +). The sulfate ions may remain in this form or combine with o th er ca tion s. However. the A1 3 + cations react immediately with water to form a var ie ty of 3quometallic ion s and hydrogen. AI.l +
A1 7A I
J
3
'
+
+
H 20
----'--+
+ 2H 2 0 +
17 H 2 0
AIOH 2 +
+
H '
~
o
a.
'"
" o"~N
~~~~~~~~~~
U. ~
~
particles
~
(4-13a)
Al(OH 2 )! + :!W
(4-13&)
AJ 7 (OH)! " ~ ' -/- 171-1 '
(4- 13c) (b)
AI"
-/- 3 H 2 0
---~
Al(OH), -/- 3 11 •
__________~~ Distanc~ between
(4-IJn)
Figure 4-14 Ionic compression: (a) reduction of thickness in diffused layer; (b) reduction of net force .
136
ENG INEERED SYSTEMS FOR WATER PUR IFI CATION
WATER
The aquometallic ions thu s fo rm ed become part of the ionic cloud surrounding the colloid and, because they have a great affinity for surfaces, are ad sorbed o nto the surface of the colloid whe re they neutra lize the surface c harge. Once .the surface charge has been neutralized , the ionic cloud di ssipates and the electrostatic potential disappears so that contact occurs fre e ly. Overdosing with coagulants can result in rest a bilizing the suspens ion. If enough a quomet a llic ions are form ed and adsorbed: the charges on the ' particles beco me reversed and the ionic clo ud s reform , with nega tive ions bein g the cbunter io ns. This phen omenon will be discussed more fully in a later section.
137
Inili a l odsorpll o n 01 Ihe optimum polym e r dosage
~
+
0
~~
F loc forination Flo ccul a tion
•
(pe rik inetic or orthokin e ti c)
Destabili zed pa rti cles
Secondary adsor pll on of po ly mer
~
~ Destabi li zed pa rt iele
Pori icl e
Polymer
Sweep coagulation According to Eq. (4-13n), th e la st produ ct fo rmed in the hydrolysis of alum is aluminum hydroxide , AI(OHh. The AI(OHh form s in amorphous, gelatinous flocs th a t a re heavier than water and se ttle by gravity. Colloids may become entrapped in a floc as it is formed , or th ey may beco me enmeshed b y its "stick y" surface as th e floes sett le. The process by which colloid s are swept from su spension in thi s manner is known as sweep coagulation.
•
~"iO" Restab ili zed padicle
~
No contact wit h vacant site s on ano lher particle
@
Destabiliz ed particl e
Interparticle bridging Large m o lec ul es may be formed when aluminum or fe rri c sa lts dissociate in water. Equati o n (4-13c) is an exa mple , although larger o nes are probably formed also. Synthetic polymers a lso may be used in stead of, or in addition to, metallic salts. These polymers ma y be linear or branched a nd are highly surface re a ctive. Thu s, seve ral colloids m ay become a tt ac hed to o ne polymer and seve ral of the polymer-co lloid groups may becom e en meshed (Fi g. 4-15), resulting in a se ttleable m ass. In addition to the adsorption forces, charges o n the pol ymer ma y assist in ' the coagulation process. Metallic polymers form ed by the addition o f aluminum .. .- ..... ............or.ferr.icsalts are positively cha rged , while synthetic polymers ma y carry positive or negative charges or may be neutral. Judicious choice of appropriate charges may do much to enhance the effectiveness of coagulation.
5 it 7
\
\
"
Excess polymers
~ Floc part ide
Ini tial adsor pti on excess poly mer dosage
+
- ~
0
Stabl e particle (no vacant sit es)
Part ide Ruplurc of !loc I
•
Intense or pro longed agi tatio n
-96~
. . ..
Floc fragments
Secondary adsorp tio n oj pol y mer
.~
Jar Tests for Optimum Coagulant Dosage Coagulation is no t yet an exact sc ience. a lthou g h rece nt advances have been m ade in understanding the mechanics o f the process . Therefore. selection and optinlum dosages of coagulant s are determin ed experimentally by the jar tes t instead o f quantitatively b y fo rmula. The j ar test must be performed on eac h wa ter th a t is to be coagulated and must be repea ted with each significa nt cha n ge in the q ua lit y of a given. water. The jar test is p erfo r.med using a se ries of g lass co ntainers that ho ld at leas t I L and are of uniFo rm size and s ha pe. Normall y. s ix jars are used w ith a stirrin g device (Fig. 4-16) th a t simultaneollsly mixes th e co nt ents o f each jar with a unifo rm power input. Each of the s ix. jars is filled to the I-L mark wit h water whose turbidity, pH, and a lk a linit y ha ve been predetermined. One jar is used as a co ntro l, while the rem a inin g fi ve are do sed wi th different amou nt s of coagulan t(s) a t different pH va lu es until the minimum valu es o f residual turbidit y are obta ined.
Ftoc fra gmen t
-'
cY
Restabilized !loc fragment
'Figure 4-15 Irll crparticle bridging with polymers. (Alier O' M elia [4'-41].)
I ~
Aft er chemical addi t io n. th e \vater IS mixed rap idl y for about j min to ensure co mplete di spersion of the ·c hemicals. then mix ed slow ly for. 15 to 20 min to aid in tlie formati on of flocs. The water is nex. t a ll owed to se ttl e for app ro ximately 30 min , or' un til clar ifica ti on 'h a s occ urred P o rtions o f th e se ttl ed wa ter are then removed a nd tes ted to d eter min e th e re maini ng turbidity. Test res ults are used to calc ul a te th e type and quantity of coagu la n t to be u sed in th e wate r-trea tm en t plan t.
138
ENGINEERED SYSTEMS FOR WATER PURIFICATION
WATER
Figure 4-16 J ar test apparatus.
Jar tests also serve to illu stra te the me'c banics of coagulation. Ge nera lized curves for residual turbidit y as a fury:tionof coagu lant dosages and initia l turbidit y appear Il1 FIg. 4-17 . Co ll o Id concen tra tIOn? S are measured in terms of surfac e area per liter a nd increase from SI to S4' A-t low coll oidal concen t ra ti ons (Fig. 4-170), insufficient numbers of colloids are present to form settleable masses (zo ne I), even if the surface charges la re neutralized. In such a case. co agu lat io n is no t initiated until enough coagulant has been added to precipitate as a metallic hydroxide . At low collo idal concentrations. the preqominant mechanism is-sweep co agu lation. I ' '!
At higher co lloida l concen trations (Fig. 4-17b), destabilization by adsorption and charge neutra lizati o n OCC llrs early (zone 2), but continued addition of the coagulant results in c harge reversal and restabilization (zone 3). Still greater colloidal concentrations (Fig. 4-17c) resu lt in more chances of collisions, and thus better coagulation over a wider range of concen tra ti on. Continued addition of coagulants result s first in restabilization and eventually in hydro xide f1bc formation and sweep coagu la ti on (zo ne 4). . Extremely large co lloidal concentrati o ns (Fig. 4-I7d) theo retically provide eno ugh co lloids to result in coagulation by adsorption and charge neutra lizati o n (zone 2), a lthough it is probab le that polymer bridging and sweep coagu lation also occur. Restab ilizat ion of highly turbid waters is seldom a problem. Information from many curves similar to th ose in Fi g. 4-17 can be summarized as show n in Fig. 4-18. Two very useful observations can be made from this figure. First. coagulation by adsorp ti on an d charge neutralizati on (zone 2) is impractical unless coagu lant dosage can be ve ry carefully con trolled . As illustrated by the coagula ti on region for t he colloid concentra tions S 2, a very sli gh t overdose results in restabilization . Second , highly turbid wa ters may require a lesser a mount o f coagulan t for good coagu la ti o n th a n waters wi th slig ht turbidit y. For this reason it is sometimes adva nt ageo us to odd turbidit y to re lati vely clear water. For example, if water with an initial turbidity correspond ing to SI were made more turbid ,
Coll o id co nc. = S.
Zone I c:
(d )
!!
~
Co ll oid conc.
Optimum
'8 coagu lan I ~
~
Zone I
~
(losage to produce "sweep noc"
(3
; ; ~---. . ;. .:;.:, ~--=--------~,...,
a
Zone 1
"
e>:
Zo ne 3 I
( b)
I
Co lloid co ne . =St
Zone I (a)
Dosage of coagulant Figure 4-17 Results of ja r tests at increasing turbidities . (Af ter O'Jleliu [4 . 4 1).)
139
Benl oni te I addi ti on I
0plimum coagula nl dusage I in s[oichiomerric I destabiliza tion : I I
s;
S4
Co ll oid concen t ration , e x pressed as co ncen lration of surface (S) Figure 4-18 Coagulant dosage as a funcliu n of lurbidil Y. (A/ ter 0' Melia [4·4 I l)
]40
ENG INEERED SYSTHIS FOR WATER PURlFICATION 141
WATER
say ro S3 or beyond , savings In coagulants could be realized. Bentonite clay is generally used for this purpose.
Alkalinity-Coagulation Relationships As noted in Eq. (4-13), the coagulation of metallic salts releases hydroge n ion s as well as coagulant species. These hydroge n ions neutralize alkalinity. H ydrogen resulting from the addition of I mg/ L of alum will neutrali ze 0.5 mg/ L of a lkalinit y. If the'initial alkalinity of a water is low. further reduction will destro y it s buffering capacity and the pH will drop rapidly. Since optimum pH values must be maintained for best coagulation and since alkalinity must be prese nt for hyd roxide floc formation, low alkalinity wate rs must be artificially buffered. This is usuall y accomplished by the addition of lime [Ca(OH)2J or soda ash (Na 2 CO).
Coagulation Practice Aluminum sulfate. the most commonly used coagulant in water purifica tion , is most eflective between pH ranges of 5.0 and 7.5. Ferric chloride. effective down to pH 4.5. and ferrous sulfate. effective on ly' above pH 9.5. are sometimes used. [4- 54J Although less expensive than alum . these coagulants can cause color problems if the precipitate is not removed completely. It is sometimes advantageous to use synthetic polymers in addition to alum. These pol ymers bind small fl ocs togethcr to make larger masses for faster settling. Alum dosage may range from 5 mg/ L to 50 mg/ L. depending upon the turbidity and nature of the water. At low turbidity and high dosage, AI(OH) , is almost certain to form so ·that· the·predominant · tubidity~removal' meclTanis~y is 'sweep-" coagulation . At high turbidity and lower dosage·s. adsorption and charge neu tra lization will be the predominant mechanis m. although interparticle bonding probably plays a significant role. Ionic layer compression woilld not be significant at these concentrations. With regard to coagulation. surface waters can be gro uped into the four general categories described below. [4-41
J
Group 1: High turbidity-low alkalinity. With relatively sma ll dosages of coag ulant. water of this type should be easily coagulated by adsorption and charge neutrali zation . Depression of pH m akes this method more elfecti ve. since the aquometallic ions are more effeclive at lower pH values. Howeve r. care should be used to prevent excessively low pH. Group 2: ,High turbidity-high alkalinity. The pH will be relatively unaffected by coagulant addition. Because of the higli alkalinity, adsorption and charge neutralization will be less effective than in waters of· low alkalinity. Higher coagulant dosage should be used to e ns ure sweep coagulation. Group 3: LOll" fIIrbiliily-hiyh allwlinity. The small number of colloid s mak e coag ulation difficult. even if the particle charge has been neutralized. The principal coagulation mechanism is sweep coag ulati o n with moderate coagu lant
do sage. Addition of so me turbidit y may decrease the amount of coag ul a nt needed. Group 4: Lo\\' liIrniditr-low alkalinilY. Again. the small number of colloids make coag ul a tion difTicult.
Rapid mixing Thorough mixing is essentia l if uniform coagulation is to occur. Conseq uently. careful attention must be paid to the design of rapid-mix units. Design param cter s for rapid-mix unit s are mixing time I and ve locit y gradient G. The velocity gradient is a measure of th e relative veloc it y of tw o particles of fluid ...·.. ,uJd .. the.distance .. bctwee n.... As .an examp le, two water particles m oving I m /s rela tivc to cach othcr ai a dista nce 0.1 111 apart would have a ve loci ty grad ient of: 1.0 m/s 0. 1 m
I ()
S
I
A more useful concept of velocity gradi e nt s. however. is given in term s o f power di ssipation per vo lum e. [4- 12J
Ci
(4-14)
\\'jlerc (i. = vclocity g.radient. s . I P '= polYcr iilput. W (N . m /s) I' = \o luill c ,)f"lllixing basin. Ill .' 2 /1 = \'isc0s it y, N s/ m Ibp iLi mi'\ln g ca n he aC Cn side u f pumps. ups tr ea m rl"Olll hydr a ulic Jumps. or in
I
ENGINEERED SYSTEMS FOR WATER PURIFICATION 143
142 WATER C lar lfi~ r
Fl oct'ulalor
.. '
.-
• D'
,
· 2J . , \.!.ts
Flatblade impeller
~
.'.
:~
D •
:..:~
Impeller
.
'., D'
_Chemical feed (a)
(b)
Figure 4-20 Typical rapid-mixing tanks: (a) back-mix impeller and (b) flat-blade impeller.
Ge:.H
Chain
motor
(a)
..... Haodrail Turntable
Tor of tank
Sludge discharge pipe (0)
Figure 4-19 Rapid mixing and flocculatio n followed by a sq uare settling ba sin: (a) plan and (b) sectional elevati o n (cuurtesy
0/ Dorr-Olin:T , In c.).
flow-throu g h basins where head loss around baffles provides power input. Most modcrn designs. howevcL use either mixing tanks with back-mix impellers or in -lin e flash mixers .. In-line fla sh mixers· Illay ha ve mechanically dri ven impellers or may rel y o n head loss created by static constrictions in the pipe. Rapid -mixing tanks operate best at C va lu es from 700 to 1000, wi th detention times of ap prox imately 2 min . [4-10,4- 36J Numerolls config urati ons of ta nks and impel lers are used, with the most popular units being square tank s with back-mix
b
I
impellers (Fig. 4-20a). A more effective unit might be a square tank with baffles and flat blade impellers as shown in Fig. 4-20b. [4-4J In-line blenders are designed for complete mixing in less than 1 s. Values of C for in-line blenders, calculated from flow rate and power input or head loss, range from 3000 to 50000 s - I [4-32J Several configurations of in-line blenders are available ; two models are shown in Fig. 4-21.
Flocculation The flocculation process relies on turbulence to promote collisions. Velocity gradients are also a convenient way of measuring this turbulence. Time is an important factor, and the design parameter for flocculation is Gt, a dimensionless number. Values of Ct from 104 to 10 5 are commonly used, with t ranging from 10 to 30 min. [4-28J Large G values with'short times tend to produce small, den'se floes, while low C 'values and long times produce larger, lighter floes. Since large, dense floes are more easily removed in the settling basin, it may be advantageous to vary the G values over the length of the flocculation basin. The small, gense floes produced at high G values subsequently combine into larger floes at" the lower G values. Reduction in G values by a factor of 2 from the influent end to the effluent end of the ftocculator has been shown to be effective. [4-33J Traditional flocculator design is illustrated in Fig. 4-22. These units consist of long-rectangular basis equipped with mechanically operated paddles to provide power input. The paddles are usually constructed of redwood or aluminum slats and may operate either transverse or parallel to the longitudinal axis of the basin. . More recent design tends toward units which combine rapid mixing, flocculatIon, and settling in one tank. Such a unit is shown in Fig. 4-23. The principles of mixing and floccu lation in this unit are the same as those for the long-rectangular
144 WATER
(b)
(a)
(II)
(b)
(c)
Figure 4-21 In-line blenders: (a) powe r-driven (courtesy of Walker Process Corp.); (b) sialic mixer pipe section (courtesy of Kom ax Systems, In c.); (c) static mi xe r in·2.5 x 2.5-m-squarc channel secl io n This mixer processes 530.000 m 3 /d a t Ih e Val Visla waler-Irealment pJa~1 in Ph oenix, Arizona. NOle chemical reed lines in front o r vanes (courtesy of Komax Systems, Inc.)
basin. although the method of operation is some~hat different. While these unit s are limited in the quantity of flow th at they can handle. l11ulti ple units ca n he provided in parallel to meet any demand . . . 'The G value for mechanicall y dri ven flocculators is calculated as follows. First. the power input is determined by P
where P = power input. W (N . l11 /s) D = drag. force on paddles. N t:p = \'e locity of paddles. mis
(c)
(4- 15)
Figun' 4-22 Flnccuialnr unil'. (a) paJdk arr:tngemcnl III long "Ink (pho to cOl/rtesy of Emirex Illc., a Iinnor" Cllmp/my): (h) mulllStage 11111" (pito to m:"lt £'.'.1' or WlIlkl'r Profess Corp.) : (c) lurbine-type unIt:-, (pho/(J ('UIIr!('.\'Y oj /:,,,,-irex. /11(' . (J RI· .\ non A q'0 111/70'11 ').
./.
'
'1·
1~ 5
I
I
i
ENGINEERED SYSTEMS FOR WATER PURIFICATION
147
The drag force on the paddle is given by C ., :J
c: .E
(4-16)
-0 0
0.
t·
1 ~
where CD = dimensionless coefficient of drag, 1.8 for flat blades Ap = area of paddle blades, m 2 p = density of water.kg/ m 3
"
Ir
.5 ~
"0 "0
I
c
~
.0
r~
i
~
£:
.....
0
00
0'"
"
I
""'"
r
~
"U
c
C)
'"coc '"c 0 x
"
~. .... 'c-
N
C
0
c'~ '" u
"0 '" C .,
sO:: .,
U)
~
~
.,
~
.~ "0
...
~
-0 ., '"c "U
"
0
0.
N
E
C
.2
... ....... '''>
e'"
~
o(j
v
0.
Of>
.~ 0 0
"E x
0::
~
,.'"
u"c 'c ' .
"
"0
c
C
§
'"c
;;:
ct
E
OJ)
~
~ u'"
0'"
u ;:'" 0
~
"
:J
E w
146
•~
1.
,l f
~
~
iI ~
I ,
r
I
c: ~
,j.
a<: "
~
~
(Z
c
3 A ) 1{ 2 _ D_ p_.-!!.
(
pu
2V,u
The area of the paddle A p refers to the combined area of the slats that are perpendicular to the cylinder of rotation. This area should not exceed 40 percent of the total area encompassed by the paddle. The veloci ty of the paddle tip up is the veloc it y relative to the water and is about 75 percent of the actual paddle speed. Paddle velocity should be less than I mis, and a minimum distance of 0.3 m shou ld be maintained between paddle tips and all o ther structures in the f1occulator . Tei 'pteveiifTocal":rr"t:as' of'ex'ce-ssive'vetoc1ty" gradients: ..... . .... . It should be noted th at in transferri ng water from the flocculator basin to the settling basin, extreme care must be exercised to avoid turbulence that can break up the floc. T his is usua :ly not a problem in units in which mixing, fl occu lation, and settling are comb ined. In the long- rectangula r units, the settling basin is often constructed adjacent to the floccu)ator, with the common wall omitted. Necessary baffles are designed for low G va lues. Design of fl occu lation units is illustrated in the following example.
,,~,
.g i"
N
.
t,
."2 C VJ
c
! t
0
G=
'.
~ U u
-5
<= .;::
~
f~
a::
Subst ituting into Eq. (4-14)
[
.2
'1<
"0
1
..!: !;
"§
Equation (4-15) now becomes
t
:::'"0
.,
~
Example 4-6: Designing a flocculator A water-treatment plant is being designed to process 50,000 m' /d of water. Jar testing and pilot-plant ana lysis indicate tha t an alum dosage of 40 mg/ L with flocculation at a GI va lu e of 4.0 x 10' produces op tim al results at t he expected waler temperatures of 15°C. Determ ine:
I. The mon th ly alum requ irement. 2. The fl occulati o n ba sin dimens ions if three cross· flow horizontal paddles are to be used. The flocculator should be a maximum of 12 m wide and 5 m deep in order to co nn ect
appropriat e ly with the sett ling basin . 3. The power requirement. 4. The paddle conligur:ltioll.
148
ENGINEERED SYSTEMS FOR WAIER PURIFICATIO N
WATER
E
SOLUTION
": 0
I. Monthly alum requirements:
,I
40 mg/L = 0.04 kg/m 3 and 3
0.04 kg ' m _ _ x 50000 - x 30 d/mo = 60,000 kg/mo . ' m3 d 2. Basin dimensions:
E
a. Assume an average. G value of 30 s -
'"
0
1
GI = 4.0 x 10'
4.0 1= 1
X
QI
~
= 22.22 min
b. Volume of the tank is V =
'0
\0 4 I min
30
:IC1
0
1""'" ...
----,
r--I
r---
I
I I
I
I
I IL ___
I
___ J
I
I
I L ___
r--- - - - , r - - I I
I
I I I L ___
I I I I
l
___ ...J
IL ___
I I IL ___
I I I I
---,
r--I I I
I I I I ___ .JI
I I I
I
I
___ J I
IL ___
---,
r--- ---..,
I
I I I I I I
I I
r--- ---"I r--I
---.,
I I I ___ ...JI
___ JI IL ___
,---
---,I
---.,
I I I
I I
I I I I I, I I I ___ J .IL ___ ___ .J L ___
E
___ JI
PI
""
= 50,000 m 3/d x 22.22 rrT)'!fl x I d/ 1440 min
0
= 771.5 m 3
3. Power requirements:
c. The tank will contain three cross-flow paddles. so its length will be divided into
a. Assume G value tapered as follows.
three compartments. For equal distribution of velocity gradients, the end area of each compartment should be square, i.e., depth equals t length. Assuming maximum depth of 5 m, length is
First compartment. G = 40 s Second compartment, G = 30 s Third compartment, G = 20 s -
3x5=15m
1 1 I
I
b. Calculate power requirements for compartments 1,2, and 3:
and width is
P = G' V/I
5xI5xw=771.5
V =: 771.5 m 3 /3 = 257.2 m 3
.... ";'; ~·iO.3· m'
c
At 15 C d. The configuration of the tanks and paddles should be as follows:
J1 = 1.139 x
E
61.~.~~·_.~~~
E 0
on
.
,
k
P2
i C>() iEB k9;) . 6t ~
~
0.8 m
0.8 m J
5m
Profile
=
30 2 x 2572 x 1.139
X
10- 3 x 10- 3
P, = 20' x 257.2 x 1.139 x 10 -
3
x 10 -
3
=
0.26 kW'
=
0.12kW
4. Paddle configuration u. Assume paddle design as shown below.
~l~/: --/
~j · H
10- P N '5/m'
P, = 40 2 / S 2 x 2572m 3 x 1.139 x 1O - 3 N.s/m 2 = 468.7 N . m)s x 10 " 3 kW,f N· mls = 0.47 kW , I
'0:4;;;""
+
~/. ~--T-
1r-rrr77""TTT:~ ~~
V////////////
_~____
(.
1 4.2 m ~_
]49
ENGINEERED SYSTEMS FOR WATER PURIFICATION
151
150 WATER
Third compartment:
Each paddle wheel has four b oa rds 2.5 m long and w wide- three paddle wheels per compa rtment. b. Calc ulate IV from power input and paddle velocity.
= (120 N. m /s x
l'
= (0.03
At 15°C
L' p
'p
=
999.1 kg/ m J
m) /s) ' !) = 0.32
"p =
·s'
mrs
J (
. . I actual = 0.32 m/s x = 0.42 m/s . 0.75 (<)
Assume
_1_~)'{3 3777 N
p
CDAppv~ p = - - -2
, -'
= 1.91 rev/min
0.67 m/s x 0.75 = 0.5 m/s and CD = I.lI. Ap = length of boards x
IV
x number of boards
4-7 SOFTENING
.\ paddles at 4 boards per paddle = 12 boards 12 x 2.5 x
P,
= JOw = Ap
IV
= 468.7 N·m/s = (1.8 x 30w m x 999.1 kg/m) x N· s'/kg·m x 05) m)/ s) / 2
937.4 m = 1.8 x 30 x 999.1
X
0.5)w
674~w
937.4 m =
IV
= 0.14 m
c. Calculate rotational speed of paddles.
Hardness as a water-quali ty parameter was discussed in Sec. 2- 10. The reduction of hardness. or sojiening, is a process commonly practiced in water treatment. Softening may be done by the water utility at the treatment plant or by the consumer at the point of use, depending on the economics of the situation and the public desire for soft water. Generally, softening of moderately hard water ·(50 to 150 mg/ L hardness) is best left to the consumer, while harder water should be softened at the water-t reatment plant. Soften ing processes commonly used are chemical precipitation and ion exchange, either of which may be employed at the utilityowned treatment plant. Home-use softeners are almost exclusively ion-exchange units.
Fir sl compartment: m
v = nD -
rev
P
= 1l4.2 -
x
m
rev
Chemical Precipitation
(lJ
x (;.)
rev 60 s 0.67 m/s x - - - - x - -. = J05 rev/ min = w 4.21l m min
Lime- soda ash All forms of carbonate hardness as "well as magnesium noncarbonate hardness can be converted to the precipitating species by the addition of lime (CaO). In the following equations, the symbol s is used to indicate that a solid precipitate forms and that it is sufficiently dense to settle by gravity.
Second compartment:
) N . m/ s p = 0.26 kW x 10 - - · kW
Ca2+ = 1.8 (30 x 0 14)m ' x 999. 1 kg /mJ x N s,lg ' m x 1' ~12
260 N . m!s = -37}7 N
----)
The different species of hanJness have different solubility limits, as shown in Table 4-2. The least soluble forms are calcium carbonate and magnesi'lim ·hYdroxlde·.......... . Chemical precipitation is accomplished by converting calcium hardness to calcium carbonate and magnesium hardness to magnesium hydroxide. This can be accomplished by the lime -· soda ash process or by the caustic soda process.
5' j m
up =
x rF
( 260 N . m/s x '3
77 17-mN'~ s:-'_)
= (007 m ·' /s.1 )") = 0.41 m is AClual speed = u,,/075 = 0.55 mi s (() = 2.5 rev/ min
1: J
Mg 2+
+ 2(HC01r + + 2(HCO,)- +
CaO CaO
+ +
2CaC0 3 $ + 2H 2 0
H20
-----+
(4-17)
H20
~ CaCO)$ + Mg2+ + CO/(4-18)
152
WATER
ENGINEERED SYSTEMS FOR WATER P U IUFI CA TI ON
Table 4-2 Equilibrium of solid and dissolved species of common ions Mineral Calcium Calcium . Calcium Calcium Calcium
bica r bonate carbonate chlo ri de sulfa te hydroxide
M agnes ium Magnesium Magnesium Magnesium Magnesium
bicarbonate carbo nate chloride hydrox ide sulfate
SodwlILbica rbon ate Sodiu m ca rbona te Sodium ch lo ride Sodium hydroxide Sodium sulfate
F o rmula
Soillbility, .. mg/L CaCO) at
Ca(HCO,h CaCO, CaCl, CaSO. Ca (OHl,
1,620 15 336,000 1.290 2,390
Mg(H CO J ), MgCO , MgC l, Mg(OH ), MgSO.
37, 100 101 362, 000 17 170,000
NaHCO J Na ,CO) NaCI NaOH Na ,SO.
38.700 61.400 225,000 370,000 33,600
153
200
rf
oec
r,. t
!
100
-
"-
I
,2 E.
i
E
<
I
160
0.
140
".., V;
co
120
~
'"c
'"
Concentralion 01 calcium io n in t:'qui li briul1l wilh
IOC)
ca lcium l'arhonah'
~
"i::;
:; '-
".., ~
Source: Adapte·d from Loe wenth a l and Marais. [4 ·38J
xo hI)
., 0
As seen in Eq. (4-20). removal o f magnesium no ncarb o nate ha rdness results in the formation of ca lcium noncarbonate hardness. Thi s calcium no nca rbon a te hardness. as well as any initially present in the water, can be removed by the ad· dition of soda ash (Na 2 CO): .
Iv
4()
\ 20
0
.......
f'.,..
..... -.
...,/
~ l)
... .
t i -.O llcentlatlonol magnesium
~. io n in cqlulibrruill wIth ma g JlC' slllll1 hydrox](je
... ····I········ j··· ···· J····· ···
II 10 pH ,crlU e"
0
\3
14
Figure 4.24 E4uilibrium concent ration of ca lciu m and magnesi~m io ns a s a function o f pH (From I'o,,·ell [4.43].)
The sodium in Eq. (4-21) is so luble a nd . unless excessive a m o unt s are added . is permiss ib le in p o table water.
';.... -
The precipi ta t ion of CaCO J and Mg(OH }z is pH-depend ent. as is illustrat ed in F ig. 4-24. The optimum pH fo r CaCO) precipita tion by line addition is from 9 t o 9.5, wh ile effective precipitati o n o f Mg(OH) 2 under water-treatment p lant condit ions requires a pH of ab o ut J 1.0. Since mo st n a tural waters have a pH cons iderab ly below t hese va lues. it is often necessar y to a rti fi c iall y r:lise the pH . Th is ca n be accomp li s hed by the additi o n o f an excess am o unt o f lime :
Although thi s reaclion does !l o t redu ce hardn ess. it d oes co ns um e lime. Rem ova l o f super sal urated CO 2 by ae ratio n is o ft en pracliced to reduce lun e reljUlrements. If CO 2 exceeds 10 m g/ L it ma y be eco n o mi ca ll y advanlageous to remove tt prIor lo soft ening. Caustic soda. All forms of hardn ess can also be conve rt ed t o th e precipit aling species by the addil ion of caus li c soda (N aOH ).
CO 2 + 2Na O l1
---->
2Na-
+ ('0,/ - +
H20
(4· 24)
Ca: ' + 2( HCO,) ' + 2N aOH
CaC0 3 $ + 2Na + + ('0/ - +
(4·22) .-"'-,
The addition of about 1.25 mequi\ !L of lime is sufli c ient to raise the pH to 11.0. If di sso lved ca rb o n di ox ide is present in water it wi ll a lso react wi th lime.
- '\
i\ lg'; + 2( HC O ,)
~ H;O
(4·25)
+ 2 H20
(4·26)
+ 4 NaO H M g.( OI1) 2$
+ -iNa ' +
2(0 / -
(4·:27)
ENGINEERED SYSTEMS FOR WATER PURIFICATION
154
155
WATER
The soda ash formed [Egs. (4-24), (4-25), and (4-26)] will react with calcium noncarbonate hardness as previou sly shown in Eg. (4-21). As in the lime - soda ash process, it is necessary to raise pH to 11.0 to precipitate magnesium hydroxide. An excess of 1.25 meguiv/ L of sodium hydroxide is added for this purpose. (4-28)
NaOH
Stabilization Complete removal of hardness cannot be accomplished by chemical precipitation. Under conditiolls normally prevailing in water-treatment plants, up to 40 mgjL CaCO) and 10 mg/ L Mg(OHh usually remain in the softened water. Precipitation of the supersaturated solution of CaCO } will continue slowly. however. resulting in deposits in water lines and storage facilities. It is therefore necessary to "stabilize " the water by converting the supersaturated CaCO) back to the soluble form. Ca,2+ + 2(HCO»-. Stabilization can be accomplished by the addition of anyone of several acids. Using sulfuric acid as an example: 2Ca 2
2CaCO} + H 2 S0 4
'
+ 2(HC01 f
-I- SO/··
The most common practice. however. is to make the cOllyersion with carbon dioxide: CaCO) + CO 2 + H 2 0 Mg(OH)2 + 2CO l
Ca 2 + -I- 2(HCO}) Mg 2'
+
2 (HCO)
I
Ca 2 +
CO~
HCO; mequiv/ L
I
80
60
50
to
mequiv/ L
I
Mg 2 +
Na+
SO~-
8.0
3.5
SOLUTION
The following treatment scheme will be used.
(4-31 )
Lime
(4- 32)
Soda ash
This process is .generally called recarbollalioll. If the pH has been raised to facilitate the precipitation of magnesium. it will be necessary to neutralize. theexcess..bydroxy.l..ions. PL·jQ[. 10. stabilization.· This· ·· necessitates a two-stage treatment process. Typical reactions are:
Settling basin
Flocculator
Recarbonation
.... Rapi~ . . . . . . . . . .. ... . mix
l. Calculate chemical requirements using appropriate formulas.
With sulfuric acid Ca 2 + +'20H~ + H 2 S0 4
Ca2+ -I- SO/ - + '21:-1 2 0
(4-33)
2Na+ + .20H - + H 2 S0 4
2Na + + SO/- -I- 2H 2 ()
(4-34 )
CaC03~ + H 2 0
(4-35)
2Na + -I- CO/- + H 2 0
(4- 36)
Cal f -I- 20H- -I- 2C0 2
+
CO 2
-
--->
I.OC0 2 + I.OCaO - - l.OCaCO,! 2.5(Ca2+ + 2HCO,) + 2.5CaO - - 5.0CaCO,! + S.OH 2 0 1.5(Ca2+ + SO/ - ) + 1.5Na 2CO, - - I.SCaCO,! + 1.5(2Na+ + S042-)
With carbon dioxide
2Na ~ -I- 20H 2
I. Calculate the chemical requirements and solids produced in milliequivalents per liter. 2. Draw a bar diagram for the finished water. 3. For a flow of 25,000 m'/d, calculate the daily chemical requirement and the mass of solids produced: Assume that the lime used is 90 percent pure and the soda ash is 85 percent pure.
(4-29)
(4-:l0)
Mg(OH)2 + Hz SO
Example 4-7: Single-stage softening A water with the ionic characteristics shown in the bar diagram below is to be softened to the minimum calcium hardness by the lime-soda ash process. Magnesium removal is not deemed necessary.
The pH must be lowered to approximately 9.5 before signiticant stabilization occurs.
Second-stage recarbonation will be required to stabilize the water. Assuming a CaC0 1 concentration of 40 mg/L in the effluent from the settling basin, 25 mg/L should be converted to reach the equilibrium of 15 mg/L of CaCO,. 05CaCO, + 0.SC0 2 + 0.5H2Sl Total chemical requirements are: Lime
Chemical requirement The CJuantity of chemicals to soften wat e r can be calculated using the appropriate formulas from Eqs. (4-17) through (4-36 ). These calculations arc illustrated in Examples 4-7 and 4-8.
- - 0.5Ca(HC0 3 h
= 1.0 + 2.5 = 3.5 mequiv/L
Soda ash
=
1.5
= 1.5 mequiv/L
CO 2 = 0.5 mequiv/L
ENG INEERED SYSTEMS FOR WATER PURIFICAT ION
157
156 WATER SOLUTION The following Irealment scheme will be used.
So lids produced are CaC0 3
= 1.0 + 5.0 + 1.5 -
0.8
= 6.7 mequ iv/ L Lim c . sod" ash
2. T he ba r diagram for the finis hed water is
o
08 Ca 2>
coj-
18
I
Mg2>
3
Na>
SO~-
· l · HCO; 0.3
53
0.8
Rapid mix Flocculation Set tling Fir st-stage
2. 3. 4.
5.3
rccarbonalion
Second-stage reca rbonation
3. The equi va lent mass of lime and soda ash is
+ 16 -2--
. 40 Lune =
2m)
.
=
28 mg/ meq ulv
+
12
I. Calculate che mical quantities using appropriate formulas.
+
3(16)
.. Soda ash = - ..- - - - - - - . - - = 53 mgj mequJ\·
2
12 + 2( 16)
--~ --
Carbon d iox ide =
. = 22 mg! meq ll iv
L
The da ily c hemica l req uirements are: ~)
I kg (1 /0.9) 28 m g!m equiv x 3.5 meq ll iv/ L x 25 x 10 Ljd x - 6 - = 2722 kg/ d 10 mg .
0.6CO ,
+
0.6CaO
0.6CaCO ) 1 6.8 CaCO )
34(Ca' >
+
2HCO) - )
+
14CaO
1. 5(Mg H
+
2HCO, - )
+
JOCaO
1.5(Mg H
+
1.5( Ca'·
+ S04 »
SO.' - )
+
1. 5CaO
+
+
1.5Na,CO,
1+
1. 5 Mg(O H ),
6.8 HP
t+
3.0CaCO,
t
1.5H , 0
6
. ___ ________ ____ . __ (I / 0 .gS)-53- m g/rnequi V")("
15-meqoiv/ t- x
23· -x
I06T/d-·x Tkg/ (Oi;-i·;;g--~ -iiiR k-gjd-
Excess lime = 1.25 mequivj L
22 mg/mequ iv x 0.5 meq uiv/ L x 25 x 10 6 Li d x J kg/ 10 6 m g = 275 kg/ d T he m ass of d ry so li ds p rod uced per day is
For first-stage recarbo Jl ation_ use CO, to neutralize excess lime.
50 mg/ mequiv x 6.7 meqlliv! L x 25 x lOb Lid x I kgllO" mg = 8375 kg;d E x ampl e 4-8: T wo-s tage soft ening A water with the ionic characteristics shown below is to be softened to th e mi n im um possible hardness by the limc - soda-ash - excess-lime process. Calcu la te the required chemica l quanti ties in milliequivalent s per liter. Draw a bar d iagram of the fin ished water.
125(Ca' +
+
201r)
40 Ca 2>
CO~
I
80
70 Mg1 •
HCOj -
I-
I soi-
N,,>
125CO ,
--
125CaCOd
+ 1.25H zO
Ass umin g 40 mg/ L CaCO J and IQ mg /L Mg(OH) , remain ing in so lution stage se ttling_ 0.2 Mg(OH) , 0.5CaCO)
06
+
+ OACO,
+ 0.5eO, +
0. 5 1-1 , 0
Total c hemi cal quantities are Lim e = 0.6
+ 3.4 + 10 + 1. 5 + 1.25
.Soda ash = 1. 5
co, =
1. 25 + 0.4
+
OJ = 2.1 5
= 'US
af~er second-
158
WATER
ENGINEERED SYSTEMS FOR WATER PURIFICATION
2. Bar diagram of final water:
08 C, a 2+
C05-\
I
10
Mg, = magnesium concentration in the raw water, mg/L Mgt = magnesium concentration remaining in the fraction of the water receiving first-stage treatment. [As previously stated, practical limits are 10 mg/ L Mg(OH)2 (as CaC0 3 )·]
35 ..
Mg2+
Na'
HCO:;
159
A typical split-treatment system for removing magnesium is shown in Fig. 4-25. The quantity of softe:1ing chemicals saved by this system is illustrated in the following example.
SO~-
0.3
3.5
Softening operations Softening operations consist of several steps and may be carried out in one or two stages. The operations include mixing of the chemicals with ' the water, flocculation· to aid in precipitate growth, settling of precipitate, and stabilization. The solids-contact system shown in Fig. 4-23 is often use'd for softening operations. These systems operate in much the same manner as the systems for coagulating and ' removing turbidity discussed in Sec. 4-6. Design criteria, however , are slightly different and are summarized in Table 4-3.
Influent
Bypassed flow Qx
Table 4-3 Typical design criteria for softening systems Parameter
Mixer
Flocculator
Detention time* Velocity gradient, s - 1 Flow-th rough velocity. I'tis Overtlow rate, gal / min / ft'
5 min 700
30·- 50 min 10- 100 o 15- 0.45
NA NA
NA
Soda ash
l.im e
Selliin g basin
Solids'contact ba sin
2- 4 h
NA
1- 4 h t
0.15- 0.45 0.85 - 1. 7 1
427t
Figure 4-25 rIow diagram for softening by split trcatment.
Example 4-9: Softening by split treatmcnt Use split treatment to soften the water with ionic strength given in Example 4-S. Assume that a final hardness of less than 100 mg/L is acceptable, provided the magnesium IS less than 45 mg/L. Calculate the chemical requirements and draw a bar diagram of the finished water.
NA
SOLUTION
• This should be confirmed by pilot-plant analysis for each water. . 't Velocit y gradient in mixer and Aocculator compone nt sho uld be approximatel y the sa me as in flow-through units. t At slurry blanket-clarifier watcr interface. SOllrce: Adapted from Recommended Standards . [4-44J
The treatment scheme shown in Fig. 4-25 wi'li ·b~-useif.·· -., ., . - - - . ............. ., . ., .. ... .. _. - .... .
J. Calculate the bypass fraction: Q = Mg f - Mg, x Mg, - Mg,
Water with high magnesium hardness is often softened by a process called split treatment. This process bypasses the first-stage softening unit with a part of the incoming water. Excess lime is added to facilitate the removal ofinagnesium in the first stage and, instead of being neutralized thereafter. is used to precipitate the calcium hardness in the bypassed water in the second stage. Since no magnesium is removed in the bypassed water, the initial magnesium hardness and the allowable magnesium hardness in the finished water govern the quantity that Olay be bypa ssed:
Qx = Mg f - Mgt Mg, - Mgt where Qx = fract ion of the total flow bypassed Mg r = Magnesium concentration in the finished (as CaC0 3 ) usually acceptable
;.
0.9 - 0.2
= 3.0 - 0.2 =
0.25
2. Calculate the quantity of chemicals added to first stage: 06CO,
+ 0.6CaO
+ 2HC0 3 - ) + 3.4CaO - - - - - t .SCaCOd + 6.SH,O (1.0 - o 25)(1.5)(Mg 2 + + 2HC0 3 - ) + (10 - 0.25)3.0CaO . -----to I.13Mg(OH),! + 2.25CaC0 3 i
3.4(Ca'+
(4-37) (1.0 - 0.25)(1.5)(Mg 2+ + SO/ - )
+ (1.0
- O.25)1.5CaO 1.13 Mg(OH),
-----t
water , 40·· 50 mg/ L
1.I 3(Ca 2 +
+
1.l3(Ca2+
+ SO/ - )
+ SO/ - ) + 113(Na,C0 3 ) -----t
I.I3CaCO}!
+ 1.13(2Na+
_I-
S04' - )
160
I!
WATER
ENGINEERED SYSTEMS FOR WATER PURIFICATION
Gas
Check to make sure extra lime is enough to provide 1.25 mequiv/L: (0.6 + 3.4)0.25 - -0-.7-5-'--- = 1.33
Air
1.33 > 1.25, so acceptable
.~ ..... -. : .:", --;, ..:.~:- " ".:
For second-stage recarbonation:
+ 2 HC0 3 -J
O,5CaC0 3 + O,5CO l + 0.5H 2 0
05(Ca 2+
0:75 x 0.2Mg(OHh + 0.30C0 2
015(Mg2+ + 2HC0 3 -
)
Total quantity of chemicals: Lime
= 0.6 +
J4 + (1.0 - 0.25)(30 + 1.5)
= 7.38 mequiv/ L
Soda ash = 1.13 mequiv/ L CO 2
=
0.30
+ 0.5
=
0.80 mequiv/ L
3. Calculate ionic strength of finished water: Ca2+ = 0.8 Mg2+ Na+
= 0.75 A 0.2 (first stage) + 0.25 = 1.0 + 1.13 = 2.13
x 3.0 (in bypass)
= 0.9
::: ~"". '.;.::
I
I
','
..
=
.. ,' '.-
.~ ~ :
' .~.
• :(7
',-.-.' ,',
.','
.'
·0
/
.'0,
2
0.3
.
.
.
'.
..
..
,',
. . ' . ':'.~ .
',.,
..... .~
..
o ','
'.",'~.
• #
..
;·~.~I' . ' . ;
'.: : • "'I'
•
.!-. :::.'" :,~ .. ')
~.' :'::::.: :.~'. :: :.
0.5(conversion "fCaCO,) t- O.15(conversion of Mg(OH),) + 0.25 x 1.5 (associated with by passed Mg) = 1.03
08
c0 3 - / .,
')" :
. ,. '.
-
.. Figure.4,26 .Subm.cr.ged.burner. for recarbonalion.
Ca 2+
'
.0
CO/- = 0.3 HCC?3 -
161
I HeO'j
1 711
Mgl+
I
113
I
Na
SOJ3.83
For a more complete descripti~n of split treatment, the reader is referred to Cleasby and Dellingham. [4-20J Recarbonation Recarbonation for pH reduction and stabilizati()n takes place in a closed reactor. Carbon dIOxIde IS added under pressure and dissolved according to gas-tra~sfer pnnclples prevIOusly dIscussed. Figure 4-26 shows a typical recarbonatlon process. . Typical recarbonation units consist of two chambers, one for mixing the CO 2 and one m whIch the reactIons occur. Detention time in the mixine. chamber should be from 3 to 5 min, with a total detention time of at least 20 ~in. [4-44J
(C.oWNS}'. or.O.zark,M.{dIQ(lirJg.CQ~IJIl{J!!Y.) ..
Where split treatment is employed it may be necessary to follow the recarbonation unit with a settling chamber if the influent to the units still contains an excess of lime. [4-45J All recarbonation units should have provisions for periodic cleaning as some precipitate will accumulate. The source of CO 2 may be the exhaust from combustion of natural gas (CH 4 + 20 2 .... CO 2 + H 2 0) or CO 2 which has been purified and shipped to the plant in containers. Walker [4-5RJ suggests that the stoichiometric quantity of CO 2 be mUltiplied by a factor of 2 to compensate for inefficiency of CO 2 transfer from the exhaust gases if submerged hurners are used. Llquified CO 2 that is essentially pure (99.S percent) can be obtained: this greatly enhances the efficiency of the recarbonation process. Storage of liquid CO 2 presents a problem since it . gasifies at 31 °C. resulting in extremely high vapor pressure. The usual procedure is to store liquid CO 2 at around - 20°C and 2000 k Pa. This necessitates strong tanks and refrigeration equipment. Large water-treatment plants often find it economically advantageous to recalcify the CaC0 3 sludge, reco'vering both lime and carbon dioxide. CaO
+
CO 2
(4-38)
162 W ATER
ENGI NEERED SYSTEMS FOR WATER PURIFICATION
Wh ere prec ipit a ted s lud ges a re esse nti a ll y pure C a C0 3 , reca lc ifyin g s ho ul d pro du ce a n excess o f bo th the lim e and the CO 2 requircm ent s fo r th e pla nt. Lime kiln s rep rese nt a sub stanti a l inves tment in ca pital equip me nt a nd maint ena nc e a nd o p e rati o n cos ts a nd a re usua ll y justifi ed o nl y thr o ug h eco n o mi es o f sca le.
Na+ +
Na+ +
N.+ +
A nio n
A n ion
Ani o n
163
Na+ + Ani o n
Ion Exchange A w ide variet y of di sso lved so lid s, including har dn ess. c:ln be re moved by io n excha n ge. The di sc uss io n he re w ill be limit ed to io n exc ha n ge fo r softenin g; a m o re gen e ra l di sc ussio n o n io n exc han ge fo r co mpl ete d em in e ra li za ti o n is co nta in ed in a lat e r sec ti o n o f thi s c ha pter. As prac ti ced in wat er so ft enin g, io n exc ha n ge in vo lves re pl ac in g ca lc ium and m ag nes ium in th e wa te r w ith a n o ther. n onhard n ess cati o n. lI su a ll y sod ium . Thi s exc ha n ge tak es place a t a so lid s inte rface. Alth o ug h th e so lid ma te ri a l d oes n o t d irec tl y e nter into th e reac ti o n. it is a necessar y a nd impo rtant part o f the io n exc han ge process. Earl y applicati o ns o f ion exchan ge Ll sed zeolite. a naturallv occurring sodium alumino-silicat e mater~al sometim es call ed yre ensand. M od e r~ a pplicati o n s more oft en use a synthetic resin coated w ith th e de sirable exc han ge ma teriaL The synthetic resin s ha ve the advantage of a g rea ter num be r o f exc hange s it es and' are more easily rege nerat ed. In equal quantities. ca lcium and ma gnesium a re a d so r bed mo re stron gly to th e medium than is sodium. As th e hard water is co nt ac ted with the med ium. th e fo llo w in g generali zed rea c ti o n occ urs.
{ca}+ Mg
[ani o n]
+ 2 Na[RJ
{ca}
M g [R ]
+
2 Na
+
[anio n]
(4- 39)
The reaction is virtuall y in s tantaneo us and co mplet e as long a s ex chan ge s it es a re a vailable. The process is depicted graphicall y in Fig. 4-27 . When all of th e exc han ge sites have been utili zed. ha rdn ess beg in s to appear in the effluent. Referred to as hreaklhrough. thi s necess itat es tile rege nerati o n o f the medium by co nt ac tin g it with a stro ng sodium -c hl o ride so luti o n. Th e strength o f the so lution ove rrid es the selec tivit y onhe a d so rpti o n sit e. alld calcium 'a nd m agnesium a re rem oved and replaced by the sodium.
{ca} Mg
[R] .
+ 2 NaC I (excess)
{ca} Mg
2 CI
+ 2 Na[R ]
(4-40)
Th e sys te m can a gain fun c ti o n a s a soften e r a cco rdin g to cq . (4-31)). Th e capacit y and effi c ie nc y o f ion-exchange so ft ene rs va ry w ith man y fact o rs. in c luciing type o f so lid m ediulll . type o f exc han ge mat c l'ia l used fo r coa tin g. qu a ntit y of rege n erati o n ma te ri a ls, and regen erati o n co nt ac t tim e. Th e overall qu a lit y o f th e wa te r to be so ft en ed is al so a n impo rtant facto r. ;\ co mplet e d isc ussio n o f th ese fac to rs is heyo n cl th e sco pe o f thi s text and t he read er is referred to Refs. [4-47J and [4-53J fo r g rea ter d e ta ils. Ge nerall y. th e ca pac it \' o f ion-excha nge j llIat er ia ls r:lll ges fro lll 2 to 10 m equ iv /g o r aboll t 15 t(> 100 kgi /ll Regenera ti o n
. .. . .. :' ,
.
.: ....
:.: : .:
+ A nio n
D''.' . .
Re!in w ith N" + R . adso rbed .
.
. .'
,'
::
+
+
+
An ion
Anion
Anion
Act ive _ excha nge zo ne
Figure 4-27 lon-exchange process .
is a ccomplished using from 80 to 160 kg of sodium chloride per cubic meter of resin in 5 to 20% solution at a flow rate of about 40 L/min . ni 2 . The effluent from the regeneration cycle will contain the hardness accumulated during the softening cycl e a s well as excess sodium chloride. After regeneration, th e medium s ho uld be flu shed with softened water to remove the excess .sodium . c hloride. These highl y min e rali zed waters constitute a waste stream that ~uSt , be di sposed of pro perly. . lon-exchange o perati o n s a re usually conducted in enclosed structures conta ining the medium. W a te r is fo rced through the material under pressure at up to OA m3 / m in . m 2 Sin g le o r m ultiple units may be used and the medium may be co ntained in either a fix ed o r a moving bed. Where continuous operation is necessary. multiple units or mo vin g beds are used . Single-stage fixed beds can be used when the flow of tre a ted wa ter can be interrupted for regeneration. Most treatmentpl a nt operations are o f th e co ntinu o us type, while h o me softeners are serviced ill te rmi !ten tl y. Io n- exch a nge soft enin g a t wa te r-treatm ent pla nts is becoming more commonplace as m o re effi c ient resins a re de ve loped a nd as the process is better understood by d es ign eng in ee rs. Io n exc ha n ge p roduces a softer water th a n chemical pre-
ENG I NEERED SYSTEMS FOR WATER PURIFICA TION
164
165
WATER
cipitat ion and avo id s the large q uantit y of sludges encou ntered in the li me-soda process. The physica l and mechanical a ppa ratu s is m uch smaller and simpler to operate. Th ere are seve ral disadva ntages, however. The water must be essentiall y free of turbidi ty and particulate matter or the resin will fun ction as a fil ter and become plugged. Surfaces of the medi um may act as an adso rben t for organ ic mo lecu les and become coa ted . Iro n and manganese Pfecipil ates can also foul the su rfaces if oxid ation occurs in, or pri or to. the io n-exchange un it. Soft eni ng of clea r grou nd wa ter should be done immedi ately (befo re aerati on occurs), whi le surface wa ter should receive a ll necessar y trea tment, incl udin g filtration, pri or to softening by ion exc hange. Th e wa ter should not be chl ori na ted prio r to ion-exchange soft ening. Exa mple 4- 10: Designing an ion-exchan ge soft ener An ion -e xchange so ftener is to be used to treat the wa ter described in EX3mpie 4-7. The medi um se lec ted has an ad so rpti ve capacity o f 90 kg/m' at a flow rate of 04 m' /min . m 'Regenera tion is accompli shed usi ng 150 kg of sodium ch lo ride per cubic meter of res in in 1 0n ~ so lutio n. Dete rmine the volume of med ium required and the physical arrange ment for co ntinuo us ope ration in fixed beds. Al so det ermine the chemica l requirement and the regeneration c\'ele time .
d. Add three extra tanks for use during regenerati on cycle. To ta l vo lume of exc ha nge
resin is:
v= =
75.4 m
3. Deter min e chemi ca l requirements for rege nera tions. Q.
Vo lume of one unit
v = 3. 14
x 2.0
= 6.28
m3
b. Sa lt requirement
150 kg/ m 3 x 6.28 m.1
=
942 kg
Regenerating 9 un its/c1 wi ll req ui re 9 x 942 = 8,478 kg/ d o f Na e !. c. Using a 10 % so luti o n. th e vo lume of rcgenerate liq ui d is 942 kg/O.I app ro ximate ly 9 Ill' for each unit. d . At a loading ra tc of 0.04 m 3 / m 2 . min. the regenera ti on tim e is 1 =
= SOLUTIOr-:
No. of tan ks x area x height
= 12 x 3.14m 2 x 2.0m
=
9.4 20 kg. o r
9.0 m 3 /(0.04 m 3 / m 1 ·min x 3.14 m 2 ) 72 min
Ass umin g a tota l of 2 h fo r all opera ti ons necessary to regen erate units in gro ups of three. all 12 unit s ca n be regenerated in an R-h wo rk day .
1. Determi ne vo lume of medium .
T o ta l hard ness = 6 mequiv i L x 50 mg/mequ i\' = 250 mg/ L. Ass ume 75 mg/ L hardness is acce ptab le. By pass 75/ 250 = 0. 30. or 30 pe rcen t o f the flow . Treat 0.70 x 25,OOO m ' /d,or 17.500 m' /d b. Hard ness to be removed :
Q.
c. Vo lum e o f medium fo r I-d o pera tio n4375 kg/d x I m'/90 kg = 4X .6 m' medillm/d opera tion 2. Determine surface area a nd he ight of medium. Q. 17,500 m'/d x d/ 1440 min = 12. 15 m' /min Area = 12. 15 m' / mi n x min j04
III
=
30.38 m
b. Use tank s 2.0 m in di ameter.
A = rrd 2;4 = 3. 14 No . of tank s
Ill '
} ()}8 = ---- =
3-14
9.67 : use 9 tank s.
Height of medium
to ta l vo !L;me
= '--~-.- ---.~
tota l area
48.6 m 3 9 x -3j4 - ~2 =
I. 72
Ill.
sa y 2 m
2
4-8 F ILT R ATION As prac ticed In mode rn water-treatment plan ts. fi lt rati on is most often a polis hin g step to remove smal l Il ocs or precipitan t pa rticles not remo ved in the sett li ng of coa gula ted or so ftened wa ters. Und er cert ain cond iti ons. fi ltrati on may serve as the primary tu rbidi ty-remova l process. e.g .. in direc t fi lt ration of ra w wa ter. Alth o ugh fi ltra tion removes many pa th ogenic orga ni sms from water. fi ltra tio n · should not be reli ed up on for complete hea lth protec tion. PreCOO l ji/(ralion . a process in which a thin shee t of d ia tomaceous eart h. or othe r ve ry Mnc media. comb in e wit h so lids in the wa ter to form a "ca ke " on a mi crosc ree n. ma y ha ve ad va n tages under ce rtai n circ u mstanccs. A discu ssion or precoa t fi ltrat ion is beyo nd the scope of th is text a nd the reade r is referred to Baumann [4- 7J fo r a th oro ugh d isc ussion of the su bjec t. The most common ly used fi lt ra tion process in vo lves passing th e water thro ugh a stationary bed of gra nular med iulll. So lids in the wate r are retain ed by the fi lter mediulll. Several modes of opera ti on are possible in gra nula r medium fi ltra tio n. Th ese in clud e upll ow. billow. pressure. and vac uum fi ltra iion. Whi le an\' of these may li nd app li ca ti on unde r spec ialized co ndili ons, the most com lll o l~ prac tice IS gravity li ltration in a c1ownw.arcl mode, with the we igh t of the water co lumn above the filt er pro viding the d ri ving force. The ablwe o perati ons are ciep icted graphicall y in Fi g. 4-28_
166
i
WATER
Water level during filtering Water leve l during back washing
T
----=-
~:.:) ~ -.- .
Wa sh-wat er tank
'-'
Wash-water trough
7-10 m
. ----,
600 mm
freeboard .," -, ..... :- .... .•..........>. : .~.:::.: .
Innuent
A
•iM..0··~0··.t;lf;~;~iW;
'+ '"
~
I
I
-+
. 300-760 mm
F=h==A===t'
I ~
m
-
How I. 2. 3.
filt er operates Open valve A. (This allows innuent to now to filter.) Open va lve B. (This aJlows water to now throu gh filter.) During filter operati on all other valves are closed.
How I. 2. 3.
filter is back washed C lose valve A. C lose va lve B when water in filter drops down to top of ove rnow . Open valves C and D. (This aJlows water from wash-water tank to now up through the filtering medium, loosening up the sand and washing the accumulated so lids from the su rface of the sand, out of th e filter. F ilt er backwash water is return ed to head end of treatment plant.
167
basins). inertial impaction . diffu sion of colloids into areas of lower concentrations and / or lower hydraulic s hea r. [4-42J and. to a lesser extent, Brownian movement and van der Waals forces. Retention of solids once contact has occurred can be attributed primarily to electrochemical forces, van der Waals force, and physical adsorption. With chemical preconditioning of the water, a well-designed and operated filter s hould remove virtually all solids down to the submicron size. Removal . begins in the top portion of the filter. As pore openings are filled by the filtered material, increased hydraulic shear sweeps particles farther into the bed. When the storage capacity of bed has thus become exhausted, the filter must be cleaned. Modern practice is to clean the filter i:Jy hydraulic backwashing. Backwash water containing the accumulated solids is disposed of and the filter returned to se rvice. Many va riables influence th e performance of granular media filters. An understanding of filter hydraulics, media characteristics, and operating procedures is necessary for the design of effective granular medium filters.
Filter Hydraulics
Wash water Underdrain sy s tem
ENGINEERED SYSTEMS FOR WATER PURIFICATION
Filter hydrau lics falls into two separate categories, the actual filtration process by which the water is cleaned and the back washing operation by which the filter is cleaned. These operations are equally important in the overall filtration process. Fl ow throu gh. the packed bed can be ana lyzed by classic hydraulic theory. Carmen [4-14J modified the Darcy - Wiesbach equations for head loss in a pipe to reflect conditions in a bed of por.OllS media .of.un.ifor.m.size. .Development-of this equ~tioN .... .. ..... . is presented in several texts (Refs. [4-16J, [4-29J, [4-53J) and will not be repeated here. The resulting equation. known as the Carmen- Kozeny equation, is:
(4-41 )
How to filter to waste (if used) I . Open valves A and E. All o th er valves closed. Ernuent is sometimes filtered to waste for a few minutes after filter has been washed to condition the filter before it is pu t into service.
where hf = friction loss thr ough bed of particles of uniform size dp' m L = depth of the filter. m e = porosity of bed V, = filtering velocity. i.e .. the ve locity of the water just above the bed (total flow Q to th e filter divided by the area of the filter), m/s !J = gravitational acce leration, m/s2 d p = diameter o f filter meuia grains. m
Figure 4-28 Typical gravity flow filter operation:(From Metcalf & Eddy, In c. [4-40].)
The so lid s- remo va l operation with granular-medium filters invo lves seve ral complicated processes. The most obvious process is the physical strainin g of particles too large to pass between filter grains. Other processes are also important. since most of the so lid material contained in se ttl ed water is too sma ll to be removed by straining. Removal o f particles and flocs in the filter bed d epends o n mechanisms that transport the solids thr ough the water to the surface of the filt e r grains. and on retention of the so lid s by the medium once con ta ct has occurred Transport mec hani sms include sett lin g (pore openings act as miniature se ttling
The remaining term!' is a friction factor related to the coefficient of drag around the particles. In the usual range of filter velocities (laminar tlow) this can be ca leulateu by
r '"
(I - e)
=
150 ----- + 1.75 Re
(4-42)
ENG INEERED SYSTEMS f OR \VATER P URIfI CATION
168
169
WATER
3. Ca lculate head loss by Eg . (4-41).
where Re
=
Reynolds number
¢Pw Jl,d
"f
(4-43)
= ---
J1
and Pw and J1 are the density and dynamic viscosity, respectively, of the water. The units of Pw are kilograms per cubic meter, and the units of J1 are newtonseconds per square meter. The shape factor ¢ ranges from 0.75 to 0.85 for most filter material. . Equation (4-41) can be modified for abed of nonuniform medium. From a sieve analysis of the medium, the weight fraction xij between adjacent sieve sizes is determined. The average particle size d'j is assumed to be halfway between the sieve sizes. The depth of the particles between adjacent sieve sizes can be taken as xijL and Eq. (4-41) can be rewritten as follows:
hJ
L(l - e)V; ,,fijx,,, 3 L - de9 ij
=
(4-44)
Equation (4-44) assumes that the bed is stratified by size and that the porosity is uniform throughout. Calcula'tion of head loss across a uniform and a stratified media is illustrated in the following examples. Example 4-11: Determining head loss across a bed of uniform-size particles Clean water at 20°C is passed through a bed of uniform sand at a 111tering velocity of 5.0 mlh (1.39 x 10- 3 m /s) . The sand grains are OA mm in diamete r with a shape factor of 0.85 and a . specific gravity of 2.65. The depth of the bed is 0.67 m and the.porosity is OA. Det ermine the head loss through the bed.
193.24 = -- .
x 0.67 m(1
- 0.4)
x (Ull x 10 - ' )2 m ' /s2
OA3;9'8I-;;/s274~O-x~4-;;----
= 0.60 m
Example 4-12: Determining head loss across a bed of nonuniform . stratified particles Wat e r a.! 20 is passed thr o ugh a fi lt er bed at 1. 2 x 10 .' m /' (4 .32 m/h) Th e bed is 0.75 m deep and is composed of no nuniform sand (s pec il1c gra vity of 2.65) strat ified so that the smallest particles are o n to p and the largest on bottom. The p o rosity and shape factors are 0.4 and D.X5 throughout the depth of the bed. The size di stribution of the granules is give n in th e table helow Determine the head loss for clea n water flow throu gh G
e
the bed.
Sieve analysis Particle size range. US. sieve no .
Passing
I~
20 2S 30
35
Rcta ineel 14 20 2S 30 35 40
'''40
S ile
Ma ss fra c t. in size ran ge
/\ \T'rage
111m
Pa ss ing
Rdain ~J
ti,) ,111 m
X,}
1.41 I.~I
(j . S ~
(l.~~
0 .7 1 0 .60 050 042
141 I 13 0/8 0.66 0.55
0. 01 0.1 1 0 .20 0.32 0.21 0.1 3
0 .7 1 0 .60 0.50
O. ~ 6
. '6.42'"
. '042"
662'
SOLUTION
I. Calculate the Reynolds number by Eq, (4-43). At 20°C
p = 998.2 kg/ m]
SOL UT tO t'
kg· m II = 1.002 x 10- 3 N . s/ m2 .x -2-s .N
I. From Eg. (4-43):
= 1.002
X
0.85 x 998 .2 x 1. 2 x 10 ' J m 's Re = _._._._._. ___ . ...... . d, ) m 1.002 x 10 - 3 k gj l1l ' S
1O - 3 ~_
m· s
Re = 0.85 =
998.2 kg/ m' ·
x 4.0 x 10 -
m x 1.39 ,. kg/ m . s 1.002 x 10 4
OA7 < 1.0 (laminar flow confirmed)
x
IO -
J
m/s
I01 6d,j 2. From Eg . (4-42):
"
2. Calculate f' by Eq. (::1-42).
I i) f
.
,
(I - OA) 0.47
= 150 --·_·-
= 19124
=
I SO(I - 04) '-' 10 16'[ - + 1.7~ 'J
+
1.75
ENGINEERED SYSTEMS FOR WATER PURIFICATION
170 WATER
a porous bed, the head loss must be at least equal to the buoyant weight of the par ticles in the fluid. For a unit area of filter this is expressed by
3. Determine L;;j x'i as follow s: dij
hfb fij ~ l ' m
P art icle size. d'J
111
x Ill'
141 I 13 078 0 .66 0.55 0.46 (J42
0.0 1
65.6
0: II
81.4
il .20 032 0.2 1 0. 13 0.02
11 7.1 138.1 16 5.4 197.4 " 16.0
465 7.924 30.026 66.958 63, 153 55.787 10.286
"
(4~45)
'Pw
or
fr om Eq. (4-44):
L fb
m x (I.______ - 0.4) x .(1.2 x iO ___")'. ----m'/s' x 7'4 "N I 1m _ 0.73 __ . ____ _ __ 1 0.4 " x 9.~1 mis' --.- ,
where = 0.24 JJ1
- L (l - e) (l - e fb)
(4-46)
= the depth of the fluidized bed e fb = the porosity of the fluidized bed
Lfb
The quantity efb is a function of the terminal settling vdocity of the particles and the backwash velocity. An increase in the backwash velocity will result in a greater expansion of the bed . The expression _commonly used to relate the bed expansion to backwash velocity and particle settling velocity is [4-28]:
...... 1t. sllot.J!dbe .noted.. lhatEqs. (4~41)ancJ ,(4-44) are ap'plic~tbleonl\, to. c lean .. filt er beds. Once so lid s begin to acc umul ,it e. the porosi ly of the bed c han ges. As the poros ity decre;]ses. th e head loss increases. The rat e at which so lids accllmulate in th e filt e r. and therefore the rale o f head-loss change. is a functi o n of the nature of th e suspension. Ih e characteristics of the media . and filter uperatIon. Although alte~pt5 to formulate a mathe mati ca l express ion ofa general nature to quantify changes in head loss with so lids rem ova l have not been very successful. some general observations can be mad e. To maintain a constant filt er ing ve locit y ~';. an in crement in driving force must be applied to match e:lC h in cre ment in head loss res ult ing fr om decreased porosity. Conversely. if a constant driVIng force IS app lied. the filtering ve locit y wi ll diminish as the porosity decreases. In fllt er operation s. a run is ter minat ed when sutn c ient so lid s have accum ulat ed to ( 1) use LIp th e a\ailable driving forc e: (2) cause th e flltering velocity to drop below a pred ete rmincd leve l: or (3) exh:wst the sto rage ca p ac ity of th e bed so i.hat solids . begin to "break thI OUgh:' into th e ef]-1 l1 ent. At this point, the.filt er must be backwas hed. BacK\\'ashIIlI! ()f £ranlilar-medi lllll hlt ers is accom plished hy rnersIIlg the flo\\' and f,)J"c Ill g~c l ea; water upward thr o ugh th e media. To clean th e interior of th e bed . it is necessary to expand it so that th e granu les are nl! lon gcr In contact wi th c,Ic h nthe r. thus exposing all s urfa ces for cle:lIlin g. To hydr;llI!i c tll v expand
elm - Pw
Weight of packed bed = weight of bed fluidized
d, )
"I
L(l -
The head loss through an expanded bed is essentially unchanged because the total buoyant weight of the bed is constant. Therefore:
Lf;j ::~? = 234.599
4. Calculate
=
. where h fb = head loss required to initiate expansion, m L = bed depth, m I ~ e = fraction of the packed bed composed of medium Pm = density of the medium, kg/m 3 Pw = density of the water, kg/ m 3
~/ij
j,j
x!J
171
_ ·(VB)O.22
efb -
(4-47)
Vc
VB
where is the backwash velocity (backwash flow Q divided by the total fiiter area). The depth of the fluidized bed and the backwash velocity for a given size. medium (with known vc) can now be related as follows :'
L(I - e)
L fb
=
-_-'-(~~-v~~)""O'-c.2'"'2'-(4-48)
tl;
This equation can also be modified for a stratified bed of nonuniform particles where
ENGINEERED SYSTEMS FOR WATER PURIFICATION
172
173
WATER
Again xij is the weight fraction between adjacent sieve sizes. Assuming uniform porosity in the packed bed, Lij will be the depth of the layer of media represented by Xij' The expansion of this layer is represented by
6. From Eq. (4-47) : ~ ) 0.22
0.7 = ( O.O~ ..
L(I - e) Ljb,ij = xij . (V . '8)0.22 . 1 - '-.
VB = 0.7 4 . 5 5 x 0.07 m / s = 1.4
l't,i)
3
mls
7. From Eq. (4-46):
The total expansion is the sum of th e individual layers Ljb = L(l
x 10 -
0.67 m (I - 0.4) =
L fb
-e)I-l--'(X;-::·~'-:-8--;-.)"O''''22
1 - 0.7
(4-49)
1.34 m
=
L r , iJ
Total expanded depth should range from 120 to 155 percent of the unexpanded depth. [4-7J Amirtharajah [4-5J has shown that the optimum expansion for hydraulic backwashing occurs at expanded porosities of from 0.65 to 0.70.
Example 4- 14: Finding the expanded depth of a nonuniform stratified bed The filter bed described in Example 4-12 is to be back washed at a velocity of 1.5 x 10 - 2 m /s Determine the depth of the expanded bed. SOLUTION
Example 4-13: Finding the expanded depth of a uniform medium The filter medium described in Example 4-11 is to be expanded to a porosity of 0.7 by hydraulic backwash. Determine the required backwash velocity and the resulting expanded depth. SOLUTION
I. The terminal settling velocity for the medium is first calculated from Stokes' law [Eq. (4-9)] 9.81 m / s (2650 - 998.2) kg/ m 3 x (4 x 10-
...... . ..... .. . ........ . .... v~.~.. .. . .... .... 1.8.x .l.om.x..
4
Each "layer" of particles defined by the sieve analysis of Example 4 - 12 must be treated separate ly and the results summed. For the bottom layer. dij = 1.41 and x ij = 0.01. I. Estimate an initial velocity assuming turbulent flow [Eq. (4.4) WIth CD
v, = (4~ x _ ._ ~Pm ~-'". x dijmm x . 3 0.4 Pw
a.
= (5 .4 X .= 0.28
2. Check Reynolds number [Eq. (4-43)] b.
0.14 m/s x 4 x 10 - 4 m x 99S.2 kg/m3 --~-1.002 x 10 -' j N s/m2
Re
,
=
=
= 3.
1
f
,'
,
847 x
= 847
3
+ 47.4 112 +
x
10- 2 x
4/3 x 9.81 m / s2 (2650 -'998.2) kg / m 3 x 4.0 x 10= - -- --------...-- . . 1.28 x 998.2 kg/ m 3'
4
141)1 /2
m!s
O.SS x 998.2 kg!m 3 x v,
=---
- -~ -----
3
*
1', X .
dij
o.n
x 1.41
24 - - --.
