ONLY QUIZ 1 CHAPTERS ARE INCLUDED IN THIS BOOK
Water Systems
1.1.
INTRODUcnON
The five essential requirements (or human existence arc: (I) air (it) ~ter (iil) food (iv) heat and (II) light Contamination of
these elements may cause seriQus health hazards not only to man but also to "animal and plant life. Environmental Engineuing deals with all these essential elements. The use of water by man, plants and animals is universal. Without it, there can be no life. Every living thing requires water.
Man and animals not only oonsume water, but they also consume vegetation (or their food. Vegetation, in tum, cannot grow without water. Growth of vegetation also depends upon bacterial action, while bacteria need waler in 'o rder to thrive. The bacterial action can cooven vegetable maner into productive soU. New plants, which grow in this soil, grow by sucking nutrients through their roots in the form of solution in water. Thus an eoologjcal chain is maintained. Water maintains an ecological balance - balance in the relationship between living "things and environment in which they live. The use of water is increasing rapidly with our growing population. Already there are acute shortages of both surface and under ground waten in many ~rts of the country. Careless poUution and contamination of tbe streams, lakes, reservoirs. wells aDd otber uDder ground sources has greatly impaired tbe q~ty of available water. This poUution results because of improper disposal of waster water -both domestic as well as industrial. Organised community ute require twin services of water supply and SCWBF 1Jisposal. Good sanitation cannot be maintained without adequate water supply system. Without (1) uJP'l
nljUIl
,
WATER SUPPLY ENGINEER ING
2
proper disposal, the wastes of a community can creale into lerable ~uisance. spread diseases and creaie other health hazards. The planning,
designing, financing and opera lion of waler and waste water systems are complex undertakings, and they require a high degree o f skill and judgement. The work of conslruction and maintenance o f water
supply .and waste water disposal systems is generally undertaken by Government agencies - mostly through Public Health Engl.'neering or Environmental ~ngineering Departments consisting of Civil Engineers.
1.2. HISTORICAL DEVELOPMENT
Man's search for pure water began is prehistoric times. The
or
Story of water supply begins with the growth ancient capital cities, or religious and trade centres. In olden days, most of com munity settlements througho ut the ~orld were made near sp rings, lakes and rivers Crom where the water supply for drinking and irrigation purposes was obtained. Rig Veda (4(XX) years S.C) makes a mentio n of digging of wells. Similarly, Ramayana, Mahabhartha and Puranas make mention of wells as the principal source of water supply. These wells \lrr'Cre mostly of shallow depth, dug near river banks. Water was lifted from the -wells through indegenous methods. However, no water treatment or distribution works existed. Apart from India (Bharat), other majo r civilisations of the World, such as Greece, Egypt, Assyria etc. used wells for their settlements which were located slightly away from springs, lakes and rivers. Joseph's well at Cairo is one of the oldest deep wells excavated in rock to a depth of about · 300 feet. lbese \lrr'Clls, however, caused water s upply problems during periods of drought. It became necessary, therefore, to store water. Cisterns \lrr'Cre constructed for collecting rain waler while reservoirs were oonSlructed to start'· water from streams and rivers during monsoon period. lbe stored water was conveyed 10 towns through masonry conduits a nd aqueducts. The earlier examples are the aqueducts built by Appius Claudius in about 312 S.c. for water supply to Rome. Lyons in Pa~, Metz i~ Germany and Segovia and Serille in Spain tiuilt similar aqueducts and syphoRs for water supply used for drinking, bathing and other pur~. Sextus Julius FroDlinus, Water Commissioner of Rome (AD. ~~ported the existence of nine aqueducts supplying water to Rome ail'd,~aryi n g in length from 10 to over SO miles and in cross-section from ,) to over SO sq. ft. , witb an estimated aggregate capacity of 84 mgd. The great sewer, known 15 tbe clOOCil maxima and constructed to drain tbe Roman Forum, is sliU in service. lbere was p~Uy no improvement in water supply systems ill the middle ·ages. Tbe earlier water supply structures got destroyp1 with the (aU of Rome. ·In the Dintb century, few impqrtant water
,
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mater~1
WATER SYSTEMS
3
s upply structures were constructed by the Moors in Spain. In the twelfth century, small aqueduct was conslructed in Paris. In London, spring water was brought by mea ns of lead pipes and masonry conduits in the thirteenth century. In Germany. water works were constructed in 1412 and pumps were introduced in 1527 in Hanover. Franciscan monk constructed aqueduct of Zempola in Mexico in ihe middle of 16th century. In 1582. a pump was erected 'o n Ihe o ld Londo n bridge for the s upply of waler from the Thames. The water was conveyed thro ugh lead pipes. In Paris, pumps operaled by water power were erected in 1608. Pumps operating from steam were in Iroduced in the 18th century in London and Paris. In the United States, spring water was conveyed by gravity to Boslon in 1652. Pumps etc. were inlroduccd at Bethlehem in 1754. However, purposeful quality control of waters upply is quite recent in origin. The scientific discoveries and engineering inventio ns of the eighteenth and ninetecth centuries created centralised industries to which people fl ocked for employment. This caused serio us water s upply and waste disposal problems in the industrial towns. No great schemes of water supply were started until the Indus trial Revolutio n had well passed its first half century. The development o f the large impounding reservoir was largely due to the necessity o f feeding canals constructed during the first phase of the Industrial Revolutio n.
'II
The fi rst water filter was constructed in 1804 by John Gibb at Paisley in Scotland. It was a slow sand filter and worked in conjunctio n with a settling basin and roughening filter. Next s uccessful filters were constructed in 1827 by Robert Thorn at Greenock. In 1829, James Simpson built sizable fillers for the Chelsea Water Company to improve ils supply from the Thames river. By 1870, the mechanica l fill er of the pressure type began to be employed, the earliest being the Halliday filters installed at Crl.we (1888), Bridlington and elsewhere. In 1894 pre-filters were successfully built. In the first decade of 20th century, mechanical pressure filters were introduced, Hastings being an early pioneer with Canndy filters built in 1900. In India, Calcutta was the first city where a modern water supply system was constructed in 1870. The technique of clarification and filtration soon grew. By 1939, mechanically-sludged sedimentation tanks were in general use,' ;''' The micro-strainer, fo r the removal of plankton (rom the impoundedwater was developed by Boucher, and was introduced by Glenfield and Kennedy in 1945. Coagulation of water with sulphate of alumina began experime~talJy in 1827, but was adapted practically only in 1881 to treat Bolton's water supply. Activated silica was introduced by Bayliss in U.S.A during 1937. Tbefirst permanent useofchlorination originated under tbe direction of Sir Alexander Houston at lincoln
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WA'reR SUPPLY ENG IN EERING
4
in 1905. In 19 17, Paterson Engineer.hg Company ins.alled the first gaseous chlo rinator at (he Rye Common Works. Super-chlorination and dechlorination was first applied in 1922 at the Deptford works o f the Metropolitan Water Board. The art of softening water was also first developed in Greal Britain. The first municipal softener was ronstructed by Plumslead in 1854. Development o f the softener took a novel tum in 1912 by- the construction, at the Hooten wo rks of the West Cheshire Water Board, of a base exchange softener. Since India was under British occupation, water supply schemes in India were undertaken practically about the same lime as in England, though with a slower rate. In 1870, a water supply system was co nstructed at calcutta. Till Independence, only few cities had protected water supply systems.
1.3. SOURCES OF WATER The following are common sources of water (i) Rain Water (ii) Surface water (iii) Ground water (iv) Water obtained from reclamation.