C
329
D -
3
+ .---c+ 112
0.34
Pm - p". dij
10 3 111m x - .. -~ .. 111m
329
= 0.58
I.',
=
--l
~ X
_,
' .2
9.81 m/ s .. - -- ..
. PH"
= (2.158
x lo- 3
_
~
~12 __
S2. nlln
CD = 1.85 = 0.07 mis
x 10 N si111 <
111 0'S X "ij
. --.--- -~-~ . -
1.002 x 10 -
5. Repeat steps 2. 3. and 4.
l',
O.4l
mm
m
d.
Re = 26.6
=
m _ )I /2
m/s
c
v, = O.OS mls .
3
= 329 (transitIol1al flow)
0.34
= 1.28
4.
(/>1' dp/I.l
47.4 (transitional flow) 24 CD = 4- 7-.4
10 -
m)2
.1.0: ] N s/m2
= 0.14 m/s (rounded)
Rc = 0.85 x
12
9· 81
=
on m/s
--;-
(D
(!,j Illnl) I , ' C/1
:l
m/mm .
ENGINEERED SYSTEMS FOR WATER PURIFICATION
175
174 WATER
The repetitive nature of the above example suggests solution by computer. The principal cleaning mechanism in backwashing filters is hydrodynamic shear. which tears adhered material away from medium grains. While increased backwash velocity might increase this shear. the resulting expansion could result tn several undesirable effects. Jets of water aimed at the surface of the filter and/or mechanically powered rakes are often employed to create turbulence in the expanded bed during backwash. In addition to increasing the shear forces without increasing backwash velocity, these operations also promote collision of media grains, with the inherent abrasion assistiJ1g in the cleaning process_ Another technique, air scour. is also use;;ful in increasing shear forces in backwashing filters. Air is introduced along with the backwash water and creates additional turbulence without substantially increasing expansion. Cleasby et al. [4-22. 4-19. 4-21] and Amirtharajah [4-5] have shown thai air scour at su bfluidizing water flows may provide more effective cleaning of granular-medium filters.
e. Repeat steps b, c, and d using * expansIOns. Final solution is: Re = 274
CD = 0.61 v, = 0.22 m / s
f
Determine expanded porosity of layer by Eq. (4-47).
e
=
lb
(~~_ )022
=
(~~ _ ~_~~m/s)o.12
v,. ij
v,. iJ
= 0.55
Xij!(1 _(~~)O.21)
~~_.
=
I
V,-ij
=
002
0.55
2. Repeat all pr'eceding steps for each layer of particles. Again, * expression can be used directly with proper values inserted. The results are tabulated below
Average part icle
1.41 I 13 078 0.66 0.55 0.;:16 0.42
V'ii'
m/s 022 0. 19 0.14' 0.12
0.10 0.08 0.07
X 'j
-u-~r21
(-~~r"
I
0.55 0.57 0.61 0.63 0.66 0.69
0.45 0.43 039 0.37 0.34 0.31
lUI
0.29
Xi}
t'.ij
{;r.IJ
0.01 0. 11 0.20
0.32 0.21 013 0.02
" - ______(--~'-j'7')"--0.7C22 = l~~
- (~ir 2 /' 1 i j
0.02 0.26
051 (U6 0.62 0.42 0.07
2.76
-
Filter Components A tyrical granular-medium filter system used in water treatment was shown in Fig. 4-28. Filter components include the containment structure (filter box). an underdrain system. and filtering media. Additionally. piping systems. pumps, valves. backwash troughs, and other appurtenances for controlling the flow of water 10 and from the filter are necessary. Filter box Containment structures for filters are usually constructed of reinforced concrete, although corrosion-resistant steel or other suitable material may be used: StructoraHy; the' filter'bux must· be 'strong'enough ·to -suppor-t..t·he wetght·of.· -... .......... _.. the underdrain system, filter medium, and water column. Additionally, the structure must be watertight at pressures corresponding to the height of the maximum water column expected. Usually square or rectangular in shape. filter boxes are arranged facing each other across an access corridor containing common piping and other appurtenances. If more than two filters are necessary. a series of multiples of two provides the economy of common walls and minimized piping, These filter galleries. as they are commonly called, are usually enclosed in suitable hOllsing with the controls located for central operations.
3. The expanded bed depth is found by Eq. (4-49).
LIb =' L(l - e) x
--(:--'~--"i:-')-'(O-)1""2 1.
=
Vl.ij
l.24 m
and 1.24 - -- x 100 = 165 ~~ , of original bed depth 0.75
Underdrain systems The purpose of the undcrdrain system is to collect and remove the tiltered water and to disperse the backwash water. Underdrain systems in tilters may consist of built-in-place main and lateral pipe arrangements or of proprietary units manufactured elsewhere and assembled on site. Figure 4-29 shows several types of underdrain systems. Many systems of this type require a graded gravel packing to prevent loss of filter media into the underdrain system. Figure 4-29a is illustrative of the sizing of the gravel. No gravel packing is required for underdrain systems such as the ones shown in Fig_ 4-29c and 29d. These
ENGINEERED SYSTEMS FOR WATER PURIFICATION
. : .. ~:-l--------'---
•....
Dispersion orifices Control orifices
e"
.f C)
Level up top of depressions with gravel
~~~~~~~~~
75'0101 spheres 30-0101 spheres 35-0101 spheres Ca)
Cb)
Clay tile filter block
Camp nozzle bolt
~rl~:::;'( Camp nozzle cap Gasket
Coupling
Nozzle assembly
(e)
Slotted
?"~~2/:;:;;?7'- nozzle
metering slot
Back wash
i i- '
water
Figure 4-29 Proprietary filter underdralns: tal BIF, Unil of General Signal Corp., (h) F. B. Leopold Company, subsidiary of Moeller Company; (el Walker Process Corp.; Cd) Inlileo Degremont. Inc.
L
176
177
Filter medium
systems have slit openings which are too narrow for grains of filter media to pass through. Underclrain systems of all types contribute significantly to heacl Joss clue to friction during filter runs and during backwash. Hydraulically, underdrain systems must be deSigned to handle backwash flow rates, which usually exceed filter ing rates by at least a factor of 2. An excellent discussion of underdrain design is presen ted by Cleasby. [4-17J Filter media Traditionally, silica sand has been the medium most commonly used in granular-medium filters. Modern filter applications often make use of anthracite coal and garnet sand in place of, or in combination with, silica sand. The important properties of these materials are size, size distribution, and dens i ty. The sm;]ller the size of granular media, the smaller the pore openings through which the water must pass. Small pore openings incr~ase filtration efficiency not only because of straining but also because of other removal mechanisms. However, as size of pore openings decreases. head loss through the medium increases, resulting in a diminished flow rate. Larger media increase pore size. reduce head loss. and increase flow rate. but at a sacrifice of filtration efficiency. Since large quantities of filter medium of any uniform size would be difficult to obtain ami therefore quite expensive. filter media vary in diameter within a selected size range. In modern filtration practice, the effect of varying size ranges becomes important because of stratification during backwashing operations. When the bed is expanded, small grains are lifted farther than larger grains and settle more slowly once the wash cycle is ended. Thus, a bed of nonuniform medi um will stratify with smaller particles. and therefore smaller pore openings. at the top, an inetncient arrangement because most of the removal and most of the head . 1655 d lli'ing the' filit:
178
ENGINEERED SYSTEMS FOR WATER PURIFICATION
WATER
filters. Cleaning was accomplished by periodically (usually no more frequently than once a month) draining the filters and mechanically removing the top few centimeters of sand, along with the accumulated solids and the biological mat. Slow sand filters have large space requirement and are capita l-intensive. Additionally, they do not function well with highly turbid water since the surface plugs quickly, requiring frequent c leaning. The rapid sand filter was developed in the mid-1800s to alleviate th ese difficulties. Rapid sand filter The rapid sand filter utilizes a bed of silica sand ranging from 0.6 to 0.75 m in depth. Sizes may range from 0.35 to 1.0 mm or even larger. with effective sizes from 0.45 to 0.55 mm. A uniformity coefficient (60 percent less than size/ IO percent size) of 1.65 is commonly specified . These larger sizes, coupled with frequent cleaning and the absence of a biological mat, result in a rate of filtration an order of magnitude larger than that of the slow sand filter. Common filtration rates in rapid sand filters range from 2.5 to 5.0 m j h. An important feature of the rapid sand filter is that it is cleaned by hydraulic backwashing w ith resulting stratification of the medium. Filtration of relatively clean water presents few problems. However, filtration of turbid water necessitates frequent backwashing. Coagulated water with large, strong Hocs cau ses binding at the fine-grained surface and results in a rapid buildup of head loss, necessitating frequent cleaning. This situ ation co uld be a lleviated if the gradation of the filter could be reversed so that larger grains were deposited on top with media of progressively decreasing size below, so that the sma llest grains were on the bottom. Such an arrangement would mean the large pores on top would retain mostly larger suspended material, while subsequent ly smaller pores would retain succeed ingly finer material. Thus, the entire depth of the filter would fun ction efllcientiy. . .ancllarger ..volumes.Qf .sllspended. solids..could.be .retained·between. backwashes. The overall result wo uld be longer filter runs, less head loss, and greater filtering rates. By carefu l selection of medium with regard to size and den s ity. it I S possible to approximate this reverse gradation. Dual-media filters do this to some ex tent, and mixed-media filters essentially approximate reverse gradation. Dual-media filters Dual-media filters are usually constructed of sil ica sand and anthracite coal. The depth of the sand may range from O. J 5 to 0.4 m, with the coal depth ranging fr om 0.3 to 0.6 m. Size and uniformity coefficients of the two media can be selected to produce either a distinct separation or a given degree of mixing after backwashing. These conditions are illustrated in Fig. 4-30. As an example, the foll owing material would pl'oduce a filter 0.6 m deep with approximately 0.15 m of intermixing. [4-17J
- - -- - - -Deplh.
m
Speclric gra\'il, Efleclive size, I11Ill Unifo rmity coefilc it.: l)t
Sand
l o,il
o\
(U
2." 5 () 5 (I
l..j )'
< I .f,;:;'
I.()
11') If) / I .X
179
.c 0.
o'"
(a)
.c 0.
o"
-
(b) Figure 4-30 Size gradation in dual-media filters: (a) sharp gradation and (0) partial mixing .
The large pores in the anthracite layer remove large particles and flocs, while most o f the sma ller material penetrates to the sand layer before it is removed. Dual-media filters thus have the advantage of more effectivel~ utilizing pore space for storage. This results in longer filter runs a nd greater filtration rates because of lower head losses. A disadv'lntage of dual-media filters is that the filtered material is held rather loosely in the anthracite layer. Any sudden increase in hydraulic loading dislodges the material and transports it to the surface of the sand layer, resulting in rapid binding at thi s level. Mixed-media filters As noted ea rlier, the ideal filter would consist of a medium graded evenly from large at the top to small at the bottom. This can be accomplished by using three or more types of media with carefully selected size , density, and uniformity coefflcients. A typical installation might consi.st of a 0.75-m bed with 60 percent anthracite, 30 percent si lica sand, and IO percent garnet sand, with specific gravities of 1.6, 2.6, and 4.2, respectively. 'Effective sizes ranging from a maximum of 1.0 mm for the anthracite to a minimum of 0.15 mm for the garnet, coupled with carefully selected uniformity coefficients, will produce intermixing and result in a pore-size gradation as shown in Fig. 4-31. [4-24J
...-'
180
ENG I NEERED SYSTEM S FOR WATER PURIFICATION
WATER
Thus, the mixed-media filter (perhaps" mi xed- up " media is;) mo re descriptive term) approaches an idea l filt er. Filtra tion rates range from 10 to 20 mj h, considerably higher tha n rapid san d filter s a nd about the sa me as for dual-media filters. Thereve rse gradation avo ids t he major problems of eac h of these medi a. however. Dual- and mixed-med ia fi lters make possib le the direct filtration of water of low turbidity without sett lin g opera ti ons. Coagulating che micals are often added to the influent of the filter to produce small. stro ng flocs to enha nce turbidit y remova l.
TOPrr~:----------------r---'
\" \
'\.
\
//1 /'
Sand
\1
'\
'
.
Bot tom '-":.......<:.....,:---_ _ _ _ _ _:..-..C~_ Particle distribution. 'If
Filter Operation _I
(a)
...... . . ....... ... ..... .. . . ........ -.
(b)
or
181
Figure.4-31 Size gradalion mix:d-mcdia fillcr: (a) panicle di,lriblllilln and pore media segrega led ancl mixed by backwashing ICOllrlC.IT 01 Nep/III1{' ,\IlI·rutloe. 11Ic.).
SIZC
and (b) multi-
The two basic modes of ope rat in g granular-med ium fillers a re ( I) constant head - vari able flow and (2) cons tant tlow- variable head. These two modes are often modified to obta in better res ult s. In the co nstant head -- variable flow mode. the water level above th e fi lter is kept at a prese lected level. Si nce a clean fi lter bed presents limit ed head loss. the flow rate will be ljuite large . As the filter becomes clogged. the head loss inc reases and the flOW rate dill1inishe~. When the fl ow re~lches the design minimums, th e Iilter must be backwashed. Because yerl' rapid !low through a clea n filt er results in poor effic ienc y. throttling the fl ow from the I-ilter with a !l ow-control va lve may be necessa ry This va lve is dcsigned to prov ide add iti ona l head loss in the underdrain system and decrease the flow rate to an acce pt ab le leve l. When the filt er medium is clean, the valve operates with a small o peni ng to produce a large suppl emental head loss. As the head loss increases due to medium plugging. the va lve gradu a ll y o pens to dec rease supplem ental head loss ane! to maintain a more o r less constant head loss across the ent ire system Th e resu lt is essenti a ll y a constan t heact-'- cbil"slah"i' 1I0w filte r. If wa ter is introduced inti) a clea n filt er at a constant rat e. an equilibrium will be estab li shed between the height of the wa ter column and th e application rate. At first. the head will be low due to th c minimal hea d· loss in the medium _ As th e medlllm beco mes plu gged ~I n d glTate r head loss occurs. the heigh t of the water co lumn m'ust incrcase to pro\'ide the needed1.lriving·force. When the wa ter co lumn reac hes a predeterm in ed Ic\eL the filt er is back washed and the cyc le repea ts ilself. Filters that o pera te in this wa y must be des igned to preve nt dewatering of the bed duri'ng the ini tial filt er cycle. i\ minimum depth of water ilbove the bee! ca n be asslired bv ele\atinl! thc entran ce tll the c lear well abo\ e the surface of th e fi lt er media.' , Mure n;ce nt desil!n ()I" !;Iruer lil ter plailts usua ll y makes use uf a cOlllblllation of tile dh()\"C modes Ill' [) l'er: lt l~,n . /\ consta nt now is delivered to a bank of severa l lilters tltroul!lt a 'Cllm11l011 lte:tder ~!nd is a ll owed tIl distrib ut e It se lf accordinu to the ()pel:ltin~ rate 1)1" c;;c h ind l\ idll a llilt e;·. Th c hcighi (ll" the water col uilln is' the Sdille' ~Ib(\\e :111 the lilter lIllltS. \l'ith the c!c:lncs t filter :Icccpting thc greatest 1I0w. Whcn thc !lo\\' ra te thrlllluh :111\ (.lne unit decre~ l ses tl) a predctermincd IC\'eL that fi llt;r IS taken oIT-lint: :I nd [)ack \~' ashcd. I\ C1l1U1 al pf (we liltCl" I"(su lt, in an increase
182
WATER
ENGINEERED SYSTEMS FOR WATER PURIFICATION
in flow to th e remaining filters, with a subsequent increase in head and flow rate through each filter. When backwashing is comp leted, the newly cleaned filter is returned to service and will accommodate a larger flow ra te. Water level will therefore drop slightly in a ll the filters, resulting In a decrease in flow through each filter. Regardless of the operating mode, a uniform flow rate is essent ia l to the best performance of a granular-medium filter. Any increases in flow rate must occur gradually, or the quality of the effluent will deteriorate. Large changes occurring quickly produce the greatest degree of deterioration. When automatic control valves are useu to (egulate filter output they must be maintained to ensure that they produce gradua l changes in the orifice opening. Otherwise, a rapid c hange in flow rate wi ll occur. with signif ican t deterioration of the effluent quality. As noted earlier, fluctuations in filtering rates occur in variable-declining-rate filtration each time a filter is taken off-line for backwashing. The magnitude of the fluctuations increases as the number of filter units in the system decreases. To prevent sign ificant disruptions in filter q ua lit y. a minimum of four filters shou ld be used in this mode of operation. [4-18J
183
._.
1900
1910
1905
1915
1920
Figure 4-32 Typhoid fever and treated water supplies during two decades. (From Vesilind [4-56].)
4-9 DISINFECTION As practiced in water treatment, disinfection refers to opera ti ons aimed at killing, or rendering harmless, pathogenic microorganisms. Ster ilization. the comp lete destruction of all living matter, is not usually the objective of dislIlfection. The effect of disinfection on the reduction of waterborne disease is quite dramatic, as evidenced in Fig. 4-32. [4-56J Other water-treatment processes assist in removing pathogens. irie'xcess' 6f" 90 percent of the bacteria and viruses should be removed by coagulation. sett ling, and filtration. Excess-lime softening is a~ effective uisinfectant due to th e high pH involved. However, to meet the EPA's standard o f one coliform organ ism per 100 mL and to provide protection against regrowth, addit ional disinfection must be practiced. , . . A good disinfectant must be toxic to microorgani'sms at conce ntrati ons well below the toxic thresholds to humans and high'er animals. Additiona ll y, it should have a fast rate of kill and should be persistent enough to prevent regrowth of organisms in the distribution system. The ratc of kill is often postulated as a rirstorder react ion:
Complete disinfection cannot be accomplished because Nl' the number of organisms remaining at time 1. will only approach zero asymptotically as time gets excessively large, However. since the number of organisms initially present (No) sho uld be small, 99,9 percent kill can be affected in a reasonable time. The value of the constant k must be determined experimentally. Factors which militate against effective disinfection are turbidity and resistant .. 'organisms:' TiJroiaiiy:pYod'titliig" to'lloids" 'offer"'sancwgty' to' "organis-m's, .. thus" sh ielding them from the full action of the disinfectant. Particulate matter may adsorb the disinfectant. Viruses, 'cysts. and ova are more resistant to disinfectants than are bacteria, AdditionaJ exposure time and higher concentrations of the disinfectant will be required for an effective kill of these organisms, Disinfectants include chemical agents such as the halogen group, ozone, or si lver: irradiat ion with gamma waves or ultraviolet light; and sonification, electro:': cut ion, heating. or other physical means, In America, disinfection and chlorination have become synonymous terms. while ozonation has been practiced more widely in Europe.
Chlorination liN
- k {\'
Chlorine may be applied to wa ter in gaseous form (CI 2 ) or as'a!) ionized, product of solids [Ca(OClh. NaOel]. The react ions in water are as follows:
rif
Ci 2
+
H20
- -- t
Ca(OCl) 2-----> (4- 50)
NaOCI
- - -->
H+
Ca Na
+
2f
HOC!
+ 20CI -
+ OCI -
(4-5 J) (4-52)
(4-5 3)
184
ENGINEERED SYSTEMS FOR WATER PURIFICATION
WATER
The hypochlorous acid (HOCI) and the hypochlorite ion (OCI) in the above equations are further related by (4-54)
HOC]
a relationship governed primarily by pH and temperature, as shown in Fig. 4-31 The sum of HOCI and OCI - is called the free chlorine residual and is the primary disinfectant employed. HOCI is the more effective disinfectant. As indicated in Eq. (4-51), HOCI is produced on a one-to-one basis by the addition of CI 2 gas, along with a reduction of pH which limits the conversion to OCl - [Eq. (4-54)]. Chlorine gas can be liquefied by compression and shipped to the site in compact containers. Because it can be regasified easily and has a solubility of approximately 700 mg/L in water at pH and temperatures generally found in water purification plants, this form of chlorine is usually the preferred species. The application of the hypochlorites tend s to raise the pH, thus driving the reaction more toward"the'less effective Commercially available calcium hypochlorite
ocr.
185
contains approximate ly 70 to 80 percent avai lable chlorine, while NaOCI contains only 3 to 15 percent available chlorine. [4-53J Some practical difficulty is involved in dissolving Ca(OCI)z. and both hypochlorites are more expensive on an equivalence ba sis than liquefi ed C l z · There are other considerations. however. which sometimes dictate the use of hypochlorites. Chlorine gas is a ve ry strong ox idant that is toxic to humans. Since it is heavier than air. it spreads s lowly at ground level. Therefore, extreme care must be exercised in its manufacture. shipping. and use. Accounts of evacuations of pop ulated areas because of rail or barge accidents involving chlorine gas have become common news ite ms. The use of hypochlorites is often mandated w here large quant ities cif ch lorine are needed in treatment plants located in highly popula t ed areas. At loll' concentrations. chlorine probably kills microorganisms by penetrating the cell and rea cti ng with the enzymes and protoplasm. At higher concentrations, oxidation of the cell wal l will destroy the organism. Factors affecting the process are
100 90
~
o
f\..
to
't
80
20
\\ \\
70
60
2. 3. 4. 5. (,.
30
40···· ............ . . ....... . ..... . .
.
!.-. U
v 0 :r: 50
Hypochlorous acid is mOle erTectlvc than the hypochlorite ion by approximate ly tw o orders of magnitudc Beca use the free-chlorine species is re lated to pH, one .. \v(!lI10..eli.p.ec.t.a. relalionship. between .efficiency and pH. Empirically. it has been found that ch lorin e dosa~es mu st be increased to compensate for higher pH . ('hh)rine concentrati~)n and contact time relationship is often expressed by
50 0
20
0
e
40
( ,0
\O°C
:30
\~
20
70
,
110
~
10 0 4
Form of c hlnrillc pH COllcent ra t ion Contact timc Type of o rgan ism Temperature
6
8
7
where C= concentra ti on of ch lorine, mg/ L 11' = tim e required for given percent kill , min n. /; = ex pe rimental derived constants for a given system An ex ample of thi s re lation s hip was reported by Be rg and is s hown in Fig. 4-34. [ 4-49J The c!Teets of temperature va ria t ion s can be modeled by the following equation deriled from the I
90
I
III ..- = I,
~
9
I 00 10
II
pH Figure 4-33 Distribution of HOCL and OCL as" function o f pH . (F,om .\mrycr (tIlll Mclartl" [4·481-)
(4-55)
C"l" = k
where
1 1.1,
£,(Tz - T
I) .-.----~
(4-56)
.R TI . T2
= tIme required for gil'en kills
7"1 "/ : =
temperature corresponding to
11
and
[2'
K
R "" gasconstanl. 1.0cal/ K-mol I-. ~ activ:ltion energy. related to pH (as shown in Table 4-4)
WATER
{,tJ
....J
0.4
I
I
E
-
...:
'"
::l '0 ''';
E
1
~..ti'<.
I chlmo-organic I
compou nds not destroyed
compounds ~----!~~-------,,,
Formati on of chloro-organic compounds and chI o ra mines
0.3
~
~
c:
_f("
Destruction of Formation of free chlorine and chloramines and presence of chloro·organic
I by reducing " I com pounds
cO
....J eo
Destruction of cht orine residu al
0.5
8
~
~~t,c?'
~ ~" J}r~'~'Th' "U''''C'''ON 187 ( ', V'
186
J&C
0. 10
t>
c
u
.2.c
:r:
U
0
0.2
0.1
o
1.0 Chlorine added, mg/L
Figure 4-35 Gene rali zed curve o btained during breakpoint chlorination. (From Metcalf & Eddy, ll1c.
[4-40].) Contact ! illle for
9 9~j
ine to form severe taste and odor problems. The original organics must be removed before chlorination, and undesirable compounds must be removed after chlorination, o r the co mpounds mu st be prevented from forming. The compounds can be re mo ved by adsorption ont.o activated carbon, or their formation can be prevented by the substitution of chloramines, which do not react with the organicsor phenols, for tree chlorine. Chloramines can be formed by first adding a small quantity of'ammonia to the water. then adding chlorine. The reactions of chlorine with ammonia are as follows:
Figure 4-34 Concentration 01' free residual chlorine and cuntac! lime necessary fo r 99 pe rcent kill at a nd 6"C (From Schro" ier [4-49l)
o
Being a s tr o ng ox idant. chlorine will react wilh almost any material that is in a reduced stat e. In water, this us ually consists of Fe " + , Mn " r , H 2 S, and organics. Ammonia (NH)) is so metimes prese nt in small quantities or ma y be added for purposes to be presently discussed. These o xidi za ble mate rials will consume . "c hlbfiile 'befo're it hi s'
a'
NH3 NH 2 C1 NHCl 2
C. .
'cal
~ .40()
I ~ , O ()U I S.!Jf.JO
10.7 .\'0/1/"(' ('
c! ,iI . [ 4-3 UJ
I· r ~ )Ill
t:;l1r
HOCI
-->
HOCI HOC!
NH 2 C! (monochloramine) NHCl z (dichloramine)
-->
+
+
H 20
(4-58)
H 20
NCI 3 (nitrogen trichloride)
+
(4-57)
H 20
(4-59)
These reactions are dependent on seve ral factors, the most important 'ofwhich
8.200
8.5 9.8
+ + +
~re pI-I, temperature, and reactant quantities. At pH greater than 6.5 mono-
Table 4-4 Actiyation energies for aqueous chlorine pH 7.0
.
kill, !nin
~
chloramine will be the predomitl
188
ENGINEEKEI ) SYSTEMS FOK WATEK PUR IFI CATION
WATER
added just pri o r to filtration to keep algae from growi ng at the medium sur face and to pre ven t large populations · of bacteria fro m developing w ithin the filter medium. . Safe and effective app li ca ti on of ch lorine requires spec ia li zed equipment and considerable care a nd sk ill o n th e part of the plant operat o r. Liquefied ch lorine is delivered to water-treatment plants in tanks con taining anywhere from 75 to 1000 k g. Large plants m ay be designed to a llow use o f chlorine directly from a tank car. In su c h cases, designers sho uid be aware of the Inters tate Commerce Comm ission (I CC) and Occupational H ealth and Sa fety Agency JO HSA) regu la tions for sh ippin g and handling ch lor ine. Mixing is o n e o f the most impor tant aspect s of th e c hl or in a ti on process. [4-40J A s uffi c ient ve loc ity g ra dient mu st be applied to ens ure uniform co ncentrati on o f ch lorine thr o u g h o ut the water and to break up a ny remaining flo cc ulent material that might shield mi c roorga ni sms from co nt ac t wi th th e c hl or ine. Any of the rapid-mixing devices discussed in Sec. 4-6 may be u sed for thi s purpose. A con tac t chamber mu st be provided t o ensure an adequate k ill ti me. In water- trea tm ent plant o p erati ons. mixing 'and con tac t ope r a ti ons m ay be accompli shed by sec ti o ning off part of t he clea r well. Safet y considerations m a n da te storing of.ch lo rin e tanks in a se parate rool11. S torage a nd oper a tin g rooms sh o uld not be direc tl y co nn ec ted. nor sho ul d th ey be directly connec ted to o ther enc losed areas of the trea tm ent plant. All doors to th ese faciliti es s h ou ld b e open to th e o ut s id e. and wi ndo ws sho uld be provided for visual ins p ec ti o n from the o ut s ide. Safety eq uipm ent , including ma sk s w ith a ir tank s. c hl o rin e detecti o n de vices. a nd e m e rge ncy re pair equ ipment: s ho uld be provided in stra teg ic loca ti o n s.
Otber Means of Disinfection Given the pro blems associated w ith c hl o rinati on. it IS n o t surpri sin g that a sea rc h for a substitut e means of di s infec ti on has been in progress for yea rs. H owever. t he lis t of candidates fo r replacement remain s q ui te s malL w ith ch lOl:ine dio xid e and ozone bei ng the leading conte nders. A lt hough both o f !hcse are effec ti ve in. destroying pa th ogens, ozo ne does not leave a disinfec ting residual that can guard aga in st pathoge n regrowth in th e distribution sys tem , a nd both are more expensive th a n c hl o rin e :md have prac ti ca l problems assoc iat ed w it h th eir use.
Ozone O zo ne , the a ll otropi c form of oxygen. ca n be produced in a hi gh-strength e lec trical field from oxygen in pure form or from the io ni zation o f clean. dry air. high
O2
- ~~~
0
+0 (4-60)
Ozone is a powerful ox id ant w hich react s with red uced in organic compounds and wi th o rgan ic material. The difference. h oweve r. is that a n c1xygen ,! tonl.
189
ins tead of a c hl OI'ide atom. is added to the o rga nics. th e elld res ult being an e n vironmenta ll y acceptable corhpo und. Once thi s ozo ne demand has been mel. th e ozo ne react s vigorous ly wi t h bacter ia and viru ses. It is repo rt ed to be more effec tive than c hl orine in ina ctiva ting resi stant strains of bacteria and viru ses .
[ 4-58J . Becau se ozone is chem ica ll y unstable it mu st be produ ced on-s ite and u sed 3 immediate ly. Typica l dosages range from 1.0 to 5.3 kg/ IOOO m [4-58], with powe r consum pt io ns of from 10 to 20 k W · h/ kg of ozone [4-53J Cost o r ozona tion IS two to three tim es h ig her than t he cost o f ch lor in atio n . Since no res idual remain s; i.l lVil l be necessar y to use a s ma ll amo unt o r ch lor ine ofler ozonatio n to provide con t inued pr()te~ ti ()n again st regrow th in th e di stribution syste m . Becau se ozo ne has a low solubilit y in water. it must be mixed th o ro ughly wi th the wa ter to ensure adequa te co nt act. Thi s ca n be a prob lem when air is used as the ox yge n sou rce. s ince large vo lu me s o f ni trogen mu st al so be hand led. In spite of t hese proble m s. ozo ne is w id ely used in Europe for disinfecting water con ta inin g co lo r
190
WATER
ENGINEERED S YSTEMS FOR WATER PURI F ICATION
A va ri e ty o f o th er disi nfecti o n methods may be u sed in specia l circumstances. Th ese includ e o ther halogens (iodin e. bro mine). m e tal s (copper, s ilver), o th e r ox id a nt s ( KMn0 4 ) . so nifi ca ti o n . e lec tri ca l curr ent. a nd gamma- ra y ir radiation. It is unlikel y, howeve r. that any o f these processes will find widespread use in di si nfec tin g pu b lic wa te r s uppli es in th e foreseea ble future.
Other Water-Treatment Processes Th e water treatment proc~sses di sc ussed in the prev ious sec tion s o f thi s chap te r a re suffici e nt to rend er mo st natura l surface water or g ro undwa ter p o tabl e. In some ins tances, howeve r. the wat er suppl y may co ntain materials that a re not rem oved by th e co n ve ntional water-treatment processes. Examp les ar e g ro und wat er with ex cess ive disso lved solids and surface waters that contain orga ni c compounds fr om d o m es ti c o r in dustria l wastewaters o r naturally occu rrin g o rga nics s uc h as humi c a nd ful vic acids or produc ts o f a lgal bl oo m s. Processes are a vailable fo r removing the se c o nt a minants. Th ese processes in vo lve so phi s ti ca ted equipm e nt. require h ighly skill ed o p e rato rs, a nd a re th e refo re qui te expens ive. Th e ir u se in p o tabl e wat e r pre parati o n s hould be co nsid ered o nl y wh en a bett erqualit y wa ter supp ly is no t a vai la ble. The fo ll owing sec ti o ns w ill di sc u ss processes fo r re m ov in g in organ ic and o rga nic di sso lve d so li d s from water inte nd ed fo r p o tabl e use. These sa me processes m ay ac t as te rt iary trea t ment fo r wastewater wi th so m e m odi ti ca t io n . Th e discuss io n is arran ged according to targe t co ntaminants rath e r than process type.
4-10 DISSOLVED-SOLIDS REMOVAL Targe t con tamina nt s ill disso lved-so lid s re mo va l processes may be Inorgani c min e ral s or refr ac to ry o r g~\Ili c compounds. Severa l processes a re av a ilabl e for reduc in g the leve ls o f th ese co mpound s in' wa ter intended for potab lc use. and process se lecti o n mu st be based o n eco nomi cs and d e pendabilit y.
Inorganic Material Demil1E'ralizmiu/i :lIld desa/inizatiol1 are sy n o n ym o us terms app li ed to th e removal o f in o rgani c rntn era l subs tances from water Thi s is mo's t uften ;I cco mpli s hecl by se lective. s taged io n- excha nge unit s o r bv processes e mpl oy in g the use of se mi permeable mem bra n es. Bo th proced llTes req uir e vir tuall y co mpl ete re mO Ved of sus pend ed so lid s prior to their app lication. Ion exc hange The pr in ciples o f th e io n -e xchange process we re desc ribed in Sec. 4-7 as re lat ed to water so ft e n ing. In th ci t process. sod ium ions we re exchanged for ca lci ulll and m3gne s ium io ns o n a n eqUiva lence basis and th e l'e was no ne t d ec rease In di ssolved so lid s. F o r d e miner a li zat io n . h owever. th e e xchan ge d ion s mu st not co ntribut e disso lvccl so lid s to the e fllu en t Thi S IS accomplished by
191
exc hangin g h ydr oge n for the di sso lved cations and hydroxide for the dissolved anio ns. The tw o th en co mbine in equal amounts to form H 2 0, leaving no residual ane! not affecting the pH. The resins are rege nerated w ith acids and bases , respective ly. The io n-exc h a nge process mu st be carried out in two or more steps. Generally, the cations a re removed fir st, followed by the an ions. The process and related chemical reactions are shown in Fi g. 4-36 . Becau se completely demineralized wa te r is und cs ira ble , a porti o n of th e wat e r is bypassed and blended with the process effluent to provide a stable water. Microporous membranes Deminerali za tion of wate r can be accomplished using th in. micro poro u s m embran es. Th e re a re two bas ic modes of operation in use. One sys tem uses press ure to drive water thro ugh th e membrane against the force o f os motic press ure and is ca lled rerene osmosis. eve n though the pressure applied is seve ral o rders of m ag nitud e in excess of th e nat ural osmo tic pressure . The other process. called electrodialysis, use s electr ica l force s to drive ion s through ionselec ti ve m embra nes. Th e me mbr a ne com m o nl y used in reverse osmos is is composed of cellulose aceta te ane! is ab o ut 100 tun thick . Spec ial tec hniques of casting result in an asymme tr ic arrangement, wi th o nc s id e of th e m e mbran e having a thin (0.2 11m), dense fi lm. wh ile th e re mainder is more p o ro us. The film con tain s microscopic o penin gs that allow water mol ec ul es to pass thr o ugh but reject di sso lved so lid s by either mo lec ular sievi ng o r by so m e o ther m ec ha ni s ms n o t yet co mpletely understood. r4-25 ] The process res ults in a concentrated so lution of th e ion s o n the pressure side of th e membrane and a pro duct water wh ich is relat ive ly free o f io n s. Three basic membrane configuration s are used in reverse-osmosis sys tems. .. Th-ese are' t he 'spira: IC\vo lt'nds'ys!crn' (Fi g~ '4': J7a)~ 'iIi 'wiiich 'r-rleri-ibra r1es' imd support materi al are placed in a lt ernate layers. roll ed into a cylindrical s hape, and placed in tubes o f suit ab le mat eriaL The supp o rt m a terial is porous and serves as a tran sport medium for the liquid strea ms. Separat ion o f th e product wa ter and con centrate is accomp li s hed by int ernal a rrangement within the containment tube. T ub ular sys tem s (Fig. 4-37b ) are avai la ble in w hich the m e mbrane and its porous sup po rt sys tem are fo rmed to fit ins id e a co ntainment tube of up to 125 mm in dia meter. Product wa te r is w ithdra wn from the p o rou s support medium, while the co ncent ra te passes thr o ugh t he core of th e membrane. H o ll ow-fiber membranes (Fig. 4-3 7c) are extreme ly sma ll tub es. diameters of 1.0 pm o r less being common. T he la rge wa ll-thi c kn ess- to-diam ete r rati o provides a good radial s tren gth, and the fibers ca n be suspend ed in the fluid w ithout the use of the s upport medium. Th e feed wa ter is usually 011 th e o ut side of th e fib er. w hil e the product wa ter is withdraw n thr o ugh th e cen ter. ' . The spira l-wound and hollow'- fib e r systems' genera ll y provide higher fl ow rat es but are m ore susce ptib le to fou lin g than are th e tubul ar systems and are mo re o ften used fo r d emin erali z in g po tabl e water. Tubula r unit s are better su ited fo r was tewa ter trea tm ent because membrane fou ling ca n be minimized by increas in g th e fl ow ra te thr o ug h th e tu be.
ENGINEERED SYSTEMS FOR WATER PURIFICATION
193
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181
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194
ENGINEERED SYSTEMS fOR WATER PURIFIC ATION
Wi\TER
Flu x ra tes of a pproximately 0.1 m J j m 2 . d are typical for spiral -wo und a nd tubul ar systems, with considerably lower rates for the hollow-fiber units. However, mu ch more membrane surface area is availab le in m o dul es packed with holl ow fib ers than in comparably sized spiral-wound mo dules, and product water per mod ule unit is approximately the same. Modules a re placed in parall el to provide the necessa ry ca pac ity and in series to increase efficiency. . Reverse osmosis systems can operate a t 90 percent efficiency or better with respec t to to tal di ssolved solid s. In addition to inorganic ion s, the membranes a lso rem ove res idual o rganic molecules. turbidit y. bacteria, a nd viruses. Th e electr o dial ys is process uses a se ri es o f membran es made fr o m ion-exchan ge resins. T hese membranes will selective ly tran sfer io n s. One membra ne is cati o npermea blT.""that is, it wi ll pass cation s but wtll~ons. while the o th er membra ne is an io n-permea ble and rejects ca ti ons. Wh en p a ra llel c hanne ls are const ru cted by a lterna tin g membranes and an electrica l cur rent is passed across them, an e lectrodia lysis cell is formed as shown in Fig. 4- 38. Ca tions are drawn towa rd the cat hode, passing thr ough the cation-se lective membrane but being stopped by th e anion -selective membrane. The o ppos ite ac tion occurs with a ni ons, resu ltin g in io n s being rem oved from one channe l a nd co ncentrated in the adjoinin g chan n el. Me mbran es in elec tr od ialys is unit s are approxima tely 0.5 mm thick a nd are separated by po ro ll s spacers about I mm thick . Water flow s through th e porous spacers. Severa l membran es and spacers a re sa ndwi ched together into one electro dialvsis ce iL 'A contac t time o f 10 to 20 s is required wit h rem oval effic iencies o f abollt 25 - 60 percent. [4 -25J Ce lls are pl aced in seri es to increase efficiency and in para lle l to meet tl ow requirement s. Under id ea l co ndition s. approxima tely 90 . . . . . . . .. .... ... . . P(oolicnv'it e r
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(From
Lllcey [4-]5].)
195
percent o f the feed water is deionized, with the ions being concentrated in the remaining 10 percent. Both reverse osmosis and electrodialysis require a high degree of trea tment prior to th eir application. Suspended so lid s removal is absolutely necessary, and dissolved organics should be removed to prevent fouling. Adjustment of pH to the slightly acidic ran ge may be necessary to prevent inorganic precipitation. Reverse osmosis generally produces a higher-qualiti effluent than does electrodialysis, although at a higher cost. D esign parameters for demineraliza tion processes are given in C ulp et al. [4-25J Both reverse osmosis and electrodialysis produce a waste stream that may range from 10 to 25 percent of the feed water. In potable water supply systems, an ad ditional volume of water must be processed to offset this loss. In both wateran d wastewater~treatment systems the concentrated wastewater streams must be disposed o f properly.
Organic Material Refractory organics can be removed from water an·d wastewater by adsorpti·on processes or by chemical oxidation. The processes are essentially the same for both water and wastewater trea tment, although the applications may · differ somewhat. Adsorption Adsorption can be defi ned as the accumulation of substances at the interface between two phases. [4-53J In wa ter a nd wastewater treatment, the in terface is between the liquid and solid surfaces that are artificially provided . The material removed from the liquid phase is called the adsorbate, and the material providing the solid surfaces is called th e adsorbent. ............ . The adso rbent most commonly used in water and wastewater trea tment is ac ti va ted carbon. Activated carbon is manufactured from carbonaceous material such as wood, coal, petroleum residues, etc. Achar is made by burning the material in the absence of air. The char is then oxidized a t higher ·temperatures to create a very porous structure. This "activation" step provides irregular channels and pores in the solid mass, resulting in a very large suiface-a rea-per-m ass ratio. Surface areas ranging fr o m 500 to 1500 m 2/g have been reported [4-53]. with a ll but a. small fraction of the surface area being associated with the pores . . Once fo rmed , activated carbon is crushed int o gra nules ranging fr om 0.1 to 2 mm in diameter or is pul verized to a very fine powder. Disso lved organic material adsorbs to both exterior and interi o r surfaces of the carbon . When these surfaces become covered , th e ca rbon must be regenerated . Although adsorption properties and mechani sms are essentially the same. ·applica ti on techniques for granular ac tiva ted carbon and powdered act ivated carb.on a re co nsiderab ly different. The contact system for granular acti va ted carbon (GAC) consists of a cylindrica l ta nk which contains a bed of th e material (Fig. 4-39). The water is passed through th e bed with sufficien t residen ce time a ll owed for completion of the adsorption process. The sys tem may be operated in e ith er a fixed-bed or mov in g-bed
196
ENG I NEERED SYSTEM S FOR WATER PUR IFICAT ION
197
WATER
F ull o pell cove r with portho le Bo lUing
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Figure 4-39 T ypica l activated-carbon ad so rpti on co lumn. (From M e/ccil! & Eddy, Inc. [4-40]. )
mode. Fixed-bed systems are ba tch o perati o n s th a t are ta k en o fT th e lin e w hen the ad s orp ti ve capac it y of the carbo'n is used up. Alth o ugh fixed g ranu lar carb o n bed s c an be c lea n ed in a place w it h s upe rheated steam, the most co mm on prac tice is to rem o ve th e ca rb o n fo r c leanin g in a furnace . The regeneration process is esse nti ? lI y the same as th e o ri ginal ac t iva tion a process. The ad so r bed organics ar e fir s t burned a t about 800 C ·in. th e absence o f oxygen. An o xidiz in g agen t, usually steam. is then a pplied a t s li g htly hi g he r temper a ture s to rem o ve the residu e and reac ti va te t he ca rb o n . In a moving-bed sys tem, spent carbon is co ntinu o us ly re m oved fr o m th e bottom of th e bed, wit h regenera ted carbo n bein g re p la ced at th e top. M os t modern app lica ti o n s u se th e m o vin g- bed sys tem w ith a co unt e rcurrent Aow; th a t is, the water is introduced a t th e bottom of the bed and moves upward again st t he Aow of ca rbon. The major proble m associa ted with g ra nular-acti vated -carb o n-contact systems is plugg in g o f the bed by su sp ended so li d s in the water. Provis io ns ma'y be made in the d esign of the ve ssel fo r backwa shin g the bed in a fas hi o n simil a r to filter backwas hing. O th er design s a vo id plugging by operat in g with the bed in a fluidized stat e. Sufficient upA o w ve loc ity is prov id ed t o m a intain th e heu at ah o ut
10 p ercent expa n sio n a t a ll tim es so sus p end ed solids in th e in tlu en t can pass throu g h. TIllS m ode o f o perati o n ha s a n added adv a ntage in that a d so r bed o rga n i c, II1 c rea se th e d en sit y o f th e carbo n , and th e spent carbon mi g ra tes to th e b o tt om o f th e Auiui zed bed fo r rem ova l to th e rege nerati o n process. D es ign o f g ranu la r-acti va ted-ca rb o n sys tem s is based on Ao w ra tes and co nt ac t t imes. Fl ow ra tes o f 0.08 to 0.4 m J j m 2 . min and cont ac t tim es o f 10 to 50 m i n based o n empt y- tank cross secti o n a nd vo lume, a re common p rac ti ce Dow nt i m~ o f up to 40 perce nt s ho uld be included in the pla nt capac ity, wit h 5 to 10 perce nt m a keu p car bo n be ing prov id ed aft e r eac h rege ne rati o n cyc le. T he int eres t ed read er is referred to C u lp, W es ner, and C ulp [4- 25 J for amore'de ta iled disc ussi o n o f d es ig n. . C arbo n co lumn s can be a rran ged in pa ra llel to inc rea se th e ca p ac it y and in se n es to In crease t he co nt ac t tim e. T o a pp r oximate th e co un te rcurre nt a ppl'oach m a se ri es o f hxed -bed co lumn s, wa ter. proceed s fr o m th e column whic h has been used th e lo nges t to th e o ne in use fo r th e sh o rt es t tim e. Powdered ac ti vate d car bo n (PA C ) ca nn o t be used in a fi xed -bed arra ngement becau se o f It s slll~ t11 size anu th e subsequ e n t hi g h head loss th a t wou ld result from pass in g wa ter th ro ug h it. Powde red ac ti va ted ca rb o n is co nt acte d with th e wa t er in o pen vesse ls w here it is maint a in ed in s us pen s io n fo r th e n ecessa ry Cllllt ac t tim e a nd th e n rem oved by co n ve nti o nal so lids- re moval processes . Fl occ ul a tio n eq u ip m ent descri bcd in Sec. 4-6 is suffic ie nt fo r thi s purpose. Powde red ac ti va ted ca rbo n is much m o r e diffi cult to regen e ra te th a n g ra nu la r. Mos t systems cmrl oy a flu id ized bed a rr a n ge m e nt in w hich a m ixt u re of steam Cln u (l th e l' ho t S~l ses ho lu s th e ca r bo n in s us p ens io n while th e rege nerat io n processes occur III so m e cases, sa nd is Auidi zed al o n g with the carbo n to help ho ld hea t in t he sys tem.
III was tewat e r treatmen.L. pow.d ef.ed. a ctiva ted .eacb.on. .can .be. a.d ued .. to. t11e. ae ra ti o n basi n a nc! re m oveu w it h th e bi o log ica l so lid s in the seco n darv c la rifi er In thi s case, bo th refra c to ry and biod eg ra dabl e o rgani cs a re a d so rbeJ 8i o mass gm wth o n th e ca rb o n surfa ce ut ili zes t he b io d egra dab le fr ac ti on. Re m oval effi c iency for biod egrada ble orga ni cs may be Improved by thi s process. but usua II v at th e ex pense o f refra c to ry organi c removal e ffi c iency. U se of powd ereu a C l i va t e ~1 car b o n in secon d a ry was tewa ter sys te m s res ult s in an inse p a ra bl e m ixtu re o f Qlo lo g lcal so li ds and ca rbo n . Th er m a l rege n era ti o n o f th e car bo n a lso res ult s III des tru ct io n o rth e bi o m ass, elimin a tin g the need fo r othe r s lud ge pr ocess in g an d d is posa l tech niqu es, b ut ·incre a si ng th e size o f t he carbo n rege n e l'a ti o n sys te m. . In current pra c ti ces, mos t sys te m s treating potable wa ter use p o wd er e d ac tl vat.ed car bo n, w hile ad vall ced wa ste wa te r sys te m s use m ovi n g-bed. g ra n ula r . ac ti vat ed ca rbo n. Bett er rege nerati o n procedures woule! g reatl y enh a nce t he li se of powdered uct ivated carbo n in was tewa te r trea tment , partic ul a rl y III the seco n da ry processes. . Che mi ca l ox id at io n C he m ica l oxid ati o n 01' re fra cto ry o rgan ic compo u nds can be llser! a s an alt e l'l1ati ve to th e ad so rpti o n process in bo th po tabl e wa ter · a nd \V
ENGINEERED SYSTEMS FOR WATER PURIFICATION
198 WATER
detergents, and phenolic and humic compounds can be broken into simpler compounds by strong oxidants such as ozone or chlorine. Further oxidation by chemical or biological means may result in stable end products. Added advantages of this process may include ammonia removal , oxidation of inorganic substances such as iron and manganese, and disinfection. The discovery that chlorine react s with some organic compounds to form undesirable haloforms makes its use as a chemical oxidant questionable . The application of ozone for both disinfection and chemical oxidation in potable water treatment has been a long-standing practice in Europe-The destruction of taste and odor compounds and color-producing organics by ozonation is quite effective. Application of ozone to wastewater organics is less etIicient. Some of the biologically resistant compounds in secondary effluents are also chemically resistant. Generally. a 3-1 ratio of ozone to organics on a mass basis is sutIicient to reduce the COD by approximatel y 70 percent. [4-40J Better efficiencies can be obtained only by significantly increasing the dosage. Application of ozone for chemical oxidation and disinfection is a simultaneous operation in potable water systems. I n wastewater systems, chemical oxidation is more cost-effective when applied alter secondary treatment, or after tertiary processes if these are included 111 the sys tem. The specific characteristics of ozone and systems for contacting it with water were discussed in Sec. 4-9 and will not be repeated here.
199
4-10 A particle with a diameter of 1.0 mm and a specific gravity of 3,0 is released in water at 30"e. How long will it take the particle to travel 2 m? 4-11 A particle with a diameter of 0,5 mm and a specific gravity of2.5 i's released in water with a temperature of 25 °C. How far does the particle travel in 3 s? 4-12 Two particles are released in water at the same time, Particle A has a diameter dA of 0.4 mm. Particle B has a diameter dB' of 0,9 mm. What is the ratio of the.settling velocity of particle A to that of particle B? Assume equal densities. .. 4-13 Suppose that a column is filled with water containing a uniform suspension of particles ' A and B as described in Prob. 4-12. Particle B is removed with 100 percent efficiency in exactly 10 s. What is the percent removal of particle A? 4-14 Name three types of settling basins employed for solids removal in water-treatment plants. 4-15 Describe the four functional zones of a long-rectangular settling tank. 4-16 A settling column analysis is run on a type-I suspension, The settling column is 2 ill taIl, and the initial concentration of the well-mixed sample is 650 mg/ L. Results of the analysis are shown below. Time. min Cone remaining, mg/ L
o
58
77
91
114
154
250
650
560
415
325
215
130
52
What is the theoretical efficiency of the settling basins that receive this suspension if the loading rate is 2.4 x 10- 2 m/ min? 4-17 Using the data from Prob. 4-16, determine the theoretical efficiency of a settling basin with a loading rate of 3.0 x 10 - 2 m/min.
DISCUSSION TOPICS AND PROBLEMS
,,,,!ler
eo.
4-1 Briefly discuss the diflerences in !b.e. qLFlJi.t,Y. .Qf o b.t
4-18 Using the data from Prob. 4-16, determine the theoretical efficiency of a settling basin with a surface area of 500 m 2 and an inflow of 14.400 m 3 /d. ..4,19..Determine.the.theoretical-efficiency gf.the settling basin in Prob, 4-18 if Vi
= 0.04x.
4-20 A settling column analysis is run on a type-2 suspension with the following results, (Entries are suspended-solids concentrations at stated times,)
4-2 What kind oT treat'ment is needed for well water intended for agri~ultural use? Surface waters intcnded for agricultural use') 4-3 WOldd ordinary tap water from a city water supply be adequate, without further treatment. for all industrial uses? Why or why not ') 4-4 Why is aeration used in water-tre;ltll1ent plants? Is it mo re commonly used with ground water or surface wat er ') Why" 4-5 Name and describe three communly uscd water -in -air systems found in water purificatio n plants. 4-6. Describe an air-in -water system cOJllmunly used in wate r purification plants. 4-7 Define (0) discrete particles. (h) l'locc Lllating particles, (c) dilute suspen sion, and (d) con. centrat ed suspen siun . 4-8 What is a type- I suspe nsion " 4-9 De te rmine the settling vckKlt y of a spherical parti c le witli a diameter of 100 I,m and a speciti c gravity of 2.3 in wat e r at l y e .
D;termine the theoretical efficiency of a settling basin with a depth of 3.5 m, a volume of 1400 ,and an II1flow of 11.200 m)/d.
III
'-
i,
ENGtNEERED SYSTEMS FOR WATER PURIFl CATtON 20 1
200 WATER 4-21 Using the data from Pro b. 4-20. dete rmine the th eo retica l efficiency of a sett ling bas in with a depth of 2.5 m, a volume of 2,200 m
3
,
and an infl ow o f 13,200 m/d l.
4-3 7 i\ fl occ ul a to r padd le of th e design and dim e nsio ns sho wn be low is ro ta ted th ro ugh wa ter at 20 v wi th an angul a r speed of 4.0 r/ min.
e
3
j
ii,
JL
III IlL Iil1:1
1·1
'---.
4-22 A settling basin processi ng 14.400 m /d of wa ter has a dep th of 4.0 m a nd a vo lume of 1200 m] Using the data from Prob. 4-20, determine th e theoretica l efficiency. 3.0
4-23 What is a type-2 suspensio n? 4-24 A water-treatment pla nt is to process 19.000 m'jd. A se ttling basi n for a type-2 suspen sion is to operate at 0.75 m/ h. Determine the dimension of th e basin for (a) a lo ng-rec tangu lar unit a nd (b) a circular unit. Check detenti o n times. hor izo nt a l ve locities. and weir ove rfl ow rat es. 4-25 Determine t he appropriate number of units a nd dimensio ns for settl in g basin s to treat 75.000 m 3 /d a t a n overflow ra te of 0.8 m/ h. 4-26 Assume that th e sett ling bas in s in Prob. 4-25 will be const ru cted of re mforced co ncre te a nd th at the cos t of fo rm ing and pou rin g circular walls is 1. 25 tim es th e cos t of forming and po uring slraight wa lls. Wh a t will be th e relati ve costs of using circular tank s com pared to rectan gu lar tank s uti lizing common wa ll s where poss ible~
I ~'
I
T
0. 1 !11 E
E
'"
0. 1 m
~
4-27 C hemica l coagu lati o n in water-trea tmen l planls is acco mplished by the additi o n of tri vale nt meta lli c sa lt s. Name tw o of these. 4-28 Name a nd di sc uss the fo ur mecha ni sms th o ug ht
10
I
occur durin g coag ulati o n.
4-29 Explain the imp o rt ance ofth eja r tes t in coag ul a ti o n opera li o ns and desc ribe Ih e test.
l. ~
'Ir-
l
t ~
i·;L. "
,. j ." .,.-.
4-30 Un"der wha t conditi o ns mi ght it be desi rable to ad d turbidi lY In wa ter in a treatment plant 0 4-31 Under wha t circum sta nces a re lime and/ or soda ash ad ded to waters d uring coagu la tio n opera t ion s? 4-32 Name and describe the fo ur genera l ca tego nes int o which surface wa te rs a re gro uped with regard to coag u la t ion. 4-33 Define (0) r3pid mixing and (b) fl occ ulati nn.
. .4-':H .~ .,:".<*r:\~e~tI11~n.t p.1 a nt is to process 30.000 111"'fd. Th e rapid mixing ta nk will ble nd 35 mg/ L of a lum with the ti ; ~; a'n'd 'i; i o' l~ a'~e'~ 'de'ten i'ori 't lin e iln riii'ri : llie l ::i iik is' fo 'nav'e a square cross sec ti o n wi th ve rtical baffles and a Rat blade impeller sim il ar to Fi g. 4-20b. De ter min e the fo ll ow ing : (a) Quantit y (kilogra ms per da y) o f a lum added (b) Dime ns io ns 'o f t.he tank (e) Powe r input (kilow.at ts) necessary fo r a C va lu e of90() s- I. Th e wa ter tempera ture is 22 '·C. 4-35 The fl ow described in Prob. 4-34 is to be Ilocc ul a ted in a basin hav ing fo ur fl occu la tors with tran sverse padd le units. (See F ig. 4-22a.) The basin may be a maximum of 10 m wide and 4 m d eep to co nn ec t to the settlin g basi n. Dete rmin e' (,,) Bas in dimen sions (h) P ower req uirements (e) Padd le config ura ti on and ro tati o na l speed 4 The best (;1 va lue fo r th is system has been fo und to be ~.5 x 10 4-36 The fl ow through a fl o.ccu la to r processes 16.800 m 3 /d nf water at 17°C. The padd les are 4 arra nged lo ngi tudin a lly. The o ptimum CI va lu e ha s bee n fo und frolll jar tests to be 4.5' x 10 Dete rmin e (0) Bas in dimension s (h) Powe r appl ied 10 th e wa te r ( e) Paddle co nfig urati on and ro tational speed
()
Fro nt
,-:r
Vl eW
. Sldt' View
(a) How mu c h po wer is di ss ipa ted int o the wat er ') (h) Iflh e tallk in whi c h thi s padd le is ro tating ha s th e dimensi o ns o f4 x 4 x -1m a nd t he fl ow through the tank is SOOO 1l1 .\ (d. determin e th e (;1 value fo r th e fl occ ulat or
4-38 Soft ening of hard wa ter ma y be do ne a t a wat e r utility trea tment pl a nt or by th e co nsume r. .As a gen e ral rul e of thum b. what ha rdness le ve l ind ica tes the need 't o so ft en a t the trea tment plant 'I 4-39 Difl"ere ntiat c betwet:n sin gle-s tage a nel two-s ta ge softening processes. 4-40 A wa ter has the fo llowi ng ionie constituent s (meq ui v/ L):
Ca ' , • N,,'
~tg2
=
-u
HCO, - = 2.5 SO, ' = 2. 9 CI = 2.5
1. 0 2.2
el l ,
--
06
(oi) ·Calculate the chemica l requiremcnt s (l11illicq ui va lenl s per liter ) required to rCl11 o\e as mueh of Ihe cal ciulll as pO SS ib le and to res tabili l e t he wat er. (N o Mg" rel11 ol'a l is re. quired.) In) I)r ~I\\' a ha r d i: lg r:llll of Ihe fin ished \\" ate l (c) Cd cu la tc I he d:lil y q L1 a lll il y (k ilogra ill s per day) o f lime Cl nd sod :1 :hh (as, u nl e a r>u rit\ J of 'n perccn l for Ih c li llle a nd ';I() perce nt fo r th e su d a ash) to treat 17.5nO Ill ·d of Ihis wat e r. (iI) Dct erm ine the dr) Illa ss (kil og ram s pc r d a) ) " f th e slud ges prl)ci uu:d .
ENGINEERED SYSTEMS FOR WATER PURIFICATION
202
203
W ATE R
4-41 A water-treatment plant processes 24.500 m 3 jd or water with the foll owing ionic concentration :
4-57 A rapid san? filter has a bed depth of 0.7 m. It is composed of sand grains that have a graVIty 01 2.65 and a shape factor of 0.82. The porosity of the bed is 0.45 throughout. 1 he sieve.analYSIS 01 the sand is shown below. s~eGlfic
0.5
M ass retained,
Na'
HCO l
cr
0.5 (a) Determine the quantities or chemicals (kilograms per day) required to so rt en this water to the minimum possible hardriess by two-sta ge lime -soda ash sortening. (b) Draw a bar diagram ro r the fini shed water. ( e) Calculate the dry mass of the so lids in the sludge.
4-42 What is split treatment ') 4-43 Rewo rk Pro b. 4-41 ll sing a split-trea tment approach in which 1.0 meq ui v/ L or M g' • is accep table in the finished water. 4-44 Determine the percent sav ings in chemicals ir two-stage treatment (Prob. 4-41) is replaced with split treatment (Prob. 4-43). 4-45 What is reca rbonation and und er what conditions is recarbo nati on necessa ry in a watertreatment system ? 4-46 An ion-exchange system is to he used to soften the water described in Prob.3 4-40. The resin has an exc han ge capacity o r9 5 kg/ Ill) when operated at a Aow rate orO.35 m / m ' · min. Deter mine the vo lume or resin needed and a tank configuration to allow con tinuous o peration ir the regenerati o n time is 2 h. 4-47 Determine the chemi ca l req uirement ror regeneration or the ion-exc h.ange system in 'Prob'. A.46 if rege neration is accomplished using 140 kg or sodium chloride per cubic meter of resin. What volume or back wash Auid mu st be disposed or ir the sa lt used is in I 0 ~,~ so lut ion? 4-48 Define" breakthrough" as it relates to treatment or hardness and discu s~ what steps must be taken arter breakthrough po int is reached 4-49 A bed or filter sa nd 0.75 m deep is composed or uniform particles with diameter 0.5 mm , spec ific gravity 2.64, and shape ractor 0.9. The po rosit y or the packed bed is 0.45. Plot a curve for head loss vs. filtering velocity over the filter velocity range or 2.0 to 7.0 rrij h at a water temper a ture o r l3 e C. 4-50 Discuss filtration as a mea ns o r wate r treatment. What is precoa t filtrati on') 4-51 What is the principal cleaning mec hanism in backwashing filters ? 4-52 Differentiate between slow sa nel filters and rapid sand filters. 4-5-3 What are elual-media filter s" Wh ilt are their advantages a nd disadvantages" 4-54 What are mixed-media filters " What are their advantages and disad va ntages " 4-55 A hydrostatic head or 2 m is maintained above a 0.6-m-eleep bed or filter sand. The sand is uniror mly sized with di a mete r 0.4 111m. spec ific gravit y 2.65. a:lel shape ractor 0.85. Determine the How rate th ro ugh the bed if the w~l t e r temperat ure is I sec. 4-56 An experim ental filt er co nsi sts Dr a 2-m depth or unirorm sand with a diameter orO. 85 mm and a shape fa cto r 01'0.7. The pur us,t\ of tile bed is 0,35 and th e specific gl'av ity of the sa nci is 2.65. Determine thc head (meters of water column anel kil o pasca ls) to maintain a fl ow of water u through the bed ;It a !low ra te of I() m ·h. The wate r temperature IS 15 C.
Sieve no .·
~o
14 -20 20- 28 28- 32 32-35 35--42 42-48 48 - 60 60- 65 65- 100
0.87 8.63 21.J0 28.10 23.64 7.09 3. 19 2.16 1.02
Average particle ·size, mm 1.0 0.71 ·054 0.46 0.38 0.32 0.27 0.23 0.18
Determine the head loss through the bed irthe flow rate is 5.0 m/ s and the water temperature is 17 ' e . 4-58 A constant head or 2.5 m is maintained above the filter bed described in Prob. 4-57. . . Determine the Aow rate through the filter. 4-59 Write a computer program anel rework Prob. 4-57 by computer (or a hand-held programmable calculator). ~O . Determine the backwash velocity (V,) at which the filter bed in Prob. 4-49 will just begin to flUIdize.
go~d disinrectant.
4-69 Name severa l commonly used disinfectants and discuss the adval1laees and disadvantages posed by each. Which is the mos t commonly used in the United State~? In Europe') 4-70 What ractors militate against effective di sinfection ?
204
ENG I NEER ED SYSTEMS ro R WATER PUR I FICATION
WATER
4-71 What methods are common ly used for desalinizati on of water? 4-72 How are refractory organics removed from water and wastewater? 4-73 Why is powdered activated qr.PQn(PAC). unsuitable Jor.u se. in a fixed-bed adsorption arrangement? 4-74 A city draws its water supply from a large reservoir. The water has consistent quality throughout the year. It has a turbidity ranging from 20 to 50 unit s. and its maximum hardness is less than 100 mg/L as CaC0 3 . Refractory orga nics are not a problem and the TDS is low . Draw a schema tic diagram of a treatment plant that may he ll sed to render this water potabl e. Identify each unit and briefly state its purpose. Show point s of chem ica l add iti ons and identify the chemicals.
-.
4-75 A city water supply is obtained from a deep aquifer. The water has uniform quality. It is clear and free of organics; hardness is in excess of 300 mg/ L and cons ists or both calc ium and magnesium. Dissolved CO 2 is approximate ly 15 mg/ L and iron (Fe z.) is abollt 1.0 mg/L. Other dissolved constituents are below problem levels. Draw a schematic diagram of a treatment plant that will render this water potable. Iden tify each unit and briefly state its purpose. Show points of chemical addition and identify the chemicals. 4-76 A large st ream flowing through a highly industri a li zed area mll st serve as a raw water supply for a community. The water is consistently turbid, has hardness in excess of 300 mg/ L, . and has refractory organics that are known precursors of trih alometha·nes. Draw a schematic diagram ofa treatmen t plant that shou ld render this water potable. Id entify all units. state their purpose. and show points of chemical addition. Identify all chem icals.
REFERENCES
.-
4-1 American Society of Civil Engineers, American Water Works Association, "nd Conference of State Sanitary Engineers: 'Waler Trealm enl Planl Design, A WWA . New Y o rk. t 969 . 4-2 American Water Works Association : Waler QualilY and Tr ea ltll enr. A H andhook 0/ Public Waler Supplies, 3d ed., McGraw-Hill , New York , 1971. 4-3 Amirt harajah , A.' "Des ign of Flocculation Systems," In R . L. Sank s (ed.). Wal er Trealmenl Planl De!ign, Ann Arbor Science, Ann Arbor, Mich ., 1978. 4-4 - - - - : "Design of Rapid Mix Units," in, R. L. Sanks (ed.) , WaleI' Trealm elll PIOIlI DeSig n, Ann Arbor Science, Ann Arbor, Mich. , 1978. 4-5 - - - : "Op timum Backwa s hing of Sand Filters." J Enl' Enlj Dil' II E . I 04(EE 5). 91 7 (October 1978). 4-6 Baker, M. N.: The Quesi/o'r Pur.e Wal er, AWWA, New Yo rk . 1948. 4-7 Baumann , E. R .: "Granular Medi a Dee p Bed Filtration." in R . L. Sonks (cd.), WaleI' Trealmenl Plant Design, Ann Arbor Science, Ann Arbor, Mich ., 1978. 4-8 - - - : "Precoat Filtrati o n ," in R . L. Sa nk s (ed.), Wa ler Treollllelli PlolIl Desi,elll . !\ nn Ar bor Sc ience, Ann Arbo r', Mich ., 1978. 4-9 Bernado , L. D., and J. R . Cleasby: "Declining-Rat e vs. Co ns ta nt-Ratc F il trat io n." J EliI' Eng
sr
Dir, ASCE, 106(E E6): 102 3 (Decembe r 1~80) 4-10 Camp. T. R.: "Floc Volume Concentration," J A WWA , 60(6) ' 6 5b (196 8) . 4-11 - - - : "Velocity Gradients and Intern a l Work in Fluid M o ti o n ," 1. BoslOn SocielY o( Ciuil Engineering, 30: 219 (1943) . 4-1 2 - - - and P . C. Stein . .. Sedimentation and D esign of Sett li ng 1>"1b." '/'rOIlS A S CE. 11 I ' 895 ( 1946). 4-13 Ca rl , K. J ., R. A . Young, and G . C. Anderso n: "Guidelin es t'or DetCfmining Fire F lows," J AWWA, 65( 5):335 (1973). 4-14 Ca rmen. P . c.: " Fluid Fl ow Thro u g h Grallular Beds." Tran s 111.11 Che lll Eml (Lon d o n ). IS , ISO ( 1937).
205
4- 15 Chanletl. E. T .. !:'lI l'irunlll ellwl Pr{)( ec li()lI , 2d ed" M c Graw- H ili , N ew Yo rk , 1979. 4-16 Clark. J . W .. W . V,e >S Ill'"1. Jr ., and M. J. Hammer : IYaler S upply alld Pol/ulion COIllI'OI, 3d ed., H arpe r '& R ow, New York. 1'!77 . 4-17 Clcasby. J. L .. "Filtr"t,,)n" in W . J . Webe r. Jr . (cd.) , Physiochemical Processes/or W aler QualilY COlll ru/. Wiley Inlerscicncc. New York , 1972 4- 18 _.-.- by pc rso nat eo rlllnuni ca ti o n . Jan uary 1980. 4-1 9 - - , J . Arboleda. D . E. Burn s, P. W. Pre ndiville. and E. S. Sava ge'" Backwa s hin g of Granular Filters ,",/ AWWA. 69 : 11 5'( February 19 77). ·' 4-20 "-'--- . and J . H . Del tingham' " R" tional Aspects o f S p lit Trea tm ent." Proe A SCI.' . .I Sail Eng Ole. 92 (S ;\ 2) 1 ( 1%6) 4-21 --- - . "nd .I . C. Lore ncc "Effectiveness 01 Bac kwas hin g for Wa s tewate r Filt ers .. J Ellt' Fnc; Dil . .·ISCE. I04( EE4) ' 749 (August 1978) 4-22 - ---' . L IN. Stangl. and G . H . R ice . . Develop me nt s in l3ackwash ln g o f Granular Filters."
,/ Elle Enel Dil'. AS CE. 101 (EE5) . 7 t .1 (Octobe r 1975). 4-23 Co hen. J . !\ t.. and S . II. ~I annah ' "Coagulation and Flocculation ," in W(t{ rr Quallf}' ond Treolml'lIl. 3d cd .. McGraw- H ilI. New Yllr k. 1971. 4-24 Conley. IV R . "Waler Poll nti 0n T echnology Report," NepICme k/ croF/oc fil e., 2( 1). F ebruar y 1968 4-25 ('ulp. R . L .. G . IVI. Wesner. a nd G. L. Culp. H andbook oj Advall ci'ci WaSII' Il'O ln' Trealmenl, Van NLlstr"nd RCilil lO ld , New Y,)I'k. 197R 4- 26 Da VIS, S. N .. and R J . M . De Wi es t J-/)'rlroqeoloq)', Wile y, N ew Yo rk. 1966 4-27 De.llgll 1'vIC/nCiolfe'r SClsp" ICIled Solids Rel/rowl, U.S. Environme ntal Pr o lec ti o n Agency. 19 75. 4·28 Fair. G. IVI .. and J C Geyer W(lf l'r Supply and Wa Slell'(ffCY Disposal, W iley. New Y o rk , 1961 4-29 - --- - ---- - - .. ant! D . A. Okun ' nelllenlS of Wale/' Supply and W aslewaler Disposal, 2d ed., Wiley. New Yo rk . 19 7 1 4-30 - - - - . F. C. M,m". S. L. Chang. I. Weil. a nd R. A . Bu rden " The Be ha vior or Chlorine as a Water D isinfectant . . ,/ A WWA. 40 ' 105 1 (1948) . 4·3 1 Gehm. H. W. and .I . I Bregman (cd s.): J-/olldlJOok oj "Jimer Resource.> IInel POl/lliioli Conllol, Van No s trand Rcinh o ld. Ne\\· York. t9 7h. 4-32 H lld son, H . 1::.. Jlld J. P . Wotrner : .. Dc ' ign of Mixing a nd Fl OCC illati o n BJ sl n s." J A WWA , 59 : 1257 ( 191,7) 4-33 IV'es: T K.· "iicl 'i\' T . 'OI;,;le" " tilc',iii 'ofJ':ioccu t',il; OI; 'f~; 'Co ~i;',;~ ~',; ~ ·Fi ~ ~· Systems" J EI1I' Ell!! D" AS(E 9~ ' 11 (IYn). 4·.1 4 Kammere r J . C. "lVater Quantit v R eqllirements for Publi c Suppli es and Other Uses." in H . W . G ehm and .1. I Rregrn'"1 (c,b) . [[olldbo'ok 0/ IYoll'r Reso urces ond POl/ulloli COl1lrol. Van Noq,,",, 1 Reinl",ld. Ne\\' York. 197(, . 4-35 La ce)'. R E ... ,,,lem hr,,nc Separall o n P ,,;cesses, " Chern Enq. 4 :56 (Se pt e mber 19 72) -1 · 36 Lettcrman . RD .. J E. VII'"'. and R . S . Gem m ell' " Inll ue nce o f R apid-Mix Paramete rs on Fl oecul,"I OII." .I .·1 1I'1I'.,t. 65 : 7 16 (19 73) . 4·J7 Lilld s le y. R K .. "nd .I . 13 . h'"1711l1 W il li'/' Re'.wurefs Ellgineering, 3d ed .. McGraw· H ill, New Yo rk . 197 9 . 4- .\ 8 L.O\\·cllth,d . R . t . . ,",,1 B. \'. R Mar",, · ('orhonale Chelllislry 0/ Aqu(;'ic' SYSlems . Theory ond IlppIiCO/lOII ,\ . :\ nll ,\rhp i Science. Ann Arbo r. Nl ich .. 19 76 4-.19 M eWh inlic . R ( .. and I' . R . .I l) tll "o n · ., Water Storage ant! D istrib ut"l11. " in A . \V . Gehm and J. I. r1 rcglllan (cd;>, . ). num/boo/... 0/ J J "(Il l'/" Nes() urcl's and Pol/lIIion lon tr ol. Van No s! rand R einhold. Ncw Yo rk. ]tJ76 4·40 M etcal f & Edd,. IIl l' New YprK. 1\.)79 . 4·41 O ·Mcl ia. ( 1)/"0( "(\\.\'('.\ III !
4· -1 2
R
Il osle" 'al{'/' F".C/lI/ eC'l'inq. Tr" olmenl, Dispos(i/, Reusr, 2d ed . M c Graw-HIli.
"( ' «"~III
If"lI f l ' l" (ju lIlll l"
;] n.l I) K . ( r; lpp' " S"nl C ( 'he mic,,1 !\ spects of R" piJ Sand Filtrallun.".1 .·IIVI-I A. 56(I OI. f " : 6( kt"hc ll <)(,-I 1. 4 ,,4 ~ P\l\h:l 1. S 'I 11 '(lI e/ ( (l lIdllll) /I /1/O /0/ 11Il.'I/Hr r. ~lc(jr~ I\\'-ll ill. Nc\\' York, 1954, 4 - ~ _1 Ul ' / fJ lI l lIIl Jldl'd,\)/dlldli r d S /1l 1 II -cl lll 1,1"(/...,, Health Fduca l ioll Sen'icc. Alban y, N , Y. i ()l h .
206
WATER
CHAPTER 4-45 Rch. Ca rl W.: .. Lime-Soda So ftening Processes," In R . L. Sanks (ed.). Water Treatment Plant
Design. Ann Arbor Science. Ann Arbo r. Mich., 1978. 4-46 Rich. L. G.: Environmental Systems Engineering, McGraw-Hili, New York, 1973 . 4-47 Sanks. R . L.. " Io n Exchange," in R. L. Sanks (ed.) , Water Treatment Plant Desi, 3d cd .. McGraw-Hili,
FIVE ENGINEERED SYSTEMS FOR WASTEWATER TREATMENT AND DISPOSAL
New York, 1979. 4 -49 Schroeder, E. D. : Water and Wastewater Treatment, McGraw-Hili , New York , 1977. 4-50 Scott, G. R.· "Ae rati o n ," in Water Quality and Tr('{/tment , 3d cd ., M cG raw- Hili , New York , 1971 . 4-5 1 Slee l. E. W .. and T. J. McGhee. Wat er S lipply and Sewerage . 9th ed., M cGraw- Hili , 1979. 4-52 Stumm, W, and C R . O'Melia . .• Stoichiomet r y of Coagu la tion," J A WWA, 60 514 ( 1968) . . 4-53 Sundcrs tro n , D . W .. an d H . E. Klei W astewater Treatment , Pre ntice- Hall, Englewood Cliffs,
N. L 1979. 4-54 Tebbutt, T.H. Y . . Principles of Water Quality Control. 2d cd. , Pergamon. Oxford, England , 1977. 4-55 Todd. D. K. Grou ndwater Hydrolo
4-'~ Walker. Rodger
N ..I . 1978 . 4-59 \Vebt:r. "Y . J. (el1.) : Phrsioc/temical ?ruct's.ws
New York. 1972.
.ItJ{ ~V(ltt'r
Qllolity
('Of/Ero!,
Wile y Interscience .
In modern soc ieli es pro per management of wastewater is a necessity. not an opt illn . The public health consequences of poor wastewater management have been di scussed in prev ious chapters. Hi stor ically. the practice of collecting aild treating was tewa te r pri or to disposal is a relatively recent undertaking. Although remains of sewe rs have been fo und in :.Jncient c ities. the extent of their use for wastewater carriage is not known. The elaborate drainage system of ancient Rome was not used for was te disposal. a nd wasles were speci fically excluded from the sewerage sys tems of London . Pari s. and Bo ston until well after the turn of the nineteenth centur y. Prior to this time, city residents placed" nig ht soil" in buckets along the streets "a-lid' \voi-ke;s"~mi;1 ;~~r 'tl~e . ~vaste into "honeywagon" tanks. The waste was transported to rural areas for disposal over agricultural lands. The invention of the flu sh loilet in Ihe nineteenth century drastically changed waste-disposal practices. Existing sys tems for transporting urb a n wastes for disposal on agricultural lands we re not adeq uate to handle the large vo lume of liquid generated by the flush toilets. Faced w ilh this tran sportation problem, cities began to use na tural drainage systems and storm sewers for wastewater carriage. against the advice of such men as Edwin C hadwick. who in 1842 recommended "rain to the river and sewage to th e so il.·' [5-21] Construction of co mbined sewers was commonplace in large ci ties during th e la tter half o f the nineteenth century. Since storm drain systems naturall y e nd ed at wa tercourses. waterborne wastes were discharged directly to stre:.Jll1s, lakes. and estuaries without treatment. Gross pollution often resulted , an el'hea lth pro.blems were transfe rred from th\} sewered community to downstream users of th e wa te r. The 'first "modern " se\~erage system for wastewater carriage was built in Hamburg. Germany. in 1842 by a n innovative English engineer named Lindley. Lindley's sys tem included many o f the principles th a t are still in use today. [5-10] 1\10S1 or th e improvements in wastewater collection sys tems over the last 100 years 207
ENG INEERED SYSTEMS FOR WASTEWATER TREATMroNT AND DISPOSAL
208 WATER
,.;'":-
have consisted of improvJd materials and the inclusion o f manholes, pumping stations, and other appurtenances, The treatment of wastewater lagged consideHlbly behind its co llectIOn. Treatment was considered necessary only after the se lf-purification capac ity of the receiving w2ters was exceeded and nuisance conditions became intolerable. Various treatment processes were tried in the late 1800s and ea rly 1900s, a.nd by the 1920s, wastewater treatment had evo lved to tho se processes in common use toda y. Design of wastewater-treatment fa cil iti es remained empirical, bowever, ulltil midcentury. In th e last 30 to 40 ye ars, great advances have been Imide in understanding wastewater treatment, and the original processes have been formulated and quantified. The science of wastewater treatment is far from stati c, however. Advanced wastewater-treat ment processes are currently being developed that will produce potable water from domestic wastewater. Problems associated with wastewater reuse will no doubt challenge the imagination of engineers for many years to come. Philosophies concerning the ultimate disposal ofwaqewater have also evolved over the years. As previously mentioned, the practice of land disposal was replaced by the convenience of the water carriage system with direct di sc harge to surface waters. Operating under the assumption that the "solution to pollution is dilution," the assimilative capacity of streams was utili zed before treatment was deemed necessary. For many years. little, if any. treatment was required of small com· munities located on large streams, while a high level of treatment was required by large cities discharging to small streams. In more recent times, the policy has shifted to require a minimum level of treatment of all waste discharges, regardless of the capacity of the receiving stream. Under current practice in the United States, all dischargers are given a permit sfating the maximum amount of each pollutant .. ..... ..... thaI ·they·are 'at]Dwed' to discharge:' DischaTge'permits' aTe-'rro'longer-intended to just prevent discharges that exceed the self-purification capacity of the streams, but are concerned with obtaining the " fishable. swimmable" goals mentioned in Sec. 2-17. Where extensive treatment of wastewater is necessary to meet stringent discharge permits: the quality of the treated effluent ofte n approaches that of the receiving stream. These effluents should be considered a va luable water resource, particularly where water is scarce. Regulatory agencies enco urage utilization of these wastewaters for irrigation. non-body-contact recreational activities. groundwater recharge, some industrial processes, and other nonpotahle uses.
209
greatly from industry to industry, and, consequently, tre2. tment processes for industrial was tewater also vary, a lthough many of the processes used to treat municipal wastewater are a lso used in industrial wastewater treatment. Acomplete coverage of industrial wastewater treatment is beyond the scope of this text, and the interested reader is referred to other text s on the subject. See Refs. [5-7, 5-1 8, a nd 5-38} Water collectecI in . municipal wastewater systems, having been put to a wide variety of uses. co ntains a wide var iety of contaminants. A list of contaminants commonly found in municipal wastewater along with their sources and their environmental consequences is given in Table 5-1. Quantitatively, constituent s of wastewater may vary significantly, depending upon the percentage and type o f industrial waste present and the amount of dilution from infiltration / infl ow mto the collection system. Results of an analysis of a typical wastewater from a municipal collection system are given in Table 5-2. The compos ition of wastewater from a given collection system may change slightly on a seasona l bas is, reflec ting different water uses. Additionally, daily flu ctua tions in quality are also observable and correlate well with flow conditions as noted in Fig. 5-1. Generally. sma ller systems with more homogeneous. uses produce greater rlu ctu,ll ions in wastewa ter composition. The most s ignificant components o(wastewater are usually suspended solids. biodegradable organics. and pathogens. Suspended solids are primarily organic in nature and are composed of some of the more objectionable material in sewage.
Table 5-1 Important wastewater contaminants' Contaminant
Source
Suspended solrds
Domestic use , industrial wastes, . erosion
Environmental signifIcance
by
infiltration/in flow
Cause sludge deposits and anaerobic conditions in
aquatic environment I3iodegradab!e organics
Domes! ie
:1
nel i ndust ria I waste
Cause biologicat degradation, which may use up oxygen in . receiving water and resu lt in
undesirable conditions Pathogens
Domest ic waste
Transmit communicable disea-ses
Nutrients
Dom estic and industrial waste
May cause eutrophication
Refract ory organics
Indu strtal waste
May cause taste and odor problems, may be toxic or
Heavy metals
Illclll ~ tn,lI \\ · ;t~tc. mining, etc
Are toxic. may interfere with
5-1 WASTEWATER CHARACTERISTICS
carcinogenIc
Wastewaters are usually classified as ilidustrial wastewater 0 1: muniCipa l' wastewater. Industrial wastewater with characteristics compatible with municipal wastewater is often discharged to th e municipal se wers. Many industrial wa5te\\aters require pretreatment to remove noncompatihle substan ces prior to dis· charge into the municipal sys tem. Characteristics of industrIal wastewater va ry
effluent reuse Incr c;l~l':-'
ahll\"C level ill watt:f
sllpph hy dOl11estic and / or industrial
U Sc
May interfere wilh ettlucnt
r ellSe
210
ENGINEERED SYSTEMS FOR WASTEWATER TREATMENT AND DISPOSAL
WATER
Table 5-2 Typical analysis of municipal wastewater
soo
O.S
400
0.4
300
0.3
211
Concentration
Constituent, mgj L *
Strong
M edium
W ca k
SoJids, to tal Dissolved, total Fixed Volatile Suspended, total Fixe d Vol a tile Sett leabl e soli d s, mL/ L Biochemical oxygen demand, 5-day, 20"C (BOD,) Total organic carbon (TOC) Chemical oxygen demand (COD) Nitrogen (total ils N): Organ ic Free ammonia
1200 850 525 325 350 75 275 20
'720 500 300 200 220 55 165 10 220 160 500
350 250 145 lO S lao 20 80
Nitrites Nitrates Ph o sphorus (tota l as P) . Organic Inorganic Chlorides Alkalinity (as CaCO J )
Grease
-' cO
E
400 290 1000 85
35 SO 0 0 15
40 IS 25 a a
8
5
3
10 100 200 150
50 lOa lao
liD 80 250 20
8 12 0 a 4 I 3 30 50
SO
* Un less otherwise noted. Source : Fro m Metcalr & Eddy , Inc . [5-36J
Body was tes. food waste. paper. rags. and biological ce lls form the bulk of suspended solids in wastewater. Even inert materials such as so il p
~
:? S;
" '"f?-
~
E
'0 C
~
-
~
-0
200
0.2
100
0.1
i ...£ LL.
'0 C
0'"
0
co
o L -_ _L -_ _L -_ _L -_ _L-_----'L...-_--' 0 12 M
4 AM
8 AM
12 N
4 PM
8 PM
12M
Time or day Figure 5-1 Typicat variation in flow, suspended solids, af)d BOD , in municipal wastewater. (From Me"·,,I! & Edell', Inc. [5-36].)
Although pathogens causing some of the more exotic diseases may rarely be present, it is a safe assumption that a sufficient number of pathogens are present in all untreated wastewater to represent a substantial health hazard. Fortunately. few or the pathogens survive wastewater treatment in a viable state. Traditional wastewater.treatment processes. are.d.esigned to reduce.suspended ........... ... . . so lid s. biodegradable organics. and pathogens' to acceptable levels prior to disposal. Additional wastewater-treatment processes may be required to reduce levels of nutrients if the wastewater is to be discharged to a delicate ecosystem. Processes !o .remove refractor y organics and heavy metals and to reduce the level of lIlorganic dissolved solids are required where wastewater reuse is anticipated,.
5-2 EFFLUENT STANDARDS The Water Pollution Control Act of 1972 (Public Law 92-500) mandated the En viro nmental Protection Agency to establish standards for wastewater disCharges. Current standards require that municipal wastewater be given secondary treatment and that most effluents meet the conditions shown in Table D-7 of the appendix. Seco ndary treatment of municipal wastewater is generally assumed to include se ttl;ng. biological treatment. and disinfection. along with sludge treatment and di sposal. Thus. the principal components o f municipal wastewater, suspended So lid s. biodegradable materiaL and pathogens should be reduced to acceptable levcl s through secondary treatment. Industrial dischargers are required to treat
212
E.NG I N I: ERFIJ S\ S II MS 1·0R WASTE\\X I ER
WATER
their wastewa ter to the leve l obta in able by th e" bes t availab le techn o logy " for wastewater (rea tm en t in that particular type of industry. The EPA regulati o n s further define rece iving streams a s "effi ue nt-Iimited " a nd ~water~qtia litY ~ l irrilied".Aii e.fflue nt-limir ed stream is a stream that will mee t its in-stream sta ndards if a ll discharges to th a t stre a m meet lhesecondary-treatment a nd "best- ava ilable-techno logy" standards. Municipalities and indu stries d ischarging to effluent-limited streams a re assigned discharge permits und er t he Natio n a l Ib lluti o n Disch a rge E limin a ti o n System (N PDES); these permit s reflect th e seconda ry trea tm e nt and best-ava il a ble-tec hn ol ogy sta ndard s. A \Vater-quality-limited stream wo uld nor meet th e proposed in-stream s tandards, even if a ll di sc harges met seco nd ary -treatment and be s t ~ava ilabl e- te c hn o l ogy levels.
TRE .~n l ENT .~'JD DISPOSAL 213
Table 5-3 U nit opera tion s, unit processes, and systems for wastewater trea tm ent Contaminant
l !ni!
Suspendeo solio s
SeutmentalHHl
OPCf:11J()Il. LIllI{
Sc reeni n g
3Jld
process. o r
Ircatme n!
system
commi nu tion
Filtration yartali o ns Fl o tatIon
Chemical-p\1Jymt: r additIon
C oag ulati on sedimentati on Land trea t ment sys tems B ll)degradabk org anll:::'
.<\cll\'aled- ~ludge \'a r!at ion~
FIXed- film Irickllng filler s I-j.xt.:d.nlm ro tating bIol ogical
cont a CI OfS
La goon and oxidatIOn pond variation s:
5-3
In te rm ittent ~ and fi lt ration
TERMINOLOGY IN WASTEWATER TREATMENT
Land Ircatlll t: JlI
s ) s tt:m ~
Ph ys l c
.~-
........
The termin o logy used in wastewater trea t men t is often confusing to the UI1 initi a ted person . T er m s s uch as unit o perati o ns, unit processes. reaClOrs. systems. a nd primary, secon'Clary. a nd terti ary trea tm ent fr eq ue nt ly appear in I he lite ral ure. and th eir usage is no t a lways cons istent. The mean in gs of th ese te r ms. as us eci i'n thi s text. are discu ssed in t he fo llowing paragraph:.. M eth ods u sed fo r trea tin g muni c ipal was tewaters are often referred to as either unit o pera ti o ns or unit processes. G enerally. IIllil opl'rario/l s invo lve COll. tamin a nt remova l by phys ica l fo rces, whil e un ir proc('sses in vo il e biologica l and / or ch emica l reactions. The term reactor refers to the vessel, o r containment structure. al o ng with all . o f its a ppurt ena nces, in which th e ullit operation or unit proces, takes place'.' A lth o u g h unit o pera ti ons a nd processes a re natura l phe nomena. they llla y be initiated, enhanced. or o th e rwise con tr o ll ed by a lt ering the environment in th e reactor. Reacto r design is a very important aspect of \\astewater tre~ltmel1t and requires a thorough und ers tc)n d ing of the unit processes a nci unil operallon s invo lved. A WQsreW(lrer-O'eallneI11 syslem is composed o f a combination of unit operations and unit processes designed to reduce certain C{)nst itu e nt ~ of wastewater (0 a n accep ta ble leve l. Many differe nt co mbin a ti o n s are possib le. Al t ho ugh piau ica lly a ll wastewa ter-trea tmen t systems a re unique in some respect s. a geil e ral grouping of unit opera ti ons and unit processes acco rding to target co ntamlnan! s has evolved ove r th e years. Unit opera ti ons a n d processes commonly used in was tewa ter t"rea t men t are li sted in Tab le 5-3 and are a rra nged according 10 co n ve nti ona l grouping. Actually, o nl y a few wastclVater-trea tme:lt methods fall com ple tely int o one ca tego ry. Thu s th e usefuln ess of thi s c lassification sys tcm is so mewhat comprom ised . Munic ipal wastewater-treatm ent sys tems are llften d ivided int u primar y, secondary , and te rti a ry su bsys tems. The purpose of {)lil/lIlrr /r (,lItnJl'lI/ is ( 0 remove so li d materi a ls from the incoming was tewat er. Large debl'is may bt: rcmoved hy
system ::.
( ' hlorln alllHl Il ypochln n n:ilion 07on~ltlon
I. and IrC<.Jtm l' llt Nutrient s' Nitrogen
:' ) S ICIll S
Su~pendcd-gr O \\'lh n itnn C
!' I,\cd -fi ll1l Illtrlfi c at lon and dC!1llriti cali o n vaflatl ollS ':\JllIl1 01l1a ~ trlrping I\)Jl exc han ~t'
,Br~i,l ~p.o,ip,t, ~,h,l p.r.1I~~ ~l.Q I),
La nd t reatment sys tems
Phosphor",
Rcfr ;\ctO I y
o rg;IJli c~
ivktill-I..all addition Lime cOilculatio n sedilllt."nt
l;1IHllrcallll t'11 1 sys t C' Jll :" Chemica! pr c'( lpilall01l l Oll ~'(Chilllg.l'
Land 1[(' ;11111 (' 111 s:s tCnl S I(l n t:,\ (h an gc.' R CH:r:-;l' o...,mn:-;JS Fkct r o d i;.lly . . ls
214
ENG tNEERED SYSTEMS FOR WASTEWATER TREATMENT AND DISPOSAL 215
WATER
Screens
Prima ry cla rifier
'/~
Emu ent to secom13ry
Raw
trea tmen t
w as te wa ter
Part of I stud ge returned
Co mminut er (grinding)
,
Undernow to sludge trea tm ent Figure
5-2
Effluent to .furth er treatment or to stream
From primary -----+1 trea tmen t
Sett led sludge
r ·- -----
J
/
To sludge Irea tment
Typical primary treatment system.
(a)
sc reeilS or may be red uced in size by grindin g dev ices. In o rganic so lid s are removed in grit channe ls, a nd much of the o rganic sll spended so lid s is re moved by sedimen tati o n. A typica l prim a ry trea tmenl sys te m ( Fig. 5- 2) sho uld remove approximately one- ha lf of the suspen ded so li ds in the incoming wastcwater. Th c BOD assoc iated wi th th ese so lid s acco unt s for abo ut 30 percent of the influ ent BOD. Seconda ry treatm ent lI sua ll y con sists of bi o log ical co nversio n o f disso lved and co ll o id a l o rgani cs int o biomass that ca n subseauent ly be removed by sedimentati o n. Co nt ac t between microorganisms and the orga ni cs is o ptimized by suspendin g the biomass in the was tewa ter or by pass ing the wa stewa ter over a film of biomass att ached to so lid surfaces . The most co mm o n suspe nded biomass sys tem is the activa ted-s ludge process shown in Fi g. 5-3a. Recircu la ting a portion of the biomass maintains a large number of o rganisms in contact with th e \vastewnt er and speeds . ·lip ·t"tie· c·on·verslo n ·pr-ace·s·s·. 'The classical aHacheclcbiomass syste-m is thetTickling filter show n in Fig. 5-3b. Stones or o th er so lid media are used to increase the surface a rea fo r bi o film gro wth . M at.ure bioilims peel o fl' the surface and a re washed o ut to th e settlin g bas in with the liquid und erfl ow . P a rt of the liquid effiu ent ma y be recyc led thro ugh the system for additi o nal trea tment and to maint a in o ptima l hydrau lic fl ow ra tes. Second a ry systems produce excess bioma ss that is biodegradab le thro ugh end oge no us catabo li sm and by o ther microorgani sms. Seco nd ar y s lud ges are usua ll y combined wi th p rim a ry slud ge fo r furthe r t reatmen t by anaerobic biological processes as show n in Fig. 5-4. Th e result s are gaseo us end product s. pr in cipa lly methane (C H 4 ) and ca rb o n di o xide (C0 2 ), a nd liquid s a nd inert so ii ds. The methane has significant hea tin g value and ma y be used to mee t pa rt of the power req ui reme nt s of th e trea tment pla nt. Th e liquid s contain la rge concentr Ct ti ons of organ ic compo unds a nd are recyc led thro ugh the trea tm ent plant. Th e so lid res idue has a hig h mineral co ntent and may be used as a so il cond iti o ner a nd fer til ize r o n agric ultura l lands. Oth er means of so lid s disposa l may be by inc inerati o n or by la nd tillin g. Sometimes prim ar y a nd seco ndar y trea tmen t can be acco mpli s hed toge th er. as shown in Fi g 5-5. Th e oxid:1ti o n pO lld (F I!"-. 5-5(/ ) most nea d )' a prrnxi l11at es
From
'-~
rrim ary _-"lr_~~~~~~~~5!-------1
lrca tm cn t
Effluent to ___ further treatment or 10 stream
Trickling fitter Effluen t recycle To sludge trea tmen t (b) Figure 5-3
Secondary trea tment system : (a) act iva ted sludge system and
(b)
trickting filter system.
nalura l systems. wi th oxygen being suppliee! by a lga l photosynthesis and surface -. reitera ti on. This oxygen se ld o m penetrates to th e bo tt o m of the pond, an e! the so lid s that settle are decomposed a naerobica ll y. In the aera ted lagoon system (Fi g. 5-5b) oxygen is supplied by mechanical ae ration, a nd the entire depth of the pond is aerobic. Decompos iti o n of the biomass occu rs by aerobIc endogenous ca tabo lism. Th e sma ll quantity of excess sludge that is produced is ret a tned l!1 the bottom sediment s. In mos t ·cases . seco nda rv treatme nt of municipal wastewater is sufficient to meet ~ffi uent stan·d ard s. In ~o m e instances. however. addi ti onal treatment may be requ ired . T eniary lr('(/ tm ellt most oft en involves furt her remova l ofsuspcnded su iids ,md /or the remova l of nutrients. So lids removal may be acco mpltshed by tiltrati o n. ,; nd ph osp horus a nd nitrogen compounds ma y be rem oved by combinati ons o r ph ys ical. cbemica l. and bi o log ica l processes.
216
ENG INE ERl D SYSTEMS FOR WASTEWA TER TREATMENT AND DISPOSAL
WATER
217
'"\ Excess water to primary cla rifi er
-
Sl ud ge from primary and se'condary c1ar'i fiers
-
Gases CH 4 , CO 2 , N H 3 , e lc.
I I
\1
Diges led sludge 10 - di sposa l (o r me chani ca l dew a lering)
Figure 5-4 Slud ge Irealme nl syslem.
A carefu l inspec li o n of Figs. 5-2 through 5-5 leads to an interest ing observati on. The " remova l" processes in was tew ater treatment are essenti all y concentra tin g. o r thickenin g. processes. Suspended so li ds a re removed as slud ges. and dissolved so lid s are co nverted to suspended so lid s and subsequentl y become removable slud ges. Ham mer [5-2 5J sta tes that prima ry and second a ry treatment. fo ll owed by slud ge thi ckenin g. ma y co nce nt ra te organic materia l rep resented by 250 mg/L of suspend ed so lid s and 200 mg/L BOD in 375 L of municipal wastewater (the average per capita contributio n) to 2.0 L of slud ge co nta ining 50,000 mg/L of so lid s. M ost of Ihe o bjectio na ble materia l initiall y in the was tewa ter is co nce ntrated in the sludges a nd mu st be di sposed of in a sa fe a nd enviro nmentall y acce pt ab le manner. Vesi lind [5-5 5J no tes th a t a maj o rit y of the ex penses , effo rt . and pro bl ems of wastewa te r trea tment a nd d isposal are associa ted with the slu dges. Design of was tewa ler-treatmen t systems is an important pa rt of a n enviro nment a l engineer's wor k. A th o ro ugh underst anding of the unit ope rati ons and processes is necessary hefore the reac to rs can be designed. Th e fo ll ow in g secti o ns of thi s cha pter arc devo teclt o the va l'io us unit o perations and processes co mm o nl y used In treat ing muniCipal wastewa ter Man y of these are simi la r. if not id ent ical. tv those used in prepar ing powb le water. With the excepti on of nutri ent remova l. tertia ry treatment opera ti ons fo r lI'astewa ter invo lve essentiall y the sa me principles useci in prepal' ing wate r of poor che mica l qu a lit y fo r a po table supply. Wh ere material wo uld he clu[lli ca led. th e rea cler is referred back to C ha p. 4.
Primary Treatment
--+----........
Wastew,11 CI' CO Ilt[\II1S a wid e va ri ety of so lid s of va ri ous sha pes. sizes. a nd c1ensi t ies. lfTec ll ve remova l (lf lhcse so lids may require a co mbinat io n of unit operati o ns sli c h :ISsc ree nin g. grind ing.
To furth er Ire" lm en l Or Iu stream
(a )
) ,1/
, 1/
Raw waS l ewal er
/t' /"" ,v.
Surface ae ral ors
.
/ 1'
,1/
/I'
,~/' ~/
/1"-
To fUriher
5-4 SCR EEN I NG
Irea l menl o r XO s[ream
/1'
;,r
(b)
Figure Prima ry ~nd seco ndar y w;lIer treatme nt In comorna!lOI1 . ( (I) lag oon 5-5 .
Screenll1l( de:II CCS ,liT used to remOve coarse solid s from wa stew a ter, Coarse 'io\i ds u)~)s i st of st icks. r;lus. bo'lrei s. and o th er lar ge objec ts th a t oft en ancl. in cx [lllc lbil.lin
I allo ' n pond and (h)
O'if
acraled
,
S
218 WATER
ENGINEERED SYSTEMS FOR WASTEWATER TREATMENT AND DISPOSAL 219
Direction of flo w
~ : • • :. ~ o· - .•. - . . . . . . . .
Section
o
Plan (0)
Wastewater screens are classified as fine or coarse. depending on their constru ction. Coarse screens usually consist of vertical bars spaced I or more centimeters apart and inclined away from the incoming flow. Solids retained by the bars are usually removed by manual raking in small plants, while mechanically cleaned units are used in larger plants. Fine screens usually consist of woven-wire cloth or perforated plates mounted on a rotating disk or drum partially submerged in the flow, or on a traveling belt. Fine screens should be mechanically cleaned on a continual basis. Typical screening devices are shown in Fig. 5-6. Screening devices are contained in rectangular channels that receive the flow from the collection system. Manually cleaned devices should be readily access ible for cleaning. and mechanically cleaned systems should be enclosed in suitable housing. Proper ventilation must be provided to prevent accumulation of explosive gases. A straight channel section should be provided a few meters ah ea d of the screen to ensure good distribution of flow across the screen. Hydraulically, flow veloCity should not exceed 1.0 mls (3.3 ft /s) in the channel, with 0.3 mls (1 ft /s) considered good design. Head loss across the screen will depend on the degree of clogging. Clean bars and screens result in a head loss of less than 0.1 m. Provisions' should be made for a head loss of up to 0.3 m for manually cleaned or for manually operated , mechanically cleaned screens. The quaniity of solids removed by screening depends primarily on screenopening size. The quantity of screenings removed from a typical municipal wastewat er as a function of the screen size is illustrated in Fig. 5-7. Screened solids are coateo with organic material of a very objectionable nature and should be promptly disposed of to prevent a health hazard andl or nuisance condition . Disposal in a 'sanitary landfill. grinding and returning to the wastewater flow. and incineration are the most common disposal practices. . 100
'"~
:':! . ~
'0
E '<>
a ~ ~
90 80 70 60 50
"" S
40
c::
"~
30
'-0
20
E
10
~
0 (b)
Figure 5-6 Scree nin g dt,; \'ices used in wa Sl e\V :ll e: trea tmt' nt (u) ma nuall y clean ed ha r r; H.: k (li"om Steele
r
[/ nd M cG hee 5-501 (b ) 1l1 t:l:h a n ic all y clean ed bar scree n (co ll rfe5Y ( ~/ E1Jl"irex IlI c._ (/ R exl10 r d Com pany).
0
3
4
5
6
7
Openin g be tween bars, c m Figure 5-7 Quantit y of screening from municipal wastewat e r as a function of bar spacing using rnechani cally clea ned bar sc reen s. (From tlie/cal(& £ddr, In • . [5 ·36].)
I
(-
ENG I N EERED
SYSTE~ I S FOR WASTEWATER TREATMENT AN D
DISPOSAL
221
220 WATER
5-5 COMMINUTING As mentioned ,above. screenings are so metimes shredded a nd returned to the wastewater flow , A hammermill de vice is most oft en used for this purpose, More often, a shredding device ca lled a comminu/or is loca ted across the flow path and intercept s the coarse so lid s and shred s them to approximate ly 8 mm (i in) in size, These so lid s remain in th e wastewater. Man y kinds of comm inut o rs are avai lable. Ba sic pan s include a sc ree n and cuttin g teeth·. The screen ma y he a slo tted drum th a t rotat es in the vertica l plane. Sta ti onary teeth th en shred material' th at is inte rcepted by the sc reen. Other types use a sta tion a ry semicircu lar screen and rotating or osc illating cu tting teeth. Another device. ca lled a borminll/or. uses a ve rti ca l bar sc reen \v ith a cuttin g head that trave ls up and down the rack of bars. shredd ing the intercepted materiaL T yp ical sh redd in g unit s are show n in Fi g. 5-8. Channe l des ign for comm inut ors is similar to th a t for sc ree r~s. Since material does no t acc umul a te on the device. head loss rarel y exceeds 10 'Clll (4 in). ('0111minutors are hi gh-maint ena nce it ems. a nd provis io ns sho uld be made to bypass the unit when repa irs are needed. In sma ll plants. bYP:l sS thro ugh a bar screen is usuall y provid ed. Large r plant s may o perat e seve ra l comminutors in parallel so that flow from o ne o r more di sabled units may be proportioned through the remaining unit s.
,.,~.
Shredding de\ices should be loca ted ahead of pumping faci lities at the trea tment plant. Grit remova l ahead of the shredder will S
5-6 GR IT REMOVAL Municipal waste\vater cnnt;llns :1 \\'ide asso rtm ent of in organic so lid s such as pebbles. s:lm!. silt . egg shelb. gl ass. and metal fra gments. Operations to remove the se in (lI'ga nics \\ ill :list) rem ovc some urthe larger. heavier organics such as bone chips. seeds. and coO'ce and tea grou nd s. T oge ther. these compose the mat erial kn ow n a<; .r;ril in \\astl'wa ter treatment sys tem s, Most of the suilsta nces In gr it ,II-e abra sive in nature and wi ll ca use accelerated wear un pumps and sludge-h:lndling equipment wi th wh ich it comes III con tact. Crit deposits in :!reas of 10\\' hydraulic shear in pipes. sumps. and cl,Hifiers ma y abso rb grease and su lidify. l\dclltl o nall y. th ese materia ls are not biodegrauable ,Inc! occupy valuable spa ce in sludge digeste rs. It is therefore desirable to se parat e them fwm the organic suspended so lid s. Beca use inh ltr:ltion is a major so urce of Inorganics, th e quantity of grit va ries with th e type, age. ;Ind co nd itlun o r the pipe in th e co ll ecti o n sys te m. Th e type and quantity of industria l \\'aste ,Inti the prc\'alence of domestic garbage grind ers are 3 also cOllll'ibuting f;lctor". Q U;lIlt il ies rangi ng from 4 to 200 m / 10" m} have been reported, with a typical \a lu e llf.arll und ISm } ' 10" m' of wastewater. [ 5-36J .G.ri t . rCI1!Qya I. rac:i.Ilt.ies,. \)as lc;t1I,Y ..co.nsist ..0.1'. ,In .en Iarg(:d .cha[j[je l area where redu ced ~l ow \'elocitics ;d lo\\' t' rit to settle Ollt. Many cl;nfig urati ons' o(g'r;t ta nk s arc availahle, wit li the 1ll0,t rCl'Cn t 11lS 1,dlali uns usual ly being channel-t ype or aerated rectangular basins such as those shoWIl in Fig. 5-9. Th e deposited gri t is removed by mec hanical sc rapers.
-
'I '
I
(a)
Figure S-8 Typical shreddin g dcvices osed in wastewaler Irealmenl rOI~ lin g lcelh behind Ihe slaliona ry stollcd drum: (b) barnllnUlOr with
II I
(b)
plants : (0) con"n lllUlor, Willi Iravcltng CUlling ItraJ (Uili/'/('S)'
Fi.l:!Ufl' 5-9 T )plcal gfll I Cllhn all'qll lpnh:nl cIJallllL'I-!ypc Ch;ll11 an d hucktt grn chamher I:'l/l"Irl'X /11('
of C/O ... CorpOral/Oil),
{f
UI'\,I1f1rd ("0111/)(11/1'1
(cOllrtL\T o j
ENGINEERED SYSTEMS FOR WASTEWATER TREATMENT AND DISPOSAL 223
222 WATER
Weir plate
SOLUTION
1. Assume a rectangular cross section with depth L5x width at maximum flow Ax =
W X
1.5w = 1.5w 2
Q
Opening for
now passage
m3 s d min 10,000- x - - x xd 0.3 m. 1440 min 60 s
= 0.39 mi '
w=0.5Im
D=0.76m 2. Assuming a settling velocity of 0,02 m/s. the detention time is
I--- Theoretical
\
I
\
c"
sectIOn
=
Dlv,
= 0.76 m /(002
\
='
38
m/s)
5
3. Determine length. ~_ _---'
L = c"v. = 38 s x 0.3 mls
Practi cal sect Ion
= 11.4 m
Tank dimensions are therefore \\' =
(b)
D = 0.76 m
.. Figu
Hydraulically. grit chambers are designed to remo\·e. by type-I settling. discreie particles with diameters of 0.2 mm and specific gravity or2.65. In channeltype. horizontal-flow grit chambers (Fig. 5- 9). it is important to maintain the hori zontal velocity at approximately 0.3 m/s. A 25- percent increase ma y result in washout of grit. while a 25-percent reduction may result In retention of nontarget organics. Since a wide variation in flow rates may be encollntered. the horizontal velocity must be artificially controlled. A proportioning weir on the effluent end of the tank (Fig. 5-IOa) or a parabolic tank sect Ion (Fig. 5-1 Ob) is often used to maintain steady . flow at 0.3 m / s. The design of channel-type' grit chambers is illustrated in Example 5-1. Example 5-1: Designing a channel-type grit chamber A grit chamh'er is designed to reo move particles with a diame ter 01"0.2 mm. specific gra vity 2.65. Settling vel ocit y for the se particles ha s been found to range from 0.016 to 0.022 m /s. dcrcndin g on their shape fa ctor. A flow-through velocity of 0.3 m!'i will be maintained by a rropo i"tl onin g weir. Determine the channel dimensions for a max imum wa stewater now of IO.OOIl m ·l /d.
0.51 m
L = 11.4 m
In larger treatment plants. the' trend is toward aerated grit chambers. Turbulen ce created by the injection of compressed air keeps lighter organic material in suspension while the heavier grit falls to the bottom. Since roll velocity. rather.than horizontal velocity. serves to separate the .no.ntarget organics from the grit, artificial control of the horizontal velocity is not necessary. Adj"ustment of air quantit ies provides settling control. The design of aerated grit chambers is based on detention time at peak flow. Typical design parameters are shown in Table 5-4. Aerated grit chambers may serve another useful purpose. If tlie sewage is anaerobic when it arrives at the plant. aeration serves to strip noxious gases from the li4 Uid and to re store it immediately to an aerobic condition, which allows for better treatment. When an aerated grit chamber is used for this purpose. the aeration period is usually extended from 15 to 20 min. Grit. particularly from channel-type grit chambers. may contain a' sizable fra ction of biodegradable organics that must be removed by washing, or must be disposed of quickly to avoid nuisance problems. Grit containing organics mu st either be placed in a sanitary landfill or incinerated, along with screenings, to a sterile ash for disposal.
224
EN Gt NEERED SYST EMS Fon W ASTE WA TER TREATMENT AN D DISPOSAL
WATER
Table 5-4 Design parameter for aerated grit chambers Value Range
hem Dimensions: Depth, m Length. m Width. m Width-depth ratio Detention time at peak flow. min Air supply. m J I min . m of length Grit and scum quantities' Grit . mJ!lO J m'
2·· 5 7. 5- 20 2.5· 7.0 I : 1- 5: I
T yp ical
2. I
2- 5 U.15 - U.45
U.3
0.004- 0.200
0.01 5
SOllree: From Metcalf & Eddy. lnc . [5-36]
5-7 FLOW MEASUREMENT Although the measurement of wastewater flows does no t in itself result in removal of contaminants. it is an important adjunct to wastewater treatment. A knowledge of hydraulic loading rates is necessary for the operation of many of the reactors in a wastewater-treatment plant. Chemical additives. air volume. recirculation rat es. and many other operating parameters depend upon the hydraulic flow rate . . Additionally, records of flows should be kept to establish trends in flow quantities .. . .... for.eva+uation·of.infiitrationjinilowq·uantities and to estimate future capacity need s. The most common devices used for measuring flows in a wastewater-treatment plant are Parshal flumes and Palmer- Bowlus flumes. These devices. essentially open-channel venturi meters. have an established flow-head relationship from which the flow is determined by simply measuring the water elevation at a gi ven point. Continuous-stage recording devices can be installed to provide flow records. The hydraulic design of flow-measuring devices is beyond the sco pe of this text. and the interested reader is referred to other texts. See for instance. Refs [5-50. 5-30.5-48].
5-8 PRIMARY SEDIMENTATION Primary 's edime'l)tation is ;1 unit operation de'signed to concentrate and remove suspeqded organif.: ~olids from the wa~tewater. When primary treatment was considered sufficient as the total treatment. primary se ttling was the most important operation in the plant. Its des ign and operati o n were critical in reducing waste loads to receiving streams. With the current universal require ment for secondary treatment. primary sedimentation plays a lesser role. Indeed. many of the secondary wastewater-treatment unit processes are capable of handling the
225
organic solids if good grit and sc um removal are provided for in preliminary treatme nt. The th eo ry and prac ti ce o f primary settling operations in wastewater are esse ntiall y the same as th ose fo r clarifying water for potable supplies. and th e read er s hould review Secs. 4-4 an d 4-5 before proceeding. M os t o f the sllspended solid s in was tewater are " sticky" in nature and floc culat e na turally. Prim a ry settlin g o pe ra tions proceed essentially as type-2 settling without the addition of chemical coag ulants and mechanical mixing and flocculati o n o perations. The o rganic mat e ri a l is slightl y heavier than ' water and settles slowl y. usua ll y in the range of fr o m 1.0 to 2. 5 m/ h. Lighter materials. primarily oils a nd g rease. fl oat to th e surface a nd mu st be skimmed off. Prim a ry sedim entatio n is acco mplished in either long-rectangula r tank s o r circul a r tanks similar to those desc ribed in Sec. 4-5. Scum removal in rectangul a r tank s is accompli shed by havin g the s ludge scrapers penetrate through the surface as the y return to the effluent end o f the tank . Floating material is carried to a collec ti o n point so me distance be hind the effluent we irs where it is re mo ved o ve r a sc um weir or by a transverse sc um sc raper. Circ ular ta nks have a skimmer arm att ac hed to th e sludge-scraper dri ve mechanis ms. The scum is wiped up an inc lin ed apron and into a scum tr o ugh for removal. In botll cases, a scum bafl'le sh o uld be locat ed between SC Uitl re mov al faciliti es and the efl'luent weir. Th e m odifi cations necessar y fo r scum remova l are s ho wn in Fig. 5-11. Separated sc um is usua ll y dis posed of with sc rec nin gs, un washed g rit , or digest ed sludge. D es ig n crit eri a fo r prima ry sed i me n tat io n ta n ks are presented in Table 5· 5.
Table 5-5 . Design criteria for primary sedimentation tanks Val ue
D ete ntion time, h O ve rfl ow rate , m J/ m ' . d Ave rage 110w . Pea k fl ow \Vcir l oad i ng, m J/m · d D im t:nsio ns: m Rcctan g ula r Dcp lh Lcnglh Wid lh * Sludge scraper speed. m Ci rc uia r D e pth D iam e te r
T ) pi c al
R a ll !-!L'
Para mdc r
111111
2.0
32 - 48 80- 120 125· 50(1
250
1 ~
} .il
100
25 - 40
15 -90 } 24 11 .0 I 2
6· 10 1.0
4 .1 1° 4 1
; .h {,(I 6()
Bo tt o lll Sll.lPC, Tll m : 1ll
Sludge sc raper .s peed, r m ill
'" iVl ust divide inl\) b;l\ :;' of
I.'i 2.5
,U
IW
('I !H
00 2 Il 0\
no1 !!rclt l '\ 11l~11l
for mec han ical s ludge rCIl~oval cqui;lllc ll l SOIlI' ,,,: From M e tc alf & Fd ch . Inc. [1.36J
6.0
III
\\"1(_k
ENG INEERED SYSTEMS FOR WASTEWATER TREATMENT AND DISPOSAL
226
227
WATER
lnl el well
In large plant s. th e use of several rectangular tan ks with common walls reduces co nstru ct ion costs and sp ace requ irements. Sma ller plants tend to use circular tanks because of the simplicit y of slud ge removal. Some settling basin arrangements common ly Llsed in primary trea tment are shown in Fig. 5- 12. Sl ud ge should be removed from the primary sedimentation tank before anaerobic conditions deve lop. If the slu dge begins to decompose anaerob ica ll y. Drive unit
Drive -shaft Scum bafne
Scum trough
Supports
Budge
rr~~~g~~~~~~~~§~~~b~~~~~~d~~~~~~~~
Eftluen l weir
Ernucnl launder
12
Rep laceab le
r-------w:-:-e l-· r-p~l~a-l-e+------J
Scurn
Sludge drawoOff pipe
pil
urelhane strip
la)
(a) Alternate weir location ( ' ; \
(center takeoff)
Bridge
\..J
Maximum water
Alicrntl.te weir local ion (rim takeoff)
8
Handrailing
Efnuent pipe
12 Sludge drJw -off pipe
(b)
Traveling bridge
Bridge travel Collecting W31c r
level
Sludge collt:ctlon PO Sition
.....
Emu"nl
(b) }. j)! urt' 5- 11 Scum 1I..:muv ai from surface (If pf1lll
((II dlagLlI1l
(it .... klllllllln~ dl..:\lc~ (courtesy
In/dco f)f!yr eI1l0flf, ' /c) : (b)" SClllTl lrough arr
I h(//{h lim/ ElIllr(}nml'II/(t/ .)'cil'nc('s).
/)/'f'(1rtnlL'f1f
(~(
(c)
Figure S-12 T ypica l pr imary clorifiers: (a) circular basin, cenler feed Ufom lv/e tcalj & Eddy, In c. [5-J~)), (/» CIrcular baSin. rim feed (from Metcal} & Eddy [ 5-36J): (e) long-rectangular basin with Iraveling bridge sludge scraper (collrrnr of FMC Corpuratioll, Mat erial Hamllin" Sl'stems DiL·isionj .
ENGtNEEREO S YSTE~ IS [OR WASTEWATER TREATMENT AND D ISPOSAL
229
SOL UTIO N
80 1.
\
70
. I~","d'd
35~~3h;;>d
,olid ,
2. Using a circular tank. the d ia metcr
60
~
-;;; > 0
E
SO
~
C
"-
From Fig. 5- 13. a n overllow rate of 35 m 3/ 1ll 2 . d should yield a suspended-solids remova l efficicncy of abollt 60 pe rcent. Required surrace area is
40
'-....
30
~
, =( 4--n/1)
I 2
/2 ( :.2_~)' 1 14
14 ] x 3 = 429
~
4'
l)
m'
-~ :.. ---.-- = 0.09 ~ ll()() Ill ' ' d
~ l"'-....... .
4.
-: - --_ ... '
14 ] m 2
20
40
60
Ove rOo w rale, mid
80
100
Figure 5-13 Suspended solids and BOD remova l as a runction or ove rfl ow ratc. (Adapted }rom S teele alld M cGhee [5-50].)
gas bubbles will be produced and will adhere to so lid particles a nd lift them toward the surface. This reduces the compactness of the sludge and makes remova l much less efficient. Sludge removal systems sho uld be designed to move sl ud ge from the farthest point in the taBk·1O the sludge hopper within 30 min to I h of whe n it settles. 'Removal from the hopper to the digester should be made at frequent intervals. The quantit y 'of slud ge removed in primary sed imen ta tion Illay depend on several variables, including th e strength of the incoming waste, the efficiency of the clarifier. and th e co nditi ons of the sludge (i.e., spec ific gra vit y. wa ter conten t, etc.). Removal efficiencies of well-designed primary tanks depend upon overflow rates, as shown in Fig. 5c 13. Average suspended-solids remova l for well-operated systems should be around 50 to 60 percent. BOD removal relates on ly to the BOD of the solids removed , since no dissolved organics are remo ved and bi oox id a tion in the primary s~ ttlin g tank neg li gib le .
!s
Example 5-2: Designing a primary settling basin A municipal wastewater-treatment plant processes an average f10\\ of 5000 Ill' /d. with peak fl ows as high as 12,500 Ill ' /IJ. Design a primary clarifier to r<:mo\<.' approximately 60 percent of the suspended solids at a vcrage . flow.
d = 2. 06 h
At pc" k now conuJl ions. the o"e rr]o \\" rate is 1/500 m·l d
o
tn '
and the detention ti me at ''''crage fl0\V is
20
10
13.5 m
3. !\ ssu ming a sid n vall dept h of 3 n1. ,,,Iu me of tank is approximately
..........
~
143 m 2
IS
I
'"~
-
=
87 mid '
and the detenti on lime i<; "pprn xlIl1'l tely SO min (a lillIe low). From Fig. 5-13. the <; uspcnded-solids removal ellic icncy dr ops to about 3Rpercent for peak flow conditions.
Secondary Treatment The effluent from primary treatm ent stdl contain s 40 to 50 percent of the original suspended solid s and virtuall y all oftht: o ri glll
,
-1 r1 f
230 WATER
2;'
k
ENGINEERED SYSTEMS FOR WASTEWATER TREATMENT AND DIS POSA L
231
,';([>' I)
culture utilizes the food source most suitable to its metabolism. Most mixed cultures will also contain grazers, or organisms that prey o n other species. The newly created biomass must be removed from th e wastewater to com plete the treatment process. The microorganisms involved in wastewater treatment are essen tially the same as those that degrade organic material in natural freshwater systems. These organisms and their metabolic pathways were desc ribed in Secs. 3-7 a nd 3-8. Th processes are not allowed to proceed in their natural fa s hion , however, but are controlled in carefully engineered reactors to optimize both the rate and completeness of organic removal. Removal efficiencies that would be effected over a period of days in natural systems are accomplished in a period of hours in engineered systems. Design of biological sys tems requires an understanding of the biological principles, kinetics of metabolism, principles of mass balance, and physical operations necessary to control the environment in the reactors. Basic biological principles were disc ussed in Secs 3-7 and 3-8 and sho uld be reviewed by the reader before proceeding. The fo llowing sections describe the kinetics of biological growth aild substrate utilization and the principles of reactor des ign.
5-9 GROWTH AND FOOD UTILIZATION The relationship o f cell growth and food utilization can be illustrated by a simple batch reactor such as a stoppered bottle. A given quantity of a food containing all the necessary nutrients is placed in the bottle and inoculated with a mixed culture of microorganisms. If S represents the quantity of soluble food (in milligrams per liter) and X represents the quantity of biomass (in milligrams per liter), the rate of .. .tiiliizatlon' 6((6od dS jdi 'a'ridifie' f11ie 6fbiorriass' grow th' dX jdt can be represented by curves as shown in Fig. 5-14. There are several distinct segments in the biomass curve that warrant further examination. The microorganisms must fir st become acclimated to their surrounding environment and to the food provided . The acclimation pe riod, called the lag phase, is represented by segment 1 on the curve and will vary in length, dependlllg on the history of the seed organisms. If the organisms have been accustomed to a similar environment and similar food, the lag phase will be very brief. Once growth has been initiated, it will proceed quite rapidly. Bacterial cells reproduce by binary fission; that is, cells divide into segments that se parate to become tw o new independent cells. The regeneration time, or the time required for a cell to mature and separate, depends on environmental factors and food supply and may be as short as 20 niin. Wh!e n maximum growth is' OCcurring, the rate of reproduction is exponential according to th e equation (5-1) where N is the number of organisms produced fr o m one indi vidual after 11 regeneration times. Maximum growth thu s occurs at a loga rithmic rate, and segment 2 on the grow th c ur ve is called the log-grOlvrh phase.
Endogenous phase
c:
.2 c: 0)
u
c:
o
U
Time - - Figure 5-14 Biomoss growth ond rood utilizati o n .
Maximum growth cannot continue indefinitely. The food supply may become limiting, environmental conditions may change (i.e., overcrowding, waste-product buildup, etc.), and a population of grazers may develop. Cells that are unable to obtain food from externa l sources begin endogenous cataboJism or the catabolizing of stored protoplasm for maintenance energy. Other cells die and lyse, or break open, releasing their protoplasm, which adds to the available food. Segment 3 of the curve, the stationary phase represents the time during which the production of new cellular material· is roughly offset by death and endogenous respiration. Although some reproduction continues beyond the stationary phase, endogeno us respiration and death predominate in segment 4 of the curve. In this final S/1(Ii~g"n()tls phase, biomass s lowiy decreases, approaching zero asymptotically after a very long time. The most common method of quantifying biomass is the sus ended"solids test. When the wastewater contains only soluble organic material, this testiliould - be fairly representati ve, alt00ugh it does neit distinguish between living and dead cells. The volat ile suspe nded-so lid s test is a better test when the wastewater contains a sizable fraction of suspended inorganics. Neither test will differentiate between biolog ical so lids aJld organic particles originally in the wastewater. In the log-growth phase. the biomass increases according to
ix
--= kX dt
dX where ... - = the growt h rate of the biom ass mgj Lt
(I!
X = the conce nt ration of biomass, mgj L k
tl:: growth rate constant, r -
I
(5-2)
ENGINEERED SYSTEMS FOR WASTEWATER TREATMENT AND DISPOSAL
233
232 WATER
Direct evaluation of the growth rate constant is impossible for mixed cultures of microorganisms metabolizing mixed organics. Several models have been developed, however, which indirectly establish a _alue....£ k. The most widely accepted of these is the Monod equation. [5-37J This equation assumes that the rate of food utilization, and therefore the rate of biomass production, is limited by the rate of enzyme reactions involving the food compound that is in shortest supply relative to its need. The Monod equation is
~o S 0' ~K:,
is a first-order equation in biomass; that is, the growth rate rx is proportional to the first power of the biomass present. When S <:g Ks. the system is food-limited. InJhis case
r,
(5-3)
+S
I
where ko = maximum growth rate constant, t - 1 S = concentration of the limiting food in solution. mg/ L BOD, COD, or TOe ' Ks = half saturation constant, i.e'., concentration of limiting food when
k = ~ko, mg/ L The growth rate of biomass is therefore a hyperbolic function of the food concentration, as shown in Fig. S-\S. Several observations can be made relative to Eq. (S-3). When there is an K.s' then the growth rate constant k is approxiexcess of the limiting food, i.e. y mat~ly equal to the maximum growth rate ko 111 Eq. (S-3), and the systerri is enzymelimited. Since the enzymes are supphed by the microbial mass, the system IS essentially biomass-limited, and the equation
=
cops t au.l.-
and the growth rate is zero order i~ bioma-ss; that is, the gro')Vth rate is independent of the biomass present. When S = K s ' the growth rate constant is one-half the maximum as per the definition of Ks· Substituting Eq. (S-3) into Eq. (5-2), the rate of biomass production becomes
. "
dX dl
koSX K, + S
(5-4)
= - - = --- --
If all of the food were converted toJ2.iomass, then the rate of food ut ilizati'on would equal the rate of bioma ss producti o n in Eq. (5-4). Because catabolism converts part of the food into waste products, the rate of food utilization will be greater than the rate of biomass product ion.
dS dl
r, =-- }"
- Yr,
or r,
-
=
y
koSX _._0 ____
Y(K,
(5-S)
+ S)
where
(";7';'.. <: ' ~
..) I,.
•••••••••••••••
;
•••••••••
•
•
p
r
., ,
c
r;
~ o
ko
------------==------
u
"
~
.. j ..
' : f')
.
r,
dS
= -,- = rate of food lItllization_ mg: LI (I
The factor Y varies dependin g o n the metabolic pathway used in the conversion process. Aerobic processe;s are mqre efficient than anaerobic processes with respect to biomass conversion and thus' have a gre~t~r value for Y. Typical values of Y for aerobic reac.tions are aboutf0:4 td\'{l.8 'kg biomass per kilogram of BODs ,while anaerobic reactions range from 0.08 ~ 0.2 kg biomass per kilogram of BODs. ----...; Equation (5-4) is II1compkte without an expression to account for depletion of biomass through endogenolls respiration, Endogeneous decay is also taken to be first order in biomass concentration. 'I
::=
ilX -
ill
K
Limiting food concentration S, mg/ L
Figure 5-15 Monod growth rate constanl as a function of limiting food concentratioll.
(5-6)
(end)= ·- k
Where k~ = endogcncolls deca y r~lte cons tant {. Eq. (5-4) result s 111 •
,
d.\'
koSX
dl
t:. , + S
-
1
Incorporation of Eq , (5-6) into
-- LY d
.
(5-7)
234
ENGINEERED SYSTEMS FOR WASTEWATER TREATMENT AND DISPOSAL
WATER
Endogeneous decay has very little effect on the overall growth rate in the initial phases of the growth curve in Fig. 5-14. In the stationary phase, however, endogeneous decay is equal to the growth rate and becomes predominant in the enclogeneous phase. Several external factors may affect the rate of biomass produc tion ~ld fo C;d ut~o.n. These include Gr:m perat!!Ji plix aDd toxin). Rate constants increase " with increasing temperatures within the range of 0 to 55 °C, with a corresponding _ '[' increase in biomass producti~n and food utilizatlop Increases in reaction rates approxImately follow the van r Ho.fl-ArrhenlUs rule of doublIng wIth every lO°C / Trr(.! It1crease 111 temperature [5-47J up toa maxImum temperature. ExcessIve ea t ~' ~ denatures the enzymes and can destroy the organism. The pH o f the surrounding microorganism is also important. Enzyme systems have a fairly narrow range of tolerance. Microorganisms that degrade waste'( 'J water organics function best n!ar neutral plol, with a tolerance ran ge offrom abo ut jlH 6 to H 9. Other factors such as toxicants. salt concentration. and ox idant s influence biomass growth. T oxican ts- poison thc mi~roorganis~'i;]lt conccntrations interfere with internal-external pressure relafion ships. and oxidants destroy enzyme and cell materials. Microorganisms are capable of adjusting to a wide range of most environmental factors. provided changes occur gradually. Sudden changes. such as a rapid drop in pH or a s lug of salt. may do irreparable damage to the culture. Several types of reactors ma y be used in biological treat ment of wastewater. Although batch reactors may be useful in a few applications. those considered here will be co ntinuou s- flow sys tems. Reactors may contain s uspended cu ltures or attached cultures . In ~'vsf!n/de4 culL4I:es, the microorgani sms are sus pended 111 the wastewater either as single cells o r as clusters of ce lls callcd, pod' They 'ate' thLI~"" surrounded by the wastewater which contains their food and other essential elements. J1t.cached cu ltures consist of masses of organisms adhered to inert surfaces with wastewater passing over the microbial film.
,y,/"
-
235
1-----------------------------------1
!
Primary effluent!
Effluent
Secondary clarifier
I
Reactor
I )
I
waste
I
I
QR II Studge
--+-Q
Sludge)
Xu return . Qu' Xu I -X----'---------:--'
Sludge underflow
u
W'
I
L ___________S~stem ~ounda~______ .. ___ __.J
I
I
(a)
1-----------------1
I
I
I
Primary effluent I
QO'
SO·
Xo
Reactor
I
variable X and 5
I
Q + QR
X, S. )
I
.. . \. Sludge I
Secondary clarifier
Q - Q",
iX" 5,
<
~'l;!:~~ ..................... si;d;;l· ···f u
I
underflow
I
~~~~--l-----~~Xu-----J Qw'
Effluent
I II
XU)
. J I---------------------------------System boundary
5-10 SUSPENDED-CULTURE SYSTEMS
.'
Suspended-culture reactors may be of three basic types: (l) completely mixed witho.ut s ludge recycle, (2) completely mixed with s ludge recycle. and (3) plug!l ow with sludge recycle. Recycling sludge. which consists primarily of microorganisms. increases the biomass in the reactor and therefore directly affects the biomass production and food utili zation rates described by Eqs. (5-5) and (5-7).
(b)
Figure 5-16 Typical aClivated·sludge systems' (a) completely mixed reactor and (b) plug·flow reactor.
increase the available biomass and speed up the reactions. The a.ctivated-slugge proces~ is thus a ~~~Oed,,-c.ulture p[Qcess with sludge return and ·may be either a completely mixed or a plug-flow proces's, as depicted in FIg. 5-16a and b. The.' process is aerobic. with oxygen being supplied by dissolution' from entrained air.
5-1] ACTIVATED SLUDGE Completely Mixed Reactors The activated-sludge process is a suspended-cu ltu re system that has been in use since the early 19005. The process derives its name from thc f:lctth'lt settled sludge containIn g li vi ng. or actll'e. microoreanisl11 s is returned ttl thc reactor to • \ " c ~() (f (' Y..I .
r.)r
Equations (5-5) and (5-7) serve as a starling point for activated-sludge analysis. Reaction rate equations are coupled with system variables. and. for a completely
236
LN( ; INU·. RFIl SYST [ ~I S FUR WAST EW ATER TREAT ME NT AND DISPOSA L
WATER
mixed system, mass balance equations are written with reference to Fig. 5-16a. Mass balance equations are written around the entire system (dotted line) for biomass and food . At steady-state conditions. i.e .. no change in biomass or food concentrations with time, these equations are as follows: Bioma ss in
+
Biomass growth
=.
Combining these eqll:lti uns givcs Qo Y _.-- - (So - S) - k V X
1 )(
/. /Q"
VX
where Qo, Qw 'X 0' x. X,. X"
+
--- =
Q S
p~(>,
(1.
3
e
Y'
Ir"'" ) ;// /1
(5 -1 3)
(5-14)
Q,::''' '\ (
(5-9)
w
influent and waste-sludge flow rate, respectively, m /d = biomass· concentration in influent. reactor, effluent. and clarifier 3 underflow (waste sludge), respectively, kg/m So . S = soluble food concentration in the influent and reacto r. respectively, kg/m) V = volume K" ko, kd' Y = kinetic constants as defined in Sec. 5-9, kg/m) , d - I , d - I , kg/ kg =
I
= O ~-
is th e hyd raul iulc:Lt:lltion time ITl Ihe rc::tc tor based on intluent flow. Th e ra tio o f the total bio ma ss In the rC ~ lcto r to th e bi omass wasted per given time
= Food out
. koSX _ (Q _ Q)S QoSo - V Y(Ks + S) 0 w
(5-1 2)
d
The in ve rse of the ex prcss ion s Q",Xj VX and Qo/ V have uniqu e ph ys ical signifi cance ITl th e :tct i\ itlt::I -s ludge system modeled in Fig. 5-16a . The quantity
Bi o ma ss o u t (effluent .+.wasted slud ge)
(5-8)
Food in - Food consum ed
237
represent s the ave rage § e th at mi croo rgani sms spend in the-;:;;;ctorJ This parameter. called the IIji'dlI c('//- r es J(l c lI cc lir!.!!.'. wIll be grea ter than the hydraulic detenti on time sin ce mos t o f th e sludge from the clarifier is returned to the reactor. Substituting Eq s. (5- 13) and (5- 14) Int o Eq . (5-12): I - II, -
"
y( So - S) ---_._- -
OX
d
(5-15)
The co ncentratl OI1 of bi ll rnass. o r.~~.::...~~~~~~!:!....:l~.!i:),..lJ,k:u...~~
Equations (5-8) and (5-9) can be simplified by making the following ass umptions : 1. The influent and effluent biomass concentrations are negligible compared to
x
IJ, Y (S o- S) .-_._.-~~
-
O( I + "dOc>
(5-16)
biomass at other points in the system. 2. Th~ influent food concentration So is immediately diluted to the reactor co ncen-
. tr'ation S because of the complete-mix regime .. 3.. All reactions occur in the reactor; i.e .. neither biomass producti o ri no r food utilization occurs in the clarifier. Because of assumption 3. the volume V represents the vo lume of the reactor only. With these assumptions. Eqs. (5-8) and (5-9) a re rearranged as follows: (5-10)
~~
Ks + S
=
Qo.2::, (S - S) V X
0
(5- II)
Alth ough thi s equati o n IIldi cat es th:lt s hortening th e hydraulic det enti o n time in creases th e M LSS when th e oth er vari ab les are held constant. there is a limit beyond whi ch thi s is not tru e. Wh en th e hydraulic detenti on time approaches the regeneration tim e fo r th e mi croo rgani snls. cell s are washed out of.1h..e reac.tor before growth ca n occ LlI-.-Co nseqll cntl y. .Y dec reases and S approaches So. meaning that no treatm ent is 'lCc LlFI·in g. I
Plug-Flow Reactors . . . The plu g fl ow With slud ge recyc le rea cw·r (fi g. 5-1 6h) is oft en used in the activ::tteclslu dge pl··occss. I\ ss nn lln g c() l1lpl ~ te mixin g III th e tr:Cln sve rse plane but minimal mi xin g in the d ircc ti() n ()r 11 0\\·. the mi xtme of was tewat er and return ed slud ge tr ~t\·c l s as a unit thr()ugh tlie IT~I Cl(lr. l~c~ l c t i() n kineti cs for bi o mass produ cti on is silnilar to the b~l t c h PJ(lCC SS ·(SCl·. 5-'!). wi th tl10 exce pti on of an initiall\'. hi coher
238
ENGINEERED SYSTEMS FOR WASTEWATER TREAHIENT AND DISPOSAL
239
WATER
bi o mass co ncentration and lower food concentrati on because of sludge return. Law rence and McCarty [ 5-29J der ived ex press ion s for an a ve rage M LSS and food utilization as foll ows: (5- 17) and 1.:.0 5X Y K, + 5
r, = - - - - -
(5-18)
where X = ave rage biomass concentr ati on in the reacto r (milligrams per lit er). Th ese equation s are applic ab le onl y wh en 2: 5. Int egrating Eq . (5- 18) over the detention time in the reactor and substituting the a ppropriate bou nd ary conditi o ns and recyc le fa cto I' yie ld s the foll ow ing equation:
eele
____ k 0(_5 0_ (5 0
,-
5)
+
}1 ____
(I - 'l.)(K, In 5 j 5)
I.:.
(5- 19)
where (/. = recyc le facto r. Q/Q,
Si = concentratio n of substrate after mi xmg with recyc led sludge, mg/L 5
=
,
~ + a5 1+
CI.
most promising. Biological con stants associa ted with the wastewater and the reactor are determined , and the operating parameters that will produce the desired degree of trea t ment are quant ified . 1\ prelim in ary design of each altern at ive is made, and the one pro vin g the most cos t-effecti ve is selected for the more detailed design necessary fo r it s constructio n. Although few abso lutes apply to process and reac tor selecti on , some general observations can be made in li ght of recent experiences. ' Beca use of required reactor volume, extended aeration systems are often limited to Aows of 7500 m 3 /d (2 Mgaljd) or less. Hi gh-rate processes, except f()I' ~he pure oxygen system, produce a hard-to-settle slud ge and are not usually used where a high-quality effluent is required. Complete-mix reactors are supe rior to plug-flow r.eactors where wide fluctuation s in fl ow rates occur. In stantaneo us dilutio n in the aerator" dampens" out shock loads th at wou ld carry through plug-flow systems and result in va riable effluent characteristics. Wh ere loading is reasonably constant, plug-flow systems produce a more mature sludge with excellen t settling characteri stics. O ne fac tor in activa ted-s lud ge design that shoufd be stressed is the interdependen ce of the biolog ica l reac tor and the seco ndar y clarifier. High biomass co ncentrations ancl short aeration period s may produce good trea tment efficiencies wi th respect to so luble BOD. The sav ings in aeration tank vo lume is o ffset, howeve r, by the large secondar y clar ifier req uired to clarify the effluent and thicken the slud ge. Beca use of thickenin g limit atio ns, it is the secondary clarifier th at usua lly sets the upper limit s on th e bio mass concentratio ns in the reac tor. Design variab les for ac tivat ed slud ge reactors ha ve included (1) volumetric ' Ioading rates. (2) food-to-ma ss ratios, and (3) mea n cell-residence times. The v'o lumeiric loading rate VI- is th e Illass of BOD in the influent divided by the . vo lume of th e reac tor. or (5-20)
Process Variations In practice, severa l variations 0 1' the completel y mixed and plug-now sys tems arc oft en used . Some invo lve subtle differences. such as rates and po ints of a ir o r wastewa ter applicat ions, de tenti on tim es, reac tor shapes, and methods of introducing air . O thers in volve more drastic differences. such as sorpt io n and settling prio r to bio logica l ox ida t ion and the use of pure oxyge n rat her th an a ir. Th e most co mmon of these va riati ons are id entified in Fi g. 5- 17.
Design Considerations Th ere are man y fa ctors. which must· be con sidered in th e des ign of activatedslu dge systc ms. 'Co mbinati ons of th e process var iati o ns and reac to r types that are co mp ~l tibl e with the was tewa ter cha racteri sti cs and environ mcntal co nstraints mu st hc se lected. Ex tern a l fa cto rs such as cons tructi on cos ts, operati on and maint enan ce difficulties and C'ost, and space limitati o ns Illu st a lso be cons id ered. Usu:11I y. the enginee r se lec t · fllf de tailed ana lys i,;severa l () fth e schemes tha t a ppea r
the unit s of wh ich are kil ogra ms of BOD per cubic meter-day .. he food-to-mass rati mass of BOD removed' divided by the bioma s e reactor,
FIM
= Q(5 o -
5)
(5-21)
VX
the unit s bein g ki logra ms or BOD per kilogram of bioma ss· day. Th e mean ce ll res idence time, e, in Egs. (5-14);(5- 15), and (5- 19). is current ly th e most co mmonl y used design parameter. Both the r i M-ratio and 0" approach . to design all ow fo r trade-off between reactor vo lume and co ncen trat ion of M LSS in the reac tor. . . Typica l design parameters for ac tiva ted sludge systems are given in Table 5-6. Th e design of a co mpletely mi xed activa ted-s lud ge reactor is illustra ted in the exa mple Oll page 243.
! \
,
240
ENG INEERED SYSTEMS FOR WASTEWATER TREATMENT AND DISPOSAL 241 WATER
Effluent Efnuent
Secondary clarifier
I
Brush·lype aeralors (c)
Sludge
L_______ --:...-::'~~.:~~----------t----~~:.--(a)
Primary effluenl
Effluen t
Effluent
Prim ary effluent
Seco ndary clarifier
Reac tor
/
Effluent
Influent Secondary clarifier
I
I
Sludge return
::
ce~to.
ct
b
. 1~ roll S~ {~if» .
Primary cmuent Sludge
L ____
~~~e__ _
~ ~-----~. J.--t~-1>---+J ~
Or
raw was(c-
/
water
,
.
I
I
~__O_x_y:..g:;.e_n_re_t_u_r_n,_ _
.
Sludge return (omitted in ~~:~~~~
1 .
__________l ____
(Iv\. ~ ~
___ ~
Figure 5-)7 COIllIllon \ariatiolls o f the activated- sludge procl: s=>. (a) Step ae ration. influent adclJlion
Was te gas
provides m·ore uiliform BOD removal throughout tank . (h) Tapered aeration: added in prnportiun In BOD exerted. (c) ('ontact ~tJ.bi li zatj()n. biomass adsorbs organics in
at intermedIate points
Efflue nt Seco nd ary clarifier
Contact b
~onl,;cl b"sin . (d) Pllrc~1 " ~· II\·aled sludge oxyge n added under press ure keeps dissolved oxygcnlevcl high. Ie) OXldalion dilch. pl'ln ,·,ew U) H igh rale· shon detention time and high rood !mass . ratio in ;lenllnr 10 maint:lin clilture in log-gr o wth phase . tg) Ex tended aeration ;Ind l{l\~ f0 0 cl ma :--s ratin I n ;]l.'r
Reactor
Sludge IL _________________________ Sludge return I ________ was te ..l.. ...... (d)
LUX?4
Sl ud ge ~~~
(g)
Primary e fflu e nt
I I I I I I
\ow f/r-A
1 I·
L_________
I
Compressed· air
illT i~
I
lv~\X"
: .: .