1. . Rain Water
-
OVERFLOW
[t'......--fi'iiF"- TO
PUMP
10J FROM ROOF TOPS
tb)
FROM
PREPARED CATCHMENTS
-PR£PIIoRED
CATCHMENTS
AG. 1.1. DIRECT· COlLECIlON OF RAIN WATER
ghled
mater~1
l
WATER SYSTEMS
(a) From roofs of houses and dweUings : Water is stored in small underground tank or cistern, for small individual supplies (Fig. 1.1 a). (b) From prtpGIftI caJdtmmls : The surface of catchments is made impervious by suitable lining material, and suitable slope is given so that water is stored in moderate size reservoirs. This water is used for communal supplies. mostly for drinking purposes. 2. Surface Waler
~~E
_."i--- --it
,
INTAKE TOWER
TO PURIFICATION
RIVER OR LAKE
WORKS
• INTAKE PIPE
(0) CONTINUOUS DRAFT FROM STREAMS OR LAKES
""'0£
BANKS
~ a ~
~
(b )
jRlVER DI VIDE WAlL
MANNEL
WATER SUPf'LY -CANAL
FROM RIVER DIVERSION WORt(5
Ie J WATER FROM RESE.RVOIR STORAGE:
FJG. U. SOURCES OF SURFACE WA1CR
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WATER SUPPLY ENG IN EER ING
6
Surface water is the one which is available as run -off from a catchment area, during rainfall o r precipitatio n. This runoff nows either into streams or into undrained lakes. The runoff water Oowing inlO st reams can either be stored in a reservoir by constructing a dam across it, or be diverted into a water supply channe\. Thus. depending upon the scheme of collection, we get surface water from the fo llowing sources. From rillers b:~ conl;mwus draft: Water may be collected directly from the river, without any diversion work (Fig 1.2 a). (b) From river di~ion, A diversion work is conSlfucted across a perennial river and water is diverted into a canal which leads water to the site o f water purification wo rks (Fig. 1.2 b). (a)
(e)
From resert'o;, storage. Where supply is not ensured throughout the year. dam may be constructed across the rive r and water stored in the reservoir (Fig. 1.2 c). (4) From direct wake from tulIural lakes. Wate r may also be obtained through direct intakes from natural la kes which receive surface run-off from the adjoining catchment (Fig. 1.2 a). 3. Ground Water The largest available source of fresh water lies undergro und. The term 'ground waler ' refers to this water, which is stored by nature, unde r-ground in the water-bearing formation of earth's crust. The total groun~ water potential is estimated to be one third the capacity o f oceans. The main source of ground water is pr~ pitati on . A portion of rain falling on the earth's surface irfftfrates into. ground, travets down and when checked by im,ervious llIyer to travel further down, forms ground water. The ground water ruervoir consists of wate r held in voids within a geologic stratum. The ground water can be tapped from the follO\\oing sources. J
'"
(a) From natural springs (Fig. 13 a). (b) From wells and bore holes (Fig. 1.3 b). (c) From inflkraiWn galloUs, basins or cribs (Fig. 1.3 c). (d) From wells and galleries with flows augmented from some other sources : (i) spread on surface of the gathering ground (u) carried into charging basins or ditches, or (m) led into diffussion galleries or wells.
(e) From river side radiJll collector wells (Fig. 1.3 d)
j
maknal
1.
WATER SYSTEMS DITCH
(0) WATfR FROM SPRINGS
TOP
SOIL
TO~
TO RESERVOIR
CL ~\Y
.... ,,.
~
:.
MIN. WATER LEVEL ----------Ib_}' TUBE W£L.L
Ibl) SHALLOW DUG WELL
_
,. __ ... 0._'" -_ .....
.. WATER BEARING·.. -.~. ~
.....• .. . .
• STRATA
~.
p
.~
GAll " 'ERY
" - · co • t-
~. ~
... .. PIPE SY5T£M
I e) INFILTRATklN GALLERY
(4) RADIAL COLLECTOR WELL
F1G. 1.3.. SOURCES OF UNDERGROUND WATER.
4. Water obtained by redemetton (a) lJa4IintJJion.
Saline or brakisb water may be rendered useful for drinking purposes by installing desalination plants. The common methods used for desalination are: distillation, reveI'5C osmosis, ek:arodialysis, freezing and solar evaporation. (6) ~ of In1III«I .....,. nUr. Eftlueot or waste water tan be lreated suitably so lhat it may be re--osed. AD mmplc of \be controlled indirect re·use-is the intentional aniflcial recharge of ground water aquifers by adequately treated waste water. C JPYnghied
mater~1
Hydrology
2.1. THE WATER CYCLE Hydrologj is the science which deals with the occurrence, dis· tribution and movement of watcr on the earth, including that in the atmosphere and below tbe surface of the earth. Water occurs in the atmosphere in the (orm of vapour, on the surface as water, snow or ice and below the surface as ground water occupying all the voids within a geologic stratum..
....
--
-.I
P! 1tC'OI,. ...TIOtI
- - __ _ __fi.'_H ....T.
I
"""0''''''''
~---
GII'OUNO WATER !'"lOW
Iftt
0<", FIG. 2.1. niE WATER CYCLE
Except for tbe deep ground water, the total water supply of earth is in oonstant circulation from earth to atmosphere. and back to the "earth. The earth's water circulatory s~(em . is known as the W121U cycle or the hydroiq.* eyclt. Water circulates qaturally through five principal realms-{I) oceans, (u}atmosphere. (ill) lakes and rivers, ( JO)
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HYDROLOGY
II
(lv) ice caps and glaciers, and (v) underground. Hydrology concerns waler and its behaviour in all these realms. Hydrologic cycle or the water cycle is the process of transfer of moisture from atmosphere to the earth in the form of precipitation, conveyance of the precipitated water by streams and rivers 10 ocean and lakes etc., and evaporation of water back to the atmosphere. Fig. 2.1 illustrates. diagrammatically. the complete hydrologic cycle.
The hydrologic cycle consists of the following processes:
t. Evaporation and Transpiration (E) The water from the surfaces of ocean, rivers, lakes and also from the moist soil evaporates. The vapours are carried over the land by air in the form of clouds. Transpiration is the process of water being lost from the leaves of the plants from their porcs. Thus, the total evaporation (E), inclusive of the trarnpiralion consists of :
and 2.
(i) Surface evaporation (1I) Water s urface evaporation (a) From river surface (b) From oceans (iii) Evaporation from plants and leaves (transpiration) (iv) Atmospheric evaporation.
Pruipitation (P)
Precipitation may be defined as the fall of moisture from the atmosphere to the earth surface in any form. Precipitation may be in two forms:
(a) Liquid Precipitation : i.e. rainfall. (b) Frozen Precipitation : This consists of
(I) Snow (iiI) Sleet
3.
(ii) Hail (iv) Freezing rain.
Run--orr (R)
Run-off is that portion of precipitation that is nO( evaporated. When moisture falls to the earth's surface as evaporation, a part of It is evaporated from the water surface, soil and vegetation and through transpiration by plant, and the remainder precipitation is available as run off which ultimately runs to the ocean through surface or sub-surface streams. Thus, run off may be classified as follOM : (1)
Suif_ "'" off
Water flows over tbe land and is tint to reacII the streams and rivers, which ultimately discbarJe the water to the sea. ,too IT
rta
WATER SUPPLY ENGINEERING
12
(1) InWfIow or sub-surfG« rllII off A portJon of precipitation infiltrates into surface soil and, depending upon the geology of the basin, runs as sub-surface run· off and reaches the streams and rivers. (3) Growul waler flow or IIan? flow h is that portion of precipitation, which after infiltration , pcrcolates down.and joins the ground waler fe5ClVOir which is ultimately
connected to the ocean. Thus, the hydrologic cycle may be expressed by the following simplified equation.