(c)
Pure oxygen
Reacto r
:
lo ng oet ention time
ENG INEER ED SYSTEMS FOR W ASTEWATER TREATMENT AND DISPOSAL
~o
243
'-=0
8: eo
'"" ~
-t:
-E ell
-'"
o coee a-. 0\ a.. (1\ aI
' /')
-r
V"'l
I
I
'A
If)
.q-
I
.,.
">1"
'r,
I CO 0-
I
or, ..q
;.., U
Cl
0
":>0 'u"c E
eo "
~
fE
",
.
~
0
" E
00
G: 1:
...
~
""~
U
'"""~
et: '"
"
OJ
'"'
U
'",J,
0-
OC
00
OC
u.. u.. u.. :2: "- "- "- V
u..
6 6 ,r,I
I
L;..
0.. 0..
'" ".
X>
:2
/ .
C-
O
v:,
,A .)., ,J,
0
0
<=
o~o
o
00 0 0 0 'n
0
M
:=
r
.....E0
("',
I
I
,r \ .
r-
C
0
co.:::.
C
cooco ~OO~~ , ,
c r'l ...c
I
., ." ''-' v.
6
lllnu ent
0.
:g
SO L U Tt O~ A sc hema tic o f th e system is shown in the accompa nyin g figure.
6'" 6
E a'i '"6 '"6 '"6 '""" '"6
""
:2:
0
0
0
..,.
C-
0 U V
V',
0
''-' C
0- 0, ,J, .;\
0-
2,
OC
',-, ' '-'
0 ~
~
X
0
0
0'
,J, ,J, '" ,J,
Example 5-3: Designing an activated-sludge reactor A n activa ted-sludge system is to be used fo r seco nda ry Irea tm ent o f 10,000 m 3/d o f municipal wastewa ter. After primary clarifi cat io n, Ihe RO D is ISO m g/ L, and it is desired 10 ha ve not more th im 5 mg/L of so luble BOD in the e muent. A co mpletely mixed reactor is to be used, and pilot -plant a nal ysis has es ta blished the fo llowing kinetic valu es : Y = 0.5 kgjkg, kd = 0.05 d - 1. Assumin g an M LSS co ncentra ti o n o f 3000 mg/ L and an underflow concentration of 10,000 mg/ L fr o m the seco nd ary cl a rifier, determine (I) the volume of the reactor, (2) the mass a nd vo lu me of so lids th a t mu st be wasted each day, a nd (3) the recycle ratio .
i
00 00 '-0 '" 0, ..,
0 0 '0 0 0 0 ,,:::, 0
~ ~ fig
c OJ
§
"" !:: OJ
OJ
.~
.~ "" ;; ~
:r:'"'
~
C
C
C
E~ ~
c
.c
.D
:::,
I
, ''-'
"" "
~
C1l
..,
co
C 00
.~
,
-to c,
C"
"
=
6
.",
:=
-q;
E -;c. 0
u
·c
.",
0:;
~
E
-""""...
CJ)
0 co
'::l
~
> .2" 0
U
ell ~
-'"
-c
00
C
0 r ;l
-C
OC 0
6
,...,I 9
~.
'"-;-
-C
,
..,
-Co 00
...:.
6 6 '0 ' b' .:...'
-
~. co I -c.
-
0
I
QY(So- S)
U,
vx
'-
'"...
0. 1 c1 -
~.
~
E
'E c
""
-g
", .
"-
""...
c:
0 0
~
-r
-r-
('l
(" I
-r
""T,
,..",
(" I
'r, O
'-C;
_ _,
99cr' ~,
0. 1) d
r l
~
" C
v
or ,
f;
-r
; 'X C
,
~= ~ i3 ~
~
<= C1l ';;
:: .c 0:
Z-lZ
f-
2. At equ il ibrium co ndi tio ns. Eq. (5- 14) a pplies.
U
~.
"
<= 0:
"? 'r,
241.67 m 3 /0 = _. - -._- ._V
V = 16 11 m )
.S:
0'"
3
I O.~~3 !d~~ (0.15 kg/m - 0.005 kg/ m3) _ 0.05 c1 V 3.0 kg/ m )
C
<=
-,:
=
"
. <.1
... OJ g-
I
k"
c -
2 C'": o
2~
mass~f;olid s wa;i~
VX
.2 _
Q" X" =
c
- :.c l C~_,.J ~' ~ '-J N ~ ~ ~
c: E x ~ cr. -;L~ I
c.....
g
-It 161 1 111 ) x 3.0kgim 3 -------_._10 d
1 j"
VX
ma ss of so lids in reacto r
II, "'"
Q".X "
=
4H3. 3 kg/d
=
Q" X ;:
I
'-
ENGINEERED SYSTEMS FOR WASTEWATER TREATMENT AND DISPOSAL
244 WATER If the concentration of so lids in the underflow is 10,000 mg/ L 483.3 kg/ d 3 . Q ... = - - - - , = 48.3 m Id 10 kg/ m 3. A mass ba lance around the secondary clarifier can be wri tt en as fo ll ows'
(Q
+ Q,)X
= (Q
+
Q, - Q... )X,.
+ (Q, + Q".)X"
Assuming that the so lid s in the effluent are negligible compared to the innuent anc! underflow.
QX + Q,X
= Q,~"
Q,(X" - X)
=
+ QwX"
QX - Q... X"
Q = QX ~ Q•. X"
,
245
Air diffusers Man y types of air diffusers are avai lable from manufacturers. Fi n ebubble diffusers produce many bubbles of approximately 2.0 to 2.5 mm in diameter, while coarse-bubble diffusers inject fewer bubbles of a larger (up to 25-mm diameter) size. Both types have advantages and di sadvantages. With respect to oxygen transfer, the fine-bubble difl'user is more efficient because of the larger surface area per volume of air. However , head loss through the small pores necessitates greater compression of the air and thus greater energy requiremen t s, and compressed a ir must be filtered to remove all particulates that would plug the tiny diffuse r openings. Coarse-bubble diffusers offer less maintenance and lowe r head loss, but poorer oxygen transfer efficiencies. One compromise is t o locate a mechanical turbine just above a coarse-bubble diffuser so that the shearing action of the blade at high rotational speed breaks the large bubbles into smaller ones and disperses them through the liquid. Typical installations of air diffuser syste ms are illu strated in Fig. 5-18.
Xu - X
~
1\
.1:"' .'
The recirculati o n rati o is
Q,
4.217
Q
10,000
- = - -- = 0.4 1
~. -'
.
,0 ,
Aeration of Activated Sludge
I ~
--The' rate at· which'oxygen 'is consumed 'by ·the·microurg,mism· ilnI1c"hiological'" reactor is ca lled the oxygt'11 UTilizatiol1 rate. For the activated-sludge processes. the oxygen utilization rate will always exceed the rate of natural replenishment. thus so me art ificia l means of adding oxygen must be used. With the exception of the pure oxygen system. oxygen is supplied by aerating the mixed liqu or in th e biological reactor. . The oxygen utilization rate is a function of the characterist ics of both t hCl\
J....
-
-
-
-
-
(b)
(a)
(d)
(e)
Figun· :'- IH T ypi cal appllc:lIInn
\)f
ddfu:-l'd aCfdll()n ~y :-. tcl1l:-'. (oJ MIXlIlg action by d!n'tl s ~rs mounted d i n"use r (t'ollrtfS), nf;\'/ 0I1l0rt {/
On ~iLic nf [t) n g, 11;1 rro\\" t~lll k , ! h) 111cch:l1lical t u rhi nc it bd\ ~ a n1arsl>hu hblc ,,'{(Ill' f)t'/ lurtnWI'I1 u (
I-Ica/,IT):
Sltltill~ fr'.)l11 11l~lilll;IIHI!I III (d
(C.-, pnrpll ~ dlA'u"cr:-.;lt
bUlium uf 3Cr:llioll bi:1Sin. (d)
hubhk aClion rc-
246
\V ATl'R
Mechanical aerators M ec ha nical aerators produce turbulence at the air-liquid interface. and this turbul e nce en trai ns a ir into th e li qu id . Mechanical aera tors may have h igh-speed impe llers iha t a dd large qu a ntiti es of a ir to relatively small quantities of wa ter. Thi s aerated water is then mixed wi t ti the reactor conten ts through ve locit y gradients. Large impellers driven at slo J speeds agitate larger quantities of wate r less violent ly. Typical units of bo th types are shown in Fig. 5-1 9. ed units is co m on in exiended aera ti on system s, Use 01' th e smaller J whi le the slow-speed units are more common in eo~ventlOna l ac IV e -s ud ge sys tems. Brush-t ype aerators are used to provide both aeration and momentum to wastewater in the oxida ti o n-dit c h va ri a ti on of the ac tivated -sludge process. Their use is illustrated in Fig. 5-20.
(a)
,~.
.-
;>':~":~1'" -;f" ~_
,.,'
.~ }!:t.~~~~" #~':~'"
(a)
(b)
Fi gu re 5- 19 T y p ical mcc h a ni l:al at!rCl( o r ~ lI sed in tl c tl 'v atc d -sllldgL' pnh.:C">Sl''', (lI) I.tl\\ -specd Il H:c hani cal ;it: rato l mlHJflled on fi xe d pl atfo rm (,!lU}(O COffrln.l 11/ 1::11111'('.\ ItlL. tf R e ynfl rd C O/"fl{ IO IIY): (h) h lgh -
(b)
Fi gur E." 5-20 O xi lial iD n d ilCh "H.' ration . (a) Rot or aerat o r bru sh used in oxidation d i tch (photo ( ourtt'sy o/Kathleen M tllrr-I/{)(mf) : (h ) brush a Clio n in :.I n oxi d alion dilch (photo COllrtCS), oj Lakeside Equipm ent C"'p.): « ) 1) l' lcd arr,, "~e m cn l "r ac r " I,W, ill o xicinli o ll chi Ch lrholV COur'fSl" oj" Ll1kesidf EquipmeJlt Corp.).
' j1 lTd I l (l tl ! ln ~ ~ l\.' ratl.) r (pho/(I ('/l lIrl f>.\\ ' uj " . 111 i l"l.' \ I f!( . (/ N t'.\ 1/lInl ( ' oml'{/I1\ ). 2~7
248
5-12 PONDS In addition to the activated-sludge processes, Qther suspencleclcculture biological systems are available for treating wastewater, the most common being ponds and lagoons. A waste~ater pond, alternatj yely known as a stabilization pond 0 jdgJjO.l pond, and sewage lagoon, consists of a large, shallow earthen basin in which waste-water is retained lo;g enough for natural purification processes to provide the necessary degree of treatment. At least part of the system must be aerob ic to produce' an acceptable effluent. Although some oxygen is ,provided by difftision from the air', the bulk of the OlL)Cgen-i . p.ond p.wvided by photosynthesis. ~ are distinguished from p onds in that oxygen for lagoons is provided by -' artificialie.I:ation here are several varieties of ponds and lagoons, each uniquely suited to specific applications. , ~ in which dissolved oxygen is present at all depths are caTTed aerobic pon!!J... Most frequently used as additional treatment processes, aerobic ponds are often referred to aSJio lishing or " terciary" poncjs. Deep ponds in which oxygen is absen t except for a rel atIvely thm surface layer are called anaerobic ponds. Anaerobic ponds ca n be u sed for partial treatrrient of a strong organic ' wastewater I?ut must be foll owed by some form of aerobic treatment to produce acceptable end products. Under 'fav o rable conditions :&£~,dtative ponds in which both aerobic and anaerobic zones exist may be used as the to tal trea tment sys tem for municipal wastewater. . Lagoons are classified by the degree of mechanical mixing provided. When sufficient energy is supplied to keep the entire contents, including the sewage solid s. mixed and aerated , the reac tor is ca lled an aerobic lagoon. The effluent from an aerobic lagoon requires solids remova l in order to meet suspend ed-so lid s effluent standards. When only enough energy is supplied to mix the liquid porti o n of th e lagoon, solids se~tie to the bo ttom in areas of low ve locity gradients and proceed to degrade anaerobically. This facility is called a (l ClI tative ague and the pro,cess differs from that in the facultative pond only in t e meth od by which oxygen is supplied. I . The majority of ponds a~d.Iagoons serving municipalities are of the fa cultative type. The remainder of this discussion will relate to the facu ltati ve processes, the interested reader being referred elsewhere for more information on the other systems. See especially Refs. [5-6J and [5-36]. Facultative ponds and lagoo ns are assumed to 'be(§ompletel y mixed reactors without ~mass recychl R aw wastewa ter is tran sported into the reactor and is released near the bottom. Was tewater so lids settle near th e influent while biological solids and fl occu lated coll o ids fo rm a thin sludge blanket over the rest o f the bottom. Outlets are located so as to minimize short circuiting.
:\
r
System Biology t
1-
L
A generalized diagram of the processes that occur In facult a ti ve po nd s is sho wn in Fig. 5-21. Aero hic cond iti o ns a re maintained in the upper po ri io ns of the pond
249
ENGINE ERED SYSTEMS FOR WASTEWATER TREATMENT AND DISPOSAL
WATER
Su nlight
Wi nd
A I'"" ~
o~
A""",,
2'
po, so,
g" N U
:0
2v
~
----,--t-rt-t - )B>"m",~ __ Orga ni c acid s and
oth er reduced com pound s o f C,N.P.S
ttl 1 ! 1
Im permeab le lining Figure 5-21 G e neralized d iagram of facultati ve pond reaction.
by oxygen ge nerated by algae, and , to a lesser extent by penetra tion o f atmospheric oxygen. Stagnant cond itio ns in the sludge along the bottom prevent oxygen transfer to that region and anaero bic condition s prevail there. The bo undar y between the'aerob ic ar.i a naerobic zo nes is not stat io nary. Mixing by wind actio n and penetration by sunlight may exte nd the aerobic area downward. Conversely, ca lm wa ters a nd wea k lighting result in the anaerob ic la yer ri sing toward the surface. Diurnal changes in li ght co ndition s ma y result in diurnal fluctuati ons in th e aerobi c-a naerobi c int erface. Th e v 'Ow .ugh which the-pl'esence o f di ssolved oxygen flu ctu ates is ca lled til /ilntilClti!;e~' ne because o rga nisms in thi s Zo ne mu st be capable of adjusting t eir metabo lism to the change in oxygen conditions. Co nsid era hle int eracli o n ex ists between the zon es. Organic ac ids and gases. prod ucts of deco mpos iti on inlhe an ae robic zo ne. a re released and become so lub le
ENG INEERED SYSTEMS FOR WASTEWATER TREATMENT AND DISPOSAL 251
250 WATER
I
r
,
C'
L.
/'
food fo r o rganisms in th e ae rob ic zo ne. Bi ologica l solius prod uced in the ~ I er o bic zo ne ult imate ly settle to the bo tt o m where they di e. pro viel in g fooel fo r th e anaero bic benth ic organ isms. A specia l relat ionship exists be twee n the bac teria and algae In the aerob ic zo ne. Here the bacteria use ox ygen as an electro n acce pto r to oxidize the wastewa ter o rgan ics to stable end product s such as CO 2 , NO J . . and P0 4 .1 The a lgae in turn use th ese compo un ds as a materi a l SO Llrce a nd. with sunligh t as a n energy so urce, produce ox yge n as a n enu produ ct. Th e oxygen is th en used bv the bacteria. Such mutuall y beneficial arrangemcnt s. callt:d Sl'IlIhi(} {i c rl!i(f{ i(lll.\hi f).~. oft en occu r in nature. The process is similar fo r th e faculta tive lagoo n. In thi s case. huwevc r. \)xyge n is suppli ed primaril y by artificial ae ral ion , a nd the elreet o r al gae. exi sting here in co nsiderab ly lesser nu mbers than in pond s. is negligibl e. Th e aero bic- an
Design of Ponds and Lagoons Severa l approaches to th e des ign o r po nd s a nd lagoo ns ha lT hee n pro posed . The mode l most co mm o nl y assumed is th cOlllplet ely Illixeu reactl)r with o ut so lid s r e c y~ . In th e case of facu lta tive system s. cllInplctc mixing is ass umed to -:lpp lv o nly to th e liquid po rtion of th e react o r. Waste\\at er so lids a nd hi o logi cal so lid s that fall to the bo tt o m a re no t res uspend ed. Ik C: llI sc th e rate at which so lid s
are removed by sedimentati on is not qu antifia ble. a mass balance for solids cannot be wri tten . A mass balance for the solu ble food can be wri tten. because soluble food is assum ed to be uniformly distributed thro ugh o ut the reacto r by mixing of the liquid. If the conversio n ra te is assumed to be firs t order in food concentration. then mass balance ca n be wri tt en as fo ll ows : BOD in
=
BOD ou l
QSo = QS
+
'-
BOD consumed
+
V(kS)
(5-22)
Upon rearranging. Eq. (5-22) become s S So
I
-1+ kV/ Q = I
I
+
kB
(5-23)
whe re SISo = k = Ii = V=
fracti o n of so luble BOD remaining reac tio n ra te coe !1i cient. d - I hydraulic detenti on time, d rcactor vo lume. m J Q = fl ow rate,m) /d
If seve ral reactors a re arranged in series. the emuen t of o ne po nd beco mes the Infl uent to th e next. II substrat e ba la nce wri tten ac ross a se ries of n reac to rs result s in the fo ll ow ing equa ti o n:
(5-24) When facultativ e ponds are used to treat municipal wastewa ter. it is common prac tice to use a t least three po nds in series to minimize short circuiting. Marais [5-35J and M ara [ 5-34J dem o nstrated that maximum effic iency occurs when ponds in seri es a re of th e sa me approxima te size. When this is the case. the firs t pond. ca lled th e primary pOlld. will ret a in most of the sewage so lids a nd will thu s be the most heav ily loaded. It ma y be necessary to provide ae ration in the primary pond to prevent complete anaerobic conditions w ith their a ttend ant odor problems. The resu lt is one facultativ e lagoo n foll owed by two or mo re facultative ponds. Altho ugh the above models are useful for visua lizi ng the pond and lagoon processes, it is impractica l to expect in stantaneo us mixing of influent with such large reac to r vo lumes. In 'prac tice, a wide range of dispersio n occurs becau se of reacto'\, sha pe and size. mi xing by wind ac tio n or aera tors. and influent and . effluent ana ngemenls. Thirumurtlii [ 5-53J developed grap hical re la tionshi ps between food remova l and valu es for kB for dispersio n factors ranglllg from IIlfinit y fo r co mpl etely mixed reac to rs to iero- fo? ug-ftow react09 · These Ig. . -_2. ca n )e used for design . prov ided va lu es of k are
'-
'
ENG I NEERED SYSTEMS FOR WASTEWATER TREATMENT AND DISPOSAL
252 WATER
253
Table 5-7 T ypical design parameters for facultative ponds and lagoons
Flow r.egirnc Pond si.ze: ha Operati o n* Detention time , d* Depth. J11 pH Temperature range,
6,59,0
Mixed surrace layer 1- 4 multiples ·Ser ies o r para lIe l 7-20 1-·2 .5 6.5- 8.5
0 -50
0-50
20 15- 18
20 -XO
20 50-200 80- 95 Algae. CO" C H 4 • bacte rial ce ll tissue 5 -20
40 - 100
40 -· hO
1 -4 lIlultiples Series or
par~lllci
7- 30 l e2
"e
Optimum temperature, <':- C BOD, loading , kg/ ha dt UOD s conver~ion Princip;Ji cOll ve r si0!1 product s Alga l conccnlr~l1ioll, Ill g/ L Elllucnt slispended '(1 lids. mg / L ~
Fa cu ltative lagoon
Fa cullative pond
Parameter
80- 95 A Igae. CO,. C H.,. h
.. Oepcnd\ on climat iC cll IH.iilion s. i· T ypIcal "alucs (much higher values have been applied at vario us locati o ns).
Pe rcen t remaining, 5 /S o
Figure 5-22 Graphi c relati on,hip between S ,So anti kll in Eg. (5-c4). (From Thinmlllnhi [5-53].)
.. .. . .. .... ........... A.wi.de. range .of. Yal.tles. for A. is..~D,:;oJ.ln.t.ere.cl in .the..I.i.\e.r.aLLI!.c,.A.I.\\l(:ll!gh man y variables relating to both the re:1c tor and W:1stewater a fTect f.: , wa ter temp~;",~;'L;r~ appears to be the most signifi cant. Equations of the fo rm
L L
are commonly used . Values freq uentl y used for /.:20 range from abo ut 0.2 to 1.0. while values of the temperature coefticient ma y range frol11 1.03 to 1.1 2. These va lu es must often be determin ed experimen tall y for a given pond sysle;11. Because oflhe complexity of accuratcly assessing th e f.: constant. des ign of ponds and lagoons IS often based on loadin g factors and ot her empiricall y deri ved parameters Parameters and values frequ entl y used are given in Table 5-7. Although so mc photosynthesis undoubt edl y occ urs In facultative lal!oons. ox ygen requirement s are ass umed to bc met by aeration . Tv.;o kilograri1s 0f l~xygen should be sU'pphed for each kilogra m of BOD s in the inHu enl t'O ens urc'acl equai c oxygen for the so luble BOD released from th e anaero bi c zo ne as we ll as fo r the BOD in the raw was tew a ter. The ra te of oxyge n tran sfer is a function of water temperature. oxygen defi cit. and aerator characteristics. In co nditi o ns no rm ally encountered in wastewater lagoons. the rate of oxyge n transfer cll rrel;lt es lI'e ll
Load in g values are orte n specillcd by Slate IX)lIulion-con tr ni agenc ies. ! Includes algat.:'. microo rgani sms. and residual influent suspe nd ed so lid s. \'a lucs arc hased on ;til ,"ilue" t so luble BOD, or 200 mg/ L and an influent suspe nded-solid s concentration of 200 mg/ L . SOllre£': Frl' lll i\k"'ill r & Edel]. Inc. [5-36}
with the ene rgy diss ipated by the ae rat o r. and transfer ra tes ranging from 0. 3 to 2.0 kgj kW . h are common. More exact figures can be ob tained from the equ ipment manufact urct·s when o perati o n cond itions are kn own. The design of facultative po ncl s ancllagoons is i-Ilu Sl ratecl by the following exa mple. Example 5-4: Designing f~cult~ti"e ponds and lagoons W astewater Au,," from a small com munit y averages JOOO m J ,d during the "inter and 500 0 m ) 'd Juring the summ~r. The a\cragc lempera t ure o f the colde,t month is 'S' C. aqd th e average temperalure of t he \\:IIme s t m o nth is 25' (". Th e :twrage BOD , is 200-mg : L with 70 percent being so lub le. The reac ti o n coe ffi cien t k is 0.2 3 d - , at 20°C. ;;net th e va lu e of 4' is 1.00. Prepare a pre liminarv desig n ror a ra c ultat ive pond treatlllent sys te m for the comllluni ty to remove 90 perce nt o f t he so lttbl e HOD.
(' ,'Illpttte the rate co tt stants ,,,I.lus ted rOt temperature. Sununer'
1..:
2
"' :::.:
lL~3( I .06)1 5
2u
= 031 d "
/-, - ()~3(1.1)6/ : " ~ 0,11 d
'
ENGINEERED SYSTEMS FOR WASTEWATER TREATMENT AND D ISPOSA L 255
254 WATER
Assume aera tors transfer 1 kg 02/kW . h
2. Fr o m F ig. 5-22, determine kO when S/So = 0. 10 and the dispersio n fa c tor is 0.5.
kO
1d kW·h 1640 kg 02/d x x - -- = 68.3 kW 24 h 1 kg O 2
= 4.0
4.0 Summe r . (1 = - - . = 12. 9d 0. 3 1 4.0 Winter : = - - = 36.4 d
Use three a erators at23kW eac~
/" 5-13 ATTACHEI>-CULTURE SYSTEMS
0. 1\
Use lo nges t tim e. () = 36.4 d 3. Co mput e vo lume of p o nd s.
v = IJQ
= 36.4 d x 3
X
10 .1 m )/d
=
109,200 m J
Use three po nd s (as s ho wn in the acco mpan yin g sketc h). each 36,400 m ), () = 12 d .
~.
Innu ent
era tor
~
»'
-~ Po nd # 2
0 0
Prim ary po nd
b io-t o wers, a nd RB Cs. In the tri cklin g filter a nd bio-towe r, th e medium is stationary and ! he wastewater is passed over th e biofi lm in intermitt ent doses. In th e RBC th e medium mo ves th e biofi lm al te rn a te ly through wa ter a nd air. Because both sys tem s ma intain a ero bic conditi o n s a t th e bi o tllm s uriac.e. bo th are classified a s aerobic
I
+ Po nd # 3
processes. .--In addi ti on to th e bi o log ica l react Q.r. an attached-culture sys te m usually ll1c1udes both primary and seco ndary- darification.· The.pfimar.y. c1ar.itier . may. be .... omitted in bi o- towe rs a nd RB C installation s where plugging o f the vo id spaces can be av o ided by gri ndin g the solid s in th e wastew a ter to sufficientl y sma ll sizes
Emuent
.j .
6 ttached-culture sys tems emplo y reactors in which wastewater is contacted ~th .-microb ia l fi lm s att ac hed to surfaces. Surface area for biofilm g rowth is increased by PGCing a porous mediu m'Trlthe reactor. When random ly packed so lid medium \ IS used. th e reactor IS called a fElCkrr~he a d vent o f m od ul ar sy nthetiC ( med ia o f high poroSIty a nd low we ight en ab les a vertIcal arrangement o f medium \;;evera l meters hi g h , leading to the ' te rm ~ More recentl y, th e use of rotating disk s pa rtiall y submerged In wa stewa ter ha s led to the rotatina blO, iogJcal conlaCLQr (REe) process. A ltho ug h other attached-cu lture sys tems. IIlcluding submerged filters (a n ae ro.QiE) a nd fluidized beds, may h ave app li cat io n under certain condi ti o ns, th e discu ss ion here will b e lim ited to trick lin g filters.
Use d epth o f 1.5 01 fo r po nd s.
prio r tb applica ti on o n\ o th e medium.
36,400 Area = ' 1.-5-
= 24, 267
01
2
= 2. 4 h ~ l System Biology
(N o te: l \dd I 01 depth for sludge sto rage in primary pond .). .". Ass umm g ph o tosy nth es is will not be s ufficient to meet ox ygen require me nt s in th e primary p o nd thro ug ho ut th e year. size a era tion equipment. Fu r primary po nd s umme r conditions :
V
IJ = -
Q
36,400 m '
= ------c--. = 7 'l d
5000m 2 / d
klJ = 0.3 1 x 7.3 = 2.3 Fro m Fi g. 5-n with d = 0.5,
S/k--O. I
80 D rem oved = 0.82 x 200 = 164 mg/ L ... Ox yge n s upp lied = 2 x 0. 164 kg/ m ) x 50UO 111 3/d = 1640 kg/ d
..
Th e bi o lo gica l metab o li sm o f wastewa ter o rga nics in a tta ched-culture sys tem s is remarkab ly simi lar to th a t in suspended-culture sys tem s, the dissimilarities in react o r characterist ics n otwi thsrand in g. The biological organ isms that att ac h them selves to th e so li d surfaces of th e medium come from essen ti a lly tb e sa me gro up s as th ose tn activated-sludge systems. Most are heterotrophi c o rga ni sm s. witb facultative bacteri a bei ng predom inant. Fungi and protozoa are a lso abundan t , a nd algae are present near th e s urfa~e where light is ava ilable. Anima ls such as ro tire rs , slud ge wo rm s, insect larva e , snails, etc. may a lso be fo und . Nitrifying o rgani sms a re found In significant numbers o nl y wben th e ca rb o n content o f the wastewater is low. The o r ga ni sm s attach themselves to the medium a nd grow int o de nse films of a visco us. jell ylik e nature. W as tewater passes over thi s film in thin sheets wi th
256
ENG INEERED SYSTHfS FOR WAST EWATE R T REA TM ENT AN D D ISP OSA L
WATER
to~ncentrati~ra.9.ien~
dissolved organics pass ing j nto the biofilm due wi.Lb.W the film. SuspendeCfpai1iC les and colloids ma y be retained on the '~~!.i£kf' surfaces wheretheyareclec;omposedinto soluble products. Oxygen from the ._ ~rewater and from air in the void spaces of the medium provide oxygen for ae.robic reactions at the biofilm surface. Waste product s from the metctbolic ('/' y,..... k p ~o[esses diffuse outward and are carried awa y by t he water Or air currents moving through the void s of the medium . These processes are diagramed in Fig. 5-23. . ,~~ Growth of the biofilm is restricted .to one direction --.o utw ard from the solid ./h,c:p rface. As the film grows thicker, conceniration gradients of both oxygen and ~ food deve.lop. Eventually. both anaerobic and end ogeneou s metaboli sm .occur /7 at the blOfilm-medlum surface IIlterface. The attachment mechani sm is weak ened . and the shearing action of the wastewater flowing across the film pulls it from its 1 mooring and washes it away. Thi s process. known as sloughipg, is a funcli.on of both the hydraulic and organic loading rate. Bi ofilm "itij uickly reestabli shed in places cleared by sloughing. The rate of food removal in attached-growth systems depends on man y factors. These include wft-l ewa ler flo w a te,_ Q!ganic. loadin g raLe. ~s' of difTusivit y of food and oxygen into the biofilm. and temperature. The depth of penetration of both oxygen and fo od is increa~d at higher loading rates. Oxygen diffu sivity is usually the limiting factor. Aerobic zones of the biofilm are usually limited to a depth of 0.1 to 0.2 mm [5-10], with the remaining thickness being anaerobic. The number of variables affecting the growth of biomass. and subsequently the rate of su bstrate utilization , makes mathematical mod eling of attached-growth systems d"ifficult: Biofilm growth . sloughing. and regrowth . and it s aei'o bicanaerobic nature. prevent application of equilibrium equation s similar to those , fil(,r' r;(' 1'. . ~ fl" r ' ..... r · r ~ . r; - ,",- I 0 \. r l. " ' . ~. \
- I,""
D
/ A i r spa ce .
Anae ro bi c, endoge nous Figure 5-23 Alia ched -c ullUre processes.
ac(ive
257
fo r suspend ed -culture syst ems [ Eqs. (5 -5) a nd (5-7)]. Design equ atio ns fo r att achedgrowth sys tems have been deri ved la rge ly on an empirica l basis.
The nam e tricklil1g fil ters is a pplied to a react o r in which randoml y packed solid forms prov id e surface area fOI' bi o film growth. The system must contain equipment for distri b uting th e wastewa ter ove r the medium and for removing the effluent. Th e term fi lt er fo r thi s process is mi sleading, since few of the ph ysical processcs assoc ia ted with filtrati o n thro ugh granular media function in trickli.ng filt ers. Jnstead. so rpt ion and su bseq uen t bi o logical oxid at ion a re t he primary mea ns o f food rcmoval lmQ.Q rta nt characteristics of th e medium includ e specific surface area and C) porosity. The ~dic surface arem refer s to t he amount of surface area of the media / ~ that is a vailable for biofilm growth. Th e:J)orositr j) a measure of the void space ava ilab le fo r passage of th e was tewat er and air and fo r ve ntilatio n of produ gases. In mosl cases th e medium in tricklin g filt ers is co mposed of crushed ston [ 4fV.pr<;r ~ . :these materi a ls prov id e ha rd . durable. chemica ll y resist ant surfaces fo r.... bi ofilm gro wth. Sizes I'an ging from iO to J00 mm (2 to 4 in) provide specific surface areas o f~ to 65 'Il 2/ m 3 (1 5 to 18 ft friO). with p o r o~ iti es of40 to 50 peJ"{;ent. Plastic med Jaof vario us shapes may be used in stead of th e sto ne or slag, with sizable ad vantages in ~ pecili c surface a rea and porosity. Areas up to 200 ~ /m 3 (57 ft 2 / f(3 ) an d porosities of. 9) err.ent are a va ilable with loose- bulk packin g mat eri a l The usc 0 [' modular medi a ma de J"rom wODden slat s or pla stic . sheet s is . ' also'poss ible The applicatio n of was tewa ter o nto th e medium is acco mplished by a ro tating distribution system as shown in Fi g. 5-24. Under a hydra ulic head of about 1.0 m. Jet actio n through the nozz les is sufficient to power the rot o r. Thi s a rran gement results in IIlt ermitt ent"d o.s in g. with o pportun ity for air circulati o n through the pores betw ee n d osin g. Dispersion of the \\;astewater is accompli shed in the to p few centimeters of rand o ml y packed medium. resultin g in uniform hydraulic loading thr o ugh o ut t l~ e remaining depth. Electrical mot o rs may b
f,'r
r fI (.11'
ENG I NEERED SYSTEMS FOR WASTEWATER TREATMENT AND DISPOSA L
258
259
WATER
Table 5-8 Typical design criteria for trickling filters Ile m
Lo w-ralC fi ller
In lermedia te-ra te filter
H ig h-r a te filter
H ydr aulic toad ing. m .lj m ' · d
t- 4
4- t O
10- 40
0.08 --0.32 \ .5- 3.0
0.24- 0.48 1.25- 2.5 0- \
0.32- 1.0 1.0- 2.0 \-3; 2 ~ 1
R oc k , s la g, e lc.
R ock , slag, etc.
R ock, s lag. synthet1t mat e ri a ls
2-4
2- 8
6- 10
Many
Int e rmediate
Few, la rvae a re was hed away Cont inu o us Not m o re than 15 s (continuou s) Nilrified at low loadings
O rga n ic loadin g. kg/ m J . d Dept h . m Recirculat io n rati o Filler m edi a Powe r requ ircm e n ts. kWilO J m J Filter fli es Fi g ure 5-24 R Olal ing a r m d is tri butlf1g wastewat er ove r activa ted bio tillc r (courtesroj lI'epl u/le M icro-
Slo ughin g Dosing in te r vals
floc. fll c.) .
Effl uen t
h ~.fG
Man y fa c tors affect the o peratio n of tricklin g tilt e rs .. the .mos t impo rt ant be in g J.:....! / ~ .I21 d r a':0c fl ow rates, and)emp.eLat.ULe.oJ t.he..~er.a lld ambien t a ir. f\ high o rganic loading rate res ult s in,a rap id grow t h uf biomass. Excessive g ro wth t .~rf~ Ill ay rcs ult l n plu gging o f pores a nd s ub~equent fl ood in g of po rt io ns o rth e medium. ~l, 'll' In crea sing ~ d · a ul lLlmid~~c.rea s~s s l oll~I'in1!'"1iiiZJ he lps to kee p th e 'tC ~~. bed o pen. Ra nges o f hydraulic an o rg an ic loacli ng rate s for lrick lll1 g rllt e rs are ~')r~.(CfQ sho wn in Table 5-8. These load in g rat es limit th e d e plh n f conve nti o nal tri cklin g tilters 10 abo ut 2 m becau se o f head loss th ro ll g h th e rando ml y packed medillm.
o
I nrerrnitlenl Not m o re than 5 min (genera ll y inl e rmitt e nt ) Usua ll y rldl y nitrified
. l~nterm itt enl 15 to 60 s (co ntinu o u s) Pa rt ia ll y n itrified
So urce: F ro m Met calf & Edd y. In c. [ 5-3 6]
r
Th e bi o m ass-wa ter-a ir int erfaces mak e trickling filters extremely sensitive to tempera tur e va riati o ns. Effluent qu a lity is thu s likel y to show dr astic seasonal cha nges. du e primarily to c han ges in ambient air temper a ture. Re la tive temperatures o f th e was tewa ter and th e a ir a lso determine the direction of air flow through the medium. Cool wa ter a bso rb s heat from the air. and th e coo led air falls toward the bo tto m o f the fi lt er in a cocu rren t fa shi o n with the water. Co n versely, warm wa ter- he'ats the a ir, ca us ing it to ri se throu g h the underdrain and up through the medium . At te mp era tur e differentials o f less than about 3 to 4°C, rel a tivel y little air move m e nt re sult s, and sta gnant co nditions preve nt good ventilation . Extreme co ld may result in ic in g a nd d estru cti o n o f the bio film. Histo rically, tr ick ling tilters have played a n impo rt a nt role in wastewater treatment. The ir s implicity and low o perating cost have made them a I) aHracti've optio n for sma ll co mmunities in wa rmer clim a tes. However, modern effluent sta nd ards th a t d ema nd hi gh-qu a lit y e fflu enl b n a co ns istent basis m ak e the use of the c lass ica l tri c kling fi lt er questionab le. Although multistage, hi gh-rate filters can be des igned to meet m os t secondary e ffluent sta nd a rd s. recent adaptations of th e bas ic pr ocess. d esc rihed in th e fo llo win g sections. have proven m o re economical in the co nstru ct io n o f new faci lities.
Bio-Towers.
Fi g llTl'
5-25
'r ypical
( '(l l/lfol F( ,t!/'I{/l/Ol/)
hlock ... u ... c-d in tri ckling filter
uIH..h::rdrain
:"'I~'-IL Ill (c Oll rlt'SY oj " 'O IN I'o/hlfioll
Bi o- towe rs are esse nti a ll y d ee~jn~ Lightw! ig ht,_ m od ul a r medi a fo rm ed by wel din g co r ruga ted an d fl at ~HQe sheets to g..e ther Ifl a lte rn~ l ing palle[~i%id~t-y--for v_erlica l stackin with o ut the excesSive weig ht that wo uld res u lt fr o m s ton e o r slag med ia . Additio na ll y, th e po rosity
260
ENG I NEERED SYSTEMS FOR WASTEWATER TREA TMEN T AND D ISPOSAL
WATER
and regular shapes provided by this medium overcome the head loss problem encountered in randomly packed reactors. Modules of thi s medium similar to that shown in Fig. 5-26b may be stacked to heights of up to 12 m to provide a large volume in a relatively small containment structure. W ooden lathes in alternating patterns, as shown in Fig. 5-26c, are sometimes used instead of a plastic medium. The pertinent characteristics of these media are given in Table 5-9. Application of wastewater may be by a rotating distributor similar to that used in a trickling filter if the surface configuration of the bio-tower is round. Most often, application nozzles are stationary. with water being sprayed over the medium from a pipe grid as shown in Fig. 5-27. Underdrain systems are similar to those for trickling filters but must be designed (or higher flow rates.
Tahle 5-9 Properties of trickling filter media
volume.
Specific surface area,
Nominal si7e. mill
kg / m'
m2jm 3
2S ·65 100- 120
12'0- 14"0 800- 1000
55-70 40- 50
40- 50 50- 60
90()· 1200 800· 1000
55 ~ 70
45 - 60
40- 50 50- 60
80- 100 100- 200 40- 50
94 - 97 94- 97 70- 80
!\rIass/ urlll
Medium River rock
S·mall
Large Bl ast-furnace slag
Sl11all Large Pla st ic COllvt:ntional
High ·s pecific surface Redwood
:5 0
~O
75 - 125 000 x 600 x 1200' 600 x 600 x 1200' 12nO x 1200 x 500'
* SlIt.: o f module uf medillm . SOli,.,,· Fn,,]] Metcalf & Ed,h. In t. [5·.<6J
Inlluent Eflluent
I I
Secondary clarifier Alternate recycle
............ ...... .... ·····1··
I
Secondary sludge
1--- ---------------"" 1
1 t
Sludge disposal
Eflluent recycle
(a)
-. (b)
(el
Figure 5-26 Bio·tower system: I l l) lliagr;trllmalic , ke tch: (I,) m od ule o f plastic mediu m AlulIlers Corp.): Ie) wt)pd billtn('dia nhHllIlc ~ /t'f)lIffl'SY I~l.\('plll!le !\1icn~llof. Inc .).
(COUrlcS\"
.
o(rhe .
Void space. percent
:10·· I OU 3D-IOO
150- 175
261
262
ENGINEERED SYSTEMS FOR WASTEWATER TR EATMEN T AND DISPOSAL
WATER
Bio-towers a re operated in a fas hi o n s imilar to high-rate trickling filters. The di s persion cha rac ter is tics o f the plas tic modul es are less effect ive than with rand om packing, and the hydraulic flow rate must be mamtained at a hi g h le ve l to ens ure that a ll s urfaces are wetted thro1ughout th e entire depth. Direct recirculation of 1 to 3 tim es the inflow is com monly prac ti ced. The ,diluted substrat e res ults in ~nd ogenous respiration thLO.ugh.ou.t mo s t o f th e dept h o f th e tower. Ca r bonaceous BOD is gener a ll y sa tisfied in the upper reaches of the medium. If th e ca rb on co nte nt o f the wastewater falls below a bo ut 20 mg/ L, nitrifying bacteria beco me compet iti ve a nd ammo nia is co n verted to nitr a te. A we ll -o perated bi otower sh o uld be able to produce a nitrified effluent. Bio- towers have several advantages over c lass ica l tricklin g filt ers. The poros it y a nd nature o f the packing allow g reat er loadin g ra tes a nd \d rtuall y el iminat e 'r' plugging prob lems. Increased ventilation m i{lim izes odor proble ms und er m ;st o perating cond iti o ns. The co mp ac t na ture ot the reac tor a llows for eco no mi ca l h ousi ng fo r opera ti o n in severe climates. Disad vantages includ e a re la ti ve ly hi gh pumping cost necess itated by the large recyc le requirement a nd th e head loss thro ugh the deep bed. Design o f bio-towers is usually based on formu las deve lo ped for tric kling filters, with allowances being made fo r medium characteristi cs. The most com m on ly u sed fo rmul a was proposed by Eckenfelder [5-20J a nd is of the form
r
Se
=
e - kD1Q"
where Sa is the BOD s o f the mi xture o f raw and recycled mixture applied to the medium
s
Example 5-5 : Designing a bio-to\Ver A bio-tower conipo'sedor a modular plastic medium is to be used as the seco ndary-treatment co mpone nt in a municipa l wastewater trea tm en t . pl a nt. rl ow fr o m the primary cla rifi er is 20,000 m 3 j d with a BOD of 150 mgj L. Pil ot -plant a na lys is ha s es tabli shed a trea tabilit y.co nsta nt o f 0.055 min - 1 fo r the sys tem a t 20°C, and th e n ractor ca n be ta ken as 0.5. Two towers a re to be used , each with a square s urrace and sepa rated by 3 common w,i ll Th e medium is to have a d e pth o f 6.5 m , and the reci rculati o n ratio is to be 2 to 1 during average tlow periods. Determine the dim ens ions o f th e units required to pro du ce an efflu ent with a soluble BOD , or lO ~mg/ L. Minimum te mpe rature is expected to be 25°C.
r,....,
----SOL UTION
150 + 2 x 10 S = -----
(5-26) Trea tab ilit y fact ors s ho uld be d etermined fr om pilot-pl a nt anal ys is o f was tewater and se lected m edium. The coefficient n fo r ' m odu lar pla stiC media can be ta k en as 0. 5 witho ut s ig nifi ca nt er ro r. [5-6J I Th e above fo rmula d oes not account for recirculation o f was tewater. Becau se bi o- towers a lm ost uni ve rsally e mpl oy recirculati o n_ Eq. (5 -25) must be mo dified as fo ll ows :
"
I
+
2
56.7 mg/ L
=
2. Th e treatability co ns tant mu st be adj usted ror te mperature [Eq. (5-26)].
k ,; = k,0( 1.035)25- 20
3.
=
005 5(I03W
=
0.065 min -
1
10he loadi ng ral e is ro und by so lving Eq . (5 -27 ) ro r Q.
10
5'07 10 c - (I
+
e-0065Xh.S/QO ., = (T+Rl=R;o· 06~5~x~6~.S""/Q""o~.,
2) = (' - 0.41!Q"'
10 + __ (2)(' - 0.42/QO. ,
)67
56.7 0.53 = 1.35(' - 0 .4 2:QO
0.39 =
'
('- 0.4 2/ Q O. .'
094 = 042 /Qo ; (lo s = 045
I' - kDI Q"
R) ~R~-:::kD/ Q"
(5-28)
I. Th e Infiu e nt concentrat ion o j' BOD s is determined from Eq. (5-28).
Th e va lu es o f th e trea tability co n sta nt k range fr om 0.01 to 0.1. A ve ra ge va lu es fo r municipal was te o n modular prasiic media are around 0.06 a t 20°C. [ 5-23J Co rrecti o n fo r ot he r temperatures can be m a d e by adjus tin g th e treatabilit y fac to r a s fo ll ows [ 5-19] I
(I +
+ RSe 1+R
1nd R is ratio of the recycled fl o w to the influent flow . Th e design of bi o- lOwers is illu stra ted by the following example.
So
Sc Sa
= So
a
(5- 25)
where Se = e fflu ent subst ra te concentration, BOD s, mg/ L So = influent s ubstrate concentrati o n, BOD s, m g/ L D = depth o f the medium, m . Q = h ydra ulic loading rate, m 3 1m2 • min k = trea ta bilit y co n stant relatin g to th e wastewater and th e med ium charac teris ti cs, min - 1 n = coeffi cie nt relating to the medium c ha rac ter isti cs
(5-27)
263
Q = 0.20
1ll
3
, m2
.
mtn
264
I;:"'JGINEER ED SYSTEMS FOR W ASTEW,\ TER rREATMENT AND DISPOSAL 265 WATER
Rotating biological con t actors
4. The surface area of eac h unit is determined as follows: 1d
20.000m 3 /d x
1440
13.9 m 3 / min
3
.
= 13.9m / min
. =
2 x 0.2 m 3; m 2 . Illin
Effluent
Inlluent
m ill
34.8111' L _________ ~---------J
Each unit is square. so dimensions are
L= W Each unit is 6.0
J11
~
= (34.8 m ') 1' 2 = 5.89 m. say 6 J11
Sludge wask
x 6.0 m x 6.5 m deep'. The system is shown schemalica ll y in Ihe
(a)
accompanying ske tch. Effluent relurn
Bi o- tower
#1 o ~.
Effluen l
6.0 m
InOuent E
Bi o- towe r #2
o
I I
I I I
I
I
L--------T--------J I
-.
+ Sludge w~stc
(b)
F;gur c S-2l! Rotating biological conlactor ,ystc m .
(0)
diagram
"r
Ihe rotallng biological contactor
systenl: (h) multiple instalh1tion (n0l~ covers on units In hackgro llndJ
(/"1010 ulllflesy
0/
IVolker
Proc /;'ssf,\: Corp.).
Rotating Biological Contactors Th e rotating bi olog ical cont actor (RBC) reactor is a unique ad'U1tation of the att ac hed-growth. pLOcess. Med ia in the form of large. flat disks mounted on a ~ mon shaft are ro tat ed thro ugh specially con toured tanks in which was tewater flows on a con tinu ous basis. Th e system is shown in Fig. 5-28. . The medium cons ists of plast ic sheets ranging from 2 4 m..:li diame ter and up to ~O mm thi ck. Th in ner material-s can be used by sa nd wiching a corr uga ted sheet between two tlat disks and welding them together as a unit. Spacing between fl at disks is approximately 30 to 40 mm.-The disks ai·e mounted th rough th e ce nt er on a steel shaft in widths L!J2. to g m. Ea;;h sha ftful or medi um . along with its t
The disks are submerged in [he wastew ntinul)[ls b~I'I' ,IS desc ribed c~lrl i e r . Thickness or thc binlilm Illa) reach .2 :, 1 4 III Ill. depcndln~ 11I1 the \laSlel\·atcl- strength
266
ENGINEERED SYSTEMS FOR W ASTEWATER TREATMENT AND DISPOSAL WATER
and the r~nal speed of tbe dis k. Since the biofi lm is oxygena ted externally from th e was tewat er. anaerobic co nd itions may deve lop in the liqu id . Prov isio n for ilir inj ecti o n nea r th e bott om of the tank . is usuall y prov id ed when multiple mod ules in series are used. U nd er no rma l operating conditions. carbonaceous substra te is removed in the initia l stages of th e RBC. Carbon co nvers ion m~l y be completed in the first stage of a se ries of modules. with nitrificatlun being comp leted after the fifth stage. [5-5 IJ As in the bio-to ~e r pmc:.ess. nitrifi e~ lti QI1 proceeds o nl y aft er carbon concent ratio ns have been substantiall\" recluced. Mos t dcsi!!.ns of I{ HC svs tems wi ll include a in illimun~or four o r fi\e mZJdules in se ries to oblain nitritic~;t io n of the wa stewater. Th e RBC system is a rela ti vely new prllCess for was tewater treatme nt. and expe ri ence with full -sca le app lication s is Illnitcd. The process appears to be we ll suited to the trea tment of mun ic ipal wastew ater. however. O ne modu le of 3.7 m in diamete r by 7.6 m lo ng con ta in s app roxilllatel y 10.000 m ' of surface area for biofi lm grow th. This large alllo unt of bioma ss permits short co ntact time. main ta ins a lt meeting stable sys tem under variab le load ing. and sho ultl roduc seco ndary- trea tment standard s. Recircu lat ing el uent throu gh th e reactor is not necessa ry. T he slo ughed bio mass I S relat ively den se and se t! ies we ll in t he second ary ·c lar ifier. Ot her ad van tages in c lude low power requiremenl
50 ~ j' ...J
40
30
E'
""
E
isco
~, 20
267
0.9
0
"
~
0.8
c:
.9 ·U
~
C
0. 7
u
0.6
L--L__L--L~~
O~...J--J__~~__
35
40
45
50
T empe rature, of
55
60
Figure 5-30 Temperature correction for toading curves in Fig. 5-29. Muttipty toading rate by co rrecti on factors (courtesy oj Autotrol Corp .).
Disadvan tages of the system include a lack of documented o pera ting experience. hi gh cap ital cost. a nd a sensi ti vity to temperature. Covers must be provided to protec t th e media fr o m da mage by the elements and from excessive algal growth s. Adeq uate ho using also he lps to minimize temperature problems in co ld er climates. Design of a n RBC unit is based on hydraulic loading rates. Graphs showing relati o nsh ips betwee n load ing rates and effic iency similar to th e graph shown in Fig. 5-29 can be obtained from manufacturers for specific media and vario us was tewa ter strengths. Required surface area is then translated into the number and size of the modules necessary. Temperature corrections can then be made using Fig. 5-30. The des ign of a n RBC system is illu strated in Example 5-6. It should be emphasized that final des ign of an RB C system sho uld be based on loading rates obtained fr o m pilo t-p lan t modeling as opposed to generalized figures such as those shown in Fi gs . 5-29 a nd 5-30.
ci 30
0
co
15
25
'~
:c -=:
Example 5-6: Designing a rotating biological contactor Dete rmine the surface area re~ quired for an RBC syslem to treat the wastewater described in Example 5-5.
s; to
;::
SOLUTION
~
c: 10
o
w.:
.1. Enter Fig. 5.29 with: 0
240
320
360
fh drJutlc Il,,,dlllg rat e . I. d . m~ Fig ure 5-29 F.ftl c i~n cy and I O llJin ~ fat e relatI onshIp water (C O llrlt 'S)" o( Ali lOfro/ ( ·()rp . '
I"I..)J
Bl() · Surf medium
trl.:allllg Illunicipal wa ste-
Influent BOD = 150 mg/L Effluent soluble !:lOD = to mg/L The hydraulic loading rate is found to be 0.05 m3 /m2 . d. 2. Disk a rea is
268
ENG I N EERED SYSTEMS FOR WAStEWATER TREATMENT AND D ISPOSA L WA T ER 4
3. Ass umin g a 7.6-m s hafl for a 3.7-lll-dia meler d isk with a 10 lal s urface arca of I x 10 m 1 . 40 modu les in para ll el wi ll be req ui red to prov ide si ngle-stage treatmen t of the wastewa te r. For nit rifi cal ion. a maximum of five sla ges (200 modu les) will be requ ired.
5-1 4 SEC ON D A RY CLA RIF Th e bi o mass ge nerated by seco nd::ny trea tment represe nt s a substa nti a l o rga nic load and m ust be removed to mee t acce pt a ble efflu ent sta nda rd s. In po nd s a nd lagoo ns,. thi s remova l is accomp li shed by se ttlin g within the reac to r. In ac ti va tedslud ge a nd att ac hed -culture systems. so li ds are removed in seco nda ry clar ifie rs. Beca use the cha rac ter istics of bio logical so lid s In suspend ed a nd a ttac hed c ultu re sys tems a re significa ntl y different. the design and opera tion of secondary cla rifiers in these systems a re a lso d iffere nt.
o
~ ThiCkening zone
R;il . . '.'\. Uniform
~ compreSSion zone
Clarified zone
L&J lone (C~seliCo)ling
269
tOO
75
50
Activated-Slud ge Clarifiy s Seco ndar y cla rifi ers fo r ac ti va ted sludge must acco mp lish two objectives. First. t hey mu st prod uce an effl uent sutflc ien t Iy cla ri fied to mee t di scharge sta ndard s. Seco nd ly. they mu st co nce ntra tc the bio logica l so lids to min im ize thc q uantity o f slud ge that mu st be ha ndled. Because bo th fun ctio ns are criti ca l to s uccess ful operatio n, seco nda ry cla rifi ers must be des igned as a n int egra l pa rt of a n ac ti va tedslud ge system. The bio logica l so lids in ac ti vated sludge are f"loccukn t in natu re and, a t con.-'...., cent ra tio ns less th a n abou t 1000 mg/ L. settle as a ty pe-2 suspensio n. Mos t biolog ica l reac tors. however. o pera te a t co ncen tra tio ns in excess o f" 1000 mg/ L. a nd th ickening in the seco nd a ry c la rifier res ult s II I eve n grea ter co ncent ra tions. A .. . . . . .. .. . ' c-oncent rat ed'susfTPl1srOlr wm;' defi'ned ' jii S'ec: 4-4 as a suspensio n In whic h pa rt icles are close en o ug h toge th er so th a t t he ir ve loc it y fie lds ove rl a p with th ose of ne ighbor in g pa rt icles a nc! a s ignifi can t upward disp lacement of wa ter occurs as pa rti cles se tt le. In concentrat ed suspensio ns. these a nd o ther fac to rs ac t to preve nt in dependent settli ng. Gro ups of pa rti cles se ttle a t th e sa me ra te. rega rdless of size d ifle rences of t he indi vi du a l pa rt ic les. The co llecti ve \'e loc it y of pa rt ic les dc pends o n severa l va riables, t he mos t obv io us of whic h is th e co ncentra ti on of th e suspension. the ve loc it y be ing in verse ly pro poni o na lt o th e concentrati o n. In second a ry c la rifiers. t he solid concent ra t ion must be in c reased fro m the co ncentratio n of th e reac to r X to t he co ncent ra t io n of th e cla rifi er unde rfl ow Xu· Sett ling velocities cha nge corresponding ly. resul ting in zo nes wit h difle ren t se ttling characteristics. T his phenomenon. known as zone s<, {{ling . ca n he Ill ustra ted by a simp le ba tch ana lys is in a co lum n. a~ described below . \
~
Ba tch ana lysis If a colum n IS fi lled with a.c~nce n t ratccl suspension and al lowed to se ttle quiescentl y, the co nt ent s will soo n cii vid e,int o zo nes as show n in Fig. 5-3 1. In zone B, t he in itia l concen trat lun Co is preser\"ecl a nd sc tt les a t a un ifo rm ve loc it y cha rac teri sti c of that conce nt ra ti o n. Th:: resu lting clarificd zo ne. zo ne A, is le ngt hened a t thi s sa me ve loc it y
l
o Time
Figure 5-3 1 Zone
:-'ClliJllg .
Below th e uni fo rm ve loc ity zo ne. twO o ther Lt>nes deve lop_ As the parti cles a t the bottolll come to rest o n the floo r of the cy linder. the pa rt ic les Immed ia te ly above L:t1 1 on top of th em. form lll g'a zo ne In whi ch pm tic les a re m ec hanica ll y supported frolil belml·. T his zone. I:lbe lcd lOn.: D In F ig. 5-31. is ca lled th e CO /11press i;JI/ zOll e. and particles In thi~ zo nc halt: onl) a slight ve loci ty I'"csuiting from consolidaiion. . The a rea between zo ne [) and zo ne H cll ntain s a Cll tl Celltratiotl gradien t ran gin g from slightly gl-e;lter- than Co .l ll St beloll Zll ne B tll sligh tly less than th e concentration ;It the lup o f the com pl-essio n Zllnc. Co llectil'e \'cloc ities of particles in Wile C. appropri a tel) ca lled th l' ,h{("k(,lIilI.lI :()II<" . llccre; l,e III pr,')portion to thi s concentration gradient. ;\s time progl·esses. the intcrLlces betlleen the zones mu\e re latil'e to eac h other Rekr rlne a£;lin tu Fil:. 5-3 1. th e C [) int erf;llT nlllyes upward as parti cles frOlll ~l'nc C Int(l zun: D. /\ s iLlng' ;IS the cll nccn t r; ltin n gradien t in zo ne Cremains unchan£cd . the Width "I' tlw; Z(l il e Ilili st als" remain cllnqant. :lIld Sl' th e H- C IIHcrface i ~ dISplaced lIpw;lrd
d;OI;
ENGINEERED SYSTEMS FOR WASTEWATER TREATMENT AND DISPOSAL
270
271
WATER
Q,. X,
(Q + Q,)
X,
c} C2
L ____~==============_ ('I Time
Figure 5-32 Relationship between initial concentralion alld settling curves.
of particles in the initial concentration, zone B is eroded from both top and bottom until itbecomes nonexistent (1 = I) in Fig. 5-3 I). After this time, the newly created A- C interface settles at a decreasing rate as the interfacial solids concentration increases successively from Co Uust at the disappearance of zone B) to the concentration of the top layer of the compression zone just as zone C also disappears (1 = 15). The A-D interface thus formed will subside at a slow, uniform rate as the solids consolidate under their own weight. releasing some of the interstitial water to the clarified zone above. All of the interfaces involving the clarified zone should be readily visible if the container used in the analysis is transparent. Other interfaces (B-C, C- D) will not oevisibJe·oecai.ise coricentrilioli pClin is' ~lre' 'sfiglit' the' sei iiJiig' characteristics of activated sludge can be graphed by recording the visible interfacial height at succeeding time intervals. A plot of the interfacial height as a function of time, similar to that superimposed on Fig. 5-31, can then be drawn. The effect of varying the initial concentration of the activated sludge is illustrated
clialiges at' t1-iese'
by the family of curves shown in Fig. 5-32.
Continuous-flow analysis The zone settling principles just described for batch analysis are also applicable, within limits, to continuous-flow secondary clarifiers. .An "idealized" secondary clarifier is shown in fig. 5-33, with the appropriate zones labeled. If steady-state conditions are imposed with respect to flow rate and suspended-solids concentration for both the influent and the undertlow, all of the zone will be maintained at static levels. Because the A - B interface is stationary, water in the clarified zone rises toward the overflow at a rate equal to the collective settling velocity of the Co concentratio.n, thus satisfying the clarification function of the secondary clarifier. The thickening function is accomplished via the concentration gradient in the thickening and compression zones and is more difficult to determine. The thickening function c
Figure 5-33 Zone seltling in secondary clarifier. (AdapledJrom Vesilind [5-55].)
and Clevenger [5-12J and later modified by Yoshioka et a!. [5-56J, DiCk and EWll1g [5-16], Dick [5-15], and Dick and Young [5-17]. Solids flux is defined as the mass of solids per unit time passing through a unit area perpendicular to the direction of flow. In secondary clarifiers, it is the product of tile solids conceritration , ····(mass!vo·lmne) times the velocity (length/time). The preferred units are kilograms per cubic meter (kg/m3) times meter per hour (m/hr), or kilograms per square meter per hour (kg/ m2 . h). . The downward velocity of solids in a secondary clarifier has two components: (I) the transport velocity due to the withdrawal of sludge, and (2) the gravity. scttllllg of the solids relative to the water. The transport velocitv is a functiOn of the underflow rate and the area of the tank. .
(5-29) and the resulting solids flux for a clarifier operating at a given underfl;w rate is a linear function of the solids concentration.
G"
=
1'"X i = (Q.,/A)Xi
(5-30)
whereG u is the soiids flux at the particular depth where the solids concentration IS Xi ThiS relationship is shown graphically in fig. 5-34. The sobds nux due to gravity settling is defined by (~-31)
272
ENGINEERED SYSTEMS FOR WASTEWATER TREATMENT AND DISPOSAL
WATER
where Vg is the settling velocity of solids at Xi concentrations. As the solids concentrations increase into the thickening zone, the gravity settling velocity decreases. )nIl1ostconcentratedsuspensions, the concentration-velocity product will increase initially, because the concentration increase is more rapid than the velocity decrease in the upper part of the thickening zone. As the solids approach the compression zone, the gravity settling velocity becomes insignificant and the concentration-velocity product approaches zero. The total solids flux is the su m o f the underflow transport and 'gravity flux
Gravity flux
't ~n
A
~
"0
(/)
B XL
(5-32) and is limited by a minimum value resldting from p'rogressive gravity thickening. For a given underflow rate, the limiting gravity flux also determines the underflow concentration Xu' as shown in Fig. 5-34. Yoshioka et al. [5-56J showed that slight modifications to the graphical approach of Fig. 5-34 give greater flexibility for matching underflow concentrations to their associated limiting flux rates. As shown in Fig. 5-350. a line beglI1ning at the de s ired underflow concentration X" and drawn ta ngent to the gravity flux curve intersects the solids flux ordinate at the limiting flux rate. The Yoshioka method is verified by comparing similar triangles in Fig. 5-35b. The absolute value of the slope of the tangent line is the underflow velocity. while the abscissa value at the point of tangency is the limiting gravity flux concentration. The ordinate value corresponding to the point of tangency is the gravity solids flux , while the interc~pt. GL - Gg , is the flux du e to the underflow transport. The relationship between underflow velocity. limiting solids concentration, .and limiting tlux rate is readily demonstrated by this technique (Fig. 5-35c).
'"
::l
273
So lid s concentration
Solids concentration
(a)
(b)
1" u)
Cu ><
~
~
~
c if)
c Lt
xu)
Xu2
I
I XL)
Xl.2
XLI
Solids concenrration ..c
c/.
(e)
Underflow trallsport (p".X,)
Figure 5-35 Yoshioka's graphica l mel hod for jdelCrminillg solids flux. (a) Yoshioka's modification; (b) verification of Yoshioka's modilicatioll. Notc ;si milarity of triangles ABO and ACO , (e) eITects of underfiow ve loci ty
SoliJ : : ,.:oncen tr atiollx" mg / L
Figure 5-34 Solids nux
a~
a fUllction \)1' :"t)lids COIlCelllralinll and uIlJert1o\\" vcloclt: .
011
solids co ncentration.
I
Secondary clarifier design Seco ndary clartfiers mtlst bc' de signed for effl uent clarification and solids thickening, hoth of which re late directly to the surface area. To determil1e the rcquired surface area. an underflow concen'tration i~ selected and the overt-low rate alld limitin g solids flu x established or assumed for the particular activated sludge under consideration. Batch analysis similar to that previollsly described can he used to provide overflow rates and thickening characteristics, prmldcd arpn) prt~IIC ,amples Ill' activated sl ud ge arc available. A sing le
274
ENGINEERED SYSTEMS FOR WASTEWATER TREATMENT AND DISPOSAL
\VATER
275
te st at the ex pec ted concen tr ation Co is sufficien t to establish th e over fl o w rate. The straight-lin e portion o f th e interface vs. time grap h es tab li s hes th e se ttlin g ve locity o f the initial concentra ti o n a nd thu s es tabli shes th e overflow rat e. Beca use it is not possible to d e termin e concentra ti o n -ve loci t y relat io nships in the thi c kenin g zo ne , a serie s of tes ts, each a t different initial concen trati ons, is neccssar y to esta bli sh the so lid s tlux curve. Onl y th e str a ight-l ine portion o f each curve is used to obtain th e ve lo city Vi relating to each co ncen t ration X i' The res ultin g so lids flux is ViX i · Obtainin g a ppropriate s lud ge sa mpl es for b;ttch ana lys is is o ft e n diHi cult and so metimes impossible. In mo st cases the ac ti va ted-s lud ge reac tor that is to produce the suspension for the clarifier is also ju st be ing d es igned. Beca use any va ltd m o del must dupli ca te bo th t he design a nd ope ra ti o n variables o f th e p ro p osed reactor as well as the character istics o f th e was tewa te r, it is unlik e ly th a t an exi stin g prototype will be readil y ava ilab le for o btaining the sus pen sio n. Pil otplant stud ies o f the reactor. co upled with batch se ttlin g analysis, s ho uld Yie ld usa ble data, provided all th e variables in the wa s tewa ter-reactor sys tem have bee n modeled co rrec tl y. Wh e re a n a lytical data are n o t a vai lab le. the engin ee r Illu s t re ly o n lit era ture va lues for design data . Values w hi ch have proved successful in so me sys te ms are presented in T a ble 5-10. It sho uld be emphas ized. h oweve r. th a t ca reful cons id erati o n of reactor a nd wastewate r c harac teris t ics sho uld he mad e before selectin g ge ne ral empir ica l data fo r d es ig n. . ' . Becau se it is unlik e ly that anyone surfac e area wil l exact ly sa tI sfy both the cl a rificat io n a nd thick enin g fun c tion s, bo th areas ar e cal c ulat ed and th e more co nse r va tive o f th e two is used , Alth o u gh ne ither des ign inco rp o rate s d epth , th e e ngineer should be awa re th aI ? ep th iSinlporl anISuffi.c ie.lll,clepth nlll sl .be av~ ilah.l e. ' f6j-"(emp6r~t'ry 's t'ciiage' o'f soliJ s due to norm a l fluctuatton s o ( flow and so lids loading. Typic al d ep ths o f seco ndar y c larifiers ra nge fr o m 3 to 5 m. The physical units used fo r seco nd ary c la rifi cation are quit e similar in appearance to th ose used in pot ab le wa te r sys te ms (Sec. 4-5) and fo r prilll ~tr y c larifi ca ti o n in wastewater tre a tment (Sec. 5-8). Din'erences in so lid s cha ra c te ri sti cs demand so mewh a t different s lu dge~remova l mec hanis m s. Slu dge sh o uld be re moved as ra pidly as poss ible to en sure th a t th e hi o log ic;ti so lid s are still viab le up o n th eir
". Figure 5-36 Secondary clarine r with r a pid-sludge -return sys tem (courtesy of FM C Co rporation, ,\tIofer ia! H.(~ndlin9 Syslerns Dil:ision).
return to the aeration unit. A rapi d sludge return also prevent s anaerobic conditions from deve lop in g, with 's ubsequent slud ge fl otat ion due to the release of gases. The s lud ge- rl~ turn sys tem mu st be capable of handling a wide ra nge of flow . Underflow rat es may exceed 100 percent of the wastewater flow under upset co nditi o ns, while normal underflow rates range from 20 to 40 percent o f the wastewa ter fl ow. A typica l c irc ular-t a nk seco ndar y clarifier with rapid-sludge-return equipment is show n in Fig. 5-36. The s lud ge enters the " V ".sections o f the sc ra per as it rotates and is lifted ve rtically through the sludge-ret urn pipes to a co mm o n conduit thr o ug h w hich it is remo ved fr o m the ta nk. Sludge is thu s rem oved from the entire Ooo r o f the tank at each revolution o f the scraper. Ear ly practice has tended towa rd the use o f circ ular tank s, although the advent of ra pid -s lud ge-re mo val mechanis ms for rectangular tanks has resu lted in a n increase in th e ir use. Ph ys ical parameters associated wi th th e design o f secondary cbriliers a re g ive n in Tabl e 5- 10. The design of secondary c larifiers is illustrated in Ex ampl e 5-7. [xample 5-7: Designing a secondar)' clarilier,for acti,.ated sludge !\ column ~naly s i s was to deter mine the se tt ling c h ~racter i s tie s of an activated-sludge sll spension . The results of the analysis a re show n in the lable be low.
Table 5-10 Design data fo r clarifiers for activated-sludge systems
rUIl
On:rtlo\\ Ill
J
Loa ding . kgem' h
IClIC.
m- d
------T ype o r Irealmcnl
A\"t!ragc
Pca\..
,\ \crage .
I'eak
D epl h, m
('one M LSS. mg! L
1400
2200
31)00
3700
4500
5200
6500
8200
3. 0
1. 85
1.21
0. 76
0.4 5
0.28
0. 13
0.089-
_ I
Sell ling fo ll owi ng ai r -ac ti vated slud ge (excluding ex te nded aera li o n)
Sc.:t lling rl) lI owing cX l end~ d aerali01l
16 - 3~ ~
16
40 -l X
, I)
0.0
9.0
2.)
1.0 5.0
7. 0
Sour(,e : !\(bpteci [",m M el('olr & Eddy. I nc [5-.16]
J~
3.5 ·:1.5--5
3
T he influent co nce ntration of MLSS is 3000 mg/ L and the no", rate is 8000 m jd De termine the size of the clarifier th ai wi ll thi cken the so lid s to 10.000 mg ,L
Jlt , )
276
bl
)
It
ENG INEERED SYSTE~IS FOR \VASTEWATER TREATMENT AND DISPOSAL
WATER
277
5, Check clarification fun ction:
SOLUTION
1, Calculate the solids flux from the above data:
\
G = MLSS(kg/ m 3 ) x velocity (m/ h)
'il.
At 3000 mg/ L the settlin g ve loc it y o f the interface is 1.21 m/ h .
,l1i;
• '114
I
I{i
-
Cone mg/ L
' 1400
2200
3000
3700
4500
5200
6500
8200
G kg/m2 , h
4,20
4,07
3.6 3
2.8
2,03
1.46
0,9
0,73
2, Plot solids flux vs, MLSS concentration as shown in 'the accompany in g figure, Draw a line from the desired underflow concentration, 10,000 mg/ L tangent to the curve and intersecting the ordinate, The value of G at the intersectio~, 2.4 kg/ m ' 'h, is the limiting flux rate and governs the thickening function, .
~11
Ilr,
,'l',[-,
4
- 'it'""
.c
;~
3
)
2
'
yi ...................... ............ ········· · ·····
~1i
ltt
2
til I ~
I
ltl.' ,r
4
10
8
6
3, Determine total so lid s loading to the clarifier: d
8000 m 3 id x --- x , 24 h
f11 ~c
~1
-\
~ - -*~~-------
Attached-Culture Systems Clarifier Design of seco ndary clarifiers for attached-cu lture systems is similar to that for primary clarifiers. The clarificati o n function is the important parameter becau se sludge thickening is not a fa ctor In fa cL settling characteristics of the sloughed biofllm, o r hUrJ'IIIS as it IS often called, approach those of 'di screte particles. Overflow rates from 25 to 33 m 3 / m 2 ,d ar~ co mmonly used, with a maximum of 50 m 3 j m 2 ,d, [ 5-50J No slud ge is recycled to the reactor, so the underflow is negligible com pared to the overflow Solids are oft en pumped to the prim ary cIa-riller where they are concentrated along with the raw wastewater solids for ultimate disposal. The total quantity of solids generated by attached-culture systems is gener ally less than that generated by suspended-culture processes becau se of the endogenous nature of the biomass near the media. So lid s production can be expected to range fr om 0,2 to 0.5 kg /kg BOD 5 removed from the liquid, Well-se ttled , , . .. . ,., ... sludges ran ge from 10 to 20 percent so lids, [5-50J Liquid recirculation through high-rate trickling tilters and bio-towers may increase the required size of the secondary clarifier substantlall v, Thi S added volume may be avoided w.ith modular plastic media by direct recircu'latiQn fro m the effluent of the reac tor p~ior to (he secondary dariticr.
Concentration, gi l
5-15 DlSINFECTlON OF EFFLUENTS
3.0 kg
1000 kg/ h
m3
4, Determine the surface area of the clarifier.
~.
,
Beca use 275 m' < 416. 7 m' , th e thi ckening function govems the d es ign,
III
. !li
333 111 3 :h
--------- = 2751111, 2 1 mi h
1000 kg/I]
.,
-- ,-------- = 4167 m-
2.4 kg/ h " min 2
'
Assuming a circular shape
/4
Dia
= \ ; 416 7
)
1, 2
= 23
III
Th e disinfection of wastewa ter IS usually required where portions of the effluent ma y co me in contact with human s, Thc processes a va ilable fo r disinfecting was tewater etl1ucnt s are essen tiall y the same as those descr ibed in Chap, 4 for potable Watel', The prese ncc of Illu ch gl:ea ter concentrations of suspend ed a nd disso lved rr:aterial in the w~\s tcwat e r m:l)' result.in interferences not found in potable water. Chemical oxid:t llt s are JCllcr:t1ly cons id ered the most effec ti ve disinfectant s, with req uired dos:t!!.cs bein!!. mLich hit! leI" Jail t lOse used for c lea ner wa ter. Chlorine is thc must C();llmnn disini'ecta ntC'i n usc_ even though it may co mbine with certain constitucnts in th e wastc\\atci' {U producc haloi'orm compounds .
278 WATER
ENG INEERED SYSTEMS FO R WASTEWATER TREATMENT AND DISPOSAL
Table 5-11 C hlorine dosa ges for va ri olls wastewaters Chlo rine dosage (mg/ Ll lo yie ld 0.2 mgl L free resid ua l
after 15-min contac t time
W aslewa le r Iype
Raw: Fresh 10 slale Se p lic Se llied Fresh 10 s lale Se pl ic Effl ue nl c he m ica l p rec ipl la li on Trickling fi ll er N o rm a l Poor ACliva led slud ge No r mal Poo r Inl e rmill e nl s
6- 12 12- 25
5-16 SLUDGE CHARACTERISTICS Th e quantity and nature o f slud ge depends o n the characteristics of the wastewat er and on the nature a ll d efficiencies of the trea tment processes. Prima r y se ttling rem o ves the se ttleable fraction o f the raw was tewa ter solid s, usua lly 40 to 60 percent o f the in flu en t so lids. The quantity o f these so lid s. on a dry mass bas is, can be determined by the following equation. [5-25]
Mp= 5- 10 12- 40 .,-6 3-5 5 10 2·-4 3- 8 I· 3 3·5
Fro m Ec ke n felde r . [5-19]
C hl o rin a ti o n o f was tewat er eftlu ent s is il cco mplis hed in mu ch th e sa me mann er a s is the c hl o rination of po tab le wa ter. Larger d osages are requir ed sin ce amm o niun; a~t ~..bs l
Sludge Treatment and Dis posal Wastewa ter trea tm en t obj ecti ves are accomp li s hed by cO llcen tr a ti ng impur ities int o so lid fo rm and th en sepa ra tin g t ese so lid s rruill the bulk liquid . This co nce ntnWillLci so lid s. re re rred to :1 S shl( . co ntai ns nl
279
~ x
SS x Q
(5-33)
where M p = ma ss of rrimary so lid s. kg/ d ~ = efficienc y of primary clarifier SS = to ta l suspended so lid s in efflu ent, kg/ m ) Q = flow rate, m} Id Primary sludge con tai ns in o rganic so lid s as well as the coa rse r fract ion o f the o rgan ic colloids. It c o ntain s a sizable fraction o f the influent BOD, will become a na erobic within a few ho urs, a nd must be isolated to preve nt nuisance pro ble m s. So li d s escaping p rim ary settlin g are ei ther so lubilized o r beco me entrained in th e bio mass duri ng seco nd a ry treatment. Additional so lids are generated b y co nversi o n of d issolved o rga ni cs into cellular material. Secondary slu dge is thu s co mposed primarily of bio lo g ica l so lids, the qu a ntity o f which ca n be es timated by th e equa ti o n
M,
=
Y'
X
BOD s x Q
(5-34)
where iV/, = m ass of seco nd ary so lid s. kg/d . ........ . Y. =. biomass. conversion .f
BODs = BOD s rem o ved by secondary treatment , kg/ m} Q = tl o w rate. m}jd' Th e valu e o f y ' is a func.ti o n o f bo th the bi o mass convers ion factor [Y in Eq. (5-5 )] a lld th e phase o f th e grow th c urve (Fig. 5- 14) a t which th e p a rti cula r system ope rat es . More simply , it may be related to th e food-to-biomass ratio a s shown in Fi g. 5-37. [5-25] . The con s istency o f was tewa ter s ludges varies with the source. -Enw a r y slud ge is mo re g.rao!1 1nr in nalur.e../.haILsecondary s ludg.e-.and is gener.allY. illQre co nce nt rated. C o nsi stency o f seco nd a ry sludge is dependent o n tre a tment processes ; llld is mo re vari a ble. So lid s rro m attached-gr o wth reactors a re particulate in natur e and conso lid a te better than the lig ht , f1 00c ul ent so lids from suspeifdedculture systems. It is some tim es advantageous to mix primary .and secondary slud ge to fa c ilitate further process i~g. The solids conten t o f va ri o us sludges an d slu dge mixt u res is given in Table 5-1 2 on a mass-per-vo lume baSI S. With each percent so lid s correspo nding to 10,000 mg/ L. Th e o rga ni c co nten t of b o t h primary and secondary sludge is ab o ut 70 percent. Si nct: th c spcci fic gra vity o r th ese o rganics is o nl y sli ghtl y g reat e r th a n I. the uni t
280
Ul Ul
....
, '"
j;
'--'-
-l ~
"0 C :l
WATER
ENGINEERED SYSTDIS FOR WASTEWATER TREATMENT AND DtSPOSAL
95 o4
V
o. 3
I
o
0.
....
<1)
0.
»
o2
II
~
"0
~
For a given solids-product ion rate. the volume of sludge varies inversely wi th the solid s content as shown in Eq. (5-35). Within the concentration range of wastewaler sludges. increasing the solid s content by only a minimum percentage resu lts in drastic reductions in th e s ludge v9 lume. Because the size, and therefore cost, of sludge-disposal facilities is a function of the volume of sludge to be hand led, considerable savings can be attained by volume reduction.
Conventional and step aeration processes
J
0.1 5
/
Cl
~
0.10
Extended aeration and biological riltration
II
0.07
5-17 SLUDGE THICKENING
1
.-1.1_
0.05
o
0.1
0.2
0.3
0.4
0.5
Fraction or BOD converted to excess solids Figure 5-37 Generalized diagram or excess sludge production (Y' in Eq. 5-34) as a runction of rood·tobIOmass ratlO. Actual quantities would vary rrom plant 10 plant. (From Ham~'ler [5-25].)
mass of sludge containing less than about 10 percent solids can be assumed to be equal to that of water without introducing significant error. The volume of wet sludge can therefore be approximated by the following equation:
v= where V i\.J S 1000
281
M/ IOOO· S
= volume of sludge produced.
m
3
( 5-35)
j d....
= mass of dry solids. kg/d
= solids content expressed as a decimal fraction = density of water,
kg/m3
th e nature o r the sludge.
Table 5-12 Typical solids conten'( of sludges Sludge concentration,
Type or sludge Separate Primary sludge Trickling-tilter sludge Actiyated sludge Pure-oxygen slucf¥c Combined Primary and trll'kilng-tllief sludge Primary and n1l.)oitlcd-ilcralion ~ILldgt' Primary and air-aCli\"atcd sludge
Un thickened
2.5-5.5
4-7
0"
Thickened
8 - 10 7- 9
0.5- 1.2
2.5 - 3"3
0.8-3.0
2.5- 9
3- 6 3-4 2.o-4R
K. 3- I 1.6 4 (, \I .n
Sui;rn-" Ffl)Jn ~·I l'Il'"lr ~'< Elid\ In ,' [5-36J
Sc\eral techniques are ,wailable for volume reduction. Mechanical methods such as vac uum fiitGulOp and centrifugatic)n may be used where the s ludge is subsequently to be handled in a scmisolid state. These methods are commonly used precedtng s lud ge incineratton. Where furth e r biological treatment is int ended. volume reduction by gr,l\itv lhickening a ~ flotation is common ractice. In both case::. th e sludge rcm:lIns in a liquid state. I:.!vity thicker ' arc \'cry similar in design and operation to the secondary clarifiers use( in suspe nd eJ-gro\\'lh sys tems The thickening function is the majo r design parameler. and tank s are gener7tll y deeper than secondary clarifiers to p;-(wicle greater thickening capacity. A typical gravity thickener is shown in Fig. 5-38. The vert ical ,. picket s" on the scrape r cause a horizontal agitation which helps to release water uapped in the Ilocculent str ucture of the sludge. and are commo n ly used when suspe nded-culture syslem sludges are to be thickened. A we ll-d eS ign ed , well-operated gravity thickener should be able to, at lea st, double the so lid s cnntent or the sludge. thereby eliminating half the volume "So!idscoriieliii;fl)riiiick(;nedsILldges:iiICiiig \vifli'c6Jl1monly used loading ra tes for gravity thick eners. are includ ed as part of Table 5-12. It should be no ted that the design of gravity thick e ne rs should be based on the results of pilot-plant analysis wherev'er possible. since successful loading rates are highly dependent on
7·9
Solids loading ror grayity thickeners, kg/ m 1 d
100 150
40- 50 20-40 25 SO (,(j .
IOU
60 · 100 40~O
Fi~UTl' 5-38 T~rjcal D/ ruio ll " )
g l;l VII)
th i. . "!.:L"IH: r
( ( "t/llr1I ' \T
of F"\/C Corporalion. !\/o,er/a! Handlin!! Syst e m s
282 W ATER
-
100 0 mL
Skimm er mec han isJll
T h icke ned Inn uent
ENG INEE R ED SYSTEMS FOR WASTEWATER TREATMENT AND DISPOSAL
-r----i Recycled
Mix ing BOI tom sludge co ll ec lor
Chem icals Pr ess u re cO il lro l va lve
Chemical fee d pu mp
-' .~
500 mL
J; . .
_ fe_e_d_l_in_e--r-.... •
Che mica l mix tan k
750 mL
283
250 mL
suullatan t
(b)
(a)
Air Se il led · so lid s drawo ff
(e )
\
'-'"
No w suppose tha t a ll o f the so li d s are a llo wed to settle (b in fi gure) a nd tha t the liquid is decan ted (c in fig ure) unt il th e lo ta l vo lume is SOD mL. The new so li ds fr action is
Pressurt: la nk
S
Aux ili ary recycl e Prt' ssur i/in g
co nn ec ti o n
r um p
( primary lank or plal1 l efn ue nl)
Fi gure 5-39 Typical di sso lved-a ir fl otat io l1 sys lem for thicke ning activated sludge . (F,.olll M etcalf & Eddl', Inc. [5-36].l
As can be seen in T ab le 5- 12. waste act lva tec! slu dge d ocs no t thi cke n we ll gra vit y thicke ners a nd loadi ng ra tes are s i ~ nifi c a n tl y lowc r th a n fo r o ther sl ud ge. Also. th e effec t ive ness of gra vity thic keners fo r prima ry s lud ge is cI im in ished con siderab ly by mi xin g wi th acti vat ed slud ge . T he light. fl occule nt na ture of ac tiva ted slu dge lend s it se lf q uite \\·cll to th ic kenin g by di sso lved air fl o tati o n. howeve r. a nd the use bf the process ha s been inc reasing in rece nt yea rs 'In' dissolved 'air' flo ta tion : '3 's ma ll 'qu a ntit yo f\,'ii rei': Us'll ;t t1y 'seco nda ry eftlu en t. is subj ect ed to ae rati o n under a pressure of abhu t 40() k Pa (5R Ib/ in 2 ). Th is supersa turated liqui d is then re leased near the bo tt om of a ta nk thro ugh whic h the slud ge is passed a t a tmospher ic press ure. T he ai r is re leased in th e fo rm of ve ry sma ll bubbles th a t a tt ac h themsel ves to. o r beco me entra pped in. th e slu dge so lids, fl oa tin g the so li ds to th e surface. Th e thi c ken ed s lud ge is skimm ed o fT a t the to p of the ta nk while th e liquid is re moved near the bo tt o m a nJ is ret urn ed to t he aera to r. A d iagra m of th e sys tem is show n in F ig. 5-39. Th e capital a nd o pera tin g costs of s ludge t h ic kcners a re just ified when sludge d igestio n is prac ti ced. The ex tent of vo lu me red ucti o n by s lu dge th ic ke nin g is illu strated by the fo ll owin g exa mples. IT1
= IHfl OOOV = 0.0 1 kg/ 1000 kg/m 3 = 0.02
x O.OOOS m
3
_. --.)
Thu s, increas ing the so lids co nt ent by a fac tor o f 2 (in this ca se o nl y I percen t) d ecrea ses th e total vo lume by a fa c to r o f 2. Exam pl e 5-9 : Reducing the volume by sludge thickeni ng A was tewa ter-trea tm ent pl a nt con s ists o f pri ma ry trea tm e'nt unit s fo ll owed by a n acti vated-slud ge seco ndary sys te m . T he p rim ary a nd seco nda ry slud ges a re mixed , th ickened in a gra vit y thickener , and se nt to fu rthe r treat m ent. A 'schematic o f the system is show n belo w . - -)
Efnu ent
Innu en t
L _ ._ ·
-
Thicken ed sludge to -- sludge -disposa l fa cilities
W a s tewa ter, trea tme nt p la nt , a nd sludge cha ra cte risti cs a rc as fo ll ows: Exa mple 5-8 : S lud ge vo lum e and so lid sTontent rr hitionship Suppose Ihe I- L graduated cylin de r in the fi g u re be low' co nt a in s a slltdge l) f I pci cen l so lid s. -Fro ll1 Elf . (5 - ~5 ) th e dry ma ss o f th e so lid s is
M = 10001 ' S 1000 kg, m -' x DOO I = 0.0 1 kg
11)'\
x CUl l
InflueIH S5 I"fl uent BOD
200 mg/ L 225 mg/ L
Eftluenl
20 rn gf L 19.000 m' /J
Fl o w
BOD
Sludge
Trealme nl pl an l
\V as tewater
Prima ry clarifier di ameler Ae ralor vo lume M LSS in aeralor
25 111 2900 m 3 3500 mg/ L
5.0\ so lids Prim ary Seco nda ry o 75 ~~ so li ds T hic kened 4 . 0 ~~ sot ids
-------------------------------------
284
ENGINEERED SYSTEMS FOR WASTEW ATER TREATMENT AND DISPOSAL
WATER
Determine (a) th e solids loading (in kilgram s per day) to the slud ge disposal facilities and (b) the percent volume reduction by the thickener.
h. Fr o m Fig. 5-37, the biomass conversio n fact o r is 0.35.
c. Th e mass of the seco nd ary solids is found by Eq . (5-34). /1'1, = Y' x BODs x
= 035
I. Determine the ma ss of the primary so lids and the vo lum e o f the primary s lud ge. (I. The area of t he primary c larifi er is
= 884 kgi d
X
Q
0.133 kg/m J x 19,000m / d
SOLUTION
J
d. The vo lu me of Ihe second ary s ludge 'is
A = rrd' /4 = rr x
285
v
25 m'
= _ M."..._ 1000 x S
~---
4
884 kgj d 1000 kg/ mJ x 0.0075 II R m 3 /d
h. The overflow ra te IS 19,000 m -',!d - - - - ... -
491 m'
.
3. Delermine Ih e 10lal
.
= 387 m/ d
.
II
M
I
=
/.1 p
h. V, = J.'" c. From Fig .
5- I 3 th e etliciency of th t.' clarifier is
SS
=
o f so lIds and Ihe IOla l vo lume of sl udge 10 the thickener.
M ., = n04 + tlX4 C~ 3088 kgjcl 3 V, = 4·U + II R = 162. 1 m ;d
-1. Del er mine IhelolJIIl1
58;;:
I hickencr te) the s lud ge dispmal Llcililies. Assumi ng negligible ",lids ill Ihe Ihic ker supernata nt. th e tOlal mass of sol ids in Ihe
II.
BOD = 32 ~.~
d. The mass of primary solids removed is found by Eq . (5-33) i\l p = .: x SS x
+
Ill"SS
+
Ihickened slud ge is 30RR kg/(I h. Th e lotal vo lume l,f th e Ihi c ken ed sl udge is 30~8
Q 1,1"" =
= 0.58 x 0.200 kg/ m ' x 19.000 m J/ d = 2204 kg id
v
= _ ..
5. Delermine the percenl of volume reducli o ll ac hie ved by th e thicken er.
Mp _ _
162. 1 - 77.2
1000 ·5
p
m' Id
= 77.2
and the vo lum e o r the primary s ludge is given by Eq . (5-35).
kgi d
IOr)() kg:;; I -;;-004
1(,2.1
,
x 100 = 52;{,
2204 kg/ d
=
1000 kg/ m 3 x-50s
5-18 SLUDGE DIGESTION
= 44.1 m 3/ d
2. Determine the mass of the second a ry so lid s and the vo lum e o f the secondary s lud ge. {/. Find the food-bioma ss ra ti o: (I) The food cons umed in th e aerator is: _.... :.. 15] mgi L BOD in = ( 1.0 - 0.32)22 5 mgi L BOD OU I = e muent BOD BOD. consumed in Ihe aerator
20 mgj L 133 mg/ L
0. 133 kg im 3 x 19,000 m 3 /d = 2527 kg/d (2) The bioma ss in th e reac lor is 3.5 kg/mJ x 2900 m ' = 10,1 50 k ); (]) Th e fnod -bio mass r:ltio is 2527 kg:d _I .----.---- = 0.25 d IO,ISO kg
'o ncentratecJ wastewater s lucJl!,cs. represent a considerab le hazard to the en-
~ ronment a ncJm ust- be re nd e?ecJincrt prior to di31osa l. The mos t com mo n
'ille'ai1s (Tr it l:iilizing is by bio luglcri dcgrad at ion. Becel use·t h is process is in rerrded to conve l , cllcls "()lJKc lular end roctucts. the term digesliol1 is commo )ly app li ed to this process. Sludge cJigestlon sen es bo th to re uce the vo lume o f th e thic!u:ncd slud ge still ['urther ~lIld to render the rem:J.ining so lid s inert and re latively ge n-frec. These goals c
Ana erobic Digestion Anaerobic di g,es ti oll is h:- f:lr the 1ll<)S I COIll Ill O Il process for dealing with waste\~' a t e r slud!.!cs cnnt;rinlll g primary sludge . Primary slu dge con ta ins large a mou nts
~ b·CQuP Cord6JtJt~ ~;;Cj
286
ENGI NEERED SYSTEMS FOR WASTEWATER TREATMENT AND DISPOSAL
WATER
of readily available organics that would .incLuce a rapid growth of biomass if tJeated aerobically. An.~ b~ decomposition produces co~iderably less biom ~lSS than '" If'. t \raerobicp.rocesses. The principal function of anaerobic digestion. therefore, is to II) U\ ~ as much of the sludge as possible to end products such as liquids and gases. cl)f'r, while producing as little residual biomass as possible. Wastewater sludge contains a wide variety of organisms, and thus requires a wide variety of organisms for itsdecomposiiion. The literature relating to anaerobic sludge digestion often divides the organisms into broad groups. the 'lcid formers and the methane formers: The 2.fidformers consist of f£lcultative and anaerobic bacteria and include organisms that solubIlize l~e organic solids through hydrolysis. The soluble products are then fermented to acids and alcohols of low molecular weight. The meJhane formers consist of strict anaerobic bacteria that convert the acids and alcohols , along with hydrogen and carbon dioxide, to methane. Specific products in the metabolic process are shown in Fig. S-40.
cAC('d -foOrt-~)...:~J>Q.(c#&('
-
C{Y'Cl~(
waste
r"~Rf
100'/;15%
Typically. about 50 to (i0 percent of the organics are metabolized, with less than 10 percent being converted to biomass. Reactors for anaerobic digesters consist of closed tanks with airtight covers. The completely mixed. continllolls-flow model without solids recycle is usually assumed, although the flrst tWQ.condjtions wjll seldom be mel exactly. Although most larger installations utilize high-rate digestion, treatment . plants processing less than 4000 m 3 /day of wastewater often use standard-rate digestion for economic reasons or simplicity of operation. A lypical, standard-rate anaerobic digester consisting of a single-stage operation is shown in Fig. 5-41. The conical bottom facilitates sludge withdrawal while the" floating" cover accommodates volume changes due to sludge additions and withdrawals. The sludge separates in the reactor as shown, although some mixing oC~lrs in the zOIie oGctive '~n and in the supernatant because of the withdrawal and return of heated sludge. Sludge is fed into the digester on an intermittent basis and the supernatant is withdrawn and returned to the secondary treatment uni!. The digested sludge accumulates in the bottom, its
-r
Complex
COD
287
fJ(tJ
65 %
20O/C
Ach'tf2
Gas
slorage
I • ,r:M...' ..d{f~"' 9f ' .
. ............... . ". Supernatant _ out
~ .
Sludge heater
Digested Figure 5-40 Paltlw ays and pro duc ts
[ 5-31)' )
or
sludge out anae ro bic di gt: stiprl of \\a \ !ewtlf er slllo gt·. (Frum AJ eCarl),
ENGINEERED SYSTEMS !-OR WASTEWATER TREATMENT AND DISPOSAL
288 WATER
Digested s lud ge acc umu lation rate:
Table 5-13 Design parameters for anaerobic digesters Parameter
Standard-rate
High-rate
Solids retention time, d Volatile so li ds loadi ng, kg/ m' /d Di gested solids concentrat ion, % Vola tile solids reduction, % (}as production (m'/ kg VSS added) Methane con tent, %
30- 90 05 - 1.6 4-6 35-50 0.5-0. 55 65
10- 20 1.6-64 4- 6 45- 55 0.6-0.65 65
2. Determine the d iges te r vo lum e from Eq. (5-36). . V, + V, V = - ...- - [,
2
rem oval o ft en being de termined by s u bsequ en t sludge d isposal fac ilit ies rather than b y o p era ti o n a l n eeds o f the di ges ter. T h e standard-rate di ges ter vol u ine is d e termi ned by loading rat es, di ges tion p er iod , soli d s~\Jc t iQ!l,_a nd siud&e s torage. T hese a re re la ted by th e fo llowi ng eq u a ti o n s. 2 V - VI + 2 V II
+
V2 £2
(5-36)
w here V = vo lume of the diges ter, m 3 VI = raw s ludge load ing rate, mj/ ct V2 = d iges ted s ludge acc um ul a tio n ra te. m 3 /d 1 1 = digesti o n peri od , d 12 = d iges ted s lud ge sto rage period, d D esign pa ramete rs for sta ndard -ra te diges ters are listed in Table 5-13. The des ign p roced ure is illus trated in the fo llowing examp le. Example 5-10 : DesighIhg ':j 'sfa'ridaril~raie'
289
iinaeroblc'di"ge'ste'r' i'h~ 't ili ~k~I; ~ d ~ i~ldg~ r;~~'
Exampl e 5-9 is to be di ges ted a naerobical ly in a standard -ra te d igester. Th c sludge is known to be about 70 pe rcen t organ ic and 30 percent inorganic in nature. App rox imat ely 60 percent of the organic fracti on is converted to liqu id and gaseo us end produ cts after a 30-d period. The digested sl udge has a so lid s con ten t of 5. 0 percent a nd mu s t be stored fo r periods of up to 90 d . Determine the vo lume req u irement for a standard ra te. si ngl estage digester.
+
V'[2
= 49 17 m"
H igh-rate cii[!es ters arc mure dlicie nt and ofte n req u ire less vo lume than sin gle-;tage dige ; ter s Th e contents are mechanically mi xed to ensure better co ntact between th e o rganics and the microorganism s and the unit is heated to increase th e metab o lic r;te o f the microorga ni sm s, thu s speeding up th e digestion process. Optimum temper;llure is aroun d 35°C (95° F). Because no dewatcrinl! occurs in th e hi gh-rat e sys tem, th e vo lume o f s ludge is essentia ll y un ch a nged. ;~I th o u g h the so lids contc:nt is reduced. Dewa ter ing o f the slu dge is necessary and m~l y bc acco mplished by any of th e mechanical dewa terin g ope ra ti ons d.:sc ribed In Sec. 5- 19. An alternative d ewa terin g syste m is a seco nd-sta ge diges ter s imilar to that uscd in standard-ra te operations. A high -ril te two -stage sys tem is s hown in Fig. 5-42 .. Lit tl e gas is generated in th e seco nd stage, but the 1Illlll e nt is su persa turated with gases that are re leasedm th e seco nd -s tage react o r. Con sequently_ th e second-stage reactor is usua ll y covered and is equipped fo r gas recovery. The seco nd-stage reacto r is n ot heated. Des ign ..of. Y9.i. ume. .r.e ql.lirCflWllls. .fm. bigi)-rate. tlVo-stage d igeste rs is illustr ated in th e foll owing examp le.
t
Fixed
D'gesle r gas outlel
cov~ r
Floa t ing cove r Gas slO rage
SOLUTI ON
Suptrnata n
outlets I. Determine the raw sludge loading rate and the digested sludge accumulation rate. a. From Ex a mple 5-9 the raw s ludge loading rate is
Sludge ou tl ets
V, = 77.2 m Old
b. The digested s ludge consists of so li d s no t converted to liquid s and gases. Tota l mass of so lids = 3088 kg/ d Organic fraction = 3088 x 0.7 = 2162 kgjd Organic fraction remaining = 2162 x 0.4 = 864.8 kg/ d In organic fracti o n rema ining = 3088 x 0.3 = 926.4 kg/d Tota l mass remain ing = 864.8 + 926.4 = 1791.2 kg!d
Flfsl,tat c (complelely mixed)
Second stage (stratified)
Fj~ur e 5-l2 J)j<.tgram (If hlgh -rale, 1\\"lI-stagc all iH.: robic sl ud ge digeste r . (Frum Linsley and Frwlz/JII [5·30))
290
ENG INEERED SYSTEMS FOR WASTEWATER TREATMENT AND DISPOSAL
WATER
Examp le 5- 11 : Designing a hi gh-ra te, two-stage anaerobic digester A high-rate , two-stage digester is to be designed for the sludge described in Example 5- 10 . A digestion time o f 10 d in the first s tage results in the dest ructi on of approximately 60 percent of the o rga nics. Dewatering in the second s tage occurs within 3 d with the stored sludge ha ving a so lid s content of about 5. 0 percen t so li ds. De termine th e vol ume of the first- and seco nd -stage digesters a nd compare th e to ta l vo lume to tha t o f the single-stage digeste r in Exampl e 5- 10. SO LUTI OI'
I. Th e \'l) lume of the first-stage digester is
v=
= 772 m
+ V, --2 - 1, +
=
77.2 + 35.S 2
J
V 2 1, J
- - - - - - · m It! x 3d
= 3392 I11
+ 35.81ll J /d x 'lOd
J
3. Com pare IOta l vo lume to sin gle-sta ge vo lume . Volume o f si ng le-stage (fro l11 Example 5- 10) = 4917 m ' T ota l vo lume of two-s tage = 772
+
3392 = 4164 m "
Differe nce =
~
~V
753
Ill
J
Opera ti o n o f anaerobic digesters is complicated by the delica te nature o f the methane for mers. Th ese o rganisms are stri ct
'--------------.----------~~
Suspended so li ds BOD , COD Ammonia as NH , TOla l phosp horu s as P SOl/fee:
2. Th e volume o f th e second -stage digester is . = V,
Table 5-14 Composition of typical supernatant from anaerobic digesters Primary ptanl s, mgJL
Trickling filters, mg/ L
Activated-studge plan IS, mg/ L
200- 1000 500- 3000 1000- 5000 300- 400 50- 200
500-5000 500-5000 2000- 10,000 400-600 100-300
5000- 15.000 1000- tO,OOO 3000-30,000 500- tOOO 300- 1000
Fr om Benefie ld and Randall. [ 5-6]
V,I ,
= 77.2 m 3 /dx 10d
I
291
I m 3 of gas is produced per ki logram of so lids digested. The heat content of the me thane isapprox imately 36,000 kJ/ m 3 (970 BTU/ f( 3 ). The digester gas is usually com bu sted to pro vide space heating in the treatment plant buildings, to heat-water for laboratory use. and to heat the digester if a two-stage system is used. These uses often consume less than o ne-half of th e methane . The remainder cou ld be used to drive an electrical ge nerator and the resulting power used within the plant. Ba nerji and O 'Conn o r [ 5-5J report that a significant portion of the ene rgy necessary to ope rat e a wa stewat er-treatment plant can be derived from the methane produced by anaerobic digesters. The conversion process requires expens ive eq uipment an d is a cos tly ope ration and ma intenance item , however. Most plan ts simply narc the excess methane. The .supernatant withdraw n from th e digester con tain s large amount s o f <;ol ubilized o rganics' and so lid s. as show n in Table 5- 14. Thi s materia l mu st be circulated back thr o ugh the plant for further treatment. Solids withdraw~ fro'm th e bottom o f the digester should be re la ti vely inert. Proced ures for disposing o f this materia l are discussed in a fo ll ow ing section .
Aerobic Digestion Sludge can a~o be s tabili zed by aerobic-diwti on. GelK@ lI y restricted to' bio16giqLs.l.~ldge5- in the absence o f primary sludge. this process js essential ly a -continu ati o n o!..the aeration process. w ith the volume being reduced by thi ckening tn the secondary c1ariner and slud ge thickener. The most co mmon applicat io n o f ., obic di ges ti o n involves stabili zing slud ge wasted from ex.tendecl~on sys t~ms~ nce an ex ternal food so urce is no t ~I?I? li ed, aerobic digestion is an endl)genous ::.spirat i.£!l process in_ which the organi SJllS are [mce.d to ID~bQlize th ei r own protop lasm, Th e resu lt is a minerali zed slu dge in wh ich any remaining organics a re incipally ce ll walls and o th er ce ll fragm ent s not readi ly ·biodegradable. Aero bic digestion is not as sensiti ve to environmenlal factors as is it s a naerobic co unterpart and is not as subject to ·upsets. Unlike tile anaerobic process, ae ro bic digestion is energy-co nsumptive. The digested s ludge is relative ly in ert but cI ewa . RQQ1l.y. It is o ften necessa rv to dispose of theentire volume of sludge in a ~. rather dilut e state .
292
ENGIN[ERED SYSTEMS FOR WASTEWATER TREATMENT AND D ISPOSAL
WATER
293
Table 5-15 Typical design parameters for aerobic digestion Value
Pa ra meter Retenti on time,
Oc
Acti vat ed slud ge on ly Activated sludge plus . primary Ai r required (diffused air) . Ac tivat ed slud ge o ill y Activated slu dge plus primary Powe r requi red (surface air) So lid s loading
15-20 d 20- 25 d
55 - 6SL! min . m-' 0.02-0.03 kW /m 3 1. 6- 3.2 kg VSS/ m .l d
Source: From Slee le and .M cGhee. [5-50J
Design criter ia for aerob ic diges ti o n a re give n in Table 5- 15. The desig n approach is essentially the sa me as for ac ti va ted-s lud ge reactors. Fi gun' 5- 43 I njection of \\(lstcwatt.:r sludge benc,alh grass land s. Not~ nllnimal dl'ilU rba l1ce o f so d ( CO II"'( 'S I '
of Rickel
,\I/Of7l1j(l('{ lIrtn/J
C·ump{IfIY) ·
5-19 SLUDGE DISPOSAL Several o ption s a re ava ilab le for th e ultim ate disposa l of wastewa ter slud ges. These in clud e incinerati on. JlacemeJ1.Lio_G s3n.iuu:.y ·la.ndfiIL and incor orat io n into sojls as a feUilizf:.WlWoiLco.ocii..t io ner Raw (undigested) sludges can be in cine rated. provided the wa ter conten t is suffic ient ly redu ced. Supp leme nt a l fu el is necessa ry to initi a te ;n d maintain combusti on and Illunic ipal so lid wa ste may be used fo r thi s purpose. Ra w or digested slud ge ca n a lso be disposed of in sa nitary landfills. prov id·ed a ppro prtate measures are ta ken to co nt ain leachat e a nd to iso la te the slu dge from th e enviro nment. These subj ects are covered more full y in a later chap ter o n so lid-waste d isposa l. La nd applica tion of wa stcwater slu dges has been practiced for man-y years. modern applica ti ons being limited to digested sludge. Th e nutrient value of the slu dge .is ben efi c ia l to vegetati o n, and it s gra nular nature may se rve a s a soi l conditioner. It s a pplicatio n ha s bee n Itmit ed to gro und used for forage croris for nonhuman consumption , a lthough th e poss ibilit y of it s use o n ground used to grow edible produce is still being invest iga ted. Metal tox icity 111 plants and water po lluti9n fr o m ex cess nitra tes appear to be th e li mitin g fact o rs in la nd app licati on of sludges. Sludges may be applied in a liquid stat e by spra yin g. ridge a nd furrow , or by direct injec tion ben ea th th e soil. Injection under gra ss land s is illu stra ted in· Fig. 5-43. Dewa tered sludge Illay be spread o n the la nd a nd cuiti vateci int n the soi l by co nve nti o na l ag ri cultura l eq uipment. With the except ion of irr igation pract ices. sludge disposa l is g rea tl y facilitat ed by vo lu me redu ction thro ugh dewateri ng. Dewa terin g ma y be acco mp lished by
lllech"nicallllc; ln ~ such as cc nt rifuga tion. \;t c uum tiltl·ati o n. fil ter press ing. r by air drying. Sl) li ds co nt e nt achievable by va riull s dew3 tet"ing techniques is show il in Tahl e )- 16. ,\Ir d l·v tn g l)f digested s llld!!cs IS poss ible in clima tes with sig nitican t evapo ratlnn putcntial. SlllCC ~I \\'cl l -c1 i~ cs t cd·~hlde.e ·i~·esse nt·ill·ll-v ·ine·r-t· .. it·cftn· be·hand led· ilnd sto red in th e ope n air ~\·i tllllu t cr~a tln g nui san~e condi tio ns. Ai r-dryi ng faci litl cs In clu de drying heds slllIilar tll th ose shown in F ig. 5-44. Th e sand a nd unde rdr~ tin sys lem mar he o mitt ed in d ry c liT1l ~ lt es whe re evapo rat io n from th e surface is ~ ulJi cient to di spose nf the liLJuid. Dried s ludge is removed ·in cake form hy so lids- hand lin g equipm ell t . Another. popular form of dewalering llr digested sludge is the sludge· pond. Not tll he confu sed \\ii th oXldatlt)t] ponds used in secondary trea tment processes .
Table 5- \6 So lids cont ent of dewate rcd sludge ApprOX1J1l;11L' ... nlll.h l·f)[l\ Clll. ""
\',I (lIllrll fill r~llllln
211 .10.
CClllnrUk!C
20 25
F illl'r
rr~~.\
hcd, Pv~,d,:" . l)rvln\:t
J() 40 l()
~5
ENG IN EERED SYSTEMS FOR WASTEWATER TREATMENT AND DISPOSA L
294 WATER
295
ma y be replaced by advanced-treatment sys tem s. Advanced-treatment processes and operations are d esc rihed in the following secti o n of this chapter. Because trea t ment sys tem s a re selec ted to meet discharge or reu se criteria with respect to specific parameters. th e di sc u ss ion is arranged acco rdin g to tre a tment objectives.
5-20 NUTRIENT REMOVAL
Figure 5-44 T ypica l opell-air sludge dr yi ng bed. (Photo court esy of R. L. Sanks .)
sludge po nd s function as settlin g basins w ith long retention times. The so lids consolidate in the bo tt om whi le th e s upe rnatant ' ,s pe riodi ca ll y remo ved fr o m the top <1nci recyc led for re trea tment. When th e so lid s ha ve accumulated to a prese lec ted depth. the pond is t<1k en out o f se r vice an d all o wed to dry out. The dried slu dge is th e n removed fo r final di s posal.
Advanced Wastewater Treatment The qual it y uf e Alu en t pro vid ed by seco nd ary t rea t me n t ma y no t a Iwa ys be s u ftI c ie n t to m eet di sc harge l equirement s. Th is is o ft e n th e case when large quantities o f . e lllu c nt arc di sc hlI,,\\' s conve nti o na l seco nd al'Y treatm ent. ThiS IS lI o t ;J1wa ys tlt c c ase, , IS S(l lll C uni t o pc l',lti ol1s or unit pl'occsscs In scco lllLlr v ur eve n prim ,lI'Y tr eatm ent
The ro le of excess nutrients in entrophica tion was discussed in C·h ap. 3. Although th e q uantiti es of nutrient s co ntribut ed by wastewa ter discharges may be less than tho se contributed by agr icult ural runoff and o th er sources, the point-source nature of wastewate r.disc harges makes them more amenable to control techniques. Thus, wa stewat e r-trea tm ent plants th a t discharge to water bodies that are delicately bala nced wi t h respect to' nutrient lo ads may ha ve nutrient limitations imposed o n their etflue nt s. The nutrients n of interest are nitrogen and phosph orous compounds. Processes for removing the se nutnen s ro wa ter are discu ssed in the fo ll o wing paragraphs.
Nitrogen Removal In domestic wastewater. nitrogen compounds res ult from the biological decomposition of proteins a nd from urea di scha rged in body waste. This nitrogen ma y be bo und in comp lex organ ic molecules and is referred to simply as organic lIi~el1 rather than b y s pecific co mpound. Organic nitrogen may be biol6gkally converted to free ammon ia (NH)o) or to the ammonium ion (NH4 +) by one of .. .~ ey.eral. different m etabolic pathways,These.two species,. together termed ammonia lIil/'{}!1£ll. exist in equilibrium accord ing to the following relationship: c (5-37) Amm o nia nitroge n. the most redu ced nitrogen co mpound found in wastewa te r. w ill he b io log ically oxid ized to nitrate as follows if molecular oxyge n is present.
Nf-J 4 ; + ~ 0 2 N0 2 -
+
~O
--->
N0 1 -
--->
NO ) -
+ 2H + + H2 0
(5-38) (5-39)
These reac ti o ns res ult in th e utili za tion of ab o ut 4.6 m g o f O 2 per each mg of Nf-J ; --N ox idized , wi th a b o ut 7. 1 mg of a lk a linit y 'needed to neutra li ze the acid (1-1 t ) produ ced. '. . .' In raw w<1s tewater, th e predo niinant fOrJ~ s of nit rogen are-o rganic nitrogen a nd am lll onia. Bi o logica l tr ea tment may result in conve rsion io nitrate. provided the processess are aerob ic and pro vid ed th e treatment peri o ds are lo ng en oug h. Co ntact tim es in Illost seco ndar y treatment sys te m s. though sufficient to comp lete th e co n version from organic nitrogen to <1mmo ni a. may n o t be s ufficient for signiftcant nitlltic a t iun. Becau se of oxyge n ci e mand exer ted by amm o nia and
HH; INEE REn SYSTH IS FO R WAST EWA T ER T REA TM EN T AN D DI SPOSA L
296
297
WA TER
beca u se of o ther e n v ir o nment a l fac tors. re m ova l o f a mm o ni a m a y be requi red . T he m os t co mmo n p roces ses fo r rem ovin g a mmo ni a from was tewa ter a re (l ) s t ripp ing with a ir and (2) bi o logica l nit r ifica ti o n-d en itri fica ti o n '
-
-.--
Air st ripping Air stripping o pera tion s co n sist o f co nve rt ing amm o n ium to th e ga seo us phase and th e n di spersin g the liquid in a ir, thu s a ll o win g tr ans fe r o f th e amm o nia fr o m th e wastewa ter to lhe a ir acco rdin g to th e pri nc ip les o ut lined in Sec . 3-4. The ga seous phase NH 3° and th e a qu eo us ph a se NH 4 + ex ist to ge th er' in equilibr ium as indi ca ted in Eq . .cS ~ 3 7) . The re la ti ve a bun da nce of t he p h a ses de p e nd s u p o n bo th t he pH a nd th e ' te m pe ra ture o f the wa stewa ter. A s see n In Fi g . 5-4 5, th e p H mu st be in excess of II fo r co m p lete co nve rs io n to N H 3° Slilce thi s is well ab ove t he n o rm a l pH fo r wa stewate r, p H adjustm e nt is f) ecessa r y p ri or !o ai r s t np p ll1 ~F 6 r eco nom ic reaso ns, lime is th e most comm o n m ean s of rai sin g th e pH . A n un avo id a ble co n sequ e nce o f lim e a dditi o n is th e so ft e nin g o f th e wa stewa te r. E no ugh lime mu s t be a dd ed to prec ipita te th e a lka linit y a nd t o a dd th e 100
l'lV VI / / II
90
80
70
....6. 0
. ... . . .... .
. . ..
40°C
II
I
IH
z
7:/ I
40
30
//
20
/
6
Jj /
?~
.-/ 7
10
8
9
JO
L/ min per sq u a re m e ter o f towe r. [ S- 14J Air strippin g. is o ne o f th e Ill Os t eco n o mi ca l m e an s o f nitroge n re m ova l. pa nicu la rl y if li m e prec ip itat io n " f ph os ph a te is a lso req ui red . beca use c hem ica l c(l nui t io n in £ can be co nc urr en t. Th ere ::ne se ri o us limitati o ns to th e process . howeve r. A; 3 i r te mp era t LI re a pp rn ac hes fr eez i ng.. a d ra st ic red uc t io n 111 e ffi c ie ncy i, obse r ve d . a nd p re hea t ing o f th e :Ii r is n() t prac ti ca l becau se o f th e la rge vo lum e requ ired . Funherill o re . towers c ann o t o pe ra te in s ub freez ing we a th er beca use o f icing. In co ld c lim a tes. a lt e rn a ti ve m eth o d s of n itroge n re m ov a l illust be p rovided
"4 0'" .
50
'-J: z
20°C
60 70
/ 00C
il r! I"t e l;lllillat or~
--
J) "lrlQutlon
Wate r inle l
"y\lL'J1l
8o :::::==: F il l
9
10
i 1
o
100
pH Figure 5· 45 EfTcCIS o rp H a lld tem pl'rill ll r~ on ci is lnbuli oll o f amm on ia and am moni um ion III \.\' a lc:f .
(From FPA [5-43].)
form in g. sp lash in g. a nd refo rm in g d fOp S / D esig n p a ram ete rs fo r amm o nia -.(r rippi ng reac to rs in clu d e a ir-t o -li q uid ra t ios. to we r d e p th. and lo adi ng ra tes. C o mm o n d es ig n prac ti ce is to use a ir-to J was tewa te r ra ti os ra n gi ng fr o m ab o ut 2000 to 6000 m o f a ir per c u bic me te r o f wa stewa ter. wit h m o re a ir be in g I-eq ui red a t lowe r te m pe ra tures. T o we r d e pt hs a re se ld o m less than 7.S 111. a nd hydr a ul ic load in g ra te s va ry fr o m a bo u t 40 to 46
durin g w ill ter. Othe r proh lem s a ssoc iat ed w ith ~lm m Oll l a str ip p ing in c lud e no ise and a ir po llut io n a nd sca lin g o f th e pac kin g med ia . N o ise a nd od o r pro blem s ca used by th e roa r o f t he fa n s a nd t he d is persio n o f a Illm Olll Cl ga s c a n be III in illl izecl by Idca t ing' lhe fac il itv a way fro lll th e p o pul a ted a rea. Prec ipi tatio n o f ca lc iulll ca rb ona te sca le
20
V '
"J:.r'I 5 0
10
o
excess O H - io ns fo r pH adj u stm e n t A n a m o u n t equ iva le n t to t he a lk ali n ity p lus J.5 mequ iv/ L is u sua ll y su ffi c ie n t to brin g th e p H to app roxima tely I I. S. Once t he co n ve rs io ll to am m o ni a has been co m p leted . s tripping, o r degasificat io n . ca n p roceed . T he m os t e ffi c ien t reac to r h a s been fo u nd to be a co unt ercur re nt sp ray t owe r s imi la r to th e o ne show n in Fi g. 5-46. Large quantiti es o f a ir ar e req u ired. a nd a fan m u s t be·.inc lud ed to draw a ir thr o ug h th e tower. P a cki n g is usua ll y pro vi d ed to mi n imi ze 111m res istan ce to gas t ra nsfer by co ntinu o usly
Fi gur e' 5--l 6 DIagram of counlc.:r-
current
towt:r
for
amm OIlI<-J
Slnrping . ( Frolll E['.·I [5-431 .1
29ll
ENG I NEERED SYSTEMS FOR W ASTE WA TER TREATMENT A ND D ISPOSA L
WATER
required . The m os t co mm o nl y used external carbo n s ource is meth a n o L CH ) OH. Wh en m eth ano l is a dd ed , the de nitrifica ti o n rea ction is
o n th e pac k mg m edia as a result o f w as tew a te r so ft e nin g ca :l be min im ized by th e use o f s m oo th- s urface p o lyv in yl c hl m id e (PV C ) pi pe a s pack ing ma te ri a l, t ho ug h occas io nal clea nin g o f th e packin g m edi a is still req uir ed .
{
Theo reti call y. eac h milli gram per liter o f nitra te sho uld require 1.9 mg/ L of meth a nol. Und e r treatment plant co nditions, ho wever , about 3.0 mg/ L of met hanoi is required for each milligram per liter o f nitr a te. ma k in g this process an expensive one. Th e inte res ted rea d e r is refer red to M et ca lf & Eddy , Inc. [ 5-36J a nd EPA litera ture [ 5-4 3J fo r des ign criteria.
Nitrification-denitr ' 'on Amm o nia nit roge n ca n be co n verted to gaseo us Ilit roge n. N 2 • b y bi o log ical pro cesses. In thi s fo rm . nitr ogen is essent ia ll y inert and d oes no t reac t w ith th e was tew ate r itse lf o r w ith o th e r cons t itue n ts o f th e wa s tewaters . S in ce N2 is the pri nc ipa l co n st itu e nt o f a ir.. trea ted was tewa ter is like ly to be a lrea d y satura ted w ith m o lecul a r nitroge n a nd the a d d iti o na l N2 is si mpl y re leased to t he atm os phe re. Bi o log ica l co n vers io n o f a mm o ni a to nitr ogen g a s is no t a direc t process but co n s is ts o f t'.vo se p a rat e s teps.Th e amm onia mu stflrs t be ox id ized to nitr a te a nd th en red u ced t o m olecular nItro ge n. These rea cti o ns reqlllfe cldfe rent en Vlro n c m ent a l co nd iti o ns and mus t be ca rri ed o ut in se p a rate reac to rs. Th e orga ni sm s respo n sible fo r n itr ifica ti o n a re t he autotrop h ic bac te ri a, nit roS0 Il1 11naS a nd nitr o b ac ter. Equ at io ns (5-38) a nd (5-39) rep resen t ca tabo lic reac t io ns t hat s up p ly en e rgy. An abo lic reac t io ns use ca rbo ll d iox id e a nd / o r bi ca d,ona te as a carbo n so urce a nd may be re p rese n ted by th e fo ll ow in g eq uat io n.
Phosphorus Removal Ph os ph o ru s is a u b iquit o us constituent o f munic ipa l wastewa ter. a ve ra g in g ar o und 10 mg/ L in mo st c ases. The principal fo rm s are w ganicall y l2.o und p h osph o ru s. polyphos ph a tes. a n d o rth o ph os ph a tes . Organically ~~J:illo.ws o rigiiiares fro m 5 0dy an d food was te and . up o n bi o lo ica l d ecompositio o f th~s e ~Q lid s , is re,lea~ed as o rth o ph ospha tes Qlypl~ 7{tes ~sed--eX'lens i vely in ~tic det ergents and o f~bute lip to o ri e-h a lf the phosph o ru s 111' was tew a ter. P oly~ h a tes can be hydro lyzed to o rth o phosph a tes. Thus th e prin c ipa l fo rm. of phosph o ru s 111 wastewa ter is assum ed to be o rth o phosph a tes. alth o u g h the other tw o fo rm s m ay coexis t. Q!:ili0 pl1ospha tes co ns ist of the negative radi cals P0 4 ) - . HPO / -, and H Z P0 4 - and m ay fo rm ch emical co mbin a ti o n s with cations or POS ItI ve ra Oicals. Tn mos t cases th e co mp o und s are quit e so luble, a nd phosphate rem ova l in co nventio na l prima ry trea tment is neg lig ible . Because phosphoru s is a co mp o nent of micro bi a l cells, so me ph os phat e may be rem o ved in th e biom ass in seco ndar y
[ 5- 32J
(5-40) A lt ho u g h some a mm o ni a is co n ve rt ed to b iomass by thi s reac t io n. th e ca tab o li c reac ti ons are t he prin cipa l a mm o ni a co n ve rs io n p rocesses. Nil rifica ti o ll ca n be accompli s hed in b o th susp en d ed-c ultur e a nd a tt ac hed·cLifiLlre reiicio·rs.· Ulld er fa vo ra ble c ircu ll1sta n ces. nit ri fi ca ti o ;) ca n be a ccom plis hed a lo n g w ith ca r bo naceo us !iDD re m ova l In seco nd ary trea tm ent sys tems... In o th e r cases it is mo re e ffici ent to sepa ra te th e processes a nd fo ll ow c arb o naceo us BOD re m ova l w ith' a sep a ra te reac tor fo r nitrifi ca ti o n Opera ti on~li p a rame ters o f imp o rt a nce inc lude pH , DO , ae ra ti o n pe ri o d s. me an cell; res id c n ce time, a nd car bo n -to- ni troge n ra ti os. T empq;tt1:lre is an over rid ing var ia bl e th a t a ffec ts o ptimu m ranges o f all t he ab ove va riab les. Co m b in ed ca rbo n ox idati o n an d nit r i- . fi ea ti o n ope ra t io ns are p oss ible a t wa r me r te m pe ra tu res. whil e co lcl e r was tewa ters will requ ire se p ara te treatm ent sys te m s in m os t cases. In t he de nit l'ifica ti o n process. nitra te is reduced to !l i ~t h eJill!Il.e fac ult m;:;e. ~ero trop hi c bac teri a in vo lved in--.th e oxid at io n o f ca r bo n aceo us m a1.er iaL F or· red ue t io n to OCC lll . t he cI isso lved oxyge n leve l mu st be at or near zero . and a car bon s u p pl y'm ti st be ava ibb le to the bac tel·ia. Because a low carbo n con te n t is req u ired for th e p rev io us n itr ifica tion step,sarbo n mus t be adclecL.WG-re d~.oJ t rifjca li ()Il t:.:.lJJ-J.lwccecl . A smal l amou n t o r primary efilu ent. by p ass~~J~d seco.Di@0· :lIlci ni tri fi ca ti o n reac tors . c a ll be usccL ) SUllp l~lit::.\J.!l:m )JL H UlVcve r. th e u n nitr~ co mp o un ci s in this wa ter wi ll he u nafTectcd by the d cnitrihca tion process ~I n d \\'i ll appear in th e e fll ue nt. W he n esse nti a lly complete lI itl'ogen relll11y.ri is reqUired . an ex terna l so urce of calbon containing no nilmgen will be
299
treatment processes. Ho\vever. microorganisms need relative"ly 'littte ·p·nospncfrbs ·· ····· _.... as compared w ith car b o n a nd nitr ogen. a nd less th a n 3 mg/L of ph osph o ru s is usu ally rem oved in co nventi o nal seco nd a ry trea tment. When effluent requirement necessitates grea ter rem oval dfici encies, additional treatment must be pro vided . The principa l mean s of phosphorus remo va f is chemical precipitatio n. At ....sli g!ltly a ~ i c p~. o rt ho ph os ph a tes co mbin e with tri valent aluminum o r iron cati ons to fo rm a prec Ipitate. . . riA.
_I. ('
lRU.()'J
.
r"'~;:J . AI J + (H " P0 4 y .l-n) Fe3 +
+ ( H" P0 4 y.l -
( 5-42) (5 -43)
n) -
Beca use d o mes tic was tewa te r usu a ll Y co nt a in s o nl y trace a mo unt s o f iro n and aluminu m. th e addi tio n o f these mat ~rials i's necessary. Salts o f th ese meta ls. such as th ose d iscussed in Sec. 4-6. can be add ed fo r thi s purpose. At h igher pH valu es. c a lcium fo rm s an insoluble com ple x With ph os pha te. The add it io n of lilll e can prov id e bo th the ca lcium a nd th e pH a dju stm e nt neces-
sary. 5Ca(OH )2
+
3( H " Po).,)"' - nl C a ,(O H )(P0 4 M
+
I1 H 2 0
+
(9 - I/)O H -
(:' -44)
"-
300
ENG INEERED SYSTEMS FOR WASTEWATER TREATMENT AND DISPOSAL
WATER
This re'a ction requires a pH o f at least 9.0 for significant ph osphoru s removaL Higher pH values generally increase removal efficiencies. H owever. recarbonation may be necessary to lower the pH after.the. precipitation process . has removed the phosphorus. Chemical requirements for phosphate precipitation exceed the stoichiometric requirements indicated in Eqs. (5-42) through (5-44). Aluminum and iron sa lts react with alkalinity in the wastewater to produce me ta llic hydroxide Aoes [Al(OHh and Fe(OHhJ and may increase the required d osages by up to a fa c tor of 3. Fortunately. this increase is not totally wasted. as th e m eta llic hydroxides . assist in the flocculati on and removal of the metallic-phos phat e precipitate. along with other su spended and colloidal so lid s in the wastewater. a nd a re thu s useful in the treatment process. At high pH values calcium reac ts co mpl e te ly w ith wastewater alkalinity to form ca lcium carbonate. Lime addit ions equivalent to the alkalinity plus that required for phosphate precipita ti o n a nd pH adju stment are required. Phosphorus removal can be incorporated into primary or seco ndary treatment or may be added as a tertiary process. Selection of th e point of application depends on efficiency requirements. wastewater characterist ics. and t·he type of secondary treatment employed . The advantages and di sadvantages of each system are summarized in Table 5-17. Where effluent ph osph o ru s concentrat io ns of up to 1.0 mgj L are accep table. the use of iron or alum inum salts in t he secondary
Table 5-17 Comparison of point of application for phosphorus removal systems Primary
Secondary
T e rtiar y Advantages
Applicable to all plants
Lowes t capital
Lowes t ph os ph o rus in effiuent
Increased BOD and suspended solids removal
Lowe r chemical dosage than primary
M ost e ffi c ie nt me tal use
Lowest degree of m eta l leakage
Im prgved stabilIt y of activated slud ge Polyme r not required
Lime recovery poss ibk Separatio n of o rganic and inorga ni e slud ge
Disadvantages Least etficient utili zation of me tal
Ca re ful pH control to get ph ospho rus < I mg! L
H ig hest capita l cost
Polymer re quired for Aocculation
O ve rd ose o f mctalmay cau se low pH tox icity
Hi g hes t met al Jc:l kage
Sludge more ditfi cu lt to dewattr than primar y sludge
Can no t use lime bec:lU se of excess i ve pH
Source: Adapt ed fr om Ku gel ma n . ( 5-28]
301
system is often th e process o f cho ice. while hi g h pH precipitati o n b y lime in a te rtiary unit is req uired to o bt a in ve ry low le ve ls o f effluent phosph o ru s. Wh ere nitroge n rem ova l b y ammonia stripping is also pract iced. terti ary lime precipitati o n at a pH of 11 .5 serves in both processes.
5-21 SOLIDS REMOVAL Remova l .s f..,suspended ::9 lid s. an ~Ole tim es di sso lved so~s.~y be necessa ry ill advanced was temter- trea tm cnt sys tem s. Th e so lid s rem ova l processes emPloyed in ad van ced was tewa te r tr ea tm e nt ~ re esse nti a ll y th e same a s tho se used in the treatment o f potah le w::l te r. a lth o ugh application is mad e m o re difficu lt b y th e overa ll poorer quality o f th e was tewa ter.
Suspended Solids Removal As an advanced Ueatment process. suspend ed -so lid s rem o val impli es the remov a l o f partic les and fl ocs tCio small 0 [' too li ghtwe ig ht to be remov ed in gravity settling ope rat ions. Th ese so lid s ma y be carried over fr o m th e secondar y c Ia rine r or fr o m terti a ry sys tem s In ivhlch so lid s were prec ipitat ed . Severa l met hods ::Ire ::Ivailable for re m ov ing ['esidual suspend ed so lid s fr o m \\·astewater. Re mova l hy ce ntrifu gat io n. air !l o ta ti o n. mech ani cal microscreening. anel granula r-m ed ia filt ra ti o n have a ll been used s uccessfull y. In c urrent practice. l!ra nul a r- media filtration is th e most com m o n Iv used process. Bas ica ll y. th e same ~) rinc ipl es th a t a ppl y to liltra tl on of particles' fr o m pot a ble wa ter appl y to th e remova l of residua l so lids in wastewa ter These princ iples we re di sc ussed in Sec. 4-8, and that di sc ll'ss i'clIi ~v';l1 iib"tbe repeate"dhere. Ditrerencesinoperat·ional modes· for application o f these prin ci ples to wastewat e r fil tration vs. potable water iiltration may ran ge fr o m s li ghi to drastic. however . and the most commonly used wastewater filtrati o n tech r1lq ues are discu ssed be low. Sand filters ha ve bee n used to poli sh effluents fr o m se ptic ta nk s. lmhoff tank s. and other anaerobic trea tm ent unit s for decad es. Beca use the y are alterna tely dosed a nd allowed to dry. th e ter m il11erlllifll.'l1l sa nd .fillers has been applied to thi s type of unit. The process is essen tiall y th e s low sand filt er described in Sec. 4-8. More recen t Iy. thi s type o f filt e r has bee n appli ed to the effluent fr o m oxidation po nds with cl) ns iderab le success. Effluent co ncentrat ions o f less than 10 mgj L of BOD and suspe nd ed su lids ha ve been reported a t filtering ra tes of 0.37 to 0. 56 m 3 i m 2 . d. Filt er run s in excess of I month are possible. [5-26J Use of intermitt ent sand filt et's in ta ndem with co nventional seco ndar y treatment has not bee n .I'erv successful. [5-141 Th e natur e of th e so lid s from these processes res ult s in rar id plu gg ll1 g at th'e S~ltlJ s urf~l ce. necess it a tin g frequent clean ing a III I thu s high maint enanc e cos ts. Th e li se of inl ermitl enlfi lt e rs for te rti ary trca tmcnt IS usua ll y res t ricted ttl plants with sma ll fl ows. Gra nul ar-media flltrat[un is us ually th e prnccss of choice in I3rger seconda ry systems. Du al or tll ultltnelkt hed s preve nt sur fa ce plugging problems and allow
302
ENG IN EERED SYSTEMS FOR WASTEWATER TREATMENT AND DISPOSAL
WATER
fo r lo nger filter runs. Loadin g rates depend o n both the co ncen trati o n and nature o f so lid s in the wastewa ter. Filtering ra tes ran g ing from 12 to 30 m 3 / m 2 dav have been used w ith filter run s o f up to I d. M o re det a il ed info rm ation o n the d e; ign o f hi g h-rate filt e rs for a dvan ced was tewat e r sys tems can be fo und in C ul p e t a l. [ 5-14J and M etca lf & Eddy. [5-36J Othe r recen t inn ova ti o ns in filtrati on practices h o ld promi se fo r advanced wastewa ter tr ea tm ent. iI/l oving hed ji/lers have been devel oped which are continuou s ly cleaned , and t he ra te o f clea nin g ca n be adju sted to match th e so lids lo ading ra te. Anoth er m od ifica ti o n ca lleel th e l'u/s{'d-hed ji/ler. uses compressed air to pe ri odicall y bre a k up th e s urface mat d ep os it ed o n a thin bed of fine filt er media . Onl y a fter a thi c k s uspe ns io n o f so lid s has accumulated o n th e bed, requiring frequen t puls in g, is the filter backwas hed . Both th e m ov in g bed and the pul sed-bed filt ers ha ve t he capabi lity o f filter ing ra w was tewater. A mu c h h ig her percentage o f so lid s C,tn be I-e moved by fi ltra ti o n than can be removed in pr imary se ttlin g_ Thc fil te r et-flu e nt. co nt a inin g lowe r leve ls o f mos tly di sso lve d o rga ni cs. res pond s very we ll to co n ve ntiona l seco ndary treatme nt. The filt ered so lid s ca n be thi cke ned and trea ted by ana ero bic di ges ti o n. Wit h a res ultant increase in overa ll meth ane pro duc ti o n, a poss ible so urce of energy for use wi thin th e plant.
Dissolved Solids Removal Bo t h seco ndar y trea t ment (Secs. 5-9 to 5- 12) a nd nuui e n t remova l (Sec. 5- 19) d ec rease the di sso lved- o rgililic-so li d s co nt ent o f was tewater. Ne ither p rocess. howe\er.- co mp le tel y re m o \'es all disso lved organic con s tit uents, and neither process rem oves s ig nifi ca nt amo unt s o f in o rganic di sso lved so lid s. Further t reatmen t ,v iii be required_whe l-e. su bs tanli a l. reducti oDs. in. the .t ota l. di sso lved .solids or. W~l s t e\\' at e r mu s t be made. Io n exc han ge. mi croporolls membrane filtr a tion. adsorpti o n. and che mi ca l ux idatit>n ca n be lI sed to decrease th e di sso lved so lid s co nt e nt of wa ter. Th ese pro.ce:sse~. de scr ibed in Chap. 4 (Sec. 4-10), we re dcvel o peel to prepare po table' wa ter fr o m a p oo r-qualit y raw wa ter. Their use ca n be adopted to adva nced was tewa ter treatm e nt ir a hi g h leve l of pretreatment is provided . Th e remova l of s uspe nded so lid s is necessa ry prior to a ny o f t hc processes d escr ibed in Sec. 4- 10. Remo\al of the d isso lved o rgani c material (by ac ti v;:lt ed carbon a d so rpti o n) is necessarv prior to mi croporo lls m embra ne filtratiun to preve nt the larger o rga nic molecules rrom pl ugg ing the micro pores. Advanced wa stewater trea tm e nt for disso lved so lid s remova l is complica ted :J nL! e .~p e n s i ve. Treatment o f municipa l wastewa ter by th ese processes ca n be justified o nly when reu se o f th e wa stewa ter is anticipa tcd.
Wastewater Disposal and Reuse II1 s ig nilicant vo lum e u rth e inll uent wastewa te l- aCC(l ll1panies s ludges and o th er materia ls d isposed of durin ~ \\'aste\\atcr- tre:ltrnent l'r l1ccsscs. Til e hulk o f th e
.'\11
303
wa stewater re main s to be disposed o f a ft e r the treatment processes h ave been completed. Ult imate receptors o f tre a ted wastewaters include surface water and gro undwat e r bodies, land surfaces, a nd , in some instances, th e atmosphere. Recogniti o n o f th e va lu e o f was tewa ter as a water reso urce has result ed in an increase in th e reuse of tre a ted e ffluent s. particul a rl y in water-scarce regi o n s. Portions of th e reused was tewater may appear as effluent for di sposal after reuse. Disp osa l s ites o r reuse facil iti es mu st be fou nd w ithin a r~as o n a ble dist a nce of th e wastewater-trea tm ent plant because o f th e cos t of trans p o rting the effluent over long di sta nces. Beca use o f th e p ossibi lity .th a t was tewa ter m ay contain a few viab le p a th oge n s eve n after ex tensive t reatment. both disposal a nd reuse must be acco mplis hed w ith d u e ca ution .
5-22 W ASTEW ATER DISPOSAL Th e most co mm o n m eth o d o f wastewate r d isp osa l is by dilution in surface waters . The resp o nse of rece iving stre a m s to wastewa ter di sc~arges was discussed in Chap. 3. a nd th e effec ts a re re lated to th e dilution factor and to th e quality of the effluen t. In mos t cases, seco nd ary treat ment is sufficient to prevent pro blems . Howeve r. w he re adequate dilution is n o t availab le. or where discharge is to· a deli ca te ecosys te m . adva nced wastew a te r trea tmen t may be requi red. Advanced trea tment pri o r to di sposa l in s urface wa ters m os t often in vo lves the removal of nutrient s. In a few Instances. a d va nc ed treatment may b e necessa ry to rem ove co ll o idal so lid s. In climates whe re evaporatio n fr o m wa ter surfaces exceed s precipitation. it may be poss ib le to dispo se of wastewater by clisch a rge to th e a tm osp here in -.. va por' form. ' Evapo rat·ton ·systems. a re essen ti a ll y ox id a tion ponel s. wi th surface areas being designed 1'01' to ta l influ en t evap o ration. Except for ar id a reas where th e net evaporat io n is significant. la rge surface areas are required, thu s limitin g _ evapo rati o n sys tems to small fl ows in rura l se ttings. Fo r c ities in coas ta l areas. ocean d isp osa l o ffer s an economically attracti ve form o f di sp osa l. The e ffluent is tran sp o rt ed o ut to sea by pipelines a long the ocea n noo r and discharged at multiple points through a manifold. The leng th of the pipeline wi ll depen d primarily o n ocean currents and th e quantity of waste in vo lved. Alth o ug h raw wastewa ter has been di sposed o f in this fa shi o n without caus ing app reciab le pro b lem s. it is desira ble to e limin ate fl oa ting debris, o il s a nd greases, ~nd recognizab le objects from the wastewater prio r to di sposa l. It is essential to remove large objec ts which cou ld plug the pipeline o r the m a nifold orifices. ·La nd app lica ti o n o f wastewater may. b~ co ns idered a disposa l techniqu e, a form o f waStewa ter reuse. or both. The m o's t common fo rm s of land a pplicatio n are irrigatio n' a nd rap id·infiltra ti o n·. Wastew a ter m ay be u sed to su ppl y both th e Water and nutrient needs of plant s. Use for this purpose may be pro mpted by eco no mics re lat in g t o ei th er th e agr ic ultural as pects o r to th e was tewa ter disposa l as pec ts. l n e ith e l~ case. direct disc harge to s urface streams is avo id ed. R a pid infiltra t ion res ult s ill the d isc harge of the wastewa ters to gro undwater b od ies
304
ENG IN EERED SYSTE ~ t S FO R W ASTEW AT ER TREATMENT AND D ISPOS AL
WATER
rather than to surface waters. In addition to wastewa ter di spos al. o bj ectives ma y include gro undwate r recharge as described in a later sec ti o n of thi s cha pter.
Irrigation Wastewater may be a pplied to la nd surfaces to prov ide bo th water a nd nutrient s to enhance plant growth. Ait hough so me of the efflu ent may be los t t o evaporatio n or to perco lation beyond the reach of pl ant roots, most of the wa ter is in corpo ra ted into plant- tissue o r is tran spired to th e atm osphere.·Was tewa ter efflu ents have heen used successfully in both argiculture and sil viculture a nd have been lIsed to maintain vegetatio n in park s, o n golf co urses, and alo ng freeways and a irport run -
(a )
Comple tely flooded
305
ways. Land app li ca tio n has beco me com monpl ace in semia rid areas where irrigatio n IS necessa ry to sustain des ira ble vege ta ti o n. La nd applic ati o n of wastewa ter can be by sprinkling, fl ooding, o r ridge-andfurr ow techniqu es, as show n in Fig. 5-47. Spr inkle irrigation is the most co mmo n me th od, wi th a pplica tio n ra tes vary ing fr o m 2. 5 to 10 em per week , depend ing o n climate, so il c hara cteri stics. and the wa ter and nutrient requirement of th e plant s. T he deg ree of pretreatment prior to land application va ries with the nature of the crop to be grown. Crops grown for a nimal consumpti on o r fo r seed produ cti o n ca n genera ll y acce pt lowe r-q ual ity eftlu ent than cro ps grown for human co nsumpti o n. In most cases, seco ndar y tn:atm cnt is requ ired. Wa stewa ter should not be used to irri ga te vege tables tha t are eat en raw. Was tewa ter irr iga tio n systems may be ow ned by the mincip alit y, o r co ntractu al arrangement s can be made with loca l farm ers for effluent utilizatio n. In either case, it is oft en poss ible to I'ecover part of th e cost of pretreatm ent of th e wastewa ter fr om th e cas h cro p or fro m sa le of the effluent. When was tewat er is reused for la nd scape irri ga tion of pu bli c property, sav ings of potable wa ter supplies may be a sig nifica nt ad va nt age. T he l'e are, ho weve r. seve ra I d isad va n tages tII t he use of wastewa ter efflu en t fo r irriga tio n pur poses. Th e seaso na l na ture of irri.ga tion wa ter needs may result Ii1 la rge sto rage req ui re ment s. If th e syste m is to be o perated by the municipalit y, land and equipment mu st be purchased at con siderable expense, Wh ere large, high- press ure sprinklers are used, ae roso ls can be formed which may transport viral path ogens, Large-sca le irrigation systems mu st be loca ted away fr om hea vily populated areas, and the cost of conveyance sys tems to the site is often significant. Distributi o n syst ems for irriga ti on of pa rk s, greenbelt s, a'nd other publicly owned areas ca n be ex pensive if suc h a reas are widel y di spersed a nd if th e transpo rt system mu st be co nst ru cted thro ugh deve lo ped a reas.
Rapid Infiltration
(r)
Figurf 5-47 Irri ga tio ll I ~c hniqu es usi ll g municipal was tewaler ' (II) srrillklillg: ,"HJ -I"urrow· tcc hn iq ucs . (From POI/lids ond Criles [5-4/ ]. )
(h)
n'h"lillg. (e) ridge,
Thc ra pi d Iflfiltrati oll process in\'olvcs spreadlfl g wastewat er in shallow, unlined earthen baS in s a nd all owin g th e liqui d to pass tl~ r o ugh th e po ro us bot tom and pe rcola te towa rd the gro und wat er, ~l S show n in Fi g. 5-48. Was tew ater is a pplied at the max imum rate a t which th e soi l ca n ca rr y It away. Int ermitt ent " res ting" peri ods mu st be prov id ed In whi ch th e soil is all owed to dry and 'reestabl is h ae robic co nci it io ns. A ppli ca ti o n cyc les 01' I() to 20 d with 1- to 2-week res t ing periods are commo n. Th e bo tt om surface may be rak ed o r disked prio r,t o each ap plica ti o n cyc le to d ispe rse so lid s a nd preve nt ~1I1 im permea ble la yer fr om fo rm ing. Ma ny of th e ra pid infiltra ti o n systems in current use were des igned pI'im aril y to dispose of un wa nt ed was tewat e r. Mo re recc ntl y th e process has been used as a means of aquifer rec ha rgc or as an alh 'anced wastewat er treatment. wit h the' pe rcola te be ing collected fOI' reu,e Co ll ec tio n may be by ho ri zo nt al Ao\\' to surrace strea ms, or by we lls or dr ~ liIl til cs IIl stalled for thi s purpose. T he so il acts essentia ll y as a filt er rll r tertiary treatmen t
306
ENG tNEE RED SYSTEMS FOR WASTEWATER TR EATMENT AND DISPOSAL
WATER
o !, ,"'"'"'"
reused for several purposes, which include creation or enhancement of recreational facilities. industrial water sup p lies. g roundwa ter recharge. a nd direct reu se in potable s uppli es.