Precipitation = Evaporation (P) = (£)
+ Run off
+ (R) provided adjustment is made (or the moisture beld in storage al the beginning and at the end of the period. 2.2. PRECIPITATION
To the hydrologist, precipitation is the general term for all forms of moisture emanating from the clouds and falling to the
ground. The following are the essential requirements for precipitation to occur : I. Some mechanism is required to cool the air sufficiently to cause condensation and droplet growth.
2. Condensation nuclii are also necessary for formation of droplets. They are usually present in the atmosphere in adequate quantities. 3. Large scale COOling is essential for significant amount of precipitation. This is achieved by Hfting of ai!. Thus a meteorological ' phenomenon of lifting of air masses is essential to result precipitation. Types of Precipitation
Precipitation is often classified according to the factors responsible for lirting. Broadly speaking, there are four types of precipitation. (1) (2) (3) (4)
i.
Cyclonic precipitation. Convective precipitation · ~...Orographic precipitation Precipitation due to turbulent ascent.
Cyclonic Precipitation
Cyclonic precipitation results from lifting of air masseS converging into low pressure area or cyclone. The cyclonic precipitation may be divided into (lI) frontal precipitation, and (b) non-frontal precipitation.
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138
WATER SUPPLY ENGINEER IN G
R.L of original water surface, before pumping Sianed
= 122.0
ffi.
R.L of water in tbe well at constant pumping = 171.1 m RL of wolter in the observation well = 121.3 m. R.L of impervious layer = 92.0 m Radial distance of observation well from the tube well
=50
m
Determine: (a) the field permeability coefficient of the free aquifer, and (b) radius of zero ?rawdown. [Ans. (a) 60.7 mJday (b) 157 m)
17. Design a tube well for the following data
(i) (ii) (iiI) (iv) (v)
Yield required = 0.2 cumec Thickness of confined aquifer 40 m Radius of circle of influence = 300 m Permeability coefficient = 80 m/day
=
Drawdown
=6
m
lABs. 28 em, or say 30 em)
18. During a recuperation lest, the waler in an open well was depressed recuperale~ 1.5 m in I hoor. Estimate the yield
by pumping by 2 m and il
from a well of 2 m diameter under a depression head of 2 m silumed in
the same area. Derive the expression your use.
3
[ADs. 8.7 m /hourj
19. A tube wetl penetrates fully a 8 m thick water bearing stratum (confined) of medium sand having coefficient of permeability of 0.004 rnjsec. The well radius is 15 em and is to be worked under a drawdown of 3 m 8t the well face. Calculate the discharge from the well. What will be percentage increase in the discharge if the radius of the well .is doubled ? Take radius of zero drawdown equal to 400 · m in each case. 3
IAIls. (i) 275 m /hour (li) 9.6%) 20. Design an open well in filJe sand to give a discharge of 0.005 cumecs when worked under a depression head of 3 metres. Take the value of the specific yield for fine sand as 05 m3lhou r per square metre of area, lADs. Dia. 3.9 m I under unit depression head.
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Water Demand and Quantity
S.l. INTRODUCTION
Before designing a proper water works project, it is essential to determine the quantity of water thai is required daily. This involves the determination of the following items. 1. PopullJlion determinaJion. Del~rminalion of population is onc of the most important factors in the planning. if the project has
to serve the community for a cenain design period. Normally. a design period 0(20 to 40 years is selected. What will be the population at the e nd of the design period, is the basic question. This ca n
be achieved by using various methods for population forecast. 2. RaU 0/ demand. The water consumption in a city may be convenienlly divided into the followin g categoric.<) : (i) domestic (it) trade (iil) agricultural (iv) public and (v) losses. The 101a\ consumption of water depends upon several factors, ,such as climatic condition, cost of water, living standards of the inhabitants, pressure in the pipelines, type of supply etc. The total quantity of water required divided by the total population givespercapica water demand. The accurate measurement of consumption is often very difficult because standards of supply and maintenance· vary widely. S.2. DESIGN PERIOD
Generally, water supply projects are designed for a design period of 20 to 40 years, after their completion. The lime lay between the design and completion should not be more than 2 years. In some specific components of the project, the design period may be modified. Different segments of water treatment and distribution sys('39)
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WATER SUPPLY EN"G IN EERING
140
terns may be approximately designed fo r differing periods of time using diffcring capacity criteri a, so that expenditu re far ahead of utility is avoided. Table 5. 1 gives the design periods far various components o f a water supply projcci. TARLE 5.1
D£SIGN PERIODS FO R PROJ ECT COMroNEr'roTS C-.ptHI~tfl
f)~$ig,. period (yN~)
, 2 3.
4.
,. 6. 7.
••
Storage by da ms
I"mlnu ion worb Pump sets (,) All prime moYen o:cqH electric motors (il) Electric mOlon and pum ps Wal e r treatmen t units Pipe: co nnect ions the several trea tment units and other small appun enanccs Raw water and clear wa ter conveying maiM Ocar water rescrvoin al the: head ""orb, bal ancing tanks and service reservoirs (O\ler head or ground level)
"
Distribution system
SO 30 30 IS IS 30 30 IS
30
5.3. POPULATION FORECAST
The data about the present populatio.n or a city under quest ion can always be obtained from Ihe records of the municipality or civic body. However, a water supply project is designed to cate r the needs of the community upto the e nd the design period which may extend upto 2 to 4 decades, berorc the project is abandoned or enlarged by reason of inadequacy. There are several methods for population forecast, but the judgment must be exercised by the engineer as to which method is most applicable fo r a panicular location. The increase in populatio n o f city depends upon several facto rs such 3S living conditions of the city and its environs, industrial potential, state o f development, loca tion with respect to road and rail links, clima tic conditio ns etc. The entire population o f a city may not be evenly distributed , due to variations in the land use pattern and available facilities etc. The populalion density, indicating the number of persons per unit area, and the distribution o f population sho uld also be studied for efficient design of the distributio n system. Following are some of Ihe important met hods of population forecasts or population projections : Of)
r htedm
na
WATER DEMAND AND QUANTITY
141
I. Arithmctical increase method .
2. Geometrical increase method. 3. Incremental increase method. 4. Decreased rate o f growth method .
5. G raphical extension method. 6. G raphical comparison method. 7. Zoning method o r master plan method. S. Ratio and correlation rnClhod. 9. Growth composition an;llysis method. 1. Arithmetical Increase Method
This is .the most simple method of population forecast, though In this method, the increase in population from decade to decade is assumed constant. Mathematically. this hypothesis may be expressed as
it generally gives lower results.
dP =K
... (5 .1 a)
dt
where
~
is the rate of change of population and K is a constant.
From the census data of past 3 or 4 decades, the increase in population fo r each decade is found, and from that an average increment is found . For each successive future decade, this average increm~nt is added. The future population P,. after n decades is thus given by P,. =P+nl ... (5.1) where
P,. = future population at the end of n decades P = present population, I = average increment for a decade.
This method should be used for forecasting population o f those large cities. which have reached their saturatio n populatio n.
2. Geometrical Increase Method or Uniform Pert'entage Growth Method In this method, it is assumed that the percenloge increase in population from decade 10 decade is constant. From the population data of previous three or four decades, the percentage increase in population is found and its average is found. If I, is the average percentage increase per decade, or is the increase per decade expressed as ratio, the p?pulation P,. after n decades is given by
r,
,
.1
WATER SUPPLY ENGINEERING
142
Pit
=P (1 +
1&
r
= P (1 + r,l"
... (5.2)
Eq. 5.2 can be derived very easily as under :
Lei P be the present population and PI be the population after one decade. Then, Similarly. population Pl after two decades is P2 = PI
Hence
+
I PI = PI (I +-.&-I )'.= P (1+1& I )' 100 P.. = P ( I
+ 1&
... (ii)
r
While the arithmetical average method is ana logous to the 'simple interest method', this method is analogous to the computation of income by the 'compound interest method'. This method gives higher results since the percent increase never remains constant but,
instead. decreases when the growth of the city reaches to saturation. The value of " can be Jound from the expression
PO)"O -1 ',= ( P
... (5.2 a)
Alternatively, " can be determined by computing the average of growth rates of several known decades of the past
r
increase in population f h d d o riginal population or eae eca e.