Applied
was tew3 ter
f
~,'-,----(a)
Flooding basins
(unsaturated zone)
(~r olln dwa[ ~ r
(h)
t t
R l'Lt.)vt:r~J
wa.ter
We ll s
-
Pe r co lation (unsa t urated zone)
-
Fi gure 5-48 Rapid infiltration o f was tewater. (a) Pe rcolatIon drain tiles: (e) recovery by wells. (From US. EPA [5·4:'].)
10
307
gro und""" er: (b) recover y by under·
5-23 W ASTEW ATER REUSE Re u se o f treated was teW:l te r may be dictated by any lIf seve ra l circ um stan ces .. In water-scarce a reas. was tewater ma y co nstitut e a maj or portion uf the availab le reso urce. Where de licate ecosys tems necessitate ~ trin ge nt etnuent t·equirements. r'euse of th e was tewa te r ma y help to offset the co~t of alhan ccd wastewa te r trea tment. or a reu se th a t wi ll ~r ccep"t a lower I c\~ 1 01" treattllen t may o bviate the need I"o r· the expense o f te rtiar y treatment pri or tll dr sch:.lrgc W;ls tew;lte r ha s bee n
Recreational Facilities Water-qu a lit y requirements for recreati o nal uses are quite strin gent. and some form o f advanced wastewater-treatment techniq ues will a lmos t invari a bl y be required prior to wastewater reuse for th is purpose. Indeed. where body-contac t ac tivities suc h as s~i mmin g and wa ter skiin g are included. th e qu a lit y of th e water reso urce mu st approac h that o f d rinkin g wa ter w ith respec t to most parameters. Recrea ti o nal wa ter shou ld be aes th e tica ll y pleasi ng and essentially free o f toxica nts a nd pa th oge nic o rga nis ms. Recrea tiona l wa ters composed chiefly o r entire ly of was tewa ter effluents are possible. provided a sufficie nt degree o f treatment is provided. T wo examples o f wastewa ter re use in recre a tion a l faciliti es o ften c ited in th e ittera ture are th e San tee projec t a nd the Indian C reek Reservo ir. bo th in California . . BOlh fa ci lities p rovide a high quality o f recrea·tiona l water. but by different treatmen t processes. At th e San tee faci lit y seco nd a ry efflue nt is fir st polished in a ter ti ary ox id a ti o n po nd and th en pumped int o a ca nyo n and a ll owed to fl ow horizo nt a ll y th ro u g h approx imate ly 1000 m o f sa nd a nd gra ve l m a te ri a l before bei ng recove red. The recove red wa ter is then routed thr o ugh a series o f three lakes su rro unded by a public park. Fi shing a nd boat ing a re a ll owed on the firs t two lakes. Th e third lake is ch lo rin a ted and used as a sw imming fac ility. The rec la imed wa ter is o f sufficient qua lit y to meet Ca lifo rni a standards fo r body-co nt ac t recreati o n. .. .. .. . . .. . . ... . ........ . .... ..' . . . . . . . [5- 14J Ind ian C reek Reservo ir rece ives trea tment e ffluent from th e So uth Tahoe Public Utilities District advanced wastewa ter-treatment pla nt The fir s t full-~ca le advanced wastewa te r-t reatm ent plant to be built in the' United States. the T a hoe fa cil ity inclu·des nutrient rem oval. fil tra ti o n. and activated ca rbo n adsorption. The reservoir co ntains about 27 x 10 6 m 3 o f wa ter. essenti a ll y a ll tre a ted effl uent . and provides ,i varie ty o f wa te r-ba sed ac t ivi ti es, includin g sw imm in g and wa ter skiing. The impoundment a lso suppo rt s excellent tro ut fi s hin g. [ 5-13J Surplus wa ters fr o m both the Santee and Tahoe faci liti es are used for irrigation purposes. Inspired by the success o f th ese two projects. other municipalities are plan nin g rec rea ti ona l use as one step in the reu se o f wastewater. The city ofOenve r has an ambit io us p lan fo r was tewa ter recycling. a porti o n of whic h includes rec rea ti o na l f'ac ilit ies. [ 5-24 J The Fairfax Cou nt y Water Author it y has included an inter media te rese rvo ir hetween its advanced was tewa ter-t rea t ment plant a nd the Occoq uan Rese r· voir. which form s a part of the W as hingt on, D.C.. water suppl y. Rt:crcationa l activities :1I"e included as a heneficia l use. [ 5-1 3J Advanced t rt:at ment of wastewater so le lv for the purpose of crea tin g a recreati o na l reso urce cuu ld se ld o m be jus tifi ed a n eco n om ic bas is. H oweve r. w hen adva nced w;t s tewat e r trea tm e nt is req uired for ot her rea so ns. int ermed ia te use o f
01;
308
ENGINEERED SYST EM S FO R WASTEWATER T REATMENT AND D ISPOSAL
WATER
reclaimed water for recrea tion ca n pro ve to be a viable sc heme and ma y improve public acceptance of waStewater reuse in genera l.
Industrial Water Supply In terms of total volume, industrial water use o utranks all o th e r wa ter-use categories in the United States. Additionally, industrial wa ter require ment s are growing more rapidly than are municipal or agricultural requirements'. An increase in th e use of wastewater effluents for industrial water supplies pa ra lle ls this growth. The quality of water required for various industrial processes va ries greatl y. Cooling water generally has the lowest quality constraints. while boiler wa ter has the highest. The degree of treatment given wastewaters wil l o bviously be dictated by the intended industrial use. Cooling processes, which co nstitute the largest water requirement in most industries, may be able to usc second a ry effluent directly, although additional solids removal is desirab le and additiona l treatment with biocides may be necessary to prevent biofo uling of surfaces. Advanced wastewater treatment may be provided by the wastewater auth o rit y prior to delivery to the industry, or industry m ay rec eive seco ndar y effluent a nd provide treatment processes designed to meet their particular need s. A wide variety of industries make use of municipal effluents, the most common being the power-generating industry and petrochemical plants. In Concord. California, an industrial complex consisting of Phillips Pe tro le um , Shel l Oil , Stauffer Chemical, Monsanto Chemical, and Pacific Gas a nd Electr ic receives effluent from the Central Contra Costa Sanitary District. The advanced was tewater-treatment plant provides about 64.000 m 3 / d of high-quality effluent to th e industries. In Odessa, Texas, a petrochemical industry receives secon dary e ffluent from the city's wastewater plant and pro vides additional treatm ent as necessary. After use in the industry, the wastewater is reu sed fo r secondar y recovery operations in the oil fields. '[5-3J
In coasta l a reas, salt wa ter from t he ocean ma y wedge undern ea th th e fr eshwa ter aq uifer because of its great er density. Drawdown fr o m wells exacerbates the prob lem a nd ca n res ult in sa lt wate r co nt a minati o n at th e well. Inj ect ion o f was tewater between th e pumpin g we ll and th e so urce of th e salt water may serve to crea te a hydrostatic barrier th a t wi ll pu s h th e sa lt water backward. This process is show n in Fig. 5-49. Land surface
G roundwater table
Fresh water
(a)
'0
---------- ~
Ocean
tb)
Groundwater Recharge Wastew<\ter can become a part of groundwater as an in advertent consequence of land applica!i6n for irrigation or from rapid in filtration systems d es ig ned fOl' wastewater disposa l. As discussed in this section, ho weve r. gro und wat e r recharge will be considered a planned activit y with we ll-defin ed o bj ec ti ves. These o bjec ti ves may i'nclude stabilizin g the groundwater table, creating hyd ros tatic ba rri ers to prevent saltwater intrusion into freshwater aquifers, and stor ing wa ter for futur e use. In areas where groundwater is used extensively for ag l'ic ultural. indu strial. a nd municipal purposes. water maybe withdrawn from aquifer Illore rapi d ly than it can be repleni shed by natural me,!ns. In additi o n to. th e dep let ion o f th e resource, the drop in the water table may res ult in s ub s id e nce o f th e area as the pores in the drain ed part of th e aquifer collapse. Should thi S occur. thc sto rage a nd hydraulic conductivity o f the acquifer may be altered. This rrocess call be s lulAcd. stop ped , or even rcvcrsed by rec hargc wit h reclaimed was tewa ter. ,'5-4 J
309
water
Wa stewater injcl"'ion
well
-
an
Fresh watf?r (cl
Fi~ur c 5-4 9 I.'se oil rCd led W;)qCW;) lCf l
v..astC\\"alc! n;lrriL'1
ENG INEERED SYST EMS FO R W ASTEW ATER T REAT MENT AND DISPOSAL
310 WATER
S to rage of was tewa te r in th e aq uife r is inc id en ta l to bo th of th e above processes, but grou n dwa te r rec ha rge sys tems may also be des ig ned w ith wa ter storage as t he ir pr ima ry fun c ti o n. T hi s sto r age may fu nct io n m uc h t he same as storage in su rface reservo irs, th e wa ter ta b le fa iling d u r in g pe r iods of h igh pu mpin g a nd r is in g d uri ng pe ri o d s of low w ithd ra wa l. When ;I LJui fer c ha racte r is ti cs a re favo rab le to st orage. thi s meth od has severa l adva nt ages ove r surface storage rese rvoi rs. E xtensive co nstru c t io n is avoided. surface use is not disturbed or res t r ic ted. eva p ora ti o n losses are m in imi zed, and t hc wa ter is iso la ted from most sources o f con tam ina nt s. L iabiliti es assoc ia ted w iih i he use of rec laimed was tewa ter for gro u ndwa ter rec ha rge re la te m os tl y to wa ter qu a lit y. Lik e surface wa te r b odi es, ac qu ife rs have self-c lea nin g m ec h a ni sm s. H oweve r. th ese m echanisms may wo rk ve ry s low ly, a nd ce rt a in co nt a min a nt s ma y rem a in in t he groundwate r for yea rs. Because lit t le can he do ne t o speed th e self-pur ifi ca t io n process. ex t reme care mu st be exe rcised to avoid aq u ife r co nt am in a ti o n . Wh e re part of an aqui fer is used for drinki n g-water supp lies. t he rec h a rge wa ter mu st be o f essen ti a ll y potab le LJua li ty. M eth od s o f aq uife r recharge in c lud e land s p rea d ing a nd subsequ ent perco latiQn (esse ntiall y t he same pro cess as rapi d intiitratiu ll dcsc ri bed ear li er ) a nd di rec t inject io n Di rec t inj ec ti c n is t he reverse o f w idtiidrawa l by a we ll and pum p syste m . as sh ow n in Fig. 5-49c. Land s p reading is usu~d l y the p referred met hod s in ce ad d itio na l a erobic treatme nt is p rov id ed in t he aer~ l ted so il ahove t he aq uifer. S usp end ed so li ds a re re mo ved ncar tli e s urfa ce \vhere th e p lugged area ca n be res tored mu c h m o re ea s il y than a plu gged a q u ikl' ~ect i o n With th e exce ptio n of la nd acq ui si t Io n. t he ca pit a l cos ts o f la nd spread Ing systems are lower than t hose of injec ti o n we ll s . a nd ope ra tin g cos ts arc a lso lower. r5-45] D irec t injec ti on may be necess iated b y im permea ble st ra ta be twee n the surface and th e aquife r, or may . p rQ.v: i.d e.ID.Q (C a CC UI.a te. placeJnellLiLLhe recl a imed .wa·ter· is Hsed· fo r· baniers ·aga inst sa lt wat e r intru s io ll . A hi g he r qu a lit y o f wa ter. p a rt ic ularl y \y ith res pec t to s us pend ed so li ds. is required fo r direc t inject io n.
Reuse in Potabl e Water Systems In co r porati o n o f was tewater info po t a ble water suppli es has a lways hee n a n in adve rt e nt co nseq ue nce o f e mu ent di sc ha rge Int ll \V a te rco u rscs. Most m aj o r st reams co n ta in a s ign ific ant pe l'Cen tage () f \V~lter that was p re\' io us ly used an d d iscarded to be d ilu ted w ith th e na tur a l fl uw "tIlel 1; ltcr wi thd rawn ~IS raw wa ter for a second or thi rd use. As wa ter dema nd s II1crcasc. t hc re use fac tN alsu increases. This sys te m has bee n co ns id ered sa t isfac tory in coun trics where adeq uate wa te rt reatme nt fac iliti es a re avail a ble. a lth o ug h in recent years th e a ppea ran ce of c he m ica l s u bs ta nces tha t a re d ifficu lt to id e nt ify allL! d il lic ult to remove has ca used c'onside rab le concern to t he water.i nd us tr y. T he int en tiona l LIse o f \\'as tewater as a p~lrt of the potahle sLIp p ly is a more recen t occurrence. This re use i..,> us ua ll y l1 eces~it;Itcd hy ~ I s lHl rt age uf n ~lt ur al wa ter Re use may be direc t o r ind irect. /)/r('o rCllse is LIsu; I1 ly I·crerred to as closeJ ilJup or pipe-Io-pipe r ('crci ill g. w hi ch ind icates t hat t li e t rea ted d ilu e nt fr u m th e
311
was tewa ter-trea tm ent system is piped directly t o the influent of the wa ter-treatment pl ant. I ndirecl reuse in vo lves sto r age o f tre ated effluent in natural.or artificial . wa ter bodies fo r a peri od o f tim e pri o r to withdra w a l and Inco rpo r a tIon mt.o th.e... wate r suppl y. Indi rect re use is th e m o re accepta ble p ractice at the present time. Di rect reu se o f w astew a ter h as been practiced a t Windhoek, Southwest Afr ica, s ince 1969. After second a ry treatment, wastew a ter is stored in m a turation (h o ld ing) p o nd s a nd the n trea ted as shown in Fig. 5 -5~. !his system is op~rated at hig h-u se perio d s o f the year and d uring drou g ht co ndItIOns a nd .h a s constItuted as m u ch as 50 perce nt o f the p o ta ble supply. [5-54]
Sru d ge diges tion a n d disposal
Mu nici pa l wHslewater
Algae scum to disposal
Collapsed foam to disposal
Goreanga b rese rvoir - - - ' - - - 1 w a te r
Municipa l wate r su ppty Figure 5-50 Schemalic or waslewaler.lreatme nl pla tH incorpo raling di rect reuse. (From Clay/Oil and Proll.\" [5-11].)
312
f jJ j
WATER
Indirect reuse separates the wastewater-treatment plant from the waterpurification plant by a carefully controlled natural link. The most common approach is by storage in surface reservoirs or in aquifers for varying periods of time. In surface reservoirs , the wastewater is subjected to sunlight, aeration, biological action, and other processes that reduce the chance of transmission of pathogens. [5-13J Dilution by runoff water mayor may not be desirable, depending on its quality. Water stored in aquifers is subjected to filtration through the soil material, biological action , and adsorption and ion-excha nge processes. Wastewater stored in aquifers is less likely to become recontaminated than is surface water. Indirect reuse of wastewater is -practiced at several places in the United States. An example is the Occoquan system near Washington, D.C. An advanced wastewater-treatment plant has replaced several smaller secondary systems and provides treatment as shown in Fig. 5-51. The terminal reservoir provides a safety factor against perturbation s in effluent quality. From the treatment plant, the treated wastewater flows through Bull Run Creek for abou t 12 km to the Occoquan Reservoir. This surface reservoir is a source of raw water for a waterpurification plant providing potable water to the surrounding area. The benefits and liabilities of using groundwater reservoirs as the natural link between wastewater and potable water are not so well defined. Although limited use of wastewater for aquifer recharge is practiced throughout the world. no large-scale use of this reclaimed water for potable supplies'is currently practiced. In southern California. where wastewater is used extensively for groundwater recharge, future reuse in potable supplies is planned, provided current research confirms the absence of health problems. [5-4J
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Planning for Wastewater Reuse Many areas of the world are present ly experiencing water shortages or expect to experience them in the foreseeable future. In these areas, wastewaters must be considered a valuable resource and integrated into the available water supply. The principal concerns involving the reuse of wastewater are public health and public acceptance. It is known that pathogen s are present in wastewaters, and the total remova l by even advanced wastewater treatment cannot be assured at all times. Additionally. some fraction of refract ory organics remains in wastewater, regardless of the extent of treatment. There may be chemical compounds present in wastewater that have not been discovered and for which there is present ly no method of measurement. Thus. human contact with wastewater. .even in nonpotable uses. carries a risk factor \vl]ich is largely unknown. It is hoped th at research currently in progr'ess will help to' quantify those risks. Public acceptance is a necessary factor in w'as tewater reuse. Experience at the Santee project in California indicates that public acceptance is greatly enhanced by informing and involvin g the public at all stages of planning ane! implementation of wastewater reuse. Following thi s lead . the City of Denver has
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ENGtNEERED SYSTEMS FOR WASTEWATER TREATMENT AND DISPOSAL
WATER
launched a mass ive drive for public acceptance of wastewater recyclin g. [5-24J Nonpotable reuse of effluent from a demo.n s t~ ':lt\W\Jillant !.~ 'p'lpnl]t;d. wit,h extens ive research on health and toxicological s tudies h e ing performed . Co ncurre ntly. a public edu ca ti o n program has been designed to g ain public accept a nce o f eventual reuse in the pot a ble sys tem , should the health s tuclies sllo'w thi s to be practical. These programs are to con tinue fo r 10 t o 15 yea rs a nd ~ if s uccess ful. will result in the construction of a full-scale plant fr om which re use will include direc t recycle t o the potable sys tem . The City of San Diego. C a lifornia , is prese ntl y embarking o n . a s imilar projec t. [5-2J Other projects have ch ose n to limit the utilization of rec laim eu was tewat er t o n o npotable u ses fo r the present tim e . In Ca lifo rni a. Lo s Ange les and Oran ge counties conducted ar: extens ive : 11\1
d S:
jb J(d
to fitting the qualit y of the, ~a\tr ,to; t1h e,lij],el). d.e~ p ~e. Curr~f1t l y. a ll water di s tri buted through pu bhc . sys~~ l:n s '+IOf ' P~ ~ltb:~~ :qual!ty , alt h 6 ugh less th a n o nehalf the water di s tribut ed : throu,gb tllcSC;!::sys te ms :i's ' us~:d in a mann e r neces-
s itatin g p o table water quclIltY' 11i~~1 ~h~ : cr~p~ rt'ftjjjt y for :' ihe , ~ se o f water o f less-than-potable qualIty tS ·a bup{faii.t, a))c! j" rec lallned wastew'~t'ler cou ld conceivably be u sed in man y instances where potable wa te r is n o w b e ing used. Such use would be in keeping with the 1958 rec o mmendation of the U.N. Economic and Social Council [5-39J: "No higher qua lit y water, unless th ere is a s urplu s o f it, should be u sed f o r a purpose that can to lerate a lower gra de ." C urre ntl y. thi s philosophy is o ften quoted , ye t se ld o m applied.
DISCUSSION TOPICS AND PROBLEMS 5-1 Name and characterize ihe three most signifi ca nt c()mp onent s of Illuni cipal wa stewater. 5-2 A mun ic ipa l wastewa ter treatment pl ant rece ives an ave rage fl ow of 11 .500 rn 3: d . Estimate the quantities (kil ogram s per day) of BOD and suspend ed so lids in the influ cnt if the waste, water is cons id ered to be (a) strong. (b) medium. and (c) weak . 5-3 A co mmunity produces an average wastewa ter fl ow of (,550 mJ;U Estim atc the nitrogen and phosph o ru s loa ding to the treatm ent plant if th e wastcwatcr is tvpiC
5-4 A muni cipal wastewa ter-trea tment plant receives a seasonal discharge fr olll a fruitprocess ing pla nt. Influ en t fl ows and strengths of the ~' a S l ewa te r when th e industry is bo th onand ofT- lin e are show n below. Determin e th e co ntribllli ()11 o f each cons tituent hy the industry.
0 n· lim'
Industry o ff-lin e
t X,750 300 420 64
13 ,275 215 240 t5
tndustr y
Flo w, m ' / d BOD " mg/ L SS , mg/ L Ammonia. mg/ L Ch lo ride , m g/ L Alkalinity , mg/ L
29
41
57
t25
315
5-5 What is an effiuent- lim ited stream ? A water-quality-limited stream? 5-6 Differentiate between unit operations and unit processes. 5-7 Define and desc ribe th e co mponents of (a) primary treatment. (b) secondary treatment, and . (e) tertiary treatment.
5-8 What are the co mm on engin ee red method s of removing so lids from wastewater? Describe and define eac h o f these method s. 5-9 What are th e maior types and S9 urces of gri t in municipal wastewaters ? Describe treatment meth ods used to remove gr it. 5-10 A channel-type grit chamber luis a flow- through velocity of 0.29 m/s. a depth of 0.8 m, and a length of to m. For inorgani c particles with specific gravity of2.5, determine the largestrliameter particle that can be removed with 100 percent effic iency. ,: 5-11 A channel-type grit chamber is to be installed in a wastewater-treatment plant processing 8550 m 3 /d. The How- through velocity is to be controlled at 0.33 m/s by a downstream propo rtioni~g weir. Determine the chann el dimension s for a depth to width ratio of I : 1.5. 5~12 Determine the appropriate dimensions for an aerated gri t chamber processing 23.500 " .. .. ... .. .............. .. ........ .. m' /d of municipa l wastewater. Also calc ulate the total air fl ow. 5-13 What ar~ ;h~ ~o~; '~~;11~~'~ d~~;~~~' ;ts'~d 'f~;~~~~'~;i~'i flov.; s· in it wastewater-trea tment plant ? 5-14 Descr ibe unit o pera ti ons used in primary sedimentation. 3 . 5-15 A municipa t wastewater-trea tm en t plant processes an average flow of 14,000 m j d. Tbe peak fl ow is 1. 75 times the average . The wastewater contains 190 mg/ L BODs and 2IO mg/L suspended so lids at average flow and 225 mg/ L BODs and 365 mg/L suspended so lIds at peak fl ow. Determine the foll ow ing for a primary clarifier with a 20-m diameter. (u) Surface overfl ow rate and the approximate rem ova l efficiency for BOD s and suspended solids at average flow (h) Surt'ace overHow rate a nd the approximate remova l efficiency for BODs and suspend ed so lids at peak now (e) Mass of so lids (k il og rams per day) that is rem oved as sludge for average and peak flow co nditi ons. 3 5-16 A wastewater:treatmcnt plant mu st process an average fl ow of 24.500 m /d , with peak Rows of up to 40.000 m 3 /d. Design criteria for surface overflow rates have been se t by the sta te reg Ulatory agency at a ma ximum of 40 mid for average conditions and 100 mid fo r maximum co nditi ons. Determine th e d imensio ns of the primary clarifier if it is a (a) Circu lar basin (h) Long-rectan gu lar basin (1. = 3 W) te) Square cross-flow tank
ENG IN FERI, f) SYSTE MS FO R WASTEW ATER TR EAHIENT AND DISPOSAL 317
316 WATER
If the influent suspended solids is 200 mg/L at average flow and 230 mg/ L at peak flow. determine the mass of solids (kilograms per day) removed by the primary clarifier. . 5~)7i\Iaq~e .w
The design overflow rate is 50 mid. and four un·its in parallel a re to be constructed . Con crete tanks are to be used and the cost of pouring circular sidewalls is 1.2 times the cost of pouring straight sidewalls. Determine the percent savings in construction costs in each in stance if (a) long-rectangular tanks (L = 4 W) or (h) square cross-flow tank s are used (with common walls) instead of circular tanks . . . 5-18 Determine the weir-loading rates in Prob. 5·.16 if a simpl e weir is used at the periphery of
the circular tank. at the 'e nd of the long-recta ngular tank . and 'along one sid e of the square tank. 5-19 Define: (a) biomass. (b) lag phase. (e) log-growth phas'e; (d) stationary phase. (f) endo· genous phase, (f) suspended cultures. (II) attached cultures. and (h) flocs. 5-20 Name, define, and describe the most common metilod of quantifying biomass. 5-21 What external factors ma y alTectthe rate of biomass producti on and food utili za tion')
5-22 Explain the basic concept of the «ui vated -s ludge process and indi cate tile advantages ::lI1d disadvantages of the two major kinds of activated-sludge react o rs.
at a con centrati on of 3000 mg/ L MLSS, and th e secondary c larifi er is designed to thicken the slud ge to 12.000 mg/ L. For a mean cell -res id ence .t ime of R d. determine (0) Th e vo lume of th e reacto r (h) Th e ma ss of the so lid s and the wet vo lum e o f sludge wasted each da y (c ) The slu dge recycl e rati o '2.. 5-28 ;\ compl etely mixed activat ed-s ludge planl is to treat 10',000 m.l/d of indu strial wa ste· . / waler. The wa stewater ha s a BOD , 01 1200 mg/ L that must be reduced to 200 mg/ L prl o l to discharge to a municipa l sewe r. Pilot-plant an a lys is indi cates that a mean cell-resid ence time of 5d maintainin g M LSS co ncentra tion of 5000 mg/ L produ ces th e desired res ult s. The valuef~r Y i~ d e t e rmined to be 0.7 kg/kg and th e va lu e of kJ is fo und to be 0.03 d - '. Determine (a ) Th e vo lume of th e rC; lct,,, (5) 1 Y;; :-;"., (h) Th e ma ss ancl vo lulll e of solids wa sted each da y ( (c) Th e sludge recirculati o n r;Jtio 5-29 Th e ac ti vat ed-slud ge sys tem shown in the skelch belln" 15 o perating at equrlibrium. Determ ine the vo lume of slu dge thai mU SI he w;J sted each day if "';J Slage is acco mplished fr om (a ) POlnt ;\ an d (b) POlnt B.
-;0
5-23 A tapered aeration syst; m similar 10 that sho wn in Fig. 5. 17h is used to treat 12.500 m'/d of municipal wastewater. The wa stewater ha s rece ived primary treatment and has a 1300, of 140 mgj L and a suspend ed solids of 125 mg /L. The system is to be o perated in the foll owing
B A erat o r
way.
Soluble BOD 5 in eltluent $ 5 mg: L Average solids concentration in the react o r = 2000 mg/ L Mean cell-retention time = 10 d The'biological constants have been determin ed by pilot-plant analysis and are :
Q = 12,000 mi d
8 e = lOci ,· =4 .000 Jll )
X
= ~ . O OO
kg biomass
Second ary clar ifl er
mg/ L
I .... .1.
.. t .
} = 0.55 ---:.: _--.. -_ .. -
kg BOD utili zed ko = 0.05 d 1
I
(a) Determine the length of the reaClor if it is 5 m wide and 5 m deep. (h) Assume an effluent suspended-so lid s concentrati on o r 30 mg/ L: the BOD; of the
A~
. solids is 0.65 mg BOD/ LO mg 55. Determine the' total BOo' in' the effluent.
'"
,
.
X, = I (J.OOO mg!1-
'" .-L------------:' ,
JI
/'" Qk'
~-24 Determine the average biomass co ncemra!ion in .a co nventional acti vated -s ludge V
reactor similar to t~lat shown in Fig. ~d e r th.e fo llowing conditions. Flow = 18.300 m 3 fd Influent BQD = 160 mg/ L Eltluent BOD = 5 mg! L kg biomass Cell yield coefficient Y = - .-- ------ .. kg BOD utilized
~
Endogeneous decay coefficient = 0.04 d Tank volume ='6100 m J M\!an cell-residence time = 9 d -
J
.
.
Determine the volumetric loading rat e ,'f the system desc ri bed ill Prob. 5-21
s"ur1)etermine the food-ma ss ratio of the systcm desc ribed in P ro h. 5-24.
\ ··
5- A wastewater flow having the c1wract cri sti cs of that in Pr o b. 5-24 is to be tre,lted in a completely mixed activated-s ludge system slnJii:l r tLI t ha t of Fig. S-16i1. The reactor is to operate
5-30 Wh y ; 11T ;J cr;lIion devices a' 11 ; 11 pMt pf hiolnglcal rea ctor s" Name and desc ribe Ih e two major "er"ti o n tcchnlC)u es. imliuling the kind s ,)f biological reactors in which they a re 1I10st often used. 5-31 Wh a t oth er suspenckd-c ulture bi o logica l systems are ava ilahl e fo r trea tin g was tewa ter besid es the ;Ictiva ted-slu dge proccsses" 5-32 An ox itia tlo n-ci itch ac ti v;Jt ed -s lu d ~e svS lem rece ives 7500 m "'d o f Ill unic ipal ,,"astewater. Th e BO D in th e wa Slcw"l", is 21() Ill'gi l a nd no pr'im;1r1 t reatment is p rovid ed . The oxidali on dit ch is 3 llllie ep. 7 III \\' id e. ami 400 111 I,)ng. The reacto r is .o peral ed at .1 800 mg/ L MLSS and tlie bi o log ical co nslant s arc )' = 0.5 kg/ kg and k" = 0.06 d - ' . D etermine th e lIlean ce ll -res id ence time for 90 perce nl ROI) rClllov;I1 . /~-33 A W;lS tcwatc r !l ow of SOO() 111 ',\1 lS trea led in;l fac ult a li' e oxid;lt lo n pL1 nd that is 2.0 m decp wi: h a sur face ;Hca of 20 li a. T he \\,a SIC\\·"tc r h;JS a so luble BOD ; of I SO Ill g 'l a nci a reacli on r;ll<: coellicie nl olI U () d ' . I)etl'rmin c Ih e sll lu bic BOD ,' f tll C ctlluent. (t\ss um e a cum· plet el\" mi \ed reac tor \\ lll )(l ut s,did s lCL"lCie )
J
31H
ENGtNEERED SYSTEMS FOR WASTEWATER TREATMENT AND DISPOSAL
WATER
5-34 Rework h ob. 5-33 with the 20- ha surface area bei ng equally divided be twee n three po nd s. 5-35 !\ wa stewa ter' How o f 3550 m "d is to be trea ted in a facu ltat r\ e Iwml system. Th e re:Jction rate coe llicrent at the average operating temperature is 0.35 rJ I Th e pond is ex pected to operate at a di spersio n fa ctor of 0,5. Deter-min e th e surface are;1 required fo r X5 perce nt removal of so lubl e BOD for a pond depth o f 2 m \V ith (a) a single -ce ll pond and (h) a four -cell sys tem .
-1
319
5-49 A settling analysis is run on the contents of an activated-sludge reactor with the following result s:
Co nce rllratloll, mg/ L
1200
2200
3800
6 t 00
8200
11,000
Veloci ty, m /h
5,8
3.2
1. 6
0.6
0.4
0 ,09
5-36 Waste\Va ter from a poullry- processing piant averages IODO rn J/d wi th ;1 $ll luhle UOD s
--r- of 1000 mgj L. This waste is to be treated In :1 three-ce ll fac ultatl\'c rond sys tem in whic h the 20"C rc'actlOfJ rate co nstilnt k ha, been found ,t u he D.5 d ' I The co ld es t month ly :Iverage temperature IS expec ted to be Iwe For a di ~ persion celdliciellt .uf IU S. deter m ine the surfa ce area required to meet an e.(fluent sta nd a rd of 50 mg/ L solu ble !lOD.
b 5-37 Rewo rk Pr ob. 5- 35h with surface :Jerato r, beIng placed in the prrrn ar y rond , Determine th e power requirement for th e aerators if the oxyge n t ransfer rate is 0.9 kg O ,/kW . h.
Gi ve n the foll owing informat ion. determine the concentration of so lids and the flo w rate of the seco ndar y clarifier underfl ow. . (a) Flow fr om the rea ctor is 9500 m 3/d with a so lids con tent of 3000 mg/L. (h) The secondary clarifier ha s a d ia meter of 17 m. 5-50 '\ se ttlinganal ysis is run o n sludge from a n extended aera tion activated-sludge reactor with the fo ll ow ing resu lt s:
~-3H
Rework Prob. 5-36 with surface aerators be in g pl aced in the primary p()nd Deter mine the power requirement for the aerators if th c oxyge n tran sfer rat e is I.OS kg 0 2/kW h.
5-39 What auvant:lge s uo bi o-IO\\C rS have over classrc:Jltrickling filter ,"
C":!ce ntralr o,, , mg/ L
tOOO
2000
3000
4000
5000
6000
2.S
14
04
0,2
0. 1
0.06
3
5-40 A municipal wastewater with a fl ow of 17,55D m / u and :1 UOD , of 150 IIlg/ L IS to be treated in a bi o- tower wit h pla str c mod ular medium . l' i1ut'p l:lIl t :J1l;Ji ys is h:" est; lhli shed a treatabilit v co nstant o f (l.OS min r for th e sys tcm at 20 C. The maximum tClllperature expec ted is 23 'c. and the minimum tcmpe ra ture is 13"C. 1'0 1' a ~ ' I recycle ra tio :lI1d a 7,0-m depth . determine the area of th e to\\'er required to produ ce a 20 Illg -L HOD , dlluent. 5-4 1 Ass ulll e that the minimum fl elW is 0.6 times th e average and th;lt the maximum flow is 2 tim es th e ~Iverage. From th e data give n in Proh . SAO. ciete rmine th e rern o v ~ Ji cllicic ncy for minimum and maxim um fl ow ra te with th e h)dra uli c fl o\\ r~ lI e Q heJd con stant h) adjus ting th e recyc le ratil' Q,.
Sl'lIli ll g vcloctly.
m/h
Under equ ilibrium conditions. flow to the secondary clarifier is 4200 m 3/d with so lids content of 2000 mg/ L. For a preselected so lids flux rate of 2,5 kg/ m 2 . h. determine the required diameter of the clarifier. 5-5 1 A secondary clarifier processes a lotal flow of 10,000 m 3 /d from a conventional activatedslu dge reac tor. The concentration o f so lids in the flow from the reactor is 2600 mg/ L. The result s of a settling anal ys is o n th e sludge is given below.
5-42 Repeat Prob. 5-4 1. bUl maintain th e 2 : I recyc le ratio of I'roh. 5·"lbn d a 11 0\\ the hyd rauli c .. ll o\(' r:i ic·Q·()\;,iry'acco raingli .. ·.... ' .. , ........ , , . " .' .. .... . 5-43 DeSC ribe a ro tat in g bi ologic al co ntactOf' reactor. What a rc th e adIClIlt;lges and disadvanta ges of such a reac tor" 5-44 A wastewater with the charact eri stics gl\c n in Pr ob. 5--10 is to he tl e,lIcel uSI ng a rllia ting biologiciJi C\"ltaCtll r sys tem Ass ume that th e informati o n in Figs. 5· 2<) a nd 5- , I) app lies to the se lec ted medium . The mediulll is' mallufactured in X-m shaft lengths, WIth 1';lc h sh; d't con taining 1. 2 x 10" In l of surfac e area , Determine th e Ilumber of modules fu r Cll ll lpietl' nllrilicatioll of I he was(ew,-I fer.
5-45 1\ \\,;rstewater with the c hara cter istIcs Ill' thai gllen III Proh. :'- 2.1 " tll he treated hyall RBC sys tem . .'\ ssu me a minimum tempe ratu re of 10 C and the RBC ' cir;rrae'tclr stics of Pwb. 5.44. I)ctermine the number ll f required ull it s for <)1)" " BOD rClllll \ ;d. 5-46
Wh ~ lt
5-47 USlllg th e Infnrm:ltio n ill Tabl e 5- 10. dc termin e the 'Ill' "I' a "ccIli!d ;lr) l ' J;1I ifi,1 to r" lIow a celnventillllili
Concc lllr:!l io n. mg/ L
1490
2600
3940
5425
6930
9 100'
12,000
SCllling velDcilY . m / h
5.5ll
3.23
1.95
1.010,550.260, 14
Fur equilibrium conditions and a so lid flux rate of 6 kg/ ril 2 . b, determine the underflow rate, th e underflow solids concentration. and the overllow rate . . 5-52 Wh en is disinfection o f wa stewater effluents required? Why has the wisdom of using ch lorine for disinfecti on of wastewate r co me under question? 5-53 What is the organic co ntent of primary and secondary sludge ? 5-54 Name and describe t he mos t co mm on method s available ror volume reduction of sludge, 5-55 i\ wa stewa ter-tre:ltmcnt plant consists of primary tre a tment fo llowed by an aet iva tedSlUd ge secondary sysle m, Sl ud ges fr o m the primary clarifier and waste-a ctivated sludge rrom the und erfl ow are mixed a nd thi ckened in a gravit y thickenerc The primary sludge contams 1250 kg of dry so lids per d ay with a .. percent so lids content. The waste-activated sludge COntains 525 kg of dry so lid s per day and has a solids co nten t of 1,2 percent. After thlckenll1g. the mi xture has a so lids co nt ent or 3.0 percent. Calcu lat e (0) the vo lum e of sludge th at mu st be processed after thi cken in g and (h) the percent vo lume reciuctio n in th e thi ckener.
320
ENG INEERED SYSTEMS FOR WASTEWATER TREATMENT AND DISPOSAL
WATER
5-56 A high-rate aeration system produces 1140 m 3 /d of waste sludge. The sludge is wasted directly from the aerator and has a solids content of 3300 mg/ L. This sludge is thickened by a dissolved air flotation unit to 3.0 percent solids. Determine the volume of the thickened sludge. 5-57 What is sludge digestion? What are the two basic types of sludge digestion units? 5-58 A wastewater-treatment plant consists of primary treatment followed by a completely mixed activated-sludge secondary system. The primary and secondary sludges are mixed. thickened. and treated by anaerobic digestion. The system is shown schematicallv in the accompanying figure. The wastewater. tr~atment plant.- and sludge characteristics interest are given below.
of
Wastewater
Treatment plant
Influent Influent Effluent Flow =
Dia. of primary Primary clarifier = 25 m sludge = 3.8~ ; solids Aeration basin Waste secondary volume = 3600 m' sludge = O. 95 ~" solids MLSS ill Thi ckened sludge = 2.6"" solids aerallon = 2800 mg/L
SS = 240 mg / L BOD = 210 mg; L BOD = 10 mgj L 14.350 m'ld
Sludges
Influent
__ l I
/
I
. .- - - - - - - ; Thickener \-- -
\
"'----/
-
.
d).
5-60 Rewo rk Prob. 5c58 for a high-rate two-stage system employing a mixed, heated first stage with a digestion period of 10 d and a second stage with a thickening period of 4 d. 5-61 A wastewater-treatment plant consists of primary treatment plus secondary treatment in a bio-tower. The underflow frolll the secon dary clarifier is returned to the influent of the primary clarifier where it assists in removing the raw solids by adsorption and settl ing. The wastewater flow is 22.500 m 3 /d with 250 mg/ Lof suspended solids. The secondary underflow con tains 1180 kg/d of biological solids. Virtually all of the secondary solid s plus 60 percent of the raw so lids is remo ved. The und erflow from the pl'imary clarifier has a so lid s content of 5 percellt. These solids a re to be furth er treated in a high-rate anaerobic digester. The first stage is heated and mixed and requires 15 d for complete digestion. The sludge is dewatered to 35 percent so lid s by a filter press before final disposal. Assuming an organic content of 70 percent and digestion of 50 percent of the organics, determine the vo lume of the reactor and the volume of the dewaterecl so lid s.
5-63 Name anel describe the most cornman methods for I'emoval of nutrients dur ing tertiary treatment of municipal wastewater 5-64 Draw a tl ow diagram in schematic for~ of a wastewater-treatment plant that includes priinary (including prelimill~lry ) treatment. secondary treatment by conventional activated sludge. and nitrogen removal by air stripping. Sludge treatment is by two-stage anaerobic digestion. Identlfy each unit In the system and brietly state ilS purpose, State the destination of all materia ls leaVing the plant Identify all points of chemical addition and name the chemical.
I
__ --L _____ .-J
'" \
3
5-62 Name and descrihe the mos t common methods of sludge disposal.
Effluent
I L __ ,
Determine (a) the required reactor volume. (b) the volatile solids loading rate (kg/ m and (e) the gas production (m 3/day).
321
~ To sludge digestion
J
Determine (0). the solids Joading to the digeste~s '(kg/d and m 3/d). (h) the percent volume requction in the, thickener, and (el the vo lum e of supernatan t returned from the thickener to the primary clarifier. .
5-59 The thickened sludge in Prob. 5-56 is processed in a standard-rate anaerobic digester The digestion period is 30 d and t he sludge must be stored for 3 mo between final disposal even ts. Organic content of the slud ge is 75 percent and 55 percent of the organics is converted to gaseous or liquid end products. The solids content of the digested slud ge is 6 percent.
5-65 Repeat Prob. 5,64 for the followi ng treatment system: (a) Primary treatment (including prel imin ary) (1)) Seconda ry treatment by rotating hiological contactors (1') Nitrogen removal bv nitrification-denitrification 5-66 Name and describe the primary methods of removing suspended and dissolved solids during tertiary·treatment of municipal wastewater. 5-67 Repeat Prob. 5-64 for the following treatment system (a) Primary (including preliminary) (b) Com pl etely mixed acti va ted-sludge secondary (J') Nitrogen and phosphorus remoVJI ,(d) Advanced solids removal by granular-medium filtr:ttlon k) Refractory orga nic removal by acti\ated carbon 5-68 Name and describe th e comJllon methods of di sposi ng of wastewater effluent from treatmcnt plants.
5-69 i\ municipa l wastewater is to be treated and dischargecilntn a stream that empties into a pristine mounta'in lake t ha t is used for recreational purposes. Draw il schematic now diagram of a treatment plant to prep;lre the w;lStewater fo r discharge. 5-70 Treat ed Illuni cip;ll waste\\";lter is to be injected into an aquifer to form a hydrostati c barrier ;Igainst ,;tltwater intrusion. Withdrawal uf the Inje):ted wat er for domestic use is not anticipated. Dr;l\i· a schematic dia~ralll of a trea tment plant to pl'epare the wastewater for this purpose. " 5-71 Disc li ss the advantage.'; :.Illd disad\antagcs nf\\,aste",;tter-treatl11ent facilit ies d eS igned to tllrn waste discharllcs into potahle water
322 WATER
REFERENCES 5-1 American Society of Civi l Engineers and Water Polluti o n Control Federati o n : Wastel"oter Treatment Plant Design •.ASCE. New York , 197.7. 5-2 American Wat e r Works Association : .. Recycl e and San Diego:' MuniciplIl WII Hell'OIer Reuse
Nelvs. A WWA Research F ound ation , Denver. Augu st 19RO. 5-3 American Water Works Association: "Reuse of Municipal Wastewater In Indu stry," Mlilliclj)(J1 Wast elt'ater Reuse News, A WWA Research Foundati on. Denver , November 1980. 5-4 Asano, T .. and K . L. Wassermann : "Groundwater Recha rge Op~ration s in Cali fornia,"'! A WWA. . 72(7): 380 (July 1980). 5-5 Banerji. S. K .• and J . T . O'Conn or' .. Designing M o re Energy Effici eTit Wa stewater Treatment Plant s." Cit· Eng. 47( 7):76 (September 1977). 5-6 !:lenefield, L. D., and C. W. Randall : Biological Process Desiqllfor Wa.\NII-ater Tre(l{mcnl. PrenticeH all. Englewood Cliffs, NJ, 1980. 5-7 Be sselievre, E . B.' The Trealmenl of Induslrial WasIl's. McGraw-Hili. New York. 1969 . )·8 Bo uwer. Herman ' "Renovating Municipal Wastewater by H igh-Ra te In fil tr.,t i;) n fo r Gro undwater Re charge," J AWWA, 66( 3): 159 (March 19 74) . 5-9 - -- . R. C. Rice , J. C. Lance , and R. G. Gilbert .. Rapid InfiltratI o n Resea rch at Flushing Meadows Project. Arizona," J WPCF, 52(10):24 57 (Octo be r 1980) 5-10 Cla rk , J . W., W arren Viessman, Jr., aTid M. J. H ammer Willa SUfiply IIl1d Poilu I IOn CO/llrol, 3d cd ., H arper & Row. New York , 1977 5-1 1 Clayton. A . .I ., aTid P. J . P ybus: "Windhoek Re claiming Sewage for Drrnkln g Water , " Cit· EII!J, 42 ' 103 (Septem ber 1972). 5- 12 Cal', H : S., and G . H . Clevenger ." Determining Thi c kener Un it ,\roa,," hllllS AIME, 55( 3).356 (1916) 5-13 Cu lp. G. L. , R . L. C ulp. and C. L. H amann:" Wat er Re so ur ce Preser va t ion hv Planned Rec ycling o f Treated Wa s tewa ter," J AWWA, 65(10) :641 (October Inl) 5- 14 - - . Wes ner , G . M ., and G . L. Culp: Handbook oj Adnll1ced lVII.\I('Irol('Y Trcolnrenl , Van Nostrand Reinho ld, New York , 19 78 . 5- 15 Dick. R. l. "Role of Activated Sludge Final Settirn g Tank s." J SIII1 Enq Oie. A .'iCE , 96 : 423 ( 1970). 5·16 ---- and B. B. Ewing ... Evaluati on "I' Activat ed Sludge Thickenin g The o ric s." j SOli Elly Di, . ASCE, 93(SA4):9 (196 7). ancl K . W . Young. "Analysis o f Thick ening Perronnanec Dr hnal SettlIng Tank s, " 5-17 - . .Pro c 27lh Ind. WasIl' Conference. P urdue Universit y. 19 72. p. 33.. 5-18 Eckcnfelder. W . W .. Jr. : In duSlrial WilIer PO//lllion COlllrol. McGra,,-Hili. New Yu rR. 1966. 5· 19 .-.- -- . Principles oj Waler Qualily l'vI anagemelll, C BI Publi sh ing, Boston, Ino . · "Trickling Filter Design and Performance," J. Sim Elly Di/:. ASCE. 87(SA6).X7 ( 196 1). 5-20 5-2 1 Finer. S. E... Tire Life lIlId l1mes of Edwin Chlldwick , Methuen. London; 1') 52 . 5-22 Gaudy , A. F ., and E. T. Gaudy : lvlicrobiology for t;lIc-irollmentlll SnenliSls lIlId Engilleers, McGraw-Hili. New York, 1980. 5-23 GermaJn. J . E. ' .. Economical Treatment of Domes tie Wa s te by Pla stIc-Media Trr c kling Filters." J WPCF. 38(2) 192 (1966). 5· 24 Hadeed . S. J .' .. Potable Water from Wa s tewater - D envers Program. " J IVI'CF, 49(X) 175 7 (Augus t 1977). 5-25 Ha mmCl, M . J . W{Jler lind WaSlell'IllCr Tcchnolo[!l' , Wil ey, New York. 19 75 5-2 6 H am s. S. E. J . H . R ey nolds, D . W . Hill , D. S. Filip . and [ J. Middlehrooks: " Intermittent Sancl FiltratIon fur Upgrading Wa ste Stabili,atJon Po nd Fmuen ts. '.1 W/'CF. 4'1( I ) K3 (January 19 77 ). 5-27 H nrnc. F . W. , R. L. Anderton. and F . ,\ . Grant · .. Water R e use' I'r (}Jectlll~ Ma r ket s and Costs," .J;I WW;I. 73(2) . 66 (Feb r uar y 1981). Kugelman. I. J .: "Sta tus of Advanced Wa ste Treatment." ill H . W . (ichllJ and .I . I. Bregman (ell:::,.). /-lane/ho ok nJ I,Vofer R eso ur ces and Po/luriotl COfllrul. Van Noslrand . New Y o rk. 19 7() ,
s-n
ENGINEE RED SYSTEMS FOR WASTEWATER TREATMENT AND DISPOSAL
323
5-29 Lawrence , A . W ., and McCarty, P. L. : "Unified Basis for Biological Treatment Design and Operation ," J Still Eng Dit', A5(,E. 96(SA 3): 757 1970. 5-30 Lin sley, R . K., and Franzini, J . B.: Waler Resol/rces Engineering, 3d ed" McGraw-HilL New Yurk . 197Q . 5-3 1 McCarty, P. L. : "Anaerobic Waste Treatment Fundamenta ls." Puhlic Works , 95: 107 (September 1964) . 5-32 ._ _ : " Bio logica l Proce%cs for Nitrogen Rem oyal: Theo r y and Applica ti o n s," Proc Tlreljih
S{Jnilary En!Jineerin!J Conference, University of Illi nois. Urbana, 1970. 5-33 McKinney, R. E.: Microbiology for S anilary Engineers, McGraw-Hili, New York. 1962 . 5-34 Mara, D . D. : Sewage Trealmelll in Hot C/imales, Wiley , New York. 1976. 5-35 Marais , G. V. R .: "Faecal Bacteria l Kinetics in Stabilization Pond s," J Em' Eng Dir. ASCE. 100 : 119 ( 19 74) . 5.36 Metcalf & Ecldy, Inc .. Waslell'aler Ellgineering: Trealmenl, Disposal, Rel/se. 2d cd., McGra,,:- Hili . New York, 1979 . 5-37 Monod, J .: "The Growth of Ba cterial Cultu r es," Ann ReI' Microhiology, voL 3, 1949. 5-38 Neme ro, N. L. , Liquid WaSles oIInduslry. Theories, Praclices, {Jnd Trealmelll. Addison-Wesley. ·Readi ng. Ma ss., 197 1. 5-39 Okun. D . /\ . "Planning for Water Reu se. " J;I WIVA, 65(10):6 17 (October 1973). 5-40 Parker, H . W .· Waslelmter Systems Engineering, Pre ntice-Hall, Englewood C liffs, N .J ., 1975. 5-4 1 Pounds, C. E ., and R. W. Crit es: WaSI",.-aler Trealmenl {Jnd Reuse hy Land Applicalion. U.S. EPA. Cincinnati. Ohio, 1973 . 5-42 Process Design lvlanllal for Land Treatment of Municiplli W{JSlell'aler,
u.s.
Environmental Pro-
tection Agency, Technology Transfer. October 1977. 5-43 Process Design r\lalllwifor Nilroqen Cvnlrol . U.S . Environmental Proteciion Agency. T echno logy Transfer. Oct l, ber 19 77 . .\- 44 Process Design Manual jiJr Swpended Solids Remoral, U.S. Environmental Protection Agency.
Technology Transfer, Januar y 1975. 5-45 Roberts, P . V.: " Water Reu se for Recharge: An Overview." J AWWA, 72(7):375 (Ju ly 1980) . 5-46 Sawyer. C. N .. and McCarty. P. L. ' ChenriSlry./or Enrironmenral En"in eerin.'!. McGraw-Hili. New York, 1978. 5-47 Schroede r, E. D . : Walei' and WaSl el""ler Treatmenl , McGraw-Hill. New York , 1977. . .5.-48. Simo n,.A. ·L,·:-Pru<'li('al H ydruuliCJ;2d ed:,·Wiley; New· York; 19-7<>.··· ·· .. · .. .... .......... . 5-49 Steel, E. W . · Wal'" Supply and ,",ell'era9(', McGraw-Hili, New York , 1960. 5.50 _ _ and J . J . McGhee' Waler Supply and Sell'era.lJe. 5th ed .. McGraw-Hili . New York. 1979. 5-5 1 Stover, E. L., a nd D . F. Kincann on: "One-Step N it rificat ion and Carbon Removal." Wale I' and
Se wage Wnrk s, 66 . J une 1975 . 5.52 Sunderstron , D . W .. and H . E. Klei : W{JSlelValel' Trealment , Prentice-Hall. Englewood C lilTs, N.J. , 1979. 5-53 Thirumurthi, D .: " Design Pri'ncip les of Waste Stabiliza ti o n Pon d s." J San Eng DiI. . ASCE. 95 311 (1969). 5-54 van Vuure n , L. R . J .. A. J . Clayton, and D . C. van der Post: "Curren t Status o f Wat e r R eclamation at Windhoek ,".1 WPCF, 52(4) : 661 (April 1980). 5-55 VesiJind. P . A. : Treatmenl alld Disposal of WaSI('lml er Sludges, 2d cd .. An n A rb o r Science , Woburn, Ma ss .. 1979. 5-56 Yoshioka, N .. et 31. ' "Contin uous T hickening of H omoge neous Flocculated S lurries" (Eng li s h abstrac t), Cheln En.'!. 21, T o ky o. 1957. 5· 57 Wong.Ch ong. G . M .. and R . C. Loehr: " The Kinetics o f Microbial Nitritication ... ·. fVIIII.'Y
Research. 9 : 1099 (1975).
, !"
ENVIRON~1 ENT Al ENGINHR ING HYD RAU LI CS DESIGN
325
Total e nergy
CHAPTER
SIX ENVIRONMENTAL ENGINEERING HYDRAULICS DESIGN (a)
T otal. ene rgy
(6)
Issues related to water and wastewater quality and treatment h ave been examined in Chaps 2 through 5. It is the purpose of this chapter to introduce the reader to the physical facilities needed to meet water-supply and wastewa ter-management objectives. To do this the' chapter is o rganized into fo ur maj o r section s dealin g with water distribution systems. wastewater collection systems. pumps and pumping stations, and treatment plant hydraulics: .......... · . . .. .. .. .... ... .......... .. .. .. . ..
T o ta l energy
.. ....... .
. Water Distribution Systems
- '.
To deliver water to indi~idu a l cons um e rs with app ro pria te qualit y: qu a ntit y. and pressure in a community setting requires an ex tensive syste m of pipes, sto rage reservoirs, pumps, a nd related appurten a nces. The term d istribution syst em is used to describe co llectively th e facilit ies used to suppl y wat er fr o m its so urce to the point of usage.
Figure 6-1 T yp ica l di str ihu tion sys te ms ' ( a) gra vil),. (hi pumped. a nd (t) co mbined .
.
6-1 METHODS OF DISTRIBUTING WATER Depending on the t0 p.0graphic rela t io ns hip between th e so urce o f s uppl y a nd th e consumer, wa ter can be transported by ca nals. f1um es. tunn els, a nd pipelines. Gravity. pumping. o r a co mbinati o n of b o th may be used to suppl y wate r to th e consumers (see Fig. 6-1) with adequ a te press ure. 324
(c)
Gravity S upply Wh ere the so urce of suppl y is a t' a 's llfTicieni elcva t ion a b~)Ve th e cons umer so th at the desired press ure can be ma int a in ed , a grav it y s uppl y can be usee! . In grav it y sys te ms, it is oft e n poss ible to suppl y wate r to o ne o r m o re sto rage rese rvoi rs wil hin th e system. Wh ere a grav ity sup ply ca n be used. it has proven to be quit e eco no mica l.
326
ENVIRONMENTAL ENGINEERING HYDRAULICS DESIGN
WATER
Pumped Supply
327
Total energy delivered by pump
In a pumped s upp ly, pumps are used t o develop the necessary head (pressure) t o distribute wa ter to th e co n sume r a nd sto ra ge reservoirs.
hit
..· . . . . . Elevated
storage reservoir
hfl
Pumped-Storage Supply In a pumped-storage suppl y sys tem , storage reservoirs Me used to main ta in ade quate pressure d urin g p erio d s o f h igh co n su m er demand and under emergency co nditi o ns s uch as fires o r power failure s. During periods of low wa ter con sumption, excess water is pumped a nd s tored in th e sto ra ge rese r vo irs. B eca u se the s t o rage reservoirs are used to provide wa ter during periods of hi gh or peak demand . t he pumps can be ope ra ted at th eir rated capaci ty.
_Q3
Pump
2. Define the three conditions of flo w that can ex ist. Ii.
6-2 DISTRIBUTION RESERVOIRS Resenllirs a re used in distribution sys tems to e qu a li ze till: rate of fl ow. to maintain pressurt.:. and for emergencies. To ortimize the ir intended use. reservoirs sho uld be loca ted as c fose to th e ce nt er o f d e mand as p o ss ibl e. In large c iti es. di s tr ibuti o n reservoirs ma y be u sed at severa l loca ti o n s within th e sys tem. Re gardl ess of the location. t he wa ter le ve l in th e reservo ir must be a t a s uffi~Fnt elevat iDn to permit grav ity tl ow at an adequate press ur e. Storage reservo irs are a lso used to reduce pressure varia ti ons within th e distribution sys tem. The a nal ys is reqlllred to dele rrnill e th e orera ti o h of an elevated reservoir is illu s trat e d III Exa mple 0-1. Example 6-1; Anal~' zing the operation of an ele"ated resc rYoir I )erlVe eq uation s Ihat ca n he used to ddine th e hyd rau li c operation of an etevated s lOr~ l ge reservoir such as shown in hg. 6-11>.
When lhe municipal demand is low, the disc harge from the pump will supply the mUtllclpal demand ; the excess pump discharge will be diverted to the storage reservo ir.
b. When th e municipal demand is high. di sc harge from both the pump and the storage reservoir will be used to meet the dem and. c. At so me point of operation. the pump discharge will just equal the municipal demand and there will be no fl ow fr om the elevated storage reservoir. 3. Write equations that can be used to solve the three flow conditions defined in step 2. Q. Low demand: ' .
h. Hi gh demand: Q, +Q)=Qo
"' + Ep
=
z, + E) + hf
Z3 = Z2.
+
£3
+
)
hJ4
c. No fl ow from storage res~rvo ir : Prep~lrc a definition sketch for the ana lysis of the reservoir ()pcr~ lti ()n . Such a sketch is shown be low. The lerms in the sketch are delined as fo llo ws
\2, \2,(Q .,1
=
pump disch;lrge. rn 3 ;"s
~ discllilrge to (from) reser vuir. rn'i s
\2/! = rnunicipal di scharge (demand). ·Ill -'; s
r,. f,
=
energy load cent er lH1~ler variou s cPlldiIIOIlS' "I' "pcralion (inc ili cles pn:ssure and \'c tocity head). m
';1
II ,,' h,: . etc. = head loss due to fr ictio n. m
:, . : ..
Q, = Qo
~
elevati ons ahlll'e a refcrence datuill.
111
COMMENT
To sol ve the equations developed in step 3 for a high-demand situation, a trial
~.alue of E3 is assum ed and the computed val ues of d ischarge are compared to the demand.
I he computation is repeated ulltil the equation of co ntinuity (Qt + Q) = Qo) is sa ti sfied. In the approac h described above. it was assumed that z) remains conSlant. In actu al practice . ") will vary with time. To so lve the problem with a varying value of z) it is necessary to develop a relationship be tween the sto rage volume and the water surface etevat ion.
~.
ENVJ\{ON~IENTAL ENG INEER ING HYDRAULI CS DESIGN
329
Types of Reservoirs
(b)
(a)
Depending on the topograph y and local envi ronme nta l conditions. storage rese rvoirs may be located above. on. o r be low the ground su rface . Unde rground reservo irs are usua ll y const ru cted of reinforced concrete. Smal l ground-leve l reservo irs are usua ll y earth- lin ed with gunite. asph~11 1. or some sy nth et ic membrane. Large surface rese rvo irs arc concre te-lined. Most large surface rese rvo irs a l-e cove red to prevent contamination by birds. animals. and humans. Open distri bution reservoirs should be fenced to keep out trespassers. To obt-ain the necessiHY' hec1d within the distribution sys tem, wa ter towe rs and elevated reservoirs arc often used. Wate r t(lwers, loca ted at ground le ve l. can be cons tructed of prestressed concret(l 01- steel. Elevated water-storage reservo irs are usuall y constructed of steel. Commo11 sha pes for eleva ted storage tanr s are illustrated in Fig. 6-2.
Capacity of S tora ge Rese rvoirs Th e capaci ty of StOl'age rcser\"() lrs C~lIl be determined an,t!ytically or gra phi ra ll y. In either case a mass balance i~ th e ba;;i s of the analysis. Both methods of analysis are illus trated in Examp le 6-2. Example 0-2: f)et e rJ1linin ~ reservo ir storage capacity Determine the capacity ora storage 6 reservoir reqtmed 10 malnlaln a conS lant \\'~Ier ,LIppi,. ldr"ft) or 2 x 10 m' /m given Ihe following monlh " mean-runotr "alues:
Rune,fr QR Ill" Ill'
Run off QR 10" Ill'
(e)
YO IO.R -I C
')
o.~
III II
(t ,
I::
()'l
c.8
I.,
t
I 2 I I ll.')
1-1
~.O
I'
U5
I'
{J.(,
I~
If,
I
t D. ~
------SOLI ; t 10"
(j)
(e)
Figure 6-2 Typical shape!> COl/i'lL'S.\'
J2X
Dr elevated
waler-storage tanks. (Photo or SI. Josep h, M I Slorage tank.
CUI/sol'r T()\\'I]Jl'lIc1 ,~ .-Js.wl'itlfl'S. 11Il'.)
I. SCI up :, wbk 1',11' Ihe compU lall on, I'c'r 111.: f!raphical and numerical Solu li on or Ihe prohkm. Th e requIred compllialirll'> :tre sho\\1l In Ihe accompal.1\"tng I~hle . The eiliries in the. c"lumns arc as fc,lIll\\', C/. Thc monlh and Ihe c()~respc)ndlng rllllntT are cnlcred III cc)IUllllb I ,md 2. rcspec· Il\c ly. h. Th e cumulall'c rlllwil "cOlllfH1Ic'd :tnd cntelCcl In cllllllllll :; The \\':lIer slIpp lv drafl I, cnlered In colllilln .j
ENV J!WNMENTA L ENG INEER ING HYD RAU
330 WATER
d. The deficit (run off - water supply draft) is compu ted and ente red in co lu mn 5. A minus sign means that the water supply draft exceeds th c runofTand a deficit exis ts. e. The cumulat ive deficit is comp ut ed a nd en tered in column 6. The numbers in parentheses represen t th e cumu lative surplus. The maximum c umulati ve deficit re p resent s the required reservoir capacity. 2 Prepare a gra ph ica l analysis of the problem . The required graphica l solution is shown 111 the figure below . Key points in the constru ction o f the graphica l so lution are as follows iI . first. th e c umulative run o ll data fr o ll1 co lum n 3 in the table are used to plot the run o fr curve. h. Next. a line is drawn fr o m th e o rig inat a slope equa l to the monthly water supply d ra ft t T o determine th e req u ired capacity of the storage rese rvoi r a lin e is drawn parallel to the water s upply draft line , but sta rtin g at the point o f tan gency at the beginning o f thc d ry period. The maximum distance betwee n the draft line from the point of tangency and cumula t ive run o ff represe nt s th e required capacity o f the storage rese rvoir. As show n In the figure, th e ca paci ty valuc is IO.S x 10' II1J . which is the sam e as thc valu e give n in .he tabl e. COM~ IE N 'I Th e graphical meth od I'or reservo'ir sizin g illu strated in th e figure wa s de -
veloped by W. Ripple som e tim e bc f'ol<-: 188 3 when he publi s hed th e me thod . [n- 12] 70r--------------------------------------------------------------.
~
60
Depl etIon of ---------4____-2~~ sto ra ge
'"
40
'3c
;:
:0
]0
'"
::J
Curve drawn parall el to re quir e d wat e r.
peri od mu st
supply curve
Interse ct flillUfr
ClH"Ve if reservoir
Sl art o r ury peri od at point 0 1 tan ge ncy
is to fiJI End or elry PL'f io d ~II
point 20
4.2
2411 26.8
2.0
2.2
2.0 2.0
0.8
1.2 I I
0.9 0.5
0.6 0.4 0.5 0.9 I I
2R .il 29.1 30.0 30 .5 31 I 31.5
32 .0 32.9 340
2.0 5.5 10. 5 3.5
41 .5 52.0
2.5
58.0
36.0
5~ .5
2.0 2.0 2.0 2.0 2.0 2.0
2.0 2.0 2.0 2.0 20 2.0 2.0
7.0 8.8
- 0.8
0.0(7.0) bO(ls.S) O.O(lS.O) OO( 18.8)* -0.8
- 0.9
- 1.7
- I I
- 2.8 - 4.3 - 5.7 -7.3
-1.5 - 14 - 1.6 - 1.5 - 1.1 - 0.9 00 3.5 8.5 1.5
0.5
- 3.8
- 9.9 - 10gt - 10 .8 - 7.3 OO( 1 .2 )~
0.0(27) 0.0(32)
• Re servoir is full at beginning of dry period . t Maximum deficit at or nea r Ihe end or the d r y pcriod. Th e cumu lalive maximum deficit represents the requi red rese rvoi r storage capacity.
6-3 DISTRIBUTION SYSTEMS o f tangen c y ;..It 'tart 01' elr y
E
C
2.0 2.0
t Re se rvoir is retilled during Ihe 161h month .
of st o rage reservoir , Vs = 10.RX 10 6 Ill )
a '-'1
Repleni shm ent or slora ge
10· m J
9.0 19.8
6
12 13 14 15 16 17 18
L(QR - Q,)
10. m J
9.0 10.8 2.8
II
(QR - Q,)
Cumulati ve runolT
4
7 8 9 10
Cum ulative defic il ,
QR 10· m J
Run olT, Mo nt h
Detici t
Water suppl y, Q, 10. m J
Required capacity
50 E
()l
Period dUring whI ch rese rvoir IS no t full
Computation of required storage
CUlllulativt.:
required lVater supply Q s. : X 10 6 mJ/ Jll o nlh
10
Curve fr u m po in! o f L..l ll gen cy (It e nd 0 1' dr y per io d mu st int ef secl rLlnotl c urv e jf res t.:rv o ir is to be f u ll :Jt " t;ln u f dry peri o d
5
10 Monlh
15
Branching System
01
tange n c y
curve
Th e ser ies of interconnected pipes used to su pp-J-y- wa ter to the consu,_ ·as.a dislriblllion network. Severa l netwo rk configurati ons have b . of these is described below.
The branch in g type of water di stributi o n network is shown in Fig.· the stru cture of such a sYs tem is similar to a tree. The trunk line is tl, or wa ter suppl y. Se r v i c~ mains are co nnected to th e trunk line, aru · connected to the service mains. In lu rn, building (;on nect iol1s service to ind ivid ual residences and buildings are con nected to the Alth ough such a svs tem is simpl e to design and build , it is not [ern wa terwor ks pract i ~e for the fo ll ow ing reaso ns: (I) bacterial gr mentation may occu r in the branch ends due to stag nati on : (2) maintain" ch lor in e residual al the dead end s of th e pipe: (3) whet.,_
/
. .!,Q
' ''~'!J~
ENV IRO NMENTAL ENGINEERING HYDRAULICS DESIGN
332
333
WATER
tion of wa ter. Loops are usua ll y added to serve business districts and o ther highrisk areas. The lo ops may be constructed with separate pipes or by enlarging some of the pipes in the existing grid. The main advantage of the du a l-main system (F ig. 6-3(1) is that brea k s in mains do no t limit the u sefulness of fire hydrants. To he lp protect against freezing , pipes are usually placed on the north and east sides of streets in th e northern hemisphere. In the so uthern hemisphere water pipes are normally placed on the so uth and east 'sides of the streets. In all cases,
Main supply line
pipes should be buried below the frost line.
6-4 DISTRIBUTION SYSTEM COMPONENTS The princ ipal components of the distribution network are pipes, valves. fire hydrant s, and se r vice (building) connections. Storage rese rvoirs and pumps are conside red separately T yp icall y, the req uiremen ts for sizes a nd placement are specified by local code. Repre se ntative values and data are reported in Table 6-1.
(b)
(a)
Pipes
I I
A variety of matertals has been lIsed for th e pipes in water distribution networks. The mo st common materials are steel. cast iron. and reinforced concrete. The type -
Table 6-1 Representative components
of distribution
data
system
.---.-.---.~----- - --.--------
Figure 6-3 Water
distr;but{on ·s·):st·e·~~·· .(~) 'b~~r~~hing ';"i;h d'~~d 'e'{1'ci;;' (b)'~rici ~a~;e'r'~;' (;.) '~;id
pa;tcrn
with loops ; (d) grid pattern with dual main s.
,?e.ma.de to an individual line. service connections beyond the point of repair will be without water until the repairs are made; and (4) the pressure at the end of the line may become undesirably low as additional extensions are made. The latter problem is common in many less-developed countries.
Grid System The distinguishing feature of the grid system is that all of the pipes are interconnected and there are no dead ends (see Fig. 6-3b). In such a system. water can reach a given point of with~rawal from several directions. The grid system overcomes all of the difficulties of the branching system discussed previously. One disadvantage is that the determination of the pipe sizes is somewhat more complicated. Several variations of the grid s"stem are also in use. Two of the most common are the grid pattern with loops (see Fig. 6-3c) and the grid pattern with dual mains (see Fig. 6-3d). In the former. additional loops are added to improve the distribu-
L .. ~_____________________
Valu e
Ilem
(d)
(e)
Pipt: ~
Smallc:-.1 plpC ~ In grid Smallest brallchlng pipes (dead cnd :-) Llrgl::';\ :-'IX1Clllg ur 150- Jl1m ((l-ln) gl'ld [2110.111111 (X-Ill) pire used be yon d Ihis \'aluc] Smallest pipes In high-value district SII"Lalks l pirc~ on principal stree ts III cClllr~iI
I gO m (600 I't)
200 mill i8 In)
:;00
di~lncl
Larg.est spa cin g of sup ply lTlaiJl~ V"llcs In singh.>
150 111m (6 in) 200 mm (1< In)
~llld
l)f
fceders
IllI1l (t
2 in)
600 III t2000 1'1 )
Til n.::e a t crosse s.
dua\·rn' lll1 sy::;lclll:-'
\\.1,.\)
l.(lrgC :-' l ~ raCillg Oil
long hranches Lu"ge:-.t spacing in high -va lu e distnl"\
al Ices
250m (~OO 1'1) 150 (500 11 )
Fire h"
Area" prot(::(tcd b) hy"dranb
Lirgesl spacing whcn lire tl o" excecds JO() L', 6tl
(5000 ga1nninl l
;lrgL':-:.1
~ r,lL'lng \\ hell lire no\\'
t \(1110 gal Illln)
IS :1 :-'
111
tclll) 1'1)
lo\\' as 6U L s 'it) III (~Of.l It)
334 WA TER
ENVIRONMENTAL ENG INEER ING HYDRAULI CS DESIGN
o f pipe m a te mli is impo rt a nt as it will a ffec t th e an c illary equipm e nt needed fo r it s in s tall a ti on a nd m a int e nan ce.
Valves The typ es o f val ves used m os t co mm o n ly a re gat e val ves, ~lir-reli e f val ves, a nd c heck va lves. As no ted .in Table 6-1 a nd s how n in F ig. 6-4 thr ee ga te val ves a re used at a ll c rosses a nd two ga te val ves ar e used a t a ll tees. Th e princ ipa l fun cti o n of ~
ifo1
,
1'-1
I
it-L
8"
} ]
[
N
t
'"
'"
8"
J
Key i\LI ins G ale v "I \'~> -+--Hyd ra nl s -----L Servi ce hC
- --
8"
-,
1
t
1
I
I
(a)
l ±I
' ~J~
} -:.
1±
.... .1.± .'
.!.j.~ L
11· 1 I
8" f------
::l
~- - ----
I I
N
1'0
T
I
I,
____ J:l '~
I
' lo
Is.
----
I I
I
,
I I
I I]
H ---- E--~ II I
~
1
I
I
-,
r I
~l+ - l - - - - - - .-
I
,;
(/ :r(Jm
rU/I
i ' I!l1
l)f gfld
I f l __""
)
di:-.l l lhll l lClIl ::, Ysl('m -
Fire Hydrants Fire hydrants are placed o n mains to pro vide locations whe~e fire hos~s and pumper trucks can be connected to the water so.urce needed for fire fighting. Three types of hyd rants are used : flu sh. wa ll. and pos t. As the name implies: flush hydrants a re placed in a chamber that is even with the surface of the street or . sidewalk . Wall hydrants project from the walls of buildings and are used ex tensively in commercia l dis tricts. Post hydrant s extend from the main to about I m above the street o r sidewa lk . Pos t hydrants are usually placed on a co ncrete block to eliminate sett ling and are braced to resi st the latera l forces of th e flowing water. T yp ically, hydrants are pro vided with one o r more 60-mm (2t- in ) hose outlets a nd a IOO-mm (4-in) pumper-truck connection. In co ld clim ates. th e. operatin g valve is loca ted below ground level so that the barrel contains no wa ter except when in use. A drain valve o pens 'automat ica ll y when the hydrant valve is closed, to permit the escape of water after use and to avoid dam a ge by freezing. In warm climates. t he hyd ra nt barrel m'ay co nt a in water a t a ll times. and a n ind ividua l valve is provided for each out let.
.':.-crvice (Building) Connections The service (building) connection is that portion of a wa ter suppl y sys tem th a t lies between the wa ter suppl y main in the str eet and the take offs for th e various plumbing fixtllres at the point of usage.
6-5 CAPACITY AND PRESSURE REQUIREMENtS
(a) sJn !,l It;.rn a i n sys t em
The capac it y ora water distribution system must be suffic ien t to', meet tl; e requirement s for fire fighting in co njunction with d o mes ti c, co mmercia l. and indu stri a l demand s and for o th er system uses and losses. It is important to note th at capacit y a nd press ure must be co ns idered s imult a neou sly. For example. water must rise to the upper stories of low-rise buildings in sufficient quantity and pressure. especia ll y when fire fighting is consid ered . It sho uld be noted th,!t a pumped supply is used in most modern high-rise buildings. Also. th e capacity and pressu re ava ila ble a t h ydrant s must be sufficient fo r fir e-fighting purposes.
~-
I
(h)
FiJ,.!lIrr 6- 4 SL'l'tI(lll
these valves is to isolate subsect ions of the system for repairs and m aintenance. Drain va lves shou ld be provided at low points in the system. To remove air from pipelin es, air-relief or release valves are placed a t high spots. in the pipeline. Check valves are used to limit the fl o w of water to a single direction .
I~
I
:1
I
:l I
335
a nd (h, d ll, d - Ill;lln . . y:-.t(,·I1l.
Capacity With th e except io n of that needed for fire fighting, the capacit y of th e dis tributi o n sys tem mu st be suffi cient to meet th e pe:1k de m a nd based o n d o mest ic, commercia L
.
-
{~
i~'
·~i
ENV IRONMENTAL ENG INEER ING HYDRAULICS DESIGN 337
336 WATER
industrial, a nd other miscella neous uses and system losses. The ultimate peak demand would be the combin a tion of the peak fire-fighting and peak conve nti onalconsumer demand. In practice, howeve r. most distribution systel11saren ot s ized to provide the ultimate peak dem and . The reasons for this are (I) the probability that the peak fire and consumer dema nd will occur simultaneous ly is low a nd (2) most distribution systems a re sized for the future so excess capacity is avai lable. In general, most distribution systems are sized to meet the fire demand and a consumer demand of ISOt o 200 L/capita . d (40 to 50 gal/cap ita · d) in excess of the yearly average value. Alternatively, the co nsumer demand ma y be taken as the maximum d a ily demand (150 percent of the average daily demand). In the Un ited States. the general fire-fightin g requ irements are based o n the recom mend ati ons of the Insurance Services Office, New Yo rk. The required fire fl ow is estimated using the foll Qw ing equation.
F
=
320C,jA
Table 6-2" Duration of required fire flows based on flow * Fif e tlow ga l/ min ?OOO or less 3.000 -l.000 5.000 6.000 7.000 ~.()OO
9.000 10.000 and greater
mJ / d
Dura tion. h
10.900 .t 6.400 21.XOO
27.300 32. 700 3.8.200 41.600 49.100 54.500
4
6
S 9 10
(6-1) Adapted f[()ln Guide j t)r D('(ermiflG/ion F hn\". 2d cd .. I nsuranct: SCC\lCC Ulliee. New York. 1'174. :0-
required fire fl ow. ml/d = . coefficient related to type of co nstructio n A = the total floor area (includin g all stor ies, but excluding basements) in the building under considera ti on, m 2 For fire-resisti ve bu ildings the six largest successive floor areas are used if the ver tica l openings are unprotected ; if the ve rtical openings are pro tected properly, o nl y the three largest success ive floor a reas a re considered.
where F C
=
Values for the coefficient Care 1.5 for wood frame construction, 1.0 for ord inary construction. 0.8 for nonco mbu stible construction , a nd 0.6 fo r fire resistive co nstruction. Interpolation between these va lues is used for constructi on th at does not fall into one of the four categories. The co mpilted value is then adjusted lip or down for (I) occupa ncy. (2) sprink ler protection , and (3) exposure. Th e maximum fire flow determined using Eq. 6-1 shall not exceed 43,600 m 3/d for wood frame construction and for ordin a ry and heavy timber constructio;l, and 32.700 for no ncombu stible construction and for fire resistive construction for anv o ne location. The required fire flow ra te must be avai lab le in add ition to tl;e coincident maximum daily fl ow rate. The duration during which th e required fire flow should be available varies fr om 2 to 10 h as su mmarized in Table 6-2. Beca use a city will be pena Iized in its fire in sura nce rates if the needed flow s cannot be met for the specified durations, most cities provide sto rage reservoirs to. meet fire demands.
Pressure For typical residential rates of demand. a sta tic press ure of275 kPa . gage (40 Ibiin z gage) is considered to be normal. The minimum recommended pressure is ,ibo ut 140 kPa. gage (20 Ib/ in 2 . gage). In business dis t ric ts, pressure va lu es in the I'an gc . of 350 to 550 kPa. gage (50 to 80 Ib 'in 2 gage) are COllllllon. For high-rise buildin g, (greater than three stor ies) water is pumped to storage tanks located o n intermediate fl oo rs. o n the roof. o r in tOIlers.
(~1 HnllfllT d Fi f t
With the adve nt of th e modern fire-fighting eq uipment. the pressure th a t must be main tained at a fire hydrant rarel y needs to be greater than 350 to 400 kPa, gage (SO to 60 Ib/ in 2 • gage). Th e exception is in sma ll towns where full-tim e fire departme nt s with new eq uipment cannot be afforded. When pumper trucks are used. the press ure at the fire hydrant sho uld not be allowed to dro p below about 70 k Pa. gage ( 10 Ib/ in 2 gage). This low pressure sho uld be ma intained to prevent untreated wa ter rrom cnteri ng the wate r distribution system by seepage o r pipe failure caused by vacuum co ll a pse. 6-6 DESIGN OF DlSTRlBUTION SYSTEMS The ciesign of a water d istributi o n system for a new area can be o utlined as follows. (The ana lys is of e:.tisti ng systems is con sidered in th e following section.) I. Obtain a detai led map of th e area to be sen 'ed on which topographic contours
2.
~. 4 5. ()
(or co ntro llin g eleva tio ns) and the locat io ns of present and future st reet s and lots are identified. Based on the topography, se lect poss ib le loca tio ns for distribution reservo irs. If the a rea to be served is large. it may be dilided into severa l subareas to be se rved with separa te di stributi on svstems. Est imate the average and peak wat~r use for the area or each subarea. allowing I'o r 11re li ghtin g and future growt h. Est imate pipe sizes on the basis of water dema nd and loca l code req uirement s. La y' ou t a ske leton syste m of supp ly mains leading from the distribution I'csel'voir o r other so urce of supp ly. i\na lyze. using one of the SCle ra l methods di scussed III the foll ow ing sec ti o n. the 1100\s ;tnd press ures in the supp ly netIl'L)I'k k)1 fire !lOll's . ;\ separa te analysis
338
7. 8.
9. 10. II. 12.
ENVIRONMENTAL ENGINEERING HYDRAULICS DESIGN
WATER
should be performed for each subarea. Also several configurations should be examined for each area under various conditions of withdrawal. Adjust pipe sizes to reduce pressure irregularities in the basic grid. Add distribution mains to the grid system. Distribution mains that serve fire hydrants should be at least 150 mm (6 in) in diameter in residential areas and 200 or 250 mm (8 or 10 in) in diameter in commercial and high-risk illdustrial areas. Reanalyze the hydraulic capacity of the system. Add street mains for domestic service. These mains usually vary in size from 50 to 100mm(1 t04 in) in diameter. Locate the necessary .valves and fire hydrants. Prepare final design drawings and quantity takeoffs.
6-7 HYDRAULIC ANALYSIS OF DISTRIBUTION SYSTEMS The purpose of a hydraulic analysis of a distrIbution system is to assess tlows (including direction) and the associated pressure distribution that develops within the system under various conditions of withdrawal. Several methods are available. These include (1) sectioning, (2) the circle method. (3) relaxation, (4) pipe equivalence. (5) digital computer analysis. and (6) electrical analogy. The characteristics of each of these methods are summarized in Table 6-3. The method of sections and the use of digital computer analysis are considered further below.
Method of Sections The method of sections was developed by Allen Hazen [6-2] as a quick method for checking the correctness of network pipe sizes. A similar procedure was proposed by Pardoe. [6-10] Although the method is approximate, it is extremely useful in analyzing pipe networks if its limitations are appreciated. The principal steps involved in the application of this method are as follows. I. Cut the network with a series of lines selected with due regard to varying pipe sizes and district characteristics. The lines need not be straight or regularly spaced. Typically the first series of lines will cut across the network at right angles to the direction of flow. Additional cut lines may be oriented in other important directions. For more than one source of supply, a curved cut line should be' used to intercept the flow from each source of supply (see Fig. 6-5). 2. Estimate the amollnt of water that must be supplied to the areas beyond each cut line (i.e., downstream). The water demand is composed of the fire demand and the normal coincident draft due to domestic, commercial, industrial and other uses. In most networks. the coincident draft will decrease from section to section. The fire demand will remain high until high-demand or high-valve areas are left behind. 3. Estimate the capacity of the distribution network at each cut line or section. This can be done as follows: a. Count ane! tabulate the number of pipes of each size that were cut. Only those pipes that provide water in the direction of flow should be counted.
Table 6-3 Methods of analysis for water distribution systems Meth o d
Method of sections
Description Water-distribution-system grid is cut with a scric:-.
...................... .
or section s. and
Pipe equivalence Digital computer analvsis
Electrical analogy
a- -
a'
t- -
A trial-and-error procedure in which systematic correci.ions are applied tn (I) an initial set nf assumed nows or (2) an illltial set lOr assumed heads until {he flow network is balanced hydraulically. The pipes ill a complex di stnbuti on system are replaccd with pipe of equivalent capacity.
;1
0"
•••••••••••
/'
/'
The pipes In a distribution system trihutary to a ccntrallire hycir,lllt or . g~OllP. of hydrants are cut \.vith a circle, anc! the capacity of· the pipes to meet the nre demand IS assessed
Relaxation
.........
the
capacity or the cut pipes is compared to the downstream demand. Clfcle methnd .
......... .
Supply main
I b - I-- -
-
t- - -
-
-
+- b'
The.distribution system is rnolkkJ \A.oith ekLtrically equivalent components. F(H ex ample, nonilliear resistors are used to ~iJllu l~llc pipe friction. If the current Illpuh and \\.:ilhdr~l\\"als :IfC prnrortiollal to the water 110v,.:, thell the hcad l()-.;:-.(~ .., \\ill be proportlOIl:d III measun.:J vd1t~lgt:: drops
/
I C_I--_
-
r- - - - -
I--c'
single
. Algorithms are "f1uen til solve Fqs. 6-2, 6-:;. and (.-4 sirnuluneoLisly throughout the netw~Hk. The algorithms arc solved using modern high-speed digital computer", . NUl1lcrnu:.. cOll1rlll'ricd programs art: available for the solutioll of \.vatcr -di~tribulI()n now problems.
I
I- -
-
-
-'-
I--
..- v"'-
I
I I \-
V
"-
"\
---:-
--- a
- r--
-- -
\
\~
\
-
I
\
\ -
v-
1/
I I \
d - I-- -
-----'b
(II)
Figu". 6-5 Dennitlon ,ketch for the application of the meThod of sections.
(b)
"--..:
"c
-b'
l- c'
340
WATER
ENV I RO N MF. N TAL EN GINEERING HYDRAULlCS:l!J.l«CN
P=4t 5 kPa
b. Determine the average availab le hydraulic gradient. This w ill d epend on
4. 5. 6.
7.
8.
system press ures a nd a llowa ble flow ve locities. F or exa mple. if a flat grid is 10,000 m wide in the direction of fl ow, if the press ure ava ilab le a t th e transmission pipe connection is 41 Sk P a, gage (60 Ib/ in 2 . gage), a nd if the minimum allowable pressure is 140 kPa, gage (20 Ib/ in 2 , gage) th e average hydra ulic gradient is 0.0028 [(41 5 - 140)/ 10,000]. H ydrau lic gradient s an d ve locities between 0.001 m/ m and 0.003m/ m and 0.6 to 125 m/ s (2 to 4 ft / s), respecti ve ly, are common. For the calculated h yd ra ulic gradient. determ ine the ca paci ty of th e ex istin g cut pipes and total capacity. Determine the difference betwee n th e req uired an d th e existing capac it y. If the existing capacity is in a dequat e, selec t pipe sizes a nd paths that will offse t any deficiencies at the required hyd ra uli c grad ient. The capac it y o f th e system can be increased by replacing sma ll pipes w ith larger pipes or addi n g pipes to the grid . Experience with the sys tem so metimes helps in selec tin g the pipe sizes, but such experience is not necessa ry. If excess ca pacity is found , p ipe s izes may be reduced u sing the same procedures. D e tefmine the size orthe equivalent pipe fo r th e re inforced sys tem a nd es ti ma te the fl ow velocity. High ve loci ti es s hould be limited to avoid wa ter- hammer p ro blem s by redu ci n g the hydra uli c grad ient. Check the pressure requirements against th e reinforced sys te m.
341
b
ISO"
300
200
150
o
o
on
V)
cOO
200
o
150
300 o o
CI
o o
150
c"
150
-- --
-
b
150
o o ,-.,
-
150
c
150 0 ;
Application of the method of sec tions is illu str a ted in Example 6-3.
0 Of>
---!. 50
200
Example 6-3: Applying the method of sections Using the meth od of sec ti ons analyze the ... 'wa"re r-oist"rib\iii o'n:pipe' g i id slio\"n in' ilie' acc6ril'p,iliYlrig ·flgU"rc. sij~Ci"(y tl;~' ~;i ~~~ '~~'d' loca ti o n of any modifications yo u thi nk shou ld be made in the pipe g rid and reanal yze it with these m o difications. A ssume t he following conditions apply.
'0
Q= where
65, ' p(t -
OO t vp)
-_
o
'"
,r,
150
150
= popUlation in thousand s
No te: The above eq uat io n was commonly used by the Nat ional Board o f Fire Und.er· writers for estim ati ng fire flows until it was replaced w it h Eq . (6- 1). 2. Co incid ent resi dent ial d emand o f 150 perce nt of average daily " 'ater dcmdnd. 3. Ave rage dai ly wat er demand is 500 L icapita d. 4. Calculate Aows and head losses \\ith Darcy - Weisbach equati on using an Ivalue of 0.02 0. 5. U se on ly pipe sizes of 150. 200 , ~IJO. 400. and (,00 mm when modd'ying the dis tribution sys tem grid.
.
- --00c,
150 millimeter s
oV", i
----150
CD
I Iv]pdif\ the I)
II =
I
l. I ' ] J 2!J
I
II
t· 2
::::- .'1 =
" "21J
I () ' .'i :.:=
iy .'l f. d
d
150
Re pl ace with 400 mm G).Q) RcpIJCC with 200 mm
Q = flow rate, Li s p
--
150
0
'r,
Pipe
1. Fi re flow demands for the downtown business dis trict a re estim a ted lI si ng th e fol lowing equation
~~
----r-
o
-
e
342
WATER
ENVIRONMENTAL ENGINEERING HYDRAULICS DESIGN
where
Q = fl ow rate. m ) /s Q.
d = pipe diameter. m
. . (15)(500 L/ capita· day)(16,500 people) COinCident demand = . 3 3 ... .... 10 L/ m
s = s lope. m / m
= 12.38 x 10) m 3 / d
y = acceleration due t o gravity. 9.81 mi s' Substitute values fo r y and
f
and so lve.
Q=
[:(~I;;J!2"5!2S I !'
b. The fire demand for the downtown business district is based on a population of 28,000. S. Determine the available hydraulic gradient acr~ss the distribution system.
P,
= 24.60d " '5 ' /'
h". co n ve nien ce. exp ress
3
343
3
Q in units uf 10 m /d.
s ·= "-- Y
L
where P, = press ure in supply main at head end of distribution system = 415 x 10 3 Pa (given)
P, = minimum pressure required at farthest end of distribution system
Rew ritin g the above eq uation in term s o f sa nd d yields
s = (2215
d = (4607 x 10
= 140 x 10 3 Pa (20 Ib/ in')
7) Q5' d
x 10-
L = length of main supply pipe across system
') ') (Q ~
= 902 5 m
II'
2. Cut the distribution -sys tem pipe grid with a series o f sect ion lines drawn approximately perpendicular to the large wat er main (see th e ligurc). J Es t ima te popubtion d ow nstrea m o f each c ut sec tion . Th e values for t he sect ions s hown in the figure are:
415 - 140 N/m' x 10 3 ------nl 0 N / m )--- ~ = = 0.0031 m/ m 9025 m 6. Co unt and tabulate the number of pipes of each size cut by each section. For example. for section bb:
2 - 0.2-m
2X.OOO 2l.fJOO 16.500 'J.OOO }. 750
<1<1
bb Cl'
dd ee
- 0.6-m diameter I - 0.3-m 4 - 0.15-m
7. Calculate the' capacity of each pipe c ut by sectiori bli .using the Darcy - Weisbach eqllation and the available hydraulic gradient. For example, for the 0.6-m-diameter pipe:
Q=
4. Es timat e w;lIer demand down stream o f each cut section. The required values are:
2125d S12 s '
2
- - - -- - - ----- - - -
-'
(see step I)
= 2125(0.6)5/2(0.0031)'"
= 32.99 x 10
3
3
m /d
The capacities of the pipes cut by sec ti o n bb are: SCUtl ) 1l
Popu lati on
roincicicnl
Fire
Total
aa
2X.OOO 2 1 .lJOO 16.)1)0 '.i.DOI) 'U SO
2100
2H 14 28 . 1-1 2X 1-1 2X . I-I 5. 45*
49.14 4')9 40. 52 }-I X') 8.27
bb cc dd l'c ~
Ba...;cJ
gal / mIn I
011
172',
12.3k 7.8X :'.81
re : -. idcll!ial fire- dl:rnarld of 63 L is (IOOU
I - 0.6
Hth e
SUIll
32.99
I - 0.3
5.83
2 - 0.2
4.23
4 - 0.15
4.12
X
10 3 m 3d
is greater th an the demand there is sufficient capacity across this section.
For section bb th e suppl y is grea te r than the demand (45.39
3
X
!O3
m /d).
1.:1 ; [ '! .
344
WATER
8. Calculate the diameter o f a sing le equivalent pipe using the d iame ter form of the Da rcy - Weis bac h eq ua ti on (see step I). . .j
d
= (4.667 x
.
'.' (4 7. 17 2)1/ 5
10- 2) -~
0.003 1
.
= 0.692 m
9. Calcula te the actual hydra ulic g radient when th e capaci ty at th e sec ti o n line eq ua ls the demand using the slope fo rm of the Darcy - Weisbach equa ti ons (see s tep i).
s
=
45.39' (2215 x 10 - 7) 0.6925
~
~
r'
= 0.00288
10. Using th e actual hydra uli c grad ient , recalc ul a te the ca pacity a t th e sec tio n line: It sho uld equal the demand. For exam ple. a t sec tion bb:
-
0-
". ....
". oC
<:0
0
C>
co
-=
CO
r, x
-=
.,.
e-
-or
"'"
r",
.,. 2? .... .,. 0 M
oC
0 0 c
.,. '" "" or, eX>
~,
i.,.
'" ....
00
~,
0
or, CC ~o 70-
r' r' -
rl
...r-:
'"::; \ .,.'"
0' r ..... :
-
0:
r ....., C N -
3
- 0.6
31.80 x 10 m'/d
I - 0.3
5.62
2 - 0.2
4.08
4 - 0.15
3.89 45.48 x 10' rn ' / d
11. Check fo r excess velocities in the pipes using the actual pipe ca paciti es fro m ste p 10.
~I
o~
\
v,
Q
v
h d 10' m' --_. (10 ') - - d 3600 s 24 h
Ld
A
2
where
Q=
o
of.,
....ofo
I{r ')~
",
I ....o~
fl ow ra te. 10' rn ' id
,I~ I~
" = pipe diameter. m
v=
'tl
.,E
4 = (1.474 x
....o
fluid ve loci ty. mls
"": r": r'! "'" -: coo c
0: ,
Ii
For example. for the 0.6-m-diameter pipe _, 3 1. 80
. V = (1.474 x 10 -) --"-'-
0.6'
=
1.30 rn /s
-g -.-:; <:
If the calculated velociti es are too hig h. modificati ons to the distribution-sys tem grid m ay be necessa ry. 12. Co mplet e the necessa ry computa ti ons fo r the remaining sec tions. The required comp utat io ns for the distribution sys tem shown in the figure are su mmarized III the accompa nying table. Based o n the calcu la tio ns in the table. sec ti ons tid and ee have' in sufficient capacity. Alth o ugh many modifications tl\ th e di stribution sysIC m grid are possible. three tha t wi ll co rrect th e insufficie nt capacit y prob lems are s hu wn in the figure . Sections dd and ee were recalculated a nd th e new resu lt s are shown in the ta ble as sec ti ons dd and ce (rc\ ised).
__ _
~ Q
-
"2 v:
.,..,.
,', or,
-
('-I
~
01
("'I
(""
~
('I
-
r·,
C'
or
.,.
x.
0
;!.
.... c ·,
,... ,...,
~
~.
]
"
i
u '0
-,;
""
:g
~
-
.~
""
346 WA T ER
ENV IRONMEN T AL ENG INE ERI NG HYDRA U LI C S DESIGN
Digital Computer Analysis
347
....)
6-9 CONSTRUCTION OF WATER DISTRIBUTION SYSTEMS
M os t di stributi o n netw o rk s a re n o w anal yzed using dig it a l co mputer p rog ra ms. In writin g a co mputer prog ram to so lve netwo rk fl ow pro blems, the fo ll ow ing equ a ti o ns mu st be satisfi ed simultan eo us ly thr o ug ho ut th e ne two rk . At eac h jun cti o n : (6-2) F or eac h co mpl ete ci rc uit :
'LH
=
0
(6-3)
F o r eac h p ipe:
H
=
k Q"
(6-4)
In t he m o re so phis ti ca ted o f t he ne t wor k comput er p rog ra m s, Eq s. 6-2, 6-3. a nd 6:4 a re so lved si mult a neo us ly us in g o ne o f severa l m a tri x in ve rs io n tec hni q ues. Severa l so luti o n tec hniqu es are prese n ted a nd a na lyzed in J e ppson [6-4J P erh a ps th e greatest ad va nt age o ffe red by th e use o f co mput e rs is that m a ny m o re so luti ons ca n be deve lo ped a t a re aso nab le cos t to assess th e res ponse of the sys te m to vary ing input s. Also. rea l-time ana lys is can be used to stu dy th e effec ts o f va ryi ng pump opera tio n p la ns. At th e prese nt tim e a lm os t a ll co ns ult ing firm s a nd m os t indus tries have in-ho use co m p ut e r p rog rams o r have access to s uc h prog ra m s o ffer ed by seve ra l o f th e nati o na l co mputin g se rvices. The key iss ue is no t in wr iting s uc h a program . b ut in un de rs ta nd in g wh a t p roblems shou ld be so lved .
The basic requirem ents o f pipes fo r water di stribution c:~~~~r~~
....)
a::m:~~1~::~
stren gth and m a ximum corros IOn reslstance't~I~r;;::lf~izes steel ~nd reinforced steel. plas tic, a nd asbes tos c~ment com~ete II1 . In cold climates, pipes should co ncrete are m o re co mpetItI ve II1 the Idfger sIzes.. . For even the coldest f eno u h belo w gro und to prevent freez1l1g II1 wmter. be ar g . d h f 15m (5 ft) is generally more than adequate. part s of the Ul1lted hStates, a e~t b: bu'r ied o nly sufficie ntly deep to avoid damage . In warm clIma tes, t e pIpes nee . t ipe are fr o m traffic load s. Service co nnec tions t.o ca st-Iron orasbestos-~~r:e: ~orporamade b y tappin g th e distributIOn m a lI1 WIth aspeC!al~ap.p~fe~dai~~~; the se~vice ti on coc k is th en installed WIth a fleXIble goosenec P I~ settlement between the · The go ose neck prevent s damage If th e re IS unequa. . pIpe. . . ' 1 d ' f m the ma1l1 to the cons umer are main and the ser vice pipe. Ser vIce pIpes ea mg/ 0 . I -family dwellings ,20- to usua lly o f copper tubing or galvafllze~ st~eL o~::~i~: sizes may be needed for 30-mm G- to It -in) pIpe IS co mm o n. ut a rger . apa rtm ent ho uses o r bu siness establI shments.
....)
--.J .....)
-..J
---
I
-..J -..J
-.J Filling A New System
.
d ra nts a nd valves are opened so that aIr can When a new pipe is fir st filled , all hy 'd ' re several days for large systems. .. . d I IV'1l1 ma y requI esca pe fr eely. FIIIlI1g IS o ne sow , < . ' . not rop edy taken out of the system. Excessive press ures can devel o p If the .a lr IS fro~ a h ydrant, it is closed. The proWhen a stead y, unlI1terrupt ed strea m Iss uesI e ' Iosed and the system is full cedure is continued until all valves and h ye rant s ar c
-..J
--.J ....)
.Df wateL. ... .....)
6-8 CROSS-CONNECTIONS IN DISTRIBUTION SYSTEMS Leakage A cross-co n nec tion occurs whe n th e drinkin g-wa ter su pp ly is con nec ted to so me so urce o f po llution . For example, if a co mmunit y ha s a d ua I' water di stributi o n sys tem , o ne fo r fi re fi g ht ing a nd the o th er fo r domes ti c co ns umpti o n. th e two m ay be interc o nnec ted so that d o mesti c wa te r ma y be used to s upplem e nt th e o th er sys tem in case o f fire. S uc h an a rr angeme nt is d a nge ro us, fo r co nt a min ated wa ter from the fi re-fi g htin g s upply may ge t int o th e drinkin g- wate r sys tem eve n th o ugh the two sys te ms a re nor mall l' sepa ra ted by c losed va lves. Th e p re ferred me tho d for int erc.o nn ec ting dual sys tems is th e a ir brea k , a lth o ugh d o ubl e c heck val ves a re sometImes used . Cross-co nn ec ti o ns may occur In priva te resid en ces, a pa rtm ent ho uses, and co mm ercia l b uil d ings. es pec iall y wi th o ld-style plumbing fixtures If th e wa ter inl e t o f a plu mb in g fix ture is be low th e ove rfl ow d rai n o r r im, a redu ced p ress ure in th e wa te r sys te m m ay cau se bac k s iph o nage. Ot he r so urces o f Cross-co nn ec t io ns a ro un d a ho use ho ld include ba thtu bs, fi s h pond s, sw immin g poo ls with un de rrim inle ts, a nd law n spr inkle rs th a t become s ub me rged whe n used .
.
. . . s stems will vary with the care exerCIsed T he am o unt o f lea ka ge fr o m di strIbutI o n y f i t m Leakage values from 5 · . d h d co ndItI o n 0 t 1e sys e . 111 construct IO n an t e age a n I' f ' diam . mi . d) are common to 25 L/ mm o rpipe dia m · km · d (SO to 250 ga / lI1 O PlelPnegth of pipe between valves . ade bv cios mg 0 ff a . for new sy ste ms. Th e test IS m ,. . . t oduced through a speCial . . the pIpe Water IS m r . . '. . d f t least 12 h while leakage IS and a ll se r vIce co nnectIOn S to · I k' . , Ire IS malI1t a 1l1e or a mlet, and norma wor mg pI eSSl II . tl'nla ted from the difference . t · th e to ta oss IS es measured. In an o pera tm g sys e m d d deliveries to the customers. . th s V 3tem a n metere between me as ure d mput to e J . 'fic leak Patented leak 'bl I d o r 10ca tlBg a specI . There a re' several pOSSI e met 10 s d 'f . g water or the disturb'. ' . k th e so un 0 escapm detecto rs use a ud lo,ph o nes to pIC up d nd near the leak. Similar . . I' fi Id ' d by S'r!urate gro u e c
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348
ENVIRON~IENTAL
WATER
ENG INEE RI NG HYDR AULICS DES IGN
349
,j
I' co nstruct .Io n. an d the constru cti on a nd ma in-
may be used to spot the loca ti o n of a lea k. The loca ti o n of all pipes. va lves, and appurtenances sho uld be entered o n maps. This informa tion is e~se nt i ,t1 III case repairs are ever required.
design of sewe l·s. t he ma terials tena nce of sewers.
Disinfection of New Systems
6-11 TYPES OF CO LLECTION S YSTEIVIS
Wh ile pipe is being handled and placed. th ere are man y opportunit ies for poll utio n. Hence, it is necessary to dis infect a new system or an ex isting sys tem after re pa irs or add itions'. Disinfect ion is usua ll y accomp lished by introdu cin g chl o ri ne, calc ium hypoch lo rite. or ch lo rin a ted lim e in a mo unt s sufficient to give an immedia te chlorine res idue of 50 mg/ L. Th e che mica l is introduced slow ly a nd permitt ed to rema in in the system fo r a t leas t 12 a nd preferab ly 24 h before it is flushed o ut. T he fl ushi ng may be accomp lished by o pening seve ra l fir e hydrants.
The three genera l types of collection systems commonly used in the United States are sa nitary. sto rmwat er. a nd combined. The characterlslics of each of th ese types of sewe rs a re d isc ussecl below.
M aintenance of Distributi on Systems T he hydra ul ic efficie ncy of pipes will diminish with tim e because of tuhercu lati on, encrusta tion . and sed iment depos it s. F lushing II ill di slodge so me of the fo reign ma tter. but to clea n a pipe effecti ve l} a sc raper mu st be run thro ugh it. The scra per may be forced t hro ugh by wate r press ure o r pu lled thr o ugh with a ca ble. Clea ni ng. even t ho ugh costly, may payoff with Illcreased hyd raulic effi ciency and in creased pressures thro ugh o ut the sys tem . Th e eflects of cleaning may last only a short time. and in ma ny cases pipes are lined with ceme nt mo rtar after clea n ing to obta in more permane nt resu It s.
6-10 P UMPI N G REQ U IR ED F OR WAT ER SU PPLY S YSTE MS In some cases. grav it y can be lI sed as th e dril'in g fo rce to bring wa ter fro m it s source to the consumer. In mos t Cases. howe\"er. some form of pumping wil l be required. Pllmps ar~ required to delile r water from we ll s and where necessary to lift wa ter to distribu tion reservoirs an d eleva ted tank s. Often booster pumps must be installed o n the mains to incrcase th e press ure. Pumps and pump stations are considered in greater deta il la ter in this chapt er. \
0
Sa nit a ry Sewers Oft en idcnt ifi ed as se parate sewe rs. sa nitar y sewers were deve loped to remove . ' .ldenlia . I areas. 0 Il" gln, . 'I lly . the fl ow III sa nitar y sewers domes lic wastes from res . , lVas by l(ral'it y. Mure rece ntl y. both presslll e a n d v acuum sewers ha ve . been used .. . lei b e Cf'ffi to .serve- areas l.vhere grav it. y sewers wou I c ult ,'ll1d cos tl y to Inst all a nd lllaintall1.
S torm wa ter Sewers Sewers Intend ed so lel y for the collection of sto rmwater are kn own as stormwa,ter , · · sewe l'S .'separate sewers. Usuallv large r tha n sa nltalY .' ' .stormwa ter . sewet S ,Ire bl ' 'lssocta ted with th e dIscharge of Ull. . . CllnstruCled to e·ltmln a te poll ution pro ems, . . ' . . " . I sewers .Int o II'aterco urses a.nd reCelVll1 e0 wa tels. treated wastewater fr om eumblne( .' developed Illt O. a se parate More recentl y. th e treatment 0 f sto rmwater Ilas . and specia lized field . Fo r this reaso n the desig n o f stormwa ter sewers IS not conSidered . 'Infurmat .lo n o n sto lm ., ter sewers mav in thi s sec ti o n. Detailed w.l. ' -be ' fo und 111 Refs. [6- 5.6-7.6- 14. a nd 6- 16].
Com bin ed Sewers , co Ile'cted toge th er in combined .. sewers. . Do mes ti c II'asteW
Wastewa ter Co ll ec tion Once used for it s intended purposes. t he wa ter suppl y of a co mmunit y j s considered to be waste wa ter. The indi vidual pipes lIsed tn co llec t ,;nd transport \vas tewa ter are ca lled sel\"t'r s. and the network of sewers lI sed to co ll ec t was tewa ter from a commu nity is known as a co /lt'Cl iofl s r Sl e lli. The purpose of this secti o n is to de fin e the types of collec tio n sys tems th a t are used . the appurtenances lI sed in conjunc ti on with sewers. the tlnw in sewers. tli e
6-12 T YPES OF SE\VERS .I-he types and sizes . ., 11'1·· tl 1 sir C of th e collect inn system and of sewers li se d WI'1 1 VJI) . ' th\: I llc~ltlon of tlte w~ls t ew(\ter-treatme nt I'aCI'1"Illes. Th e prin. clp
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T ypica l 3,O O-mm VC p ipe st ub required a t ail connec tions to manhole
J Rubber gasket o r sealan t
200-mm minimum
T y pical VC stub wi th stopper fo r future' co nnection
Class B conc rete crad le support
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Ba se to be co nstructed on undisturbed mate rial o r co mpa cted screened gravel
Brick masonry o r -7JY4)7~~ cl,ass A co ncrete Cla ss A co ncrete
Se ctio n A - A Figure 6-7 T yp ical manh ole used ror reinrorced-concre le sewer p ipe, (From MeT calf & £ddv, Inc, [6 -8].)
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See note 3
353
6-13 COLLECTION SYSTEM APPURTENANCES
Reverse VC
150 mm
150 mm
The principal app urt enances of sa nitary sewers are manho les, drop inlets to manho les_ buildin g connec ti o ns, a nd junc ti o n cha mbers. D e pendin g. on loca l topography, sp ec ial stru ct ures ma y be req utred.
Manholes
cut curves to be cut and fitted to suit conditions at each manhole (refer to note 2)
Ma nholes are used to int e rco nn ect I\VO or more sewers (see Fig. 6-6) a nd to p rovide entry for scwe r c leanin g. Fo r sewers that are 1200 mm (4:::; in ) and sma \l er, manhol es shou ld be loca ted at changes in s ize. s lo pe. or direction . In large r sewe rs these changes ca n be made without us ing a manho le. :\ typ ica l manhole fo r reinforced-concrete pipe is shown in Fig. 6-7.
Class B concre te placed against undisturbed material or sheeting
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Drop Inlets to Manholes
I
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Shallow dro p _ _ typical
Wh ere th e difference in e leva ti on between the incoming a nd ou tgo in g sewer exceeds 0.5 m (1.5 ft), flow from incoming sewer ca n be dropped to th e eleva ti on of th e o utg o in g sewe r wi th :1 drop inlet s lIch as s how n in Fig. 6-8.
Dee p drop typi ca l
(a)
I
Notes :
I. Drop pipe to be same diameter as sewer discharging into manhole for up 10 and including including 300-mm size. 2. Deeper drop may be constructed with stra ight pipe betwee n wye branch and curve 3. Extend encasement to first joint beyond excavation for drop con nection. 4. Dimensions and cons tru ction of drop manh o le to be simi lar
except as shown.
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Pipe sizes, mm Inl et sewer,
2
Invert detail at sid e drops
(b)
Drop pipe ,
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D2
300 250 200 150 125
250 200 200150 125
Notes: J. Refer to table above for sizes of drop pipes to be used with inlet sewers. 2. Dimensions and construct ion of drop manhole to be simila r 10 typical manhole excep t as shown.
Figure 6-8 Typical drop inlets for vitrified clay pipe used in collect io n sys tems: (0) o ut side drop, ih) inside drop for sewer 600 mm and smaller. (From M el<'l1 li L~ i:"dd... Ill c. ["-81.) 352
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Pipe size table
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pip" No te: I SO-min pipe nJily be Vc. AC. DI . or PVC
Main sewer size v;]ries 200- mm VC shown
S t31flless-s leel bands. with takeup screws
plas ti c body DeLlil i\ - Flexible coupling: Ita scale
354 WAT ER
ENVIRONI>1ENTAL ENGINEERING HYDRAULICS DESIGN
Building Connections The sewers leading from individual hou ses or buildings to the municip a l collcction sys tem are known as building co nnecti o ns. A typical h o use co nnection is s ho wn in Fig. 6-9.
355
where V = veloc it y, m/ s 11 = fri c tion factor R = hydraulic radius cro ss-sectional area of Aow, m 2 wetted perimeter, m
Junction Chambers
S = slope of energy grade line, m / m
As the diameter of intersecting sewers (e.g. ,. main and trunk sewers) continu es to increase as wastewater from more of the service area is collected , precast manh o les can no lo nger be used. Wh e n thi s situation occurs, special juncti o n chambers a re constructed to connect the intersecting sewers.
6-14 BASIC CONSIDERA nONS IN THE DESIGN OF SEWERS In planning and des igning sanitary sewers the following fa cto rs must be co nsid ered separately fo r ea ch install a tion: I. Estimation o·f wastewater des ign Aow rates 2. Selection of design parameters G . H ydraulic design equation b. Alternative sewer pipe materials c. Minimum sizes d. Minimum and maximum velocities 3. Selection of appropriate sewer appurtenances 4. Evaluation ·ofalternatlve· aiigI1nients ············
The recommended n value for the des ign of new and existing well-constructed sewers is 0.013. An 11 value of 0.0 ISis recommended for the analysis of older sewers. The graphs presented in Figs. 6-10 and 6-11 have been prepared to simplify the use of the Manning equation in the design of sewers. Also because many sewers do not Aow full, the relationship between hydraulic elements for flow at full depth and at other depths in circular sewers is illustrated in Fig ..6-12. Developed using the Manning equation , Fig. 6-12 is used to obtain the·values of V. Q, A, R. and n at a given depth ratio based on the corresponding values of VJ , QJ' A J' R J' and nJ when the pipe is flowing full.
Sewer Pipe Materials and Sizes The principal materials used in the manufacture of sewer pipe are asbestos cement, ductile iron, reinforced concrete, prestressed concrete, polyvinyl chloride, and vitrified clay. Information on the sizes of pipes made with these materials is presented in Table 6-5. Minimum sewer sizes are usually specified in local building codes. The smallest sewer used should be larger than the building sewer connections so that ...o.bj~q~ .P.a.~?t;q .thrQ.l,Ighth.e building sewer will not clog the municipal sewer. Building sewer connections vary in size from 100 to I SO mm (4 to 6 in). The minimum size recommended for gravity sewers is 200 mm (8 in), although ISO-mm (6-in) co nnections have been used in some communities.
S. Evaluation of the use of c urv ed sewers
Design Flow Rates
Minimum and Maximum Velocities
The total wastewater flow in sanitar y sewers is made up o f three co mp o nents : (I) res idential, commercial, and instituti o nal wastewater, (2) indu strial wastew a ter. and (3) infiltration . SanitilfY sewers are designed for the following fl ows (Ref.
[6-8J) I. Peak flow s from residential. co mm ercial, ins tituti o nal, and indu strial so urces for the entire service area 2. Peak infiltration allowance for th e entire ser vice area Hydraulic design equation Curre ntly, th e Mannin g equati o n is used mos t co mmonly for th e d es ign o f sanitar y sc wers. The M a nnin g equati o n is
V =
~ 11
R2 tJ S I / 1
(6- 5)
When the vel o city of flow in a sewer is low, there is a tendency for the solids present in wastewater to settle out. Because the deposited solids may accumulate and ultimately block the flow. sufficient velocity should be developed on a regular basis to flush out any deposited solids. Based on past experience, current practice is to design sa nitar y sewers with appropriate slopes to maintain a minimum flow velocit y of 0.6 m / s (-2.0 ft/ s) when the sewer is Aowing full or half full. To prevent tbe deposit Ion of sand and gravel a· velocity of 0.75 m /s (2.5 ft/ s) is recommended. To a void damaging sewers it is recommended that the maximum flow velocities be limited to v;t!ues equal"to or less than 30 m / s (10 ft/ s).
Minimum Slopes Minimum s lopes are o ften used to a vo id extensive excavation where the slope of th e gr o und surface is Aat. In gen eral. minimum s lopes based on Manning's equation
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358
ENVIRONMENTAL ENGINEERING HYDRAULICS DESIGN
WATER
Table 6-5 Ayailable size ranges and descriptions of commonly used pipe for grayity-
n
Values o f -
flow st'wers
nruu 1.1
1.2
1.3
- - - fI
0.8
14 Type of pipe
\'\, I 1
constan t
- - - Ind ependent of 1/
I
Avaitabte size range, mm (in)
100-900 (4-36)
Weighs less than other commonly rigid pipes. May be su sceplible to acid corrosion and hydrogen sulfide atlack, but if properly cured with steam at high pressure (autoclave process), may be used even in env ironments with moderately aggressive waters or soi ls with high-sulfate content.
Ductile iron (0 1)
100-t350 (4-54)
Often used for river crossings and where Ihe pipe must support unusua·lly high loads, where an unusually lea kproof sewe r is required, or where unusual root problems are likely to .develop. Ductite-iron pipes are susceptible to acid corrosion and hydrogen sulfide a!lack, and therefore should not be used where the groundwater is brackish, unless suitabte protective measures are taken.
Reinforced concrele IRC)
300-3600
Readit y available in most localities. Susceptible to co rr os ion of interior if th e atmosphere over wastewate r contains hydrogen sutfide, or from outside if buried in an acid of high-sulfate environment.
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Preslressed conc re te ( PC)
400 - 3600 ( 16- 144)
Espec ially suited to tong transmission mains without building connections and where precautions against lea kage are required. Susceptibility to corrosion (the s;tme as reinforced concrete).
P,)lyvinyl chloride (PYC)
100-375
A plastic pipe used for sewers as an alternative to .. asbestos-=ent·aMd 'lfitf>liecl-day pipe, Lightweight ............ . but slrong. Highly resistant to corrosion.
1.4
R R'ulJ
(4- t5)
Figure 6-12 H yd rauli c elements fnr circular sewers.
Yilrified c lay (VC)
have proved to be adequate for sma ll-diameter sewers. As the pipe sizes increase beyond 600 mm (24 in) the minimum practicable slop.e for construction is about 0.0008 Ill/ m. In wa rm areas. hydrogen sulfid e will often develop as wastewater is tran sported In sewers laid at minimum slopes. The deve lopment o f hydrogen su lfide can (I) cause odor problems, (2) lead to th e deterioration of materials co ntaining cement. and (3) result in the precipitation. as sulfides. of trace metals needed for proper bacterial growth in biological trea tment systems. A more complete analysis of hydrogen-s lilfide problem s In sewers ma y be found in Ref. [6-8].
Description
Asbeslos cement (A C)
/ A-. Hydraulic
.",
359
100- 900 (4- 36)
For many years the most widety used pipe for gravity sewe rs; st ill widely used in small and medium sizes. Res istant to corrosio n by both acids and alkalies. Not susce ptible to damage from hydrogen sulfide, but is britlle and susceptibte to breakage.
• .)Olirce: From Metcalf & Eddy, Inc . [6·8J
Another reason for not installing curved sewers is that the lise of laser-type surveying equipment for maintaining grade during construction is not feasible. Sewer Ventilation
Usc of Cuned Sewers Although not used in thc past , cu rvcd selVers have pl'ovcd to he satisf;lc tor y as lon g as the curva ture is nut severe. Before usin g cur ved sewers. the compa tibilit y of I he clean in g eq uipm ent t\l be used for sewer ma intenan cc Illust be assessed.
Ventilation in sewers is needed to avoid (1) t he danger of asphyxiation of sewer maintenance employees, (2) the buildup of odorous gases. and (3) the development of ex plos ive mixtures of sewe r gases, principally methane and oxygen. Design ConSiderations for the ventilation of sewers are discussed in detail in Ref. [6-8].
360
WATER
Sin gle- family hom es A-I (200 hal
6-15 DESIGN OF SANITARY SEWERS
4th St.
The design of sanitary se\VersinvQlves fieldwork, the preparation of maps and profiles, and detailed design computations. Each of these topics is considered briefly below. The detailed design of san itary sewers is illustrated in Example 6-4.
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.
~
7th St
Singlefamily
Industrial"-.........
III
I
A-7
"- f"--
19.0
( 300 hal
(,
"
cO.O m
hal
------ ---
homes A-6 (200 hal
6
Example 6-4: Designing a gravity-flow s3nit3r~' sewer 'Design a gravity-flow trunk san itary ~e wer for the area shown in the accompanying ligure ((II. The trunk sewer is to be laid along Peach Avenue starting at 4th Street and ending at I I th Street. Asslime that the following design criteria have been developed based on an analy sis of local co ndition s and codes.
-
6th St
Singlefa m il y
While tlie fieldwork is going on, work on the preparation of maps and profiles should proceed simultaneously. Thus. if any information is found to be missing it can be collected before the fieldwork is completed. Maps on a scale of 25 m' to ' 10 mm (200 ft to 1 in) are acceptable for most purposes. Where additional detail is needed a scale of about 5 m or less to 10 mm (40 ft or less to 1 in) is often used. In preparing design profiles, street centerline elevations are shown at least every 15 m (50 ft) and at all locations where the surface stope changes abruptly.
The detailed design of. sanitary sewers involves the se lection of appropriate pipe sizes and slopes to transport th e quantity of wastewater expected from the surrounding and upstream areas the next pipe in series, subject to t.he appropriate design constraints. The design procedure for sanitary sewers is illustrated in Example 6-4.
-
;:===============-=1-
.........
Preparatio.n of Maps and Profiles
Design Computations for Sanitary Sewers
A- S
___
A-4 (100 hQ)
........ .......
-
hOUSing
Low-ri se
apartment .................\
I
-
resid~nti31
~================~~_;::;=====::::::::.J ~
Fieldwork To·design sanitary sewers properly, accurate and detailed.maps should be available for the areas to be served. The location of streets, alleys, highways, railroads, public buildings and parks, streams, drainage ditches. and o ther features that may influence the design of the sewers should be identified. Accurate elevations are needed throughout the area to be served by the proposed sewer. Profiles are needed for all existing or proposed streets, alleys, and potential rights-of-way where sewers may be placed. In addition, detailed information must be available on the location of surface and subsurface utilities such as water and gas mains, electrical conduits. drain lines, and other underground structures. In addition to the above information, soils data should also be available. Soils borings should be made to a depth of at least 1.5 m (5 ft) below the bottom of the sewer trench.
r
'">
<:
To tltatment plant
III
Used to indica!c location or line to \',:hicll w~sk\\'att.:.'r· from con t ributing Jrea IS di sch~ r!:!t'd
\0) 3tl l
ENV IRONMENTAL ENG INEERIN G HYDRAULICS DESIGN
362 WATER
b. F or commercia l, industrial , and institutional areas, also obtain the peak infiltrati o n va lues from the figure (b) . However, to take into account that the total length
I. For design period use the saturat ion peri od (time required to reach sa turation po ulatlon). p
o f sewe rs in' these areas will generally be less than that in residential areas, use only 50 percent o f the actua l a rea to co mpute the infiltration allowance, 7. for inflow allowance assume that the steady-flow inflow is accounted for in the infiltration allowance. 8. Peaking fac tors:. o. R esidentia l- u se the curve given in the accompanying figure (e) . b. Co mmerc ial - 1.8 c. Indu strial -2. l d. In stituti ona l (school) - 4.0
2. For population densities use the data gj\'en in the table.
Saturat ion
Zoni ng
T ype or development
Reside nt ial Residential Resident ial Residential
Single-family dwellings Duplexes Low-ri se apartments Mixed ho usi ng
populat ion density. person! ha
Wastewater flow , L/capi t" d
40 60 120
380 300 220
70
250
363
(;
4
~ 00
3. For residential wastewater tl ows use the data given in the table . 4. For commercial and industrial flow s (average): a. Co mme rcial - 20 m 3 : ha d. b, Indu s trial - 30 mJ/ ha . d. 5. F o r inst ituti onal fl ows (ave rage): College - 400 nl" / d (5330 stud ents x 7S L/student . d),(IOOO L /m J) 6. F o r infiltration a ll owance: (/. For residential a l·eas. o btain the peak infiltration va lu es from the accompany in g figure (b).
50
'"
40
E
30
~
m3 /s, 2.0
I
0.004
0.01
0.5 0.1 0.05 Avecage wastewater flow (excl uding infiltration l inflow) , m 3/s
5
(c)
9. for th e hydraulic des ign equatio n u se the Manning equation with a n n value-ofO.Ol3.
pe rmi ssib le for this situation. 11. Minimum ve loci ty: To prevent the depositi on of so lids at low wastewater flow s, use a minimum velocity of 0.75 m ls (2. 5 ft /s ) during the peak flow conditions. 12. Minimum cove r (minimum depth of cover ove r the to p of the sewer): As established by th e local community building code, the minimum depth of cover is 2.0 m. Old sewe rs
20
SOLUTIO N
5,000 ha. 10 .0 mJ/ ha' d
I. Layo ut the trunk sewer. Draw a line to represent the proposed sewer [see figure (a)]. (!l changes in direction. (2) changes in slope, (3) pipe junctions. (4) upper ends o f sewe rs. and (5) int erva ls from 90 to 120 m o r less. Identify each manhole with a number [see figure (a)]. For the purpose o f thi s example only the manh o les at the maj o r junctions have been numbered. In an actual design, intermediate manholes would be loca ted and numbered. 3. Pre pare a sewer des ign comp utati o n table. Based on the experi ence of numerous engi neers. it has been found that the best approach for carrying out sewe r compu tati ons is to use a co mput a ti on table. The necessary computations fo r the sani ta ry sewer s hown in figure (II) are presented in the accompanying ta ble. Although the table is, for the most part. scl f-explanaw ry. the following comments arc presented to clarify it s
2. Locate and number th e manh o les. Loca te ma nhol es at
10
5
5
0...
T o si mplify the co mputation s, use Fig. 6-10. 10. Minimum pipe s ize:The loca l building code spec ifies 200 mm (8 in) as the smallest pipe
100
'0
c
""~
N~w sewers include existing sewers having pipe joints sea led With com press io n gaskets o f eiastomeric mat eri a ls.
5 ,000 ha. 3.3 mJ/ ha ' d ';:O:-----L--L--1--1L.l-LliL---L.-'---"--:::I~1J.....ILILLI:-:c---L_..L-L I -->
500
Servic,' area. ha (b )
1.000
5,000
deve lopment.
...'"
0-
Sewer computation table Locat ion
Residential fl ows
Lengt h of
( I)
Fmni
perso ns
m' j d
m' j d
Peaking factor
( II x 12)
(8)
(9)
( 10)
(II)
(12)
( 13)
3,040 3.040 3,040 7.415 10.055 14.615 17.655 17.655 20.695 22,445 3,800t 26,245
2.9 2.9 2.9
8,8 16 8.8 16 8.8 16 20,021 26,143 37,999 44.13 8 44.13 8 51,738 56.113 11.020 65 ,613
Subarea'
Area. ha
(3)
(4)
(5)
(6)
:(7)
A-I A-2 A- 3 A-5 A-4 A-7 A-6 A-8 A-9 A-I O A-II 707
200
: 40
8. 000
380
3040
250 100 300 200
: 70 : 120 40 ' 40
17.500 12.000 12.000 8.000
250 220 3S0 380
4375 2640 4560 3040
40 70 40
~.OOO
3S0 250 380
3040 1750 3800
, (2)
707
4
1414
,~
5
5.
(,
707 707
6 7
Cumulative average flo w.
m
707
.j
Flow increment,
To
sewer,
Line
(,
8 9
Sewer computation table
707 707 707
200 100 250 A-12
Populati on increment,
7.000 10.000
m.l 'd
Peaking faclol
Cumulative peak Aow. m' id (16 x 17)
L'lle
Fr\.)!l1
To
m
Subarea'
ha
;\ \'erage unit now. m.\ ." ha d
(II
12)
dl
(4)
( 5)
( 14)
(15)
(10)
(17)
( 18)
50
20
1.8 1.0 1.8 1.8
100
20
1000 1000 1000 1000 1000 3000 3000 3000
1.8
1800 ISOO 1800 1800 1800 5400 5400 5400
3000
1.8
5400
of Area.
.
A-J A-2 A- 3 A- 5
~
6
'~ ~
'J'
4
1~14
h
707 707
9
707 70i 707
~
i\
2.5 2.5 2.5
2.9 2.5
Indu str ial 110ws
Commc:rci,1i now~
Lcngl h
707 707
2.7
2.6 2.6 2.5
m 3jd
(Conlinued)
Loca ti on
~e\\'er
Cumulative pea k flow.
Average unit flow. Ljca pita . d
: Population ~density. : persons jha
A-~
A-7 A-6 A-S .'\-9 A-IO A-II A-12
CUITlUlatl\C
average
!low.
1.8
1.8 1.8
Cumulative peak flow,
Area. ha
A \'Crage unit flow, m' /ha d
Cumulative average flow, mJ jd
Peaking factor
(21 x 22)
(19)
(20)
(21 )
(22)
(23)
200
30
6000
2.1
m 3 jd
12,600
'.-J
""'"
Sewer computation table Lnc(
(Col1linued)
(ion
In stitutional Aows
Len gth
average tlo\v,
sev.:cr, Lin t.:
To
Frll!l1
In
Suharca*
. m" ,<1
--_._--------------III
pI
(::'1
Cumulative
Cumulative
or
(5)
( ::'4 1
Cumulat ive subtotal,
Cum ulative peak Aow.
average
factor
m·1 /d (24 x ::'5)
( II + 16 + 21 + 24)
(::'5)
(26}
Peaking
flow: m.\ /d
Inti lna tion
Cumulati ve peak Aow. m'\ /d (13+18+ 2:l + 26)
Cumulative Area. ha
200::: 50t
25+ 4 4
4
250 100 300 200 50::: 200 100 250 100:::
14 14
707 707
11 X
9
707 707 707
Sewer computation table
Cumu lative Peak unit infiltration Intiltrati on allowance, allowance m' /d m' /ha . u (30 x 31)
area,
ha
200 250 275 525 625 92-5 1125 11 75 1375 1475t 250t 1825
R.O 7.5 7.5 7.0 6.5 5.5 5.0 5.0 4.9 4.8 8.0 4.0
(Conril1ued)
Loca tion
Sewer layout
Sewer design
De sign flows:
Ground surface elevation Lengt h of Line
From
To
sewer, m
(I)
(2)
(3)
(4)
2
I 2
4
6
707 707
4
1414
6
707 707
4
6
8
7 9
707 707 707
1600 1875 2063 3675 4063 5088 5625 5875 6738 7080 2000 7300
Cumulati ve peak Aow, m' /d
+
(33)
(34) :
A·I
10,416 12,291 14,279 27,096 33,6'06 46,487 53,163 57.013 65.476 70.193 13,020 92.513
0.121: 0.142: 0.165: 0.314: 0.389: 0.538: 061)
A·II A·12
At upper manhole
At lower manhole
Upper end
Lower end
(36)
(37)
(38)
(39)
(40)
(41)
(42)
450
0.0018
0.121
0.75
20.00
19.00
17.50
16.23
750
0.0009
0.330
0.75
19.00
18.33
15.93
15.29
900 1050
0 .0009
0.540 0.770
0.85 0 .87
18.33 17.40
17.40 17.00
15.14 13 .71
U.86 13.14
0.770 0.820 0.165 1.100
0.87 0 .95 0.75 0.98
17.00 16.50 16.20 16.00
16.50 16.00 16.00 15.00
13.14 12.58 12.46 11.79
12.58 11.94 13.46 11.22
(35)
(5)
A·IO
Velocity when full, m/s
Sewer diameter, mm
(28
A·2 A·3 A·5 A·4 A·7 A·6 A·8 A·q
32)
Sl ope, m/m
Capacity when full, m 3 /s
Cumulative peak Mow ,§ m' /s:
Subarea •
Sewer pipe invert elevation
00008~
066~ 0 758: 0.81Z 0.151: 1.071;
1050 1050
525 1200
00008~:
0.0009 0.0014 00008~
• See figure (0). t Line '/ receives flow from subarea A·ll only. t 50 percent of area (see assumption 6b). ~ mJ /s = (m' /d) /(86.400 sid). ~ The minimum practical slope for construction is about 0.0008 m/m. '.-J
"'....."
(
ENV tRONMENTA L ENG INEERt NG HYDRAU LI CS DESIGN 369
368 WATER O.
The entries in columns I through 5 a re used to identify th e sewer lin es under co nsideration a nd to suinmarize the basic physical data from figure (a) .
b. The entries in co lumns 6 through 13 are used to obt ain the cumu la ti ve peak domesiic flow (column 13). The area (co lumn 6) is obtained from figure (a). The pop ulati on density (column 7) and the unit fl ow da ta (co lumn 9) were given. Peaking faC!ors , obtai ned from figure (e) , are en tered in co lumn 12. L The commerc.ial area , the co rres ponding unit fl ow. and the cumul a ti ve ave ra lle fl ows are·entered ·in co lumns 14, 15; and 16. respecti ve ly. T he give n peaking fact~r fo r the commercial area is en tered in co lu mn 17. and the comp ut ed cumul ative peak com mercial.flows·a re ent ered in .co lum n 18. d Th e en tr ies in col umn s 19 'thro ugh 23 for the industrial flows a rc th e same as desc ribed for the commerc ial !low.s (co lumn s 14 through 18) e. The instituti o nal fl ows are e nt ered in co lumn s 24 thro ugh 26. I The cumula tive a ve rage and peak tl ows are summari zed in co lu mn s 27 and 28, res pectivel y . .If. The infiltration allowance (columns 29 "thr ough 3~) is dete rmin cd usin g th e curvc for new sewe rs in fig ure (b). Ii. The total cum ul at ive peak desig n fl ow (col umn 33) is obtai ned by su mming col umn s 28 and 32. i . Sewer i:l esig n informati o n is summarized in co lumns 3S through 38. The required pipe sizes are estimated usi ng Manni.ng·s eq ua ti on with an IJ va lue of 0.0. 13 (see Fi g. 6-10) . The capacit y of the se lec ted pipe and th e ve l,)('it\ whe n rull are tabulateci in columns 37 and 38. In a ll cases th e velocity shou ld exc~eel 0.75 m/s (2.5 ft / s). j. The necessa ry la yo ut data fo r th e sewe r (co lumns :W through 42) are ob tain ed as foll ows: The gro und surfa ce eleva tio ns at th e manhol e loca t ion s en tered in colu mn s 39 a nd 40 are ob tained by int erpo latio n with th e eiev;lIi on dat a given in fig ure (a) . The sewe r inve rt eleva tions shown in col umn s 4 1 and 42 are obtained by tria l and erro r with a sewe r profile work shee t. Th e first step in preparing ~ wor k sheet is to plot the gro und-surfa ce elevation s give n in co lum ns YJ and 40. working bac kwards fr om a co nvenient point. After th e gro und -surface' r~~'filc is 'Ct ;';I',;,;): the ne'x'i 'ste'p" is to beg in sketching the in vcrt and crown (in side holtom
ground surface - depth of cover - pipe wall Ih ickncss - pire """neler 20.00 m
2.00 m
0.05111
0.45 m
The p ipe thi ckn ess wi ll va ry with the type 01' sewer For this exa mple. 0.05 III will be used fo r all pipe s izes. Th e lower elev ati on is co nlpuied.tw ,>ub tra ctlng the fall as follows: Lowe r in vert el c"~11 ion
='
Uppe r i 11\Trt devatioll
Slope of sewer
A
Length of sew~r
For line I : Lower invcr! elevation
17.50
III -
(00018 mfm)(707 m) = 16.23 m
[I' the depth or cover (rcmember to allow for th e pipe wa ll thickness above th e crown) t'or any section ha s become toO shall o\\' , repea t the process 'with a lower initial in vert elevation ,lr a steepe r slope for th at sec ti on. When a manhole IS loca ted at a sewer junction. the outlet sewe r elevation is ftxcd by the lowes t inlet scwer. If rhe pI pe size in creases, th e crowns of the two pipes Illu st be matched at t he manh ole. Thi s is done to avoid th e back ing up of wastewa ter into the ,ma iler pi pe. An example of this si tua tion is th e increase in size from 450 to 750 I11Ill atmanho\e~. For th is case. the calc ul ations are as foll ows: Lowe r invert ciev;lIion of the 450- ml11 sewer is 16.23 m. Uppcr in\'crt cleva t ion fo r the 750-IllIll-sewe r (l ine 2) is 15.93 III (16.23 m + 0.45 III 0.75 Ill) Lower l\wer t elev~lti (> n for th e 7S0- mlll sewer is 1 5.~9 III [ 15.93 m - (0.0009 m/ m) x (707 m)j. Th ese procedures ;\re rcpc," ed until the elevation,> fo r the en tire sewe r are established. CIJ~ t MI' NT A com putation :able. suc h as the o ne show n in thi s examp le. lIot only saves time but also is usl!ful ror su mmarizing bot h the data and the com put ed resu lts in an orderlv sequence I'or ,ubscLJue nl usc . Th e specific col um ns in a given co mput a tion ta ble depend on th e factors that must be consi dered In arriv in g at the peak deSIgn tl ows. Most sanItary and civil engineering con"Jillng firms have develuped tabulation forms of their .. .. .()wfl·for ·seWCF ·dch>gn .wmpulaLions .. Alt.hQ ug,h .th.e .fq pl.l.~.rn.a)l.eliff~r. in specific details and in t he order uf pres~ntat ion rrom thIs ta ble. t he same informa ti on is u s'~a 'lI'y prese nted . Some cng in en illg firlll ' have developed com put er program s ro r se wer des ign.
6-16 PREPARATI ON OF CONTRACT DRAW I NGS AND SPEC I FICATI ONS Once the sewer des ign computa ti o ll s have been comp lcted. a lt erna ti ve a li gnments ha ve been examined. and the mos t cos t- and energy-effecti ve ali gnment been select ed. th e next step is to prcpare COlltrac t drawings and specificati ons. Deta iled co nt ract drawings, In cluding plans and profiles for each scwer. 'a nd speci ficati o ns mll st be prepared before bid s can be obtall1el! to bude! the projec t. A typica l pran and profile of a segment uf a sewer· line is show n ."1 Fig. 67 13. Th e Importance of pre paring accura te and detailed draw ings and specificat ions canno t 'be overemphasized. Perh~lps the most com pellin g reaso n is tha t such careful preliminary work wi ll like ly ensure a successful pr oject with a minimum number of change orders and wi tho ut a lawsu it.
has
ENVIRONMENTAL ENGINEERING HYDRAUU
90'l5 13
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or ·,
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+
o
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~
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c? l
o
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o
+
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,
+
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There are many ways in which the actual constructionofasewerl!YS accomplished, dependin g on the soil conditions encountered and the, equipment available for the j o b. The important (hing is that the filn·.~. perfor m the function for which it is intended with a minimum of main!,! . ~~SII" To attain this end, three conditions should be met: (1) the pipe shoul\.!i}'1i_:_ . carefully, properly bedded , and backfilled in such a manner that there j ', or breakage during and after construction ; (2) joints.should be made care to eliminate excessive infiltration; and (3) the line and slope should be free of irregularities that might favor the accumulation of with resultant clogging of the pipe.
r ,'
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6-17 CONSTRUCTION OF SEWERS
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Sewer maintenance involves keeping sewers clear of obstructions an~ working order. Most stoppagesin sewers are caused by the accumulati. material. tree roots, accumulatI?n of grease, or collapse of the sewer:, ... llsually enter through the pIpe jomts. Good deSIgn and proper jomt c.~ IS the best preventIve measure .. Most CItIes have ordinances req~mT!gl;5i grease traps o n serVIce connectIOns where wastewater may contam la~~ of grease, Collapse of the pipe is unlikely if adequate cover is provided . able care is exercised to avoid breakage during and after constructio . Where flushin g is inadequate to remove an ·obstruction, sewer$. with specia l tools attached to cables or jointed rods and pushed or p ' the sewer rrom a manhole or o ther point or entry. The type of tool d _,., calise or the obstruction, Cutting tools are used to remove roots, scoo~, are lIsed to remo ve grit and sludge, and brushes are effectIve III rem ,' ., The use or a little copper sulfate in a sewer is often effective in killing r , damaging the tree. Occasionally, explosions may occur in se~ers, The most commo,~ ex plOS Ive gases are mflammable and volatile liqUIds III the wastewat <, or domestic gas from an adjacent main. Ga ses gIven off by the de or was tes are rarely the cause of explosions. However, many sewerwor kers have been asphyxiated in gas-filled sewers. In no case sho be permitted to enter a sewe r until proper tests ror the presence of da~ have been made. Whenever a worker enters a sewer, there. should ~~~ person at the surface who can give emergency aId If reqlIIfeci, [6-5, 6-4fJ ."
. 1
I
0
I
6-19 DESIGN OF STORMWATER SEWERS /'
~
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UI 'UO~II':!AJI]
Oll
E
E
6
~
:170
c:
c:
E
E
6
V )
"
6
-' '- '
>.,
000 + 0 l' IS 0+
N HW
°
,
0
--::- be-
c
S
:;;
6-18 MAINTENANCE OF SEWERS
The design procedure ror stormwater sewers is essentially the same ,~~,~ lor the deSign or sa llltary sewers. The major dIfference IS that thequant~~ " Water to be removed rrom a service area is determined on the baSIS ofa:tJ.~:
/
372
ENVIRONMENTAL ENGINEERING HYDRA U LI CS DESIGN
WATER
373
Radial now Mi xed n ow
analysis. Details on the analysis and design of stormwater sewers may be found in Refs. [6-5, 6-7,6-14, and 6-16].
Axial now
Water and Wastewater Pumping Single slage Mullis tage
Some form of pumping is used in' most water supply and wastewater collection systems. As noted earlier, in water supply systems pumps are required to deliv!!r water from wells. to lift water to distribution reservoirs and elevated tanks. and to increase pressure in distribution systems. In wastewater systems. pumps are used to avoid deep excavations. to convey wastewater over hills and other terrain where gravity sewers cannot be used. 'and at treatment plants to provide sufficient head for plant operation. The movement of water and wastewater from one location to another is the most common application of pumps ill both types of systems. Because pumping is so important in the operation of water supply and collection systems, it is the purpose of this section (I) to examine the types of pumps and pump drivers that are commonly used. (2) to review pump application terminology. (3) to review pump characteristics and their applications. (4) to review the analysis of pump systems and the selection of pumps. and (5) to review the design of water and wastewater pump stations.
J e t (ej ec tor boosted) '. Cas lift H ydrau li c ram Electromagnetic Piston Plun ger
Diaphragm
Single rotor
Multipl e rotor
6-20 PUMPS
Figure 6-14 PrinCipal types (If pumps. (Adaptcd (rom Hvdraulic Institllte Standards [6·3].)
The types of pumps used most commonly in the water and wastewater systems are described in this section. In general. pumps may be classifi ed according to their (1) principle of operation. (2) field of application (i.e .. liquid s handled). (3) opera.. .... .. tlonar'cfLliy' ({.e.·: capaci't'y): (4) type of construction. and (5) method of drive. With respect to the principle of operation, pumps may be classified as kinetic-energy pumps or positive-displacement pumps. The term lurbo machine is' also used to describe kinetic-energy pumps. The principal types of pumps included under these two classifications are shown in Fig. 6-14.
fieaci"anc(
Kinetic-Energy Pumps The principal subclassification of kinetic-energy pumps is centrifllgal. which, in turn . is divided further into three groups:
J. Radial-flow pumps 2. Mixed-flow pumps 3. Axial-flow pumps . The above classifications are derived from the manner in which th e fluid is displaced as it moves through the pump. Thus. the fluid is displaced radially in a radial-flow pump. axially in an axial-flow pump. and both radially and axially in a mixedtlnw pump.
The principal components of kinetic-energy pumps are: L The rotating element called the impeller that imparts energy to the liquid being pumped. 2. The shaft on which th e impeller IS mounted. 3. The pump casing that includ es th e inlet and outlet passages for leading the .liquid being pumped int o and ou t o f the pump. and the recuperatmg sectIOn \':hid1 receives th'e liquid discharged from the impeller and directs It to the outlet passage. The function of the recuperating section is to convert a portion of the kinetic energy of th e fluid into pressure energy. T ypica lly thiS IS accomplished by means of a vo lute or a set of diffusion vanes. In a volute casmg,-the size of the channel surrou ndin g the impeller increa ses gradually to the size of the pump discharge nozzle, and most of the conversion of veloeity to pressure occurs in the conical discharge nozzle. In a difl'usion casing, the impeller discharges mto a channel provided with guide vanes. The cOliversion of velocny to pressure occurs within the va ne ]1
374
ENV IRO NMENTAL ENGINEERIN G H YD RA ULICS DESIGN
WATE R
for untreated was tewater are usua ll y the s ingle-end suction vo lut e type. fitt ed with nonclog impellers. Nonclog pumps have open passages and a minimum number of va nes (no t exceeding tw o in th e smaller s izes and limited to three. or at thc m os t fou r. in th e la rger sizes). Wastewater pumps mu st be ab le to pass so lid s that en ter the co llccti o n sys tem. Because a 70-mm (2 .S-in) -diameter so lid ca n p ass thr o ugh m os t d o mesti c to ilets, it is co mm o n practice to req uire that pumps be able to discharge a 7S-mm (3-in) so lid . M os t IOO-mm(4-in) pumps -- Le .. pumps w ith a IOO-mm (4-in) di sc harge openll1g -:- no rmall y s ho uld be abl e to pa ss 7S-m m (3 ~ in) · -diame t er so lid s, and 200-mm (8-in) pumps should be able to pass IOO-lllm (4-in) -diam e ter so lid s, etc. N o nc log pumps s maller than 100 mm (4 in) s ho uld not be used in municipa l pumping sta ti o ns fo r hand lin g untrea ted was te wa ter.