Knowing r], rz ..... r" for each decade, the average value r, can be found either by arithmatic average method or by geometric average
method : (i) By arithmatic average method :
"
rl + rz + ...... r" n
... (5.2/1)
(b) By geometric average method
r,= (rl.r2 ...... r,,)IF"
...(5.2 c)
The field engineers use the arithmatjc average method for computing r, (or I,) since it gives slightly higher (and hence safer)
WATER DEMAND AND QUANTITI
143
values. However, the Manual on water supply and treatment recom· mends to use the geometn'c mean method.
3.
Inc~mental
Increase Method
This method combines both the arithmetic average method and the geometrical average method. From the census data for the past several decades, the actual increase in each decade is first found. Then the increment in increase for each decade is found . From these, an average increment of the increase.r (known as incremental increase) is found. The population ~n the next decade is found by adding to the present population the average increase plus the average in· cremelllal increase per decade. The process is repeated fo r the second future decade, and so on. Thus the future population at the end of n decades is given by : Pit = P + nl
where
P = present I = average r = average n = number
+ n (n +
J) r 2 population increase per decade incremental increase of decades.
... (5.3)
. Eq. 5.3 can be easily derived as under Let P be at the prc-<;em population. The poJiulation PI after one decade will be PI = P + I + I r ... {i)
Similarly, population
p~
after 2 decades is
2(2+ I) r P, =P I +I+2r=P+21+3r=P+2/+ 2 Population P1 after 3 decades is
PJ =P, +I+3r=P+31+6r=P+31+
3 (3
+
2
I)
r
Hence, population Pit after n decades is Pit = P
+ n 1+ n (n + 1) r 2
4. Decreased Rate of Growth MethOd or Logistic Method It is found that the rate of increase of population never remains constant, but varies. Fig. 5.1 shows a plot between the population P and the time T for a developing city. The population of a city will grow until it reaches a saturation"population wbich is established by limit of economic opportunity. All populations thus grow according to the logistic or $·curve. The curve ABC (Fig. 5.1 a) starts with
'44
WATER SUPPLY ENG INEERIN G
a low rate o f growth, fo llowed by a high ralc and tben at a progressively lower ra le 10 the saturation populatio n. Thus in Fig. 5. 1 (a) pa ri AB has geometric increase while there is fi rst o rder increase from B to C. From D to E, near point of inflection, there is straight line increase. The curve abc is the first derivative cUlVe indicating the rate of growth .
,
p
SATURATION POpu"'ATION PI _ p M
.t
E
z
B
Q
p.
,,
~
~
'.
0
~
. 0
p' A
C
,
0
TIME T _ (O)B,
.t ~
9
.-
$
',
.
.~
;lip
"
p'
§ c, T IME T -
,.)
FIG. 5.1. INCREASE IN POPULATION
wrrn
'n ME ; LOG ISTIC CURVE
It is seen that in the pari be of the curve, the rale of increase decreases. Fig 5.1 (b shows the same plol in which the populatio n is plo n ed on log scale. It is clear that for tbe part A I Ht. we have increasing rale of growth while for the pan B\ Cit there is decreasing rate of growth of population. Thus, as the city becomes large, a decreased rate of growth may be expected. This facto r should be taken into account while computing future population, as illustrated in Example 5, I. C JPYnghied
mater~1
WATER DEMAND AND QUANTITY
' 45
Logistic curve analysis The logistie curve used in modelling population trends has · S-shape, as shown in Fig. 5.1 (a). The Gomperlz curve and the logistic curve arc both used in establishing long term population trends of large population centres. Both of these curves are S-shapcd and have upper and lower asymptotes. According to P.E Verhulst. the logistic curve can be represented by the equation
lo~
(p,; P l- log ( PI;: PO)=_ KP, . t
P, = saluratio n population Po = population at starting point A P = population at any lime t from K = constants
where
_p_x p,PoPo
_I
P, - P
; = I + ( PI
or
P
=
loge ( - KP, . t)
;0Pol log; I(- K P, t) P,
1+
Selecting
A.
[(,;P) x (p,:-p,l] = -KP,I
.. log. or
origi~
... (5.4)
P' Po Po lO~-I(-KP,. I)
P -Po
' P.
m
... (5.5)
and - K . P, = n, where m and n are
constants, we get
P=
p, ... (5.6) 1 + m log.e- I (nl) If three pairs of characteristic values Po . Ph and P 1 at time t = If), I II and I tl 21, are selected from the useful range of census population data. the values of PI. m and n can be found fro m the following simultaneous equations
=
= =
P _2Pop,P l -n(PO +P2) ,Po P2 -P f
... (5.7 a)
m = ~P'-'n--'P-"
... (5.7. b)
p,
n = !IOg. II
[P,(P, - P,)] PI (P, - Po)
Eq. 5.6 can also alternatively expressed as P P, l+e,,+bI
... (5.7 c)
... (5.8)
C JPYnghied
mater~1
WATER SUPPLY ENGINEERING
14'
PI> a and b may be dCJerminb:! from three successive census populations and (he Eqs :
pl (Po + P z) pl
PI = 2 PaP I P 1 a = lo&c b =
!
n
... (5.9 a)
POP} p - Po I
•• .
P.
100Po (Ps ~ PI (Ps
- PI)
(5.9 b)
... (5.9 c)
Po)
where n is the lime interval between successive censuses. The values of PI> a and b so obtained may be substituted in Eq. 5.8 to estimate the population for any period, beyond the base year corresponding 10 Po- Eq 5.8 in more useful for computation with the help of electronic calculators. See example 5.4 for ill ustrat io n.
S. Graphical Extension Method In this method, a curve is drawn bctween the popu latio n P and lime T, with the help of census data of previous few decades, so that the shape ,of the population curve is obtained - upto the pr~e nt
period. The curve is [hen ca refully exte nded from the present
.
D€SIGN POPULATION. 80 000
eo
/'
10
•
V
!
/ /
20
0
-
/
,
Xl
10
Cl.fi\I[
/
60 00
E;~r;,;O
PVIIOO
V
1931
$41
. 19~
1961 19 7 1 1981 TIME T
1911
2CX>I 2011
FlG. j.2 GRAPHICAL EXTENSION MEnlOD. ; JPYnghied
mater~1
WATER DEMAND AND QUANTITY
147
to the future decades. From the extended part of the curve, the population at the end of any future decade is approximately determined.
6. Graphical Comparison Method This method is a variation of the previous method. It assumes that the city unde r consideratio n will develo p as similar cities developed in thc past. The met hod consist of plotting curves of cities that; o ne or more decades ago, had reached the present population o f the city under consideratio n. 9 0 000
V
eo 000 70 000
~ /'
000 40 000
l!6 ~
V
./
/ V
000 1930
1940
19!tO 1960 YEAR
1980
2000 (AI
1940
1960
""'.
' " , tel I96!S to) 1900 lEI
10:'.
1030
(8)
FIG. 5.3. GRAPHICAL COMPARISON METHOD
Thus, as shown in Fig. 5.3, the population of city A under consideration is plo tted upto }970 at which its population is 62,000. The city B having similar oonditiom, reached the popuJaUOa. of 62()(X) in 1930 and its curve is plo tted from 1930 onwards. Similar curves are plotted for other cities C, D and E which reacbed tM population of 62 in 1925, 1935 and 1920 respectively. The cune of city A can be then be continued (shown by dotted line). allowtD, it to be influenced by the rate of growth of the larger atka. Ia practice however. is is difficult to find identical cities with fCSpect to population growth.
7. Zoning Method or Master Plan Method This is probably a scientific metbod using tbe limitatiom lmposed by tbe town planner in tbe increase in douily ofpopu/tJtion of various parts of the city. For this, a master plan of the dty is prepared, ~
WATER SUPPLY ENG INEER ING
148
dividing it into various zones such as industrial, commercial, resident ial and ot her zones. Each zone Is allowed 10 develop as per master plan only. The future population o f each zone. when (ully developed can be easily found. For example, sector A of a residential zone has HXX> plots. Allowing 5 persons per plot. the populat io n of this sector, when (ully developed, will be 1(0) x 5 = 5(0) perso ns. Similarly. the developme nt of each zone can be estimated. This met hod is more advantageous because oflhe fact that the to tal water require me nt of the city depends not only for domestic purposes, but also for commercial, industria l, social health a nd other purposes.
Population de nsity is generally expressed as number of persons
per hectare, and their values may be estimated from data collected on existing areas and fcom zoning master plans for undevelo ped areas. Table 5.2 gives the values of common population densities.
-
TABLE 5.1.
,
1. Residential
~.
••
.....
Sinl le family units
Rc:OOenlial • mulfiple family units. Apanmenh Commerical area
S. Industrial area
'f
COMMON POPVUTION
DENSITl~
PWMN p«' "'«Ian
15 -80 80- 250 250 - 2500 40 - 75 15-40
8. RaUo and Correlation Method The population growt h of a small town or area is rclatcd to big towns or big areas. The increase in population of big cities bear a direct relationship to the population of the whole stale or country. In this method, the local to national (or sta te) population ralio is determined in the previous two to four decades. Depending upon conditions or other factors, even changing ratio may be adopted. These ratios may be used in predicting the future population. This method takes into account the regional and nat ional factors affecting poPI,l~tio n growth. This method is useful for o nly those areas whose population growth in the past is fairly consistent with that of state or nation. 9. Crowth eo.poslUon Analysis Method The change in population of a cil)' is due to three reasons: (i) binh, (u) death, and (iii) migration from .villages or other towns. The population fo recast may be made by proper analysis of these three factors. The .difference berween binh rate and death rate gives Ihe MIUra} increase in tbe population. Thus, , P. _ P + Nalural increase + Migration. C JPYnghied
mater~1
...
"
WATER DEMAND AND QUANTITY
The estimated natural increase is given by the following ex·
pression: ,Natural inaease = T(/.i-JDP)
... (5.10)
T "'" design (forecast) period. P - present population. I. = avera~ binh rate per year;
where
ID = average deltii. nte per yeuV ' 5,4. FACTORS AFFECTING POPULATION GROWI'H .... The population growth of a city depends upon rollowing (actors. These factors affect considerably the estimated .population. 1. Economic factors. Such as development of new industries, discovery of oil or other minerals in the vicinity of the cily.
2. Devtlopment programmes. Development of projects of national importance, such as river valley projects etc. 3. Social facilities. Educational , medical, recreational and other
sooal facilities. 4. Communication links. Connection of the town with other big cities, and also to the mandies of agricuhural products.
5. Tourism. Tourist facilities. religious places or historical buildings.
6. Communi'Y life. Uving habits, social customs, and general Cducat'ion in the communitf. . -- '. 7. Unforeseen faclors. Earthquakes, floods, epidemics, frequent famines etc. 5.5. DETERMINATION OF POPULATION FOR INTER·CENSAL AND POST·CENSAL YEARS Sometimes, it may be required to determine the population for the intermediate portion of a censal period, from the available · data. This can be done with the help or arithmetical increase metbod and the geometrical increase method. lei tIP be the increase in population during a time period dT. U
~= KA = . constant,
If
:~= KG . P,
then the growth wiU be arithmetic.
where KG is the proportionality factor, then
the growth will be geometric. The values or the ractors KA aDd can be · determined from the ronowing expressions :
Ka
C JPYnghied
mater~1
ISO
WATER SUI"PLY ENG IN EEKING
K.. = p, - h TL T£
... (5. 11)
K,; = log. p, - log. p, ... (5. 12) h T£ PL = population at the last census al data h P f = population at the earlier census 31 dale h.
and
where
Now if the population PM is the desired mid·year population at a date TN. its value is given by the following expressio ns.
AriIJrmdiaU I"",,", MtI/wd For inter censal period :
or
Pili =P£
+ KA.(T/II- Te}
Pill = PE
+ ~N,-- ~£, (PL -
... (5. 13 a)
Pc)
... (5.13)
PE)
... (5. 14)
For posl-censal period ,
+ ~ (T/II - Td T/II-TL PII = PL + TL T£ (PL -
PM = PL
or·
G«Hrtdrical ;trCrf!4Se Method For inter-censal period,
or
loge PM = loge PI:
+ KG(T/II -
loglo P" = JogIOP!
+ ~N, -- ~E, (IOg10PL -
... (5.15 a)
Te)
logloP,e)
... (5.15)
For post-censal period : 10g. P. '" 10g.P,
+ KG(T.- T,)
Joglo P", = IOgl,PL +
or
... (5. 16 a)
~.w, - ~L, (IOg10 PL -
loglO P£)
.
... (5.16)
ExaIIIple 5.1. The following is the population data of a city, available from past census records. iNlmnine the population of the city in lOll by (a) arithmetical incrwse method (b) geometrical increase method (e ) incnmt!ntai increa.r~ , m~thod (d) graphical mdhod (~) d«nased rat~ of growth method.
r_
.-,
,
1931
I
(PI
1941
I_ I"'"
1951
1961
1971
19."
·1991
26800
41'"
S7S00
68000
74100
Solution : The oomputations about increment, % increment and incremen· tal iDa"eae pe:- decade are arran£ed in Table 513 below ·:
Of)
r htedm
na
WATER DEMAND AND QUANTITY
lSI
TAB LE 5.3. y~
PoplI""ioII
, 1931
Z 1_
194 1
16500
1951
26800
1961
'1 500
1971
"500
1981
68000
ItlDYtltlttli
... iflCTml~1II
Itw:nrrI~trtal
lhcrNu/"
,.~~
IH"d~
i~
... i1lC1Ytrl~'"
"00
37.50
10300
62.42
6
+ '800
14700
".8S
16000
38.55
10.500
18.26
' 100
8.97
Total
62,100
220.55
AvetlIge
-,-
-,-
1991
,
+'400
7.57
+1300
16.30
- 5S00
20.29
-
4400
9.29
+ 1600 1600
53.45
74100 62, 100
-
10350
220.55
- 36.76
.-,,,.-
53.45 -'-,-
13.36
In (he above table, percentage Increase fo r the first decade (1931 10 1941) = 16500 - 12000 100 = 4500 100 = 375 % t21XX> x 12000 x . . Similarly, % increment for other decades have been calculated. 1. AriJhmdicaI 1nct'«lSe Method
where
P. = P + nl (Eq. 5.1) P = population in 1991 = 74,100 n "" number of decades
1991 - 1971 2 10 I :z average increase per decade= 10350 (from Table 5.3) p. = 74100 + 2 x 10350 = 94Il00. 2. Gt:ontttricPl Incrrase Method j>. = P [ I
Here,
+ 1&
r
I, = average per cent increase per decade = 36.76% (from Table 5.3)
..