375
Positive-Displacement Pumps Posi ti ve-displacemcnt pumps are usuall y divid ed in to tw o major categories: reciprocatin g (piston o r diaphragm) pumps and rotary pumps. Pneumatic ejectors and th e Archimedean screw pump are also included under this category. Pi ston-t ype reciprocating pumps utilize a reciprocating piston or plunger ill a cyli nd er to draw a fluid in on the suction side and to discharge it under pressure on the di sc harge side. In a diaphragm pump, the reciprocating element is a flexible diaphragm. In both of these pumps check valves a re used to control the pump sucti o n and discharge. In rotary pos itive-disp lacement pumps, the essential working element is. a rotor thaI may have th e fo rm o f an impeller. vane, lobe, or any o ther suitable configura tion . The principal types o f rotary positive-displacement pumps are (I) eccentric ro tor sc rew (progressive cav ity), (2) gear, (3) lobe, (4) peristaltic, (S) pi sto n, (6) sc rew, (7) vane, a nd (8) flexible vane. Pneumatic ejectors are o ften used for raising wastewater from building sumps. The ejector consists of an airtigh t tank into which wastewater flows by gravity and out o f which the wastewater is forced automatically whenever sufficient wastewater has accumulated to raise a float and ope n the compressed a ir-inlet valve. The screw pump is based on the Archimedean sc rew principle in which a revo lvi ng sha ft fitted with one, two, o r three heli ca l blades rotates in an inclined tr ough and pushes the wastewater up the tr o ugh (see Fig. 6-IS). Screw pumps are com mo nl y used in wastewater-treatment plant s to pump untreated wastewater and return was te activated s lud ge.
6-21 PUMP DRIVE UNITS Th e most com mo nl y used drives for pumps are direct-connected electric motors (see Fig. 6-16). Constant-speed electric motors coupled to var iab le-s peed devices are a lso used extensive ly: Interna l-comb ustion engines and turbines are ofien In stall ed to ensure that th e pumps ca n operate during electric-power ou tages or whe re was tewater gas or other gas is available fo r fuel.
Electric M otors - Direct Connected Electric direc t-conn ec ted motors may be constant -. Illulti- or variable-speed. Each is d esc ribed below.
Figure 6- 15 T y pical screw pump lI se d
10
pump wa stc\\ater.
C011SIMIl-speed pumps may be driven by squirrel-cage induction motors, wo und-I' o ln r indu ction motors, o r sy nchronous motors. Squirrel-cage inductio'n motors ;.II1d synchro nous m o tors opera te at a co ns tant speed, but wound-rotor indu cti o n motors can operate at diffe rent speed s b y va ry ing the res istance of the rot or or seco ndary ci rc uit. Squirrel-cage motors will normally be selected for constan t- speed pUlllpS because o r th e ir s impli c it y. reliabilit y, and economy.
376
E" \ ' IR O"~ I E,, T A L EN G IN EER ING HYDRA U LI CS DESIGN 377
WATER
Table 6-6 Approximate operating speeds of co nstant-speed motors on 60-cycle a lt erna tin g currcnt M ow r sreed. ri mi n P n1c~
Sy nchro no u s
I nd uct io n
I KIJu
3500 ,550 17'fI 1770 I I ) 11 117(1
1200
~7i1
7::'0
h<)()
I" 14 16
600 51 4 450
IS
400
390
20 22
3)0
.'SO
3~7
'I~
24
:;00 277 257
c'J() cr.X
26
2B Figur~ 6-16 E x" mples
or elec lric mOlors used 10 drive pumps
Muiliple-:ipeed operati o n ca n be o bt a ineci with squ irre l-cage o r wo un d-ro tor motors. For squirrel-cage mo tors. th e choice of speeds is res tri cted to two o r more of the speeds listed in Table 6-6. If the lowe r speed of a two-s peed mo to r is one- half the hig her speed, a single- o r two-wind in g mo to r can be used. If th e lower speed is not one-ha lf. a two-speed mo tor wi th·t \.V 0 willdrngs ·is · reqll ired: WlieiY ·otie(a fih-g ..• . a pump at two co nstant speeds. th e ad va ntage of the squ irrel cage (or synch ronous motor) is that the motor operates at maximum e tficieney at both speeds. However. the wound-rotor motor 0perates at maximum efficiency o nly at full speed· If the operating c.oqci it.io ns vary in pumping sta ti o ns. (,CltiL/h le-speed Qperation of. the pumps may be desirable. Variable-speed (stepless ) mo to r uperatiun has been poss ible. for man y years us in g liquid res isto rs with wo und-rot o r mot or controls. With tile develo pment of solid elec tro nic controls
Electric Constant-Speed Motors Coupled to Variable-Speed Dnices Worldwide: th e most co mnlOn wa y to o btain vari a ble-spced pUIllP ope ra ti on is to li se ,I co nstan t-s peed elec tri c motor co upled to a I':lr i'lble-s pced del·icc. Variab le. spceu devices. in se rted between the motor and th e PUIllP, Ill ay he l·lassined as rneclElni ca l. magnetic. and Iluici .
90' 705
900 10
58~
,Oil -4:; ~
24'}
--_._-T he mus t Ul l11l11U II Ill cc: il
Internal-Combustion Engines and Gas Turbines In large pumpin g sta tion s, inl crnal- l·o mbu stl o n e ng ines a re used as a source o f standby power for clril in g Ih e pumps ancith e crit ica l elect rica l con tr o ls If th e powe r fails . Int ernal-co mbu sti on cl1llin cs us uall y clri \·e ge ncrators so that powe r IS ava ilable not o nl y for the pump b'ut a lso for the
378
ENVIRONMENTAL ENGINEERING HYDRAULICS DESIGN
WATER
S. I'vlal1oll1!!crie discharge head (U md) is the discharge gage reading (expressed III meters) measli red at the disc harge nozzle of pump referenced to the centerIlIIe of the pump impe ll er. 6. tv/allometric (Hm) is the ' increase6f presstirehead (expressecl in meters) ge nerated by the pump (U ms + H md)' . . 7. Fric ~ionhcad (h f s' hfd) is the head of water that must be supplied to overcome the fnclIonal loss caused by the flow of fluid through the pipe system. The frictIOnal head loss In the suction (h f ;) and discharge (h f ,,) piping system may be comp uted WIth the Hazen - Wllh,ims or Darcy - Weisbach equations, 8. Velocity head is the kinetic energy contained in the liquid being pumped at . any POlI1t III the system and is given by
and a separate so urce of alternate fuel, such as natural or propane gas, to prov ide power when the s ludge gas is not avai lab le. Gas turbines have been used a s high-speed driv es for pumps. especially in large-capacity m ob ile pumping unit s. In larger s izes. gas turbin es are competitive wit Ii steam turbines.
Fluid-Driven Pumps Fluid-driven pumps are a lso bec o min g mo re common throughout th e world . The most common Auid drives are powered with compressed gas, pressurized water or oiL and steam .
V2 Velocity head = -
(6-6)
2g
6-22 PUMP APPLICATION TERMINOLOGY AND USAGE
where V = velocity of fluid, m/s (ft/s) 9 = acceleration due to gravity. 9.81 m/s 2 (32.2 ft / S2)
The purpose of this section is to present the bas ic terminolog y used to define pump performance and to con sider it s usage in the so lution of pump pro blems. Terminology to be considered in this discu ss ion includes (I) capacity. (2) head, (3) pump efficiency. and (4) power input to the pump. [6- 8J
9. tv! inorhead loss is the term applied to the head of water that must be supplied to overcome the loss o f head through fittings and valves. Minor losses in the
Capacity
vJ
The capacity (flow rate) of a pump is th e vol um e of fluid pumped per unit of time, which us ually is measured in cubic meters per second (m 3 /s).lit e rs per second (Lis), gallons per minnte (ga l/ min), million ga llon s per day (Mgal/d), o r c ubic feet per ·· · ··· · · ··· ·····second(ft.3./s):··
2g
................ .
Head The term head refers to the elevation of a free surfac e o f water above o r below a reference datum. Terms app lied s pecifically to the ana lys is o f pumps a nd pump sys tems are illustrated grap hica lly In Figs. 6- 17 and 6- 18 and are defined brieAy below.
l. Stalic SLiCliOI1 head (lis) is the di~Terence in e leva tion betwee n the suction liquid level and the centerline o f the pump impe ller. If the s uc ti o n liquid le ve l is below th e center lin e o f the pump impeller. it is a sllllie sli c rio l1 liJi. 2. SlllIic di sc!wrye hewl (h J ) is ~he diA'e re nce in eva lu ation between th e disc harge liquid level and the centerlin'e o f th e pLlmp impeller. 1 Stm;c head (H sta , ) is the diA'e ren'ce in ele'vation between the sta ti c discharge and s tatic s uc tion liquid levels (hd - II,). 4. M al10mel ric Sllct iOIl/, eud (H mJ is th e sucti oll g:.tge reading (expressed ill me te rs) mea sured at the suc tion non le of th e pUIllP refere nced to th e center line o f the pump impe ll er.
379
h,
v2
J---+-----1.--j~+-J._fI~\--,I-I-f-·I-'+--L---1-E ~ ~
Figure £>.17 Definition sketch for a pump in stallati on with a suc ti on head .
Datum
380
W ATER
ENV IR ON MEN T AL ENG IN EER ING H YDR AU LI CS DESIGN
381
where (6-9) (6- 10)
-H,
. - -j[-- .p,;-t-t-H-t-l1-+H+t-.L--+-- -- - - Da tum H ms
--
---
where /-I , = to tal d yna mic head, m (ft) /-I md( /-I ",,) = man o metri c d isc harge (s ucti o n) head mea sured a t discharge (s ucti o n) nozz le of pump referenced to th e centerlin e of the pump impell er, 111 (ft ) ~~i( Vs) = ve locit y In d isc ha rge (sucti o n) no zzle, m/s (ft /s) 9 = acce lera ti o n du e to gravi ty. 9.8 1 m/s2 (32 .2 ft /s" ) hd (Ii,) = static discharge (s ucti o n) head. m (ft) he n' = suction entra nce loss. m (ft) h fd (hfJ = fl'icti ona l head loss In d ischarge (suc ti o n) p iping, m (ft) h",d (II",,) = mi no r fi tting and va lve losses in discha rge (sucti on) pi ping sys tem. m (rt ) As no ted previo usly. t he reference datum fo r wr it ing Eq. (6- 10) is taken as th e eleva ti on of the ce nte rline of th e pump impeller. In accord ance wi t h th e stand ards of th e Hydrauli c Instit ut e f 6·}]. distances (head s) above dil tu m are co nsi dered pos it ive : distances be low da tum are co nsidered negati ve. In terms of th e sta t ic head. Eq . (6- 10) ca n be writt en as
~g
Figure 6- 18 Defini t io n sketc h fo r a pum p in sta llation with a sucti o n lift.
(6- 11 )
suctio n (11m,) a nd di sch a rge (hmd ) pip ing system are usua ll y estima ted as ....... ......... . '. (~;;l,<;!! o n s. of the ve loc it y head by usin g the fo llow ing ex p ression :
v2
h = K m
where 17 m K
= =
(6- 7)
2g
m ino r head loss, m (ft) head loss coeffi cient
where /-I , = to ta l dynam ic head. m (ft ) /-I s,o, = tota l static head. m (ft ) = h" - 17 , Th e energy (Bern oulli 's) equat io n ca n a lso be applied to de termine the tota l dyna mic head on the pump, Th e energ y equ'a ti o n written be tween the suctio n and ciisc ha rge nozz le of the pump is fI , =
St andard textboo ks and reference wo rk s o n hydra uli cs should be co ns ult ed fo r typica l K va lues for va ri o us pipelin e fi tt ings a nd appurtenances. 10. T otal dynamic head (H,) is the head against whi ch th e pump mu'st wor k when wil ter or was tewa ter is bein g pumped . Th e to ta l dy na mic head on a pump. co mmonly a bbreY la ted TD H , can be de termined by considerin g th e static suction a nd d ischa rge heads, the frictio na l head losses. the ve loc ity heads, a nd th e mm or head losses. T he expression fo r determin ing the to ta l d yna mic head for the pump show n in Fig. 6-1 8 is given by Eq. (6-1 0).
Hf
=
H md -
v3 V; Hnos + -- - --2g
2g
(1i-8)
J) ~ "
V'
+ '-..C i + :" .'2{/
(P-' + ~ V2 + :, ) ;.
(6-1 2)
2g
Whe re H , = to ta l dy na mic head . m (ft) 2 Pil ( Ps ) = disc ha rge (suctio n) gage press ure. kN / m2 (lbr/ft ) ~' = specifi c we ig ht of wat er. N !m 3 (lh r/ ft 3 ) v~ ( V; ) = veloc ity in d isc ha l!;e (s uct ion) nozzle. m/s (ft/s) q = accelerat io ll dll e t ~) gl'a vity. 9.8 1 m/s2 (3 2. 2 ft /s2 ) 2,1.( Z,) = eleva ti o n of disc hal'ge (s uctio n) gage ~lb ()ve cl a tuill.
III
(ft )
Pump Effici ency Pump pe rfo rm a nce IS Illeasured in term s of the c lpacit y tha t a pump can disc harge ag
31ll
WATER
ENVIRONMENTAL ENGINEERING H YDRAU LI CS DESI GN
the design. Info rmatio n on the design is furni shed by the pump manufacturer in a series of curves for a given pump. Pump effic iency Ep , the r:ltio of the useful power output to the power input, is given by
E P
power delivered to Auid power input to pump
= " - - - - - ; - --
Po PI
(6- JJ)
Pump efficiencies usua lly range from 60 to 85 percent. The energy losses in a pump may be class ified as vo lu metric. mechanical, and hydrau li c. llolu met ric losses occur becau se the sma ll clearances necessary between th e pump casing and the rota ting element can leak. Mechanical losses are ca used by mechanica l friction in the stuffing boxes and beari ngs, by internal disk friction , and by fluid shear. Frictio nal and eddy losses within the flow passages account for the hydraulic' losses.
Power Input In practice, power input to the pump is computed usin g th e fo ll ow in g equa tion : PI
power delivered to nuid
;'Q H ,
Ep
LI'
= -.--------- =
Applica ti on of the terminology and equations used to define pump performance is illus trated in Example 6-5. Example 6-5: Finding energy requirements for pumping A water pump is discharging at a rate of 0.25 m J/s. The diame ters of the di scha rge and suctio n !Jozzles are 300 and 3-50 mm , respective ly. The reading on the discha rge gage located 0.25 m a bove the cent e rlin e of the impeiler is 150 kN /m 2'; the. rea ding o n the sucti o n gage located at the ce nterline of the impe ller is 20 kN / m2 Determine (I) the to ta l dynamic hea d , (2) the power input required b y the pump, and (3) the power input to the m o tor. Assume the cmciency of the pump and m otor are 65 and 90 percent , respectively.
SOLUTION
Determine th e head on the pump using the energy eq uati o n [Eq . (6- 12)]. The reference datum is the cen ter lin e o f th e pump impeller. (/. Th e va lues for the indi vidua l ter ms in the ene rgy equations are as fo ll ows:
~
(6- 14)
V "
(3.54 m/s)2
,
(6- J 5)
Z,
= 3.)-4
/ ms
= 0.64 m
0.25 m
= 20.000 N/m ~ = 204
m
98 10 N/ m2
Q,
0.25 m)/s
= -A, = (1[ / 4)(0.35 m) 2 = (2.60 mj s)2 2(9.8 1 m/s2)
260 m/s .
= 0.34 m
= 0
h. The tota l dynamic head is obtained by s ubstituti ng the above va lues in Eq. (6-12).
H =
P" = 29 1'5 (P, v; ) y' + Z" + r + 2g + z,
= 15.2Y m
(6-17)
=
.1:1 = 2g
(6-16)
0.25 m)/s (n / 4)(0.3 m)2
2(9.8 1 m/s2 )
V
. (62.4 Ib/ ft J)(Q ft 3 !s)( H, ft ) P - - - - - . _ - -I (550 ft· Ib i s· hp)E p
8.8 14E p
Ad
JI,~
P, )'
p _ (62.4 Ib/ft~ )(Q ~~~t1/d)(H , !t~(I~(~~~y M gaQ I 86.400 sic! (7.48 gal /ft J)(550 ft · Ib/s· hp)E p
(Q ft J/ s)(H , ft) - -----
N/ m 2 = 15.29 m 98 10 N/ m 2
2g
z"
(62.4 Ib/ ft J)(Q gal/min)(H, ft) p - - _ . _ - - - - --------- - .-----J (60 s/ min)( 7.48 gal/ft J)(550 ft . Ib is· hp)EI'
(Q Mga l/d)( H, ft) 5. 696 Ep
~~,OOO
= -Q" =
-- =
When the How rate is give n in ga ll o ns per minute, milli on ga llons per day, or cubic feet per seco nd and the head is give n in feet , then th e power' inpu t to the pump can be co mputed using Eqs. (6- J5), (6- 16), and (6-17), respecti ve ly.
(Q galfmin)(H, ft) 3960 EI'
=
)'
whe re PI = power input to pump. kW (kN· m/s) )' = specific weight of liquid . kN/ mJ Q = capac ity, m J/s H , = to tal dynamic head [see Eg. (6-II)J , III Ep = pump effic iency
.
383
=
I 3.R
III
+ O.M m + 0.25 m - (2.04 m + 0.34 m + 0)
ENVI R ON~ I ENTAL ENG INEERI NG H YDRAULI CS DESIGN
384 WATER
int ersec ti o n o f th e new pump h ead-capac it y c ur ve w ith th e sys tem head-ca pac it y cur ve. a nd not h y app lica ti on o f the affinit y laws to the or ig in a l ope rating p oi nt
2. Us ing Eq . (6- 14) dete rmine t he power input required by th e pump. p
yQH
,
= --
on ly.
Ep
=
385
(9 810 ~~m 3!025 ~J/s)~~) 0.65
Cha nges in Impeller D ia meters To cove r a wid e range o f fl ows eco n omica ll y \v ith a min imum numb er o f pump sizes and impell er des ig ns. man u fac turers cus to maril y offer a range o f impeller diamete rs for eac h size ca s ing (see Pump Charac te ri sti c Cur ves helow). I n gen e ra l, these impe llers h,J\'e identica l inlets a nd o nl y th e o ut s id e diameter is changed. lI suall y by ma chinin g clown th e diame ter. Th e' fo llow ing re la tio n s hips for det e rmining the effec t of chang es in th e d iame te r o f th e impelle r ho ld a ppro xi mately. but wi th less accuracy than th e a fJinity laws.
= 52.1 kW
3. Determine th e powe r input to th e m o to r.
52.1 kW
= - --
0.90
QI Q2
DI D2
(6-21 )
HI
Df Di
(6-22)
= 57.9 kW
il l
6-23 PUMP OPERATING CHARACTERISTICS AND CU RV ES
D~ The operating c harac teris tics of pumps depend o n th e ir size, speed. and design . P u mps of si mil a r size a nd des ign are p rod uced by many m anufac t ure rs. bu t they vary somewhat because of t he design mod ification s made by eac h man ufa cturer. Im p o rt ant bas ic re la ti o n s hi ps th a t ca n be used to cha rac teri ze a nd ana lyze pump pe rfo rm a n ce un der va rying cond itions include t he affinity laws. ty r e numbers. a nd 'ne t pos it ive s uc tio n head. To aid in t he selectio n o f an appropriate rump for a g iven se r vice, pump manufacturers provide characteristic curves fo r th eir pumps . .... . . . . ... ........ .. .. . . ......... ........ ... ... ..
"
..
-
... .. ....... .
Affinity Laws
D~
In som e cases. two o r more iJll[le ll e r des igns may be ava ilable. eac h in a range o f sizes . fo r the sa me cas ing. Beca use these impellers are not geo me t rica ll y simil a r. the affinity laws do not ho ld.
Type N umber (S pecifi c S peed) For a geometrically s imilar se ri es of pumps o peratin g und er s imilar conditi o ns. th e fo ll ow in g relati ons hip ()htain ed is d e fined as th e' t ype numbe r (s pec ific speed ) . IIQI 2
For t he sa m e p u mp operating a t di fferen t speed s t he diamet er d o es not c hange. . and t ~1e fo llowing re la ti o nships can b e derived fo r.centrifuga l pumps.
Ql Q2
=
n2
nf
HI
H 2 = n~
PI P2
11
where 11,
11~ =
n~
(6- 18)
(6-23)
s
=---H J '4
or
(6-24 )
= type number = speed. r/ min Q = capaci ty. 11I 3 ;S (gal /min)
II,
11
H = head. m (ft)
( 6-1 9) (6-20)
T h ese re la t ionsh ips. k n own co llective ly as t h.e alJinil,l'./aws. are used to determine tHe effect of c han ges in speed on th e ca pacit). head . a nd po wer o f a pumr. The effect of changes in speed o n the pOmp c haracteri stic c ur ves is obtained by pl o tting new curves with the use o f the aiffinity laws. T he new o perati ng po in!. th e Inte rsec tion o f the pump and sys te m he;Jd -ca pac ity curves. wi ll he give n hy th e
Although th e seco nd form of the type-number eq uation is correct when a ppli ed with a consis tent se t of-u nit s. the fil's t fo rm is u sed in the United States. [6-1J Fo r a n y pump o peratin g at any given speed. Q a nd H are taken a t th e p o in t of ma ximum efficien cy. Wh en us in g Eq . (6-24) fo r pum ps ha ving double-suction im[le llers: o ne- hair o f t he di sc h;\r ge is used . unl ess o ther w ise noted. F o r mult istage pumps. t he head is th e head pe r stage. Th e var iations in maximum effic ien cy to he ex pec ted with va ri a ti o ns in size (ca pac it y) a nd design (t ype num be r) a re show n in Fig. 6- 1Y. The [lrogressi'e cha n ges in impeller shape as th e typ e num her inc n::ascs a re shol\n a lo ng th e b o ttllm o f Fi g. 0-1 9.
386
ENVIRONMENTAL ENGINEERING HYDRAULI CS DESIGN
WA IloR
1 00 r-------------------------.-------------~
0.031 mJ/s I
~
~ 70 r-_--,,,£--_ _ ____~----OO 13 mJ/s - - - - - ---1
387
Net Positive Suction Head (N PSH) When pumps operate at high speeds and at a capacity greater than the best efficiency po int (bep), pump cav itation is apotential danger: cavitation reduces pump capac ity ane! elTicicncy and can damage the pump. It occurs in pumps when the abso lute pressure of the inlet drops below the vapor pressure of the fluid being pumped. Under this condition, vapor bubbles form at the inlet, and when the vapor bubbles are carried into a zone of higher pressure, they collapse abruptly and the surrounding fluid rushes to fill the void with such force that a hammering action occurs. The high localized stresses that result from the hammering action ca n pit the pump impeller. When determining if cavitation will be a problem, the NPSH available (NPSH A ) at the eye of the impeller is determined. The available NPSH A is then compared to the NPSH required by the pump (NPSH R ) to prevent cavitation. Th e NPSH A is the total energy available at the inlet flange of the pump, above the va por pressure of the water. expressed in feet (meters). In effect, the NPSH A is the head available to push liquid into the pump to replace .liquid discharge by the pump. The NPSH4 is found by adding the. term (PatmiY Pva~or/Y) to the total energy head available at th e suction side of the pump. NPS H 4 = .
± IIs
-
I1,nl
-
Jlfs
-
Palm Pvapor L hms + -Y- - --Y
(6-25)
where Palm = atmospheric press ure, kN / m 2(lbr/ft2) ['va po r = vapor pressure o f water, kN/ m 2 (lbr/ft 2 ) 'i' = specific weight of water, k N/ mJ(lbr/ft3) Tile N PSH required by the pump is determined by tests of geometrically similar pumps operated at constant speed and rated capacity but with varying Sucti o n head s. The onset of cavitation is indicated by a drop in efficiency as the heau is reuuced. The application of Eq. (6-25) is illustrated in Example.6-6.
Imrc' lkr s hape
Example 6~6: Determining net positive suction head Determine the available net positive suclion head (NPSHAl for the pump installation shown in Fig. 6·17. Assume the following dala are applicab le
Figure 6-19 Pump efficiency ve rsus Iype number and pump caracilY
h, = 2.0 m
h,", = 0.10 m
Pump d esign c haracteristics. cav itation paramet e rs, ,~nd abn o rmal ope rati o n under transient co nditi o ns can be cQr relat ed satisfactor il y wi th the ty pe number. Fu rt her cons idera ti o n o f th e type- number equat io n .reveals t he fo ll ow ing:
"Is
Temp
J. If la rger units o f the same type are se lect ed fo r ab o ut th e same heac!, the operating speed mus t be reduced. 2. If unit s o f high e r s pecific speed are se lec ted fo r the same head and ca pacity, the y will o perat e at a high e r speed: hence the cum plete unit , in c lud/ll g th e driver. s hould be less expensive. H o wever. long-ter m ope rati un a nd maintenance cos ts will ge nera lly be hi g her.
= 0.25 m
L:>ms =
0.1 5 m
= 20c e
S(ll.L! nO N
Delermine th e vapor pressure at 20°e. P"",,, = 2.34 kN/ m2 (see Appendix C)
388
ENVIRONMENTAL ENGINEERING HYDRAULICS DESIGN
WATER
curves, the total dynamic head H, in meters (feet), the efficiency E in percent, and the power input P in kilowatts (horsepower) are plotted as ordinates against the capacity (flow rate) Q in cubic meters per second (gallons per minute or million gallons per day) as the abscissa (see Fig. 6-20). The general shape of these curves varies with the type number. Characteristic curves for typical radial-flow, mixedflow volute, mixed-flow propeller, and axial-flow centrifugal pumps are shown in Fig. 6-21. The variables have been plotted as a percentage of their values at the best efficiency point (bep).
2. Substitute known quantities in Eq. (6-25) and solve for NPSH" . NPSH A = h.s -e n h t - hfs _ 'h L ms
+ P"m _ P"POt }'
I'
= -2.0 m - 0.1 m - 0.25 m - 0.15 m
+ _IO_I_.3_k_N--,-/_m~2 9.789 kN/m 3
=
-
2.0 m - 0.1 m - 0.25 m - 0. 15 m
+
389
2.34 kN/m2 9.787 kN/ m 3
10.35 m - 0.24 m
= 7.61 m COMMENT The computed value of NPSH A is compared to the value required for the pump (NPSH R ) to determine if the pump can be operated safely without cavitation.
150
11 ,
Pump
150
Radial flow
= 1000
Characteri~tic_Curves
Pump manufacturers provide information on the performance of their pumps In the form .of characteristic curves, commonly called pump curves. In most pump
30
100
~
25
300
'">-
~
~llX c d
o
;;
20
:::
:r::'"'"
250
Axial flow
flow
11,
65 00
=
13,000
60 E -0
", =
0.
15
10
\ .
20
\
.
~'-
. IOO~P~o~~~ ' c~r___-__--~~,
5
0
150
0
50 0
I 0
0.1
2000
4000
I
I I
I 0.2
6000 (gnl / min)
I 0.3
I
I
J
0.4
Discilnrge , m 3j s Fi.gure 6-20 Typical pump characteristic curves for a 375-mm-diamcter impeller variable-speed pump. (Courtesy 01 SmiTh and Lorrlrss.)
Per ce nt 0 1 d isc harge at be; t effi ciency Figure 6-21 Typical characteri stic curve s fo r centrifugal pumps' (0) radial-flow; (b) mixed -flow VOlute ; (c) rrdxeLl-tl o w propeller ; (d ) axial -fl nw.
390
ENVIRONMENTAL ENGINEERING·HYDRAULiCS DESIGN
WATER
391
40,-------------------------------.
6-24 ANALYSIS OF PUMP SYSTEMS System analysis for a pumping installation is conducted to select the most suitable pumping units and to define their operating points. System analysis involves calculating system head-capacity curves for the pumping system and the use of these curves in conjunction with the head-capacity curves of available pumps. Both single-pump and mUltiple-pump systems are considered.
Pump headcapacity curve
30
E
System head-
~ 20
System Head-Capacity Curve To determine the head required ofa pump, or group of pumps, that would discharge various flow rates into a given piping system, a system head-capacity curve is prepared (see Fig. 6-22). This curve is a graphic representation of the system head and is developed by plotting-the total dynamic head (static lift plus kinetic energy losses) over a range of flows from zero to the maximum expected value with the use of Eq. (6-1 I) for pump systems such as shown in Figs. 6-17 and 6-18.
:r:" 90 ;>,
u
c
80 ~
10
I:i:i 70 OL-~
o
Single-Pump Operation As noted previously, pump characteristic curves illustrate the relationship between head, capacity, efficiency, and brake horsepower over a wide range of possible operating conditions, but they do not indicate at which point on the curves
40,---------------------------------,
____~______- L______~L__ _ _ _~ 10 Discharge. Lis
15
20
Figure 6-23 Definition sketch for determination of pump operating point.
the pump will operate. The operating point is found by plotting the pump headcapacity curve on the system head-capacity curve (see Fig. 6-23). The intersection of the pump head-capacity curve and the system head-capacity curve represents the head and capacity that the pump will produce if operated in the given piping ·Systc·riiTliis· poinf is ·alsc)"(<'iioWtyaslhe -pump opeyating· paine.
System ilc'ad-c·apacity curve
Multiple-Pump Operation
30
In most pump stations, two or more pumps usually operate in paralleL Situations will also be encountered where pumps operate in series. In pumping stations where two or more pumps may operate either individually or in parallel and discharge into the same header and force main, the following method for determining the pump operating point is recommended:
E
-g 20
minor losses
:r:
10
Total static head
OL-______
o
~
5
______
~
________L -_ _ _ _
10 Discharge, Lis
IS
~
20
Figure 6-22 Typical head-capacity curve for a pump installation .
I. The friction losses in the suction and discharge piping of individual pumps are omitted from the system head-capacity curve. . 2. Instead, these losses are subtracted from the head-capacity clIr,>;es of the individlIal pumps to obtain modified· pump head-capacity curves, which represent the he,ld-capacity capability of the puinp and i·ts individual valves and piping combines. 3. When two or more pumps operate in paralleL the combined pump head-capacity Curve is found by adding the capacities of the modified curves at the same head (,ee Fig. 6-240). The point of intersection of the combined pump-head curve with
l:NVI RO N MEN TAl ENG INEERI NG HYDRAULI CS DESIG N
392
393
WATER
PI
-0P2
PI
P2
--E)--D-
Force main (eli ameler = 3 S0 111 III , le ngth = 200 m) Pump syste m
Diameter and length or pump suction lin es
manirold
r-_ _ _ 1
I
Bend
No I 2S 0-mmq.,2 m No.2 300-mmq. , 2m
-g
Ekv .= I S.O m
Diameter and length or pump discharge line s
::c'"
No. 22 S-mmq.,3 m No.2 27 S-01mq. ,3 m Pump d isc harge lin e
. Discharge
Disc harge
(a)
(b)
Elev. = 5.0 m Bend
Figure 6-24 Head -ca paci ty curves ror pumps opera ted in (a) parall el and (b) se ri es. I so lation VJl vc
tlie system head -ca pacity CUr ves is the ope ra tin g poin t for th e two pumps opera tin g in pa rall el. By entering th e mod ified pump-head curves of each pump at the o pera tin g- po int head. th e capacity con tri b ut ed by each pump. th e effic iency of each pump. and th e brake ho rse power req uired under th ese co nditi ons can be d etermined. To find th e to ta l head a t w hi c h eac h indi vid ua l pUIllP wil l operate. proceed ve rti ca ll y a t co ns tant capac it y from the m o difi ed pump head-capac it y c urve to th e actua l head-capac it y curve. The pump spec ificat io ns o r purchase order must be drawn so th a t th e pump will prod uce thi s hcad . Each pump can o perate a t seve ral po int s on the head-capacity curve. w ith the . . ' .. ' ' heaG liicreas'in g; a ncl"rhe oisc h'a rge'dec'reas ih'g ::is'I'lW WpUi11pS go' into o pera tion: An effort s ho uld be made to limit th ese o pe ra tin g po int s to a range of fl ows between 60 and 120 percent of t~le bep.
Often o ne o r more boos te r pwnps may be ins talled in th e s ucti o n lin e or th e forc e main lead in g from a pumping stati o n to meet specific s it e co ndition s. Pumps insta lled in se ries wi th ex is ting pumps are used to increase th e head capacity of th e pumping s tation, When two o r more pumps operate in se ri es, the co mbined head-capacity cur ve is found by adding th e head of eac h pump at th e sa me capaci ty, This procedure is illustrat ed in Fi gure 6- 24b. Wh e n a boos ter pump is add ed to a for ce m a in fed by parallel pumps, th e combined head -capacit y curve is found by adding th e head of th e booster pump to th e m o difi ed head of th e parallel pumpS. at a given capac ity, The analysis ofa typical pump sys tem is illu strated in EXCllllple '
No te: Bo th pumps are se t at t he
Sucti o n
Intake
Pump sue t io n line
bel l
Pump no. I : No min a l impell er size = 225 mill Ope ralin g , pecd "Q.
III
0
:1 0n
D.I
n.u
().~
I X.S
025
X()
Pump no.
~.
No m ina l im peller size ~' ~75 111 m OJl e ralin ~ speed = 70(J r min
Q In
3
lI .tl
Example 6-7; Analyzing a pump system T wo ce nt rifu gal pumps are ava ibbJc fo r usc in th e pump sys tem shown in the acco mpan yin g f'i gure (a). Usin g Ihe data gi ven bel,,\\", det ermine the sy slem d ischa rge when eac h pump is opcrated sep.lrately a nd WIl , '11 Iw lh pumps a re opera led ill IXlra lJcI.
i-
11 50 r ' min
II .
In ·l i ':,
II. s
·m
6-7, i -
00
40.0
2
1) . 1
,I)
0.2
tU
l5 .U 26 0
(U
IO.il
sa me e leva ti o n . (a)
394
WATER
ENv tR ONMENTAL ENG INEERING HYDRAULICS DESIGN
3. H ead loss coefficients: a. Int a ke be ll = 0. 3 b. Isolatiun valves = 0.1 (open fully) c. Eccentric reducer = 0. 1 d. Co ncentric increase r = 0.05 t'. C heck va lve = 2.5 f Bend = 0.25 g. Fricti o n = 0.020 4. H ead loss co mputatio ns: u . Use Darcy- W e isbach eq uati on for head loss co mputati o ns [ se ~ Eq . (6-26)]. h. Neglect hea d loss in pump system manifol d.
395
5 0.-~~~~~~~~~~----~--~--------~--~~
O ri gin al pump heod-capacit y curve (P2) Modified pump head -ca pacity curve (mP2)
40,--_ __
System head-capacity
30 E U
'"" :r:
SOl. UT ION
--
20
I. De\"elup a nd p lo t the system head-capacity curve.
{/. The head loss in the force main. compu ted using the D arcy W eisbach eq uation , is as follow s
10~---
L V' " = j -D 2g where
r = 0.020
o
L = 250 m
0.2
0. 1
0.3
0.5
0.4
0.6
Di scha rge , m 3/s
D = 0.35 m
(b)
1(3.14 x (03W)
V = QI
I
- -·~------'---'-
4
2. Pl o t the o ri ginal pump head-capacity cu rves. The head-capacity curves fo r the given pumps are pl o tt ed o n fi gu re (b).
....... g.",; .9.R I m/ s2
3. Determine th e station lusses fo r the pumps.
b. P re pare a head loss com putati on table .
iI.
hni" '"
h ~ , a, It
m
m
m
h1{"al
0.0 0.1 0.2 0.3 0.4 0.5
000 0.7'i 3. 15 7. 09 12.60 19.68
0.00 0.0 5 0.22 0.50 0.88 138
10.0 10. 0 10.0 10 .0 10.0 10 .0
10 00 10.84 13. 37 17 .59 23 .48 31.06
h(1.011
:=
. L) V (0.3 +.0.1 + 0.1. + 0.020 D- -29 2
Q. m 3 .,s
*
a. Com pute the head loss in the suction piping as follows:
=
b. Cu mput e it L , as a fun cti u ll uf the discharge fo r the two pumps
Pum p I
Q, m' /s
V' 29
t S tat ic head = 10.0 (15.0 - 5 0)
(" The system head-capactty cline is ploned
III
ligure (h )
0.0 0.1 0.2 0.3 0.4
fiLS '
0-0 0.14 0.56
m
Pump 2
ilL , m 0.0 0.06 0.26 0.58 1 03
396
WATER
ENVIRONMENTAL ENGINEERING HYDRA ULICS DESIGN
c. Compute the head loss in the discharge piping as foll ows: hLD = (h;n<
+ h,. + h. + 2hb + 0.020~)
d. Compute hLD as a function of the discharge for the two pumps.
Pump 2
m'/s
hLD.m
hLDI
00 0.1 0.2 0.3 0.4
0.0 110 4.41
0.0 0.48 1.95 4.39 7.80
Q,
-. e.
m
Sum of the head losses in the suc ti on and discharge piping to obtain the station . losses for each pump.
Pump I
Pump 2
mJ/s
17 sl •
fl si •
00 O. I 0.2 0.3 0.4
0.0 1.24 4.97
Q, In
m
00 0. 54 2.2 I 4.97 8.83
4. Plot the station losses and develop the modified pump curves. a. The sta tion losses are plotted as.shown in figure' (b). b. The modified pump curves are ob tain ed by subtracting the station losses from the origina l pump head-capacity curves. The modified pump curves are designated (mP I) and (mP2). 5. Determine the system discharges and corresponding head s. a. Referring to figure (b) the following values are ob tain ed'
Q,
H,
Pump( s)
m"' /s
III
13. 5
(2) !I (I)
0.200 0.312 0.405
I H.I 24.0
h. Determin e the pump discharges and operating heads al the given discharge va lues.
~:
V2 (0.05 + 2.5 + 0.1 + 2(0.25) + 0.020 -L) D 2g
Pump I
397
Pump(s) I
2 (I )* (2)*
Q. m' /s
H, m
0.200 0.312 0.133 0. 272
18.0 24.2 26.0
28.0
* When pump ' (I) and (2) are operated in parallel.
A wider range of di sc harges could be achieved if the smal ler pump was convertecllo variable-speed operation
CUMMENT
6-25 PUMP STATIONS FOR WATER AND WASTEWATER Pump st~ltiolls ror wa ter and wastewater will \'ary in configuration depending 011 the ser\ice requircments. Beca usc th e deSign o r pumplll g stations is be yo nd the scope of the prese nt discuSSI()n. tile read er IS referred to Refs. [6-1. 6-S, 6- 11. and 6- 15J for a ll10re complete discussion or water and wastewater pump station s.
Hydraulic Analysis of Water and Wastewater Treatment The primary purpose of tilis sectio n IS to delineate the steps involved in th e hy draLili c analysis of wate r- and \\,~I s t e\\'a ter-trea tm e nt plants. I-!owe\e r, before conside ring th e subj ec t o f trc~ltment plant hydl·
6-26 TREATi\IENT PLANT DESIG N Once th e requir ed emuc nt qu:liilv has ocell defined. the steps involved in treat mellt plant design typic:l11\ Include (.1) s\nthesis of alternati\e fl ow sheets, (2) ben ch tcsts ~Ind pihlt-plant s tu dlc, . (,I) se lection oj design cr it eria. (..) sIzi ng PI' physica l faci lit ies. (~) prep~II' ~lti(111 "I' solids b;II:ll1ces. (6) layollt of the ph vsic al facilitie s. (7) prepar:tlioll nrllldraltilC I'rnfil cs. and (X) preparation of cons tructi 011 c1ra\\·ings. specili c:lIlllI1S. :Ind CtlSl est 11I1:ltes Re cilise o f til e imporwnce of each of these steps. e:lc h IS c()nsidcled scp: tr~ltcl\ In the f"lluII'JIlg discliss io n.
398
ENV IRONMENTAL ENG INEER ING H YDRAU LI CS DES IGN
WATER
399
Synthesis of Alternative Treatment Process Flow S heets
Plant Layout
A fl ow shee t can be defi ned as th e group in g toge ther o f unit o pcra ti o ns and processes to a c hi eve a specific t reatment o bj ec ti ve. Alt crnate fl ow s hcets wi ll be deve lo ped o n th e ba sis o f t he characteri s ti cs o f the wat er and wastewa ter to be treated , th e tr ea tm e nt objecti ves a ncL if availab le, the res ults o f be nch and pilo t-sca le te sts. Th e bes t a lterna ti ve fl ow s heet s are se lected after the y ha\'e a ll bee n eva lu a ted in ter m s o f their perfo rmance, ph ys ica l implementati o n. energy requIrement s. and cos t. T yp ica l ex amples o f s uc h fluw s hee t:; are s how n In Fi gs. 4- 1 and 4~2 a nd Fi gs 5-2 a nd 5-3 . .
Usi ng the informa t io n o n th e size o f the facilities dete rmined o n the basis of the selected cr it e ri a. va ri o u s p la nt layou ts are de vel o ped w ithin the constraints o f the ph ys ica l s ite. [n layi n g Ollt the va riou s faci lities, s pecial a ttenti o n s ho uld be g iven to minimi z ing pipe le ng t h s, to g ro upin g togeth er rel a ted facilities, and to th e need for future expan s ion.
Bench Tests and Pilot-Plant S tudies Th e purpose o f co nductin g benc h tes ts a nd pilo t-p lant studies IS ( I) to es ta b lish th e s uitab ly o f a lter na ti ve unit o pe rati o ns a nd processes fOI' trea ting a g ivcn wat er o r wa stewa ter a n d (2) to ob ta in th e data a nd Infor ma tion necessary to des ig n t he se lec ted o r e ratio ns and processes. Be nc h tes ts, ;15 th c n;lmc illlpli es. ;Irc s mallsca le te sts tha t ca n be co nducted in tlt e labora to r). T y ricalh' tltey arc used to es tabli s h app rox im a te c hemical d osages a nd to obt;lill kll1 etic coe t-fi cie nt s. Conti nu o us r il o t- pla nt st udies are co nduct ed to \'crify t he result s llr bc nch tes ts.
Selection of Design Criteria Afte r o ne o r m o re a lternati ve tl o\V s hee ts have hee n de\"eill ped . th c nex t s tep in design inv o lves se lec ti o n o f d es ig n cr it eria. Des ign cr iteria ar e selected o n the basis o f t heu ry, pu b li s hed dat a ill ·t·he· Irterat'ure-, ·t·he r e~'u Its' pf be m:h ' te ~ t s ·
Sizing of Unit .operations and Proc esses Once des ign criteria have been se lec ted. th e nex t step is to s ize t·h~ requ ired uriit o pera ti ons a nd processes so that the phys ica l fa c iliti es required fo r th e ir imp lementati o n can be de ter min ed . De pending o n s ite co nst ra int s. it Ill ay be necessary to cha nge from a c irc ular to a rec ta n g u lar bas in. fo r examr k .
Solids Balances . Afte r des ign cr it er ia ha ve been se lected and th e unit o perati o ns ;.IIld processes s ized. so lids ba la nces shou ld be prerared for each se lec ted p mcess !l o w sheet. [·deall y. so lids balances sho uld be prepared for the average and peak /l o w rates. The preparat io n o f a so li d s ba lance II1 vo lves th e det e r m inati o n ll f th e quantities o f so lid s en te rin g and lea ving each uni t o pe ra ti u n or process. These da ta are espec ia ll y impo rt a nt in th e design (s izin g) of the s lud ge-process ing facilit ies.
Hydraulic Profiles Once the treatm e nt faciliti es and interco nn ec tin g p ipi ng have been sized preliminaril y, hydrau lic profi les s ho uld be dev elo p ed fo r peak an d average fl ow rates. The piepar ati o n o f h ydrauli c profi les is co ns idered in d e tai l in th e foll owing sectio n .
Const ruction Drawiings and Specifications Th e fina l step in the des ign process in vo lves the pre para tion of const ruction dr awings. specifica t io ns. and cos t es tim a tes. Because th e clarity w ith wh ic h the co nstructi o n d raw in gs are prese nted w ill affec t b o th t he bid prices and final plant o perati o n, th e importa nce o f th is s tep ca nn o t be ove rstressed. Con s tructi on specificatio n s ha ve been m ore o r less s tand a rdized. The key iss ue is to make sure th a t specific ati ons are co mplete s o th a t cos tl y ch a n ge orde rs ca n be eliminated. Finally, the eng in eer's cos t es tim a te is used as a gu id e in eva luating the bids subm itt ed by the var io us contractors.
... 6-27 .. PREPARA nON .. OF ·HYDRAULIC PROFILES Hydra uli c profiles are prepa red for three reaso ns: (1) to e ns ure th a t the hydrau lic grJdi ent is adeq uJ te fo r fl o w thr o ugh the trea tme nt fac ilities, (2) to establish th e head needed for pu mps. w here requ ired, a nd (3) to ensure that pla nt facilities w ill not be fl ooded o r back ed lIr dur in g period s o f peak flow . Prepa ring hydra ulic p ro fil es invo lves careful co ns idera ti o n o f th e frict ional and min o r head losses that ca n occur in piping sys tem s a nd o f th e head losses assoc iated with control stru ctures. These head losses are co ns id e red separa tely bel o w. Application o f the info rma ti o n o n head lo sses in t he prepara tion o f hydra ulic profi les is illustra ted in th e fin a l part or thi s sec tion.
Frictional Head .Lo ss The fri cti o nal hea (1 loss that ·oc.curs as water a nd wastewa ter fl ows thro ugh pipes ca n be ' computed wi th seve ra l eq ua ti o ns The recommended equati o n is the O'\I"(;Y ' Weisbach as g iven b.:: lo w.
h,.
L 1' "
f-D 2g --
(6-26)
400
ENV IR ONMENTA L ENGINEERING HYDRAULI CS DEStGN
WATER
The appl ica ti on of Eqs. (6-26) ane! (6-27) is illu strated in Example 6-8. A more complete review of th e equations used for the analysis of co ntrol structures ma y be founci in Refs. [6-6. 6-X, 6- 13, and 6-15].
where h f = head loss, m (ft) f = coefficient of friction L = length of pipe, m (ft ) D = diameter of pipe, m (ft) V = mean velocity, mls (ft/s ) 9 = acceleration due to gravit y. 9.8 1 m/s2 (32.2 ft / S2) The valuesofthe friction f?ctor are obtained from a Moody diagram. A represent ative value used for most friction computations is 0.020.
Minor Head Losses
Example 6-8: Preparing a hydraulic profile Prepare a hydraulic profil e for peak fl ow conditions·and se t cont ro l eleva tions for the portion of a treatment plant shown in the accompanying figurc. The following data and assumpt ions a re applicable.
Inlet channel / Straighl (Francis) Weir
Vee notch weir (90° )
As noted earlier in the section on pumps. minor head losses are produced when various control devices are inserted in piping systems. Valves are the most common control devices used in piping systems. Minor head losses 31so occur at pipe joints, pipe interconnections, pipe expansions and contract ions. ane! pipe entrances and exits. For practical purposes minor head losses are usuall y estima ted as a fraction of the velocity head in th e downstream pipe section usi ng J::q. (6-7) V2
h
= m
K .--2g
40 m
(6-7)
O.S
III
Air
-
di" - 4 - __-l~==~I~I_
Head Losses from Control Structures The most common control structures usee! in both water- and was tewater-trea t_ ...... . ...~~~1t..I?!~.~!~ .~r~. VJ~it:s. ?f.o~.e..s.
1.84 (L - 0.1 nh)h J2
(6-27)
where Q = discharge, m 3/s (ft3 jS ) 1.84 = numerical constant L = length of crest of weir. m (ft) 11 = number of end contractions h = head on weir crest. 111 (ft) 3.33 = valu e of numerica l constant for U.S. customary unit s For 90° trian gular weirs the general equation is: Q = 0. 55 h S : z where Q = di scharge, m 1 /s (ftJ;s) 0.55 = t1umerical constant h = heae! on weir crest. 111 (ft) 2.5 = value of numerical constant for U.s. customa ry unit s.
(6-28)
Vee not ch weir (90°) (Weir cres t se t at elev. 100.0 m)
17 m
-15 m
Typical K values for various kinds of control devices ane! pipe configurations may be found in Refs. [6-6.6-8, and 6-13J and in manufacturers' lit era ture.
Q
401
Primary sedimen tation tank
. Secondary se dimentation tank
Aeration tank (0)
I. Flow rates' a. Average flow = 8000 mI ld h. Peak fl ow = 16,000 111 J Id = D.IS5 ml js
2. Primar y sedimentation tank' o. Diameter at weir circle = 15 III h. Weir spacing = 0.3 m r. Weir type = 900. vce Ilotch d. Weir depth = 0. 1 111 e. Return flow s frolll sludge·processing facilities = 0.15Q 3. Aeration tank . o. In let type = slide gates h. Number 01' gates = b c Width or slide gate = 0.20 III d. Return activated sludge discharged to influent chan nel at peak tl ow = 0.25Q e Length of aerati on tank eftlucnt \\'elr = 15 III j Weir Iype = straig ht sharp-crc,tcd
402
ENVIRONMENTAL ENGINEERING HYDRAULICS DESIGN
WATER
4. Secondary sedimentation tank: a. Weir crest elevation = 100 m b. Diameter at weir circle = 17 m c. Weir spacing = 0.3 m d. W e ir type = 90° vee notch e. Weir depth = 0.1 f Underflow = O.4Q 5. H ead loss computations: a. Head loss coefficients Pipe entrance = 0.5 Pipe bends = 0.4 Pipe exit = 1.0 b. Pipe friction factor in D arcy-Weisbach eq uation = 0.020 r. Head loss across aeration tank = 0.02 m (The head los - . . . . not well defined.) , across aeration tank s IS
d. Neglect liquid in underflow from primary sedi m entation tank. e. ~ses~:t head loss be twee n s lid e gates in aeration-tank influent channel. f ' h e the !Illet slide gates to the aeration tank ca n be modeled as a Fran CIS weir \\It two end contractions. g. Assume effluent weir in aeration tank can be modeled as a Francis weir. h. the In se tt1llg assume a free -fall of 0 .010 m betwee n tlle weir ' crest an d t weir f elevations . wa er sur ace In the downstream channel.
403
2. Determine water surface elevation in aeration-tank effluent channel. a. Summarize head losses and coefficient values .
(I) Exit
1055_
k" = LO
(2) Bend lo sses_ 2 at kb
=
0.4
(3) Friction loss in pipe, f = 0.020 (4) Entrance loss. k," = 0.5
b. Deter ri111le . velocity in pipe connecting the aeration tank to the secondary sedimentati on tank:
v=
Q/ A
= 14(0.185 m 3/ s)/ 3.14(0.3 m)2 = 0.92 m/s
c. Determine head loss in piping system connecting the aeration tank to the secondary sedimentation tank.
V , =(k" + 2kh + f L) D+ k,n 29
2
=
(
I
+
2(O A)
+
50 m 0.020 0.6 m
) (0.92)2
+
0.5
2(981)
= 0.171 m SOLUTION
d. Determine water surface elevation in aeration-tank effluent channel. Determine water surface elevation in seco nda ry clarifier
Elev.
u. Determine number of weirs.
100.081 m
+
0.171 m
100.252 m
No. of weirs = Tr D/ (d/ weir) =
114 (17)i(0.J rn/ wcir) 177.9. say 17X
h. Det er min e flow pe r weir.
3. Set the elevation of effluent discharge weir and d~i~~~i~e' ~~tei 'surfac'e'ele"Vati'o~';ri'" aeration tank near the etTIuent discharge weir. a. Se t the elevation of the effluent weir in the aeration tank. As given in the problem statement. the free-faJl distance between the weir crest and the water surface elevatio~ in the efflu ent channel' is 0.010 m. Thus
q/ weir = (16,000 mJ/ d)/ l n = 89.89 m J/d . weir
.Elev. = 100.252 m
Q = 1.84(L - 0.1 nll)113/2 1.4(0.11'5 mJ js) = 1.84[15 m - 0.1(2)I1JI1 3/2
q = 0.5511 5 ; 2
h
II = (Q!0.55) 2.' s <
Elev. = 100.262 m
= 1).081111
d. Determine water su rface elevat ion in seconc!;lry cbrilicr. 1000 m
= 0.044 m (by trial-a nd-error analysis)
c. Determine water s urface elevati o n in aeration tank near effiuent di scharge weir.
= (0.001 04:0.55)'
Ele\'.
0.010 m = 100.262 m
b. Determine'the head on the effiuent weir assuming two end contractions.
= 0.00104 111 3 Is . weir
c. Det erm ine head on vee-notc h weirs.
+
+
100.08 1 rn
O.OX I rn
+
0.044 m = 100.306 m
4. Set elevation 0 1s lide gates and determine water surface elevation in influent channel to
aeration tank . a. Ass ume 3 head loss o f 0.020 m across the aeration tank. Also assume a free fall of 0.0 10m between the crest 0 1 t he s lide gate a nd t he water surface in the aeration tank.
-... ..... ... ..
404
WATER
ENVIRONMENTAL ENGINEER I NG HYDRAULI CS DESIGN
b. Set the elevation of the crest of t he slide gate.
Elev. = 100 306 c.
In
+
0.020
rlI
405
c Detcrmine flow per weir.
+ 0.010 In
= 100.336 In
If/weir
=
Determine the head on rhe·slid e ga tes: · (I) The flow per slide gate = 1.4(0.185 m J/s )/6 = 0.043 m J /s (2) Determine head on slide ga te assuming slide gate is a Francis weir with lWO end contractions.
J
I 15(16.000 Ill /cI); 157 117.2 m ·l jd· weir
= 0.00136 111 3 /s· wClr
Ii. Determin e head on vee-nolch weirs. if = 0.55115"
Q = 1. 84(L - 0.1 nh)h J /2 0.043 mJ/s = 1. 84(0.5
In -
" = .. =
0.1(2)h)h 3 /'
= 0.091111
h = 0.139 m (by trial-and -er ror ana lys is) d. Determine water surface eleva tion in influent channel to aera ti on tank.
Elev. = 100.336m
s.
+
v=
e. D eterrnJllL: \V{l i e r
elpv···tl·on in primar y sedi men tation tank . S·lll·r'lce ~ .... u
Elcv.
0.139 m = 100.475 m
Determine water surface elevation in primary-sedimcntation-tank etfiuenr channel. (/. Summarize head losses and coefficient values. See step 20. b. Determine velocity in pipe con necting the primary sedimentalion lank 10 Ihe aeration lank inlet channel.
(QIOS5)' 5 (0001 36/055), ,5
=
p
100.7 17 m + 0.091 m
=
100.808 m
I )ns.. See th e acco mpanying 7. Preparc a hydraulic profile showing Ihe computelI e In·d" tigure (iJ). 100301)
100.808
100.08 1
100.26:
100.717
;--
Q/ A
100.707
11 5(018 5 m 3 /s)/ 3.14(025 m) ' = 108 m/ s
c. Determine head loss. in piping sys lem connecting the primary sedimenlali on lank to the aeration tank inlet channel.
. = ( 1 + 2(0.4) + 0.020 -.4+ 0 1.0' 0.5) - ---~ O.S 2(9.X I) = 0.232 m
d.
Deter~ine -:vater surface elevaiion Elc\,.
=
in primary sedimentation tank em-lIenl eha·Jln el.
100.475 m + 0.2 32 m
=
100.707 m
6. Set elevation of primary cttluent weirs and del ermine wain surface eleva l ion in primary sedimenlalion lank . Q.
+
0.010 m
=
100. 7 17 m
b. Determine number of weirs.
No. of weirs
=
rrD I(d !weir spac in g) 114( 15 m)!(OJ m/weir) 157
Aeration
lank
lank
Seconuary se diment ati on lank
(Ii)
Set elevalion of vee- n ot~h weirs in primary sedimenlation tank. EIe\'. ,= 100.707 m
n
PrIlJl~HY :-oeulllltniatioll
. each In Ihl\ e.\ampk;1 distance 0 I· 001 . n 1 'VIS . . lI sed . ao; a .free-fall.at .• . . of Ihe b . Where lhe lo ss "I. he
ENVIRONMENTAL ENGINEERING HYDRAULICS
406 WATER
DISCUSSION TOPICS AND PROBLEMS
6-8 Why IS the equation used in Example 6-3 no longer favored for estimating fire flows
6-1 Determine the maximum monthly water s upply that can be taken from a st ream using the data from Example 6-2. What is the capacity of the reqll1red sto rage reservoir'!
6-9 What is the origin of th e ter m" fire plug ""
in business districts"
6-2 Determine the maximum monthly water supply tha t ca n be obtained from a stream with the cumulative runoff record shown in th e accompanying figure. What is the capacity of the required storage reservoir'!
50r-----------------------~--------------------------__,
6-10 Referring to the figure used in Example 6-3 and assuming that the data given in that eX 3m r ie are applieahle. wh
,
of flows. determine: (a) Q and V when the sewer is flowing full .' (b) Q and V when the sewer is flowing at a depth of 0.3 m (c) Q and V when the Aow in the sewer isat 0.6 of its capacity
'<>
a
x
<:!i
30
;::'" o
(d) V and depth of flow when
<>
.
6-13 Solve Prob. 6-12 assuming that n is variable and is equal to 0.015 when the pipe is flowmg
;:'"
..";
3
Q = 1.0 m /s
fulL 6-14 A rectangular sewer 1.25 m wide and 15 m high has been laid on a slope of 0.0055. (0) What is the maximum flow rate if the Manning's n value for the sewer is 0.013? (b) What are th e dimensions of an egg-shaped sewer laid at the same slope that has the
20
'" "3 E ;:0
u
same now caracity? 6-15 Develop a preliminary design. Including flow rates, pipe sizes, and pipe slopes for a trunk sewer to be laid in the develorment shown in the accompanying figure from 6th to 1st Streets
10
along Peavy Avenue. Assume the following data apply. 12
6
18
24
30
36
I-~-
Month 6-3 Using the cumulative runolTcurve given in the figure for Prob 6-2. determine the capacity of the storage reservoir needed to provide a'constant supply ofO.:n x 10" m 3 / month. 6-4 Referring to the figure used in Example 6-1 and assllming that the following data apply. determine the maximum flow rate that can be withdrawn by the c it y while maintaining a minlIllum pressure of 140 kPa.
fp = 50
III
L, = 3,500
III
20.5
19.0
18. 5
I
\
\
Low-rise apartments
\
Commercial
= S.OOO m
iI" ,= 0.5
III
ill'
=
OS
III
Peavy Ave.
Park
treat· \ plant I
~~===T=c1=10=b=a=n=o=g=IO=U~S~A=v=e=.=====1
Lo w· risl'
6-7 Estimate the I'e(juired fir e tl ow for vour classroom huilding.
i
\
\
.
~--i-
o
500
II I I
sCJle. m
I
\
\
Highflse
apa rtments \
6-6 Estimate the required fire flow for a schoo l of wood frame cons truction with a total Aoor area of 1600 m 2 Assume the fire flow Illust be increased by 10 rercent clue to unfav orable expos ure.
Singlefamily \ dwellings
upstream area 3 0.005 m 7s
To-~==~~~~~====~~==~I
j = 0.020 (d p , d,)
6-5 Refe rring to th e sa ine figure and assulllin'g that the data gi ven in Prob. 6-4 apply, determine E .\ when the city demand is equal to 0.3 m 3 /s.
.;:
V"l
\
Jnent
L"
I
Vi
Vi
ap~rtments
~
\
I I
__\-=~=-\-~ _\_J \
Single·family \
'\
408
WATER
ENV tRONMENTAL ENGINEER ING HYDR AULICS DESIGN
r-- ------ --
(a) Saturation population and flow data:
---
409
~---- - --I
I
I Commercia l Saturation population density, person/ha
Type of development
Zoning
1 Wa stewater flow,*
L/capita d
I
206 1
2063
• Refer
Single-family dwellings Duplexes Low- rise apartments
High -rise apartments to
figure
(c)
30. 50.
10.0. 160.
380. 310. 260. 220.
in Example 6-4 for pea king factors.
·1 I
stitutional faci lities:
Industrial
... "
Commercial Industrial School Hospital Park
m 3 /ha . d m 3 ; ha ·d
L/student . d L/bed . d L/perso n . d
Pea king factor
32
1.5
60. 80.
2.2
60. 3D
3.5 3.5 4.0.
. . . . . . . . . .-.-..- .....,-.-. . -.. .,.. -..- ... --..- ..- . ...,. .. ...,. .. c.. .,....,..----,.,-,.,. ...~ ...,. .~..,..~ .. .. . ....... . . . . . . . . . . . .
(c) The average daily attendance at the schoo l is 1000 students. (d) Assume park usage wi ll be 500 persons/d. (e) Assume flow from upstream area is equal to 0.005 m 3 /s.
6-16 Develop a preliminary design'. including flow rates. pipe sizes. and pipe slo pes for a trunk sewer to be laid in the development show n in the figure shown on the opposite page frotn 10th to 13th Avenues along Ash Street to II th Avenue to Birch Street. Assume the data given in Prob. 6-15 are applicable. 6-] 7 Using the data from Prob. 6-1 5 and the contour data shown in that figure. prepa re a
profile simi lar to the one shown in Fig. 6-16 for the trunk sewe r.
6-18 Using the data from Prob. 6-16 and the elevation data show n in that figure. prepare a profile similar to the one shown in Fig. 6-13 for. the trunk sewer. 6-19 Compute the volume of excavat ion and length of pipe'of various diametns req'uired for the trunk sewer designed in Probs. 6-1<; and (,-17.' Assume thaI the width of the trench is 14 times the inside diameter of the sewe rs plus O.lnl. The minimum width of the trench is I m and to allow for pipe bedding material. the depth of the excavation is to be 0.2 m oelow the invert of t he sewer.
6-20 Compute the volume of excava tion and leng th of pipe of various diameters required for the sewer designed ill Pmos. ()-!Ii and ('-I~ . 11,<' the design constraints gi\enll1 hob. (,-1'.1.
'"
~I
Single-family dwellings
-
;;:; 1
-r '1
~
I
- .20.01
-
Cedar Sf.
I
Comrnerci,11
o Valu e
o-lo.
.<::
I
Flow basis
20.60
~
.c r ·,
;-
I
Area/ facility
~ ;;; o
.,;
i (b) Wastewater flow and peaking data for commercia l and industrial areas and in-
. ·c -= ~
Ash St
1
Residential Residential Residential Residential
1 .1
'1)c v>
500 II! til : :; caie.
-----------
.
I
.
Birch St. 19.80
19.70
19.
57
1
Hi ghrise a part· mcnts
--- ---
Low·r ise apar t men ts
1
II
----- ---- -~
III
6-21 II wastewate r pump has:1 lOO-mm d ischarge a nd a 350-mm suc ti on. The readin g on the discharge gage located at the pump cent erline .is J40 kPa (kN/ m'). The readin g on the sucti on gage loca ted 0.75 m below t he pump cen terl ine is 20 k Pa (k N/ m '). 1ft he total head 0 11 the pump is 15 m. determine (I) the pump discharge and (2) the energy input to the motor. assuming a pump efficiency of 82 percent and a mot or dficiency of 91 percent. 6-22 Solve Prob. 0-21. but assume that the total head is 10 m and the readin g on the discharge gage IS 100 k Pa (k N/ ni') . 6-231\ centrifugal pump with an impeller cliameter of 0.25 m delivers 0.02 m'/s agamst a
head of IX m at a power input or 4 kW whe n opera ting at 1170 r/ min . If it is assumed that the cfliciency remains th e same, d~tefJlline the (I) head. (2) discharge. and (3) power input for a geometrica ll y similar pump with <.ill impeller diameter of 0.30 m operating at 870 r/ min. 6-24 A mixed-flow volu te pump is to opera te at a head of 5 m and discharge 0.17 m 3 js. It is to be driven by a direct-coupled squirre l-cage inductio n motor operat ing on 6-cycle (60-H z) cur rent If the specifi c speed is not to exceed 100. what sho uld be the operat ing speed? What efficiency cou ld be expected. illld how much power will be required ~ 6-25 Ir the ciJameter of th e impell er in pump no. 2. in Exampl e 6-7 were changed from 275 to 250 mm. \\'hat would be the maximulll discharge that clJuld be expected with two pumps Operatin g ill paralleJ'l 6-26 Usin!, the d~lta give n below in conjunction with the pumpin g system sc hematic show n in figure ((]) on page 410: . (a) Determine t he sys tem pumping Cil pacity when pumps I and 2 are ope rat ing in parallel. The pump performance curves for pumps I and 2 are given in figure (b). Ignore losses othe r than rriction ill developing the system cu rve. Minor losses should be considered in deve loping th e muddied pump curves.
ENVtRONMENTAL ENGINEERING HYDRAULICS DESIGN 411
410 WATER
Elev. 10.0 m Po int c
(I» What is the kilowatt requirement for each pump at the above operati ng point? 3 (c) At what reduced speed must pump I be operated alone to pump 0.20 m / s to the
reservoir ? Wha t is the corresponding head?
L. H ev . 3 .5 m
Point b
Pipe
D, m
L,m
a- b
0.35 0.50
30 (sa me for both pumps) 800
b -c
I i. Head loss compu ta tions: Pum p 2
k,nl = 0.3
Bo th pumps are se t at th e same eleva tion .
(a)
k" ,,, kche ck yalv e
f
=
0.5
= 2.5 = 0.020 (for all piping)
6-27 Solve Pr ob. 6-26 using pump c urves 3 and 4 given in figure (b) of Prob. 6-26. 6-28 Determin e the a vaila ble net positi ve suction head (NPSH A) for the pumping system given
E
in Example 6-6. 6-29 Solve Pr o b. 6-26 for the pumping system given in Prob. 6-24. 6-30 Develop the hydraulic profile for the peak-flow condition for the portion of the wastewater-treatment plant shown in the figure o n page 412. Assume that 33 percent of the plant infl ow is recyc led from the secondary 'sedit'rierHation tanks tu the head'end'of the aeration tank :·· · · .... .. .
-0 16
The pertinen t data and informa ti o n are show n as follows:
::r:'"'"
3
Q". = 10,000 m /d 12
90
8
80
3
Qp<" = 18,000 m /d
*;:, u
"
P(imary sedimentation:
.~
u
Nu mber of tanks = 2
E
lJ..I
4
Di a meter - 13.75 m
70 Secondary sed imentation :
0 0
Number o f tanks = 2
0.6 Discharge , IIl J !>
Di a meter = 15 m
lb)
.-J :;.!:'
Aeration-tank cflluen t weir : T ype-s ha rp-crested , straight weir We ir length = 4 m
,~
.
. . ~ .... "' ~-~~~:;j'$~;':'.-' ; ~.~~~..::
-
412
ENVIRONMENTAL ENG INEER ING HYDRA UI.I CS DESIGN
WATER
413
6.6 K ing. H . W. , a nd E. F. Brat er HOlldbook o/Hydraulics, 51h cd., M cGraw- Hili , New York, 1963. 6.7 Lager, J . A., and W . G. Smith' erball SlOrnJll'Oler Manag~menl and Technology: An Assessmenl, 518.26 (secondary)
EPi\.670/2-74-040 . Ci ncin nat i. Ohio, December 1974. WaS/ elVala Engineering. Col/eclion and PlImping of WaSlell·'(Jler. rev . by
6-8 Metcalf & Eddy. Inc.
G. Tchobanoglous , McGraw-HilI. New York. 1981. 6.9 Metcaif & Eddy , Inc. Wa.HeH'llIer Engineering. Trealmenl, Disposal, R ellse, 2d ed .. McGraw-HilI. New York, 1979 6- 10 Pardoe, .W . S.· "Computing Head Loss in Grid iron D istributi on System;" Erigineerlng News-
Record, 93(1 3):5 16 (1924). ' 6. 11 Pump in!) Sialion Design fin Ihe I'raniein.'! Enl}lI1ecr Volllllle f Fundamentals. Volume II WaslewaleI'. Vol;ill1e III W(lIer. Conference P roceedings . Depanrilent o f Civil Engineering and Engineering Materials, Montana State Linrversil\'. Bozeman, 1981. 6-12 Ripple. W.: "T he Capacity of Storage Re sef\'oirs f\>r Water Supply." Prof Insl Cie Eng: vo l. 7 1, 1883 6.13 Vennard. J . K .. a nu R . L. Street Ul'fllenllJfl' FlUid .If echanics. 5th cu .. Wiley. New York, 1975. See de lai l A EI 5 1il.26
0-14 \Vanieli sta, M
P.: S'tnrnlll'oter !\l alla[lenlrr!1
,\roor , Mich .. 1978. 6-15 Water Pollution Cutltru\ FederatI on
QuantifY und QUOlifY· Ann Arbor Scie nce. Ann
Drsigll (~( ~V(J SfeH'(lle" alld SwrmH'([ler Pumping Srariolls.
Manual of Practice no. FD-4. New Yor k. 1%1 6-16 Whipple. 'Ivy" et al . Sror1Jl\\'(J/c'f .\4ollaqerncltf In l (rhani::ing Areas, Prentice -H all, Englewood
ClifTs. N.J .. 1%)
EI 5 16.00
48.0 m
EI
15 .0 m
I 51J.90 I__ 2-0.40 m
I
. 1------ - - ----1 4 0 n1 A~rali o n
Primary clarification tanks
Seco.ndary c larifica li o n lank s
I~nk
Profil e
REFERENCES 6-1 Addison I . P umps, 3d ed .. Chapman an d Hall. Lo nd on, 1966.- , H.: Cenln.IlIgal and Ollwr R O{()
ami
G,' M ., J . C. Ge ye r, and D. /\ . Oktlll: fI '(ilCI' WaSlell'Oler Engine~rinq, Volume I . Waler IIppl) and WaSlewater Remoral, Wiley. New York. 1968. . 6-3 Hydraulic In>lilul e Slm;&If d F: r C'tlllI~U.lJa.Rntary,{JndRt!('ipro('alilItIPllmpJ ·'Ii I . .' I Jlhed. Hyuraulic I nSlllute , Cleveland . Ohio. 1975 . . .." . 6-4 J ~pPsocn, R. W .: Allalysis 0/ Flo II ' ill Pipe .Velll·orks, Ann Arbor Scicnce. Ann Arbo r Mich 1976 f I A ' . . .. . 6-5 Oml F d ommJ!lee : 0 I Ie .. mencan Soclelv of C ivil Engineers and Ihe Walc r Po llu lion COnlrol 6-2
F;i~,
a.
J .
E e erallon: . Design and ConSlruction ~f SanilOrr Sellws, ASCI:' Manuals and R~p orls on ngmecflng Pracllce no : 37. Ncw Yo rk , 1969. .