(5.2)
G JPYnghtcd makrtaJ
WATER SUPPLY ENGIN EERI NG
152
p ~ 74100 ( I •
+ 36.76 )' 100
~ 1,38,590. The above computatio ns are based on the va lue o f Ix computcd
by arithmatic average mClhod. If, however, geomet ric average me lhod is used, as recommended by the Manual, we have
I,
=
( I, •. 161' ..... J,II )
1 / 11
= ( 37.50x62.42x54.85x38.55 x lS.26x8.97) II. = 30.54 (against a value o f 36.76)
P.
~ 74100 ( 1 + ~~4 )' ~ 126272
3. IflCr'emenlal IflCnase Method
p" = P + "I
+ n (n + 2
I) r
where,
I
and
r = average incremental increase
~
... (5.3)
10,350
= 320 (from Table 5.3)
P. ~ 74 100
+2 x
10350
+ 2 (2
t
I ) x 320
= '5760. 4. GrapIaicGJ Extensiim Metlwd Fig. 5.2 shows the plot between the population and the time. The dotted portion o f the curve is the extended part fIOm 199 1 to 201 1, (ollowing closely irs trend. From the extended part. the
populalion al the end of 2011 = 8O,CXXl. S. Decrm.sed 1We
of
Growl. Mdlwd
Column 6 of Table 5.3 give<; the decrease in the per cent increment found in column 4. In the initial ponion of the census reoords, there is no decrease in the percent increment, and hence this period has nOI been included in Ihe computations. The total decrease in percent increment for four decades comes out to be 53.45, giving an average rate of dtcrl!ase in the percentage growth _ 53 45 - 13.36%
4
In column 4, the average increment rate per decade was found to be 36.76%, but due to decrease in the rale of growth, Ihis figure will be modified as under :
Of)
r htedm
na
WATER DEMAND AND QUANTITY
Year
Average"increment
per decade 2001 2011
36.76
23.40
'" Average rate of dl'crease in the increment 13.36 13.36.
Nel incremenl rate (%)
23.40 10.04
Hence the population at the end of each decade will be as under : 74100 + 23,40% of 74100 = 91439 2001 2011 91439 + 10,04% of 91439 = 100619 Example S.2. In a town, il has bun decided 10 provide 200 litres per head per day in the 21# century. Estimate "the domestic water requirements of this town in the year AD 2000 by projecting the population of the town by the increm.ental increase method, from the data given below . y~
PopMlIII..",
1940
2,s0.OOO 4.80,500 5,s0,300 6,38,600 6,95,200
t950 1960
1970 1980
Solution : The computallons about Increase In populat ion per decade and incremen tal increase are done in Table 5.4 : y-
,.....,..
TABLE 54 I~ ...
,.,"-h 1940
250000
1950
""'00
1960
5S~
"""00
,-
,--, 1(-) 160700
(+) 18500
88300
1970
6l86OO
1980
'''200 Tou1 Avcrale
,"'445200 200 _ 111300
•
(-) 31700
(-) 173900 173900 (-r;=-(-)S7967 3
.1
WATER SUPPLY ENGIN EERING
154
I = 111300 and r = - 57967 Expected population in the year 200JAD (ie. after 2 decades) is Here,
P. = P + II J + n (n 2+ 1) r• where n = 2 p ... = 695200 + 2 (111300) + 2 (22+ 1)
I
-57967 )
= 695200 + 222600 - 17390 1 = 743899 Hence water requirement in 2000 AD @ 200 IitTcs/head/day
= 743899 x 200 == 148.8 x U! lilres/day = 148.8 million litres/day Example 5.3. The population of city in successive decennial census ;s given as 41500 and 57500. Assuming the census dale as 10th April,Jind the midyear population as Jdh July for (a) 3rd inter-censal year, and (b) 6th post-censal year by the arithmetical increase method and the geometn'cal increase method. Solution: (a) For 3rd inUr-censa1 year
TN - T£ = 3
+ (101h July -
101/1 April)
= 3.25
TL - T£
= 10
TM
= 3.25 = 0325
TL
-
TE
T£
years
10
Arithmetical Increase
.
Geometrical Increase
PL = 57500
IOglOPL = 4.7597
p, - 41500
logloP£ = 4.6181
PL - PE - 16(1» 0.325 (h - P,) =5200
.. PM = 41500
+ 5200
= 46700.
IOglOPL -log1oP£= 0.1416 0.325 (IOgIOPL - logIOP£) = 0.0460 logloP.v= 4.6181
+ 0.0460
= 4.6641
(b) F., 6lh ,..,......,,/ "'"
T",- TL =6.25;
TL - T£ = 10 years
G JPYnghtcd maknal
IS'
WAlCR DEMAND AND QUANTITY
Arithmetical Increase
Geometrical In crease
PL = 57500 P£ ... 41500
PI. - P£= 0.625 (P, -
loglo PL = 4.7597 10gtoPE = 4.6181 logl'PI. - 10gl' P£= 0.1416
16(XX)
Pd
0.625 (IOgt.PL - logtoP£)
= 0.0885
=10000
P. = 57500
+ 10000
10gto P", = 4.7597
+ 0.0885
= 4.8482
= 67500
p", = 70500 Note. The geometric estimates are higher for post-censal years and lower for inter
1951 1971 1991
50000 110000 160000
Estimate: (0) the saturation population, aIId (b) expected population in 20/1. Solution:
n = 20 years
Here
P. = 50,(0); P 1=I , IO,(XX); Pz= 1,60,lXK> Hence [rom Eqs. 5.9 (a), (b) and (c) P
•-
2 x 50000 x II 0000 x 160000- (II 0000)'(50000 + 160000) 50000 x 160000 - (110000),
=1_
= - 0.0673 Hence the equation or the logistic curve is P = 1'lO488 1 + e LI3J - ""7), In 2011, t - 2011 - 1951 = 60 yea". . 1'lO488 P.u = I + e LiIJ -"", . " =
190488
I
+ 0.04954
_ 181500
.
C JPYnghied
mater~1
IS.
WATER SUPPLY ENGINEERING
5.6. WATER DEMAND An average person may consume no more than 5 to 8 Iitres a day in liquid and solid foods, including 3 to 6 litres in the form of water, milk and other beverages. However, the per capita con· sumplion of Water drawn from public supply is quite large. Total water requirements may be divided into the following five categories:
1. 2. 3. 4. 5.
Residential or domestic use. Institutional use. Public or civic use. Industrial use. Water system losses.
1. Residential or domestic use
The residential or domestic use includes Water requirements for drinking, cooking, bathing, washing of clothes, utensils and house, and flushing ofwatercloscts. Provision is sometimes made for domestic animals. IS : 1172-1957 recommends a per capita water consumption of 13S Iitres per day. Table 5.5 gives the break up of water requirements for domestic purposes, which forms about 50% of the total water requirements per head per day, for all the five categories mentioned above. Table 5.6 gives the water requirementS for domestic animals. It should ·be noted that water required for lawn sprinkling and for residential gardens is over and above the values given in Table 5.5. TABLE 5.5. WATER ·REQUIREMENTS FOR DOMESTIC PURPOSES
..u-... of -,,,, ill S.No.
Iibu",.".
Dot. Ph"
-"'"
1
B.thinS
2
WlI$hini of clothea
"
,
3
FlU5hini of W.e.
30
S
WubID, of utensils
•
Cootinl
S
7
DrinkiD.
S
20
I. I.
Wuhinl the house
T","
135 .....
WATER DEMAND AND Q UANTITY
IS7
TABLE 5.,. CONSUMPTION OF WATER FOR DOMESTIC ANIMAlS AND LIVE-STOCKS
51'10.
AllimGls
Wain" OOfU""."i_ i"Iibv,- IIIU_'
,.....,
I
Cow and buffalo
2 3
H~
Dog
4
Sheep or gool
The Manual Ministry o( Urban the following rates and non·domcstic
8 •• 12 5 to 10
on water supply and treatment, prepared by the Development (MUD) New Del.hi recommends in Iitrcs per capita per day (Icpd) for domestic needs (Table 5.7) :
TABLE 5.7. WATER FOR DOMESTIC AND NON·OOMESTIC NEEDS Dncri,4kM
"'-"'o/W(lJn(/q
l. For oomm unilic:s wil h popul ation
.uplo 20,000 (a) Water su pply through stand post (b) Water supply through house scrvice connection For commu nitie. with population 2 20,000 10 100,000 3. For communilies with population above 100,000
2.
40 (min.) 70 10 100 100 10
ISO
150 10 200
Institutional Use
The manual on water supply and Treatment recommends the values of water requirements for institutional needs as given in Table 5.7. 3. Public or Civic use
Water required for public or civic uses may be for the following purposes : (i) Road washing, (iI.) Sanitati~ n, (iii) Public parks, and (iv) Fire fighting. For road washing in the municipality area, a provision of 5 Hires per head per day is made. Similarly. (or sanitary purposes, such as cleaning public sanitary blocks, flushing sewer systems etc., a provision of 3 to 5 litres per head per day may be made. Water required for maintaining public parks etc. may be 2 to 3 liues per square ·metre per day, G JPYnghtcd maknal
WAlCR SUPPLY ENG INEER ING
ISS
TABLE 5.1 WATER FO R INSTITUTIONAL NEEDS I MliluI_
Wilt"" rTfI"inmt!lII
(lilru F I.
HO!ipi la ls (including laund ry)
(a ) No. of Ixi.b elIceeding 100 (b) No: of beds not acceding 100
450 (per bed) 340 (per bed)
2
Hotels .
ISO (per bed)
3.
HOIitels
m m
4.
Jretllp"da,)
Nurse"s homes and mediCIIl quartc:~
S.
boardi ng schoolstC:Ollegcs
m
6.
Restaurants
70 (per sea t)
7.
Air pons and sea pon!
70
8.
Junction slalions and
70
intenned iatc: sta tions where mail and apress stoppage: (bot h railways and bus slat ions) is provided)
•• 10.
" " " "
Terminal slations intermediate sl1llions
(o:cludin, mail and express SlOps) II .
Day $Chools f<:OlIeces
12
Off=
13.
Factories
(cou ld be rcduttd 10 2S when: bathing facil ites are not provides)
"
(could be rnluccd \0 30 where no bathing rooms arc req uired
to be provided ) 14.
Cinema, concert a nd theatres
"'"
IS
Fire demand Water required-for nre fighting is usually known asflTt! demand. It is treated as a function of population and may be compuled from
the fallowing formulae : 1 Kuichling's fonnuls Q - 3182 v'P where Q ""' quantity of wate r in litres per minute P = Population in thousands
2.
Bwton~
... (5.17)
fomuda Q ~ 5663 v'P
... (5.18)
WATER DEMAND AND QUAJIITITY
15'
.l• .Freeman's formula
Q=
J136 ( ~+ 10 )
... (5. 19)
and
F=2.8VP
where
F = number of simultaneo us fire streams
... (5.19 a)
4. NaJiotull Board oj Fire Underwrilers formula
Q
= 4637 VP (I
-
om VP)
... (5.20)
Though the to tal demand of water for extinguishing firc is usually vel)' small, the ratc o f consumption is vel)' high. It depends upon bulk. co ngestio n and fire resistance of buildings. The minimum limit of fire dema nd is the amount and rate of supply that would extinguish the largest probable fire in the city. Fire hydrants of 15 to 20 em diameter are normally provided on all street corners, and at suitable intermediate points. These are gene rally connected to water supply mains. Whe n fire occurs, pumps installed on fire brigade trucks are rushed to the site and connected to fire hydrants from where they throw jet o f water under very high press ure. Thc pressure varies between 1 to 2 kg/ cmz (0. 1 to 0.2 N / mm 2). For a fire of moderate nature, three streams each of 1100 litres per minute are required. For a city of o ne lac population, the fire flow, required by National Boa rd o f Fire Underwriters (now known as 'American Insurance Association) comes out to be 40,500 lilres/minute. Assuming an average to tal consumption o f 150 litres per capita per day, tota l water requirement comes o ut to be 150 lac litres per day or 10,417 litres/minute. Thus the rate of flow required-·for fire demand, (i.e. 40,500 lilres/minute) is vel)' much higher than the tota l flow rate required fo r other purposes. However, the provisio n for fire stream is made o nly for 3 to 5 hours fire flow . The total quantity of water calculated o n yea rly basis is usually very small because. fire breaks o ut o nly few times in a year. Thus, for the city of population of I lac, if fire breaks out 12 times a yea r, each of a duration of 4 ho urs, the per capita water demand
= Yearly requirement of Water 365 x Population
,
= (I04l7 x60x4) 12 = 082 365 x HXXXXJ .
n I
res.
Thus for Indian conditiOns, a proVISion of I lilre per head per day will be sufficient ror fi re fighting. Each fire hydrant provided for this purpose has three streams, and each stream discharges 1100 liues per head per day. "
"'.,
160
WATER SUPPLY ENGINEERING
The above formulae for water demand do not take into account the frequency of fire that may oa;ur. II may be determined from the following expression : 4360 T o. m Q= o.7S7 liucs/ minute .. .(5.21) (I 12)
+
I = duration of fire in minutes T = period of occurrence of fire, in yea rs. The recommended minimum values for Ihe above formula arc: I = 30 minutes and T = I year The man ual on water supply and Treatment by MUD recommends that a provision in kilo Htres per day based on Eq. 5.22, where P is the population in thousands may be adoplCd for communities larger Ihan SO,(XX) :
where
Q ~ IOOVP ... (5.22) It is desirable that one third of the fire fighting requirements form the part of service slorage. Thus, for a population of 100,000, Q will be = 100 ViOO = 100 kilo litres per day. 4. Induslrial use The presence of industries in or near the cily has great impact on the water demand. The quantity of water required depends upon the type ofindustry. For a ci1}' with moderate facloric~, a provision of 20 to 25 percent of per capita consumption may be made for this purpose. The fore cast for this demand will be based on nature and magnitude of each industry and the potent ial for its expansion. Table 5.9 gives data about the needs of" some industries. TARtE
5.9. INDUSTRIAL NEED.
b"'_ 1. Automobile Dislillary 3. Fertilizer 4. S. Special qualily paper 7. Siraw board Petroleum refinery SleeI. 10. Su,ar 11. Tcnile ~
In''''' p,,,,,
•• •• ••
Uttilof ~r:I"",
Vellide I(jJo!jlre (proof alcohol) Tonne 100 .q (Ionne)
T..., Toone Tonne Tonne (erode:) T""~
Toone (cue crusbcd) 100 .. (Joodo)
WAIn- nlllli,.",."", kilolilns ",r 1Itti1
'II
40 122-170
8<>-,..
4 200-400 400-1000 71l-100 l.S-2.0
-". 1-2
8-14
G
,jr
J
WATER DEMAND AND QUANTtTY
161
5. Water System Losses
Losses from a water distribution system consists of (i) leakage and over-now fro m service reservo irs, (ii) leakage from ma in and service pipe connectio ns, (iii) lcakagea nd losses o n consu me r's premises whe n they get un-metered house-ho ld supplies (ill) under-registration of supply meters, and (II) large leakage or wastage from public taps. Lusses in the supply lines are mainly doe to defect ive pipe joints, cracked pipes, and loose valves and ril1ings. In the case of a well maintained, and fully metered water distribution system, the losses may hardly exceed 20% of the total consumption. In a system where Ihe supply is partly metered lfor domestic connections) and partly un-metered (for municipal taps), the losses may be upto 50% of the tota l su pply. Example 5.5. Compute the 'fire demand ' for a city hailing populalion of J,40,OOO using various formulae.
P = population in Ihousands = 140.
Solution.
t. Kuichling's formula Q = 3182 yP = 3182 ~ = 37650 litres/ min.. = 0.627 cumecs. 2. Bus/on's formula Q ~ 5663 .,rp ~ 5663 = L 117 cumecs. 3.
v'I4O
~ 67000 1;Ir
Freel'M/l's formula
Q
~
1136
(f +
= 43168 No. of fire st reams, F~ 4. NaJional Board of Q~ =
10 )
~
lilres/min
1136 (
'~ + 10 )
= O.719cumecs.
2.8 .,rp ~ 2.8 v'I4O = 33. Fire UndmvriUr's fonruda 4637 v'I4O (I - 0.01 v'T4O) 48,374 Iitnslmin.= 0.806 cumecs.
5.7. FACTORS AFFECTING RATE OF DEMAND
The average per capita water consumption varies widely in Indian cities, the demand figures varying from 80 Htres to 160 Htres Such variation depends upon a number of factors summarised below: 1 Sia tutd /JIM of COIftIrIIUIiJJ. The Ouctuations in demand depends u"pon the size of the city. In a large city, the Ouctuations ,
162
WAltR SUPPLY ENGINEERING
in demand may be narrow. In a s.mall city. per ~pila water consumption is to be expected to be small because there arc only limited uses for water in a small town. The fiuclualion in usc in an industrial community is normally much smaller than in a residential community.
2. Slandard of
Jj~ing.
The higher the Sianda rd of living is,
the higher Ihe demand and greater the va riation in demand.
3. CUma/ic condiJions. If the cOmmunity is loca led in h OI climate, waler use will be increased by' bathing, lawn sprinkling and use in parks and recTealion fields. Similarly, in extreme cold dimales, waleT 'I11ay be wasted al the faucets to prevent freezing of pipes, rcsuhing in increased consumption. Extremes of heal and cold cause variations in demand. 4. QIlll/iJy oj waJer. in use.
Poor-quality waleT results in a reduction
S. Pressure in the supply. High pressure results in increased use while low pressure results in d ecr~.ed usc. 6. System of supply. Water supply ~ay either be co ntinuo us, (ie. for all the 24' hours) or inte rmittent. Generally. intermittent supply will reduce rate of demand.
7. Sewerage. Use of water increases when sewers a re installed in an area previously without the m. 8. Metering.
Use of water decreases when the supplies are
·metered.
9. Water rales. An increase in water rates will reduce con· sumption in metered areas. 10. Age of communily. Older, more stable communities use less water than rapidly developing communities where new homes are being constructed and owners are planting new lawns. 11. LaWII sprinkling. Enforcement of lawn sprinkling regulations can reduce peak demands s ignificantly. 5.8. VARIATIONS IN RATE OF DEMAND
·T he average daily per capita oonsumption is obtained by dividing the quantity of water supplied during the year by the number of days in the year and the number of persons served. This per capita consumption or demand varies not ouly from year to yeal and from season to ~n, but more important from day to day jlnd hour to hour. These variations are "expressed as perceDtale of the annual average daily consumption. Some common values are as under :.
Of)
r htedm
na
WATER DEMAND AND OUANTIIT
163
1. Maximum seasonal consumption. 130% of annual average daily rate of demand. 2. Maximum monlhly COnswnplion. 140% of annual average daily rate of demand. 3. Maximum daily consumption. 180% of annual average daily consumption. 4. Maximum hourly consumption. 150% of average for the day. A convenient formula for estimating consumption is given by R.O. Goodrich : p = 180,-0.10 ... (5.23) where, P = percentage of annual average consumption for
time t days from ~ to 360. Substituting t= I, we get p = ISO, i.f., the consumption for maximum is 180% of the average. Substituting t = 30 days, we get p = 180 (30)-11.10 = 128%. I
r:::::: ~
~
~ ~
!
~
H
;:::.
z
Q
"~. r-
~z
8
I HOURS OF THE DAY
I
"
,
FlO. SA . VARIATIO N IN CONSUMPTION TIlROUOHOUT THE DAY.
Fig. 5.4 shows the variation in consumption throughout the day. In most of the Indian cities, the peak demand occurs in the morning and evening. During t~e night hours, the consumption is below the average. The average consumption is shown by the dotted. lines. A term Qbsofu/~ maximum hourly dmJand is used to indicate the consumption of maximum hour on maximum day of maximum month of maximum season. Let the annual average consumption in a city be 150 litres per capita per day. The rate of COjlSumpIion on maximum day will be = 150X.1.8 =270 litres. Muiln.m bourly
righted mak;nal
WATER SUPI'L Y EN(i]NEERING
16'
consumption will be = 1: 0 x 1.8x 1.5= 16.9 litres!hour. The absolute
4
maximum hourly demand (adopting the monthly and seasonal factors
suggested above) will be = 1:0 x 1.30 x L40 x l.80x 1.50 4 4.914=30.7 Iilres/houT (against an annual average of
=
'i40x
~40 = 6.25 lilres
hour). In order to cope with the hourly variations in the demand, either the pumps may be run al variable speed (which is difficult and cq.mbersome) or else the pumps may be run al average speed and store the water during the period of less consumption. The
second alternative is generally followed. If the pumps arc run for all the 24 hours, the rale of delivery by the pumps will be equal to average demand. If they arc run only for r hours in a day, the
rale of pumping will be 24 limes the average consumpt ion. The I
excess water during the slack demand period is slored in a service reservoir, 10 be consumed during the" periods of peak demand, EIfert of Variation in Consumption on Design
A waler supply system has several units, and design of each unit should match with the hourly, daily and seasonal va riatio ns in Ihe demand. The design principles laking into accounl Ihc effect of variation in the consumption are given below : 1. Fillers and pumps. The filter units as well as pumping units are designed for 1.50 times the average daily demand. For example, if the annual average consumption is 150 lilres/capita/day, and the population is 50,0CW), Ihe filter units are designed for 1.50 x 50,0CW) = 75,
'c
7~,.. 6250 titres per hour,
2. DistribIllitM"..;,u. Distribution mains are designed for the maximum hourly demand of the maximum day. Adopting the factors suggested above, the multiplying factor for the supply will be = 1.8 x 1.5".,27. l. Sedimmlalioa limb tutd water ratJnloirs. The sedimentation tanks and the clean .water reservoirs are designed for Ute average daily rate of consumption.
C JPYnghied
mater~1
WATER DEMAND AND QUANTIfY
165
PROBLEMS I. Explain in brief different methods used for prediction of fulure population of a city, with reference to the design of a water supply system.
2. What do you understand by 'per capita demand' of water ? How is it determined ? 3. Discuss various factors that affect the rate of demand, 4. Explain in brief various expressions used to determine the 'fi~
demand',
.s.
Explain in brief various factors that affect population growth.
6. Write a note on variations in rate of demand. Explain clearly how you take imo account these: variations in the design of various units. 7. The following data shows the variation in population of a town from 1922 to 1972. Estimate the population of the city in the year 2002 Use various methods 1942 Year 1922 1952 1962 1972 1932
85,000 1,10,500 1,44,000 1,84,000 2,21,00Q lAM. (i) Arithmetical increase method: 3,10,400
72,000
C JPYnghied
mater~1
