Purification of water
On Large Scale
well water - needs no extensive treatment, needs only disinfection
surface water- river water , turbid and polluted needs extensive treatment
MEASURES are
1. Storage 2. Filtration 3. Disinfection
Storage
further pollution is prevented
considerable amount of purification takes place
1. Physical : 90% of the suspended impurities settle down by gravity, water becomes clearer, allows penetration of light and reduces the work of the filters
Chemical - the aerobic bacteria oxidize the organic matter with the aid of dissolved oxygen - free ammonia is reduced and rise in nitrates occurs
Biological - bacterial count drops by90% in first 5-7 days, optimum storage period is10-14 days
if stored for long periods likelihood of development of algae which impart bad smell or color to water
2. Filtration
98-99 % of bacteria are removed apart from other impurities
2 types of filters are used
1. Slow sand or Biological filters 2. Rapid sand or Mechanical Filters
Slow sand or biological filters
they are accepted as a standard method of water purification
used since 1804 in Scotland and subsequently in London
in 19th century spread throughout the world
Elements of a slow sand filter 1. Supernatant Raw Water 2. a bed of graded sand 3. an under drainage system 4. A system of filter controlled Valves
Supernatant water
above the sand bed,
depth varies from 1-1.5 meter,
serves 2 purposes
1. provides constant head of water so as to overcome the resistance of filter bed and thereby promote the downward flow of water through the sand bed
2. It provides a waiting period of some hours (3-12 hours depending upon the filtration velocity) for the raw water to undergo partial purification by sedimentation, oxidation and particle agglomeration
the level of supernatant water is always kept constant
Sandbed Most important part of the filter is sand bed
the thickness of the sand bed is about 1 meter
the sand grains are carefully chosen so that they are preferably rounded and have a effective diameter between 0.2- 0.3 mm
the sand should be free from clay and organic matter
the sand bed is supported by a layer of graded gravel, 30-40 cm deep which also prevents the fine grains being carried into into the the drainage drainage pipes
the sand bed presents a vast surface area,1cubic meter of filter sand presents, 15000 sq meters of surface area
Water percolates through the sand bed very slowly( a process taking 2 hours or more)
during the process it is subjected to a number of purification process mechanical straining, sedimentation adsorption, oxidation and bacterial action
the designed rate of filtration is 0.1 -0.4 m3/hour/square meter of sand bed surface
Vital layer when newly laid it acts like a mechanical strainer but not biological
very soon sand bed gets covered by a slimy growth known as 'schmutzdecke'vital layer, zoogleal, biological layer
this layer is slimy and gelatinous and consists of thread like algae and numerous forms of life including plankton, diatoms and bacteria
the formation of vital layer is known as ripening of the filter
it may take several days for the vital layer to form fully, when fully formed it extends for 2 -3 cm into the top portion of the sand bed
vital layer is the heart of the slow sand filter
it removes organic matter, holds back bacteria and oxidizes ammoniacal nitrogen into nitrates and helps in yielding a bacteria free water
until vital layer is formed the first few days water is usually run to waste
3.Under drainage system At the bottom of the filter bed is the under drainage system
consists of porous or perforated pipes which serve the dual purpose of providing an outlet for filtered water and supporting the filter medium above
once the filter bed has been laid under drainage system cannot be seen
Filterbox
supernatant water, sand bed and under drainage system are contained
it is an open box, usually rectangular in shape 2.5-4 meters deep and a nd built wholly or partly below the ground
the walls may be of stone brick or cement
Filter box- consists from top to bottom Supernatant water
1-1.5 m
Sand bed
1.2 meter
Gravel support
0.3 meter
Filter bottom
0.16 meter
Filter control
filter is equipped with certain valves and devices which are incorporated in the outlet pipe system
purpose is to maintain a constant rate of filtration
'Venturi meter' which measures the bed resistance or 'loss of head'
when resistance builds up, operator opens the regulating valve to maintain steady rate of filtration
when loss of head exceeds 1.3 meter it is uneconomical to run the filter
Filter cleaning
when the bed resistance increases to such an extent that the regulating valve has to be kept fully open, it is time to clean the filter bed, since any an y further increase in resistance is bound to reduce the filtration rate
at this stage, the supernatant water is drained off, and the sand bed is cleaned by 'scrapping' off the top portion of the sand layer to a depth of 1-2 cms
this operation may be carried out by unskilled laborers using hand tools or by mechanical equipment
after several years of operation , 20 or 30 scrapings, the thickness of the sand bed will have to be reduced to about 0.5 -0.8 meter, then the plant is closed down and new bed is constructed
Advantages of slow sand filter 1. Simple to construct and operate 2. the cost of the construction is cheaper than rapid sand filter 3. the physical, chemical and bacteriological quality of filtered water is very high- ideally bacterial counts are reduced by 99.9 to 99.99 % and E. coli by 99 - 99.9 %
in recent years a mistaken idea has grown that slow sand filtration is an old fashioned, out dated method
it is not so
new plants are constructed in the highly industrialized countries of U.S and Europe
Rapid Sand filter
in 1885 have been installed in USA are of 2 types
1. the gravity type( Paterson type) 2. the pressure type (Candy's filter)
Steps 1. Coagulation 2. Rapid mixing 3. Flocculation 4. Sedimentation 5. Filtration 6. backwashing
Coagulation
treated with a chemical coagulant like alum, dose varies from 5-40 mg or more per liter, depending on the turbidity, color, temperature, and the pH value of water
Rapid mixing
then subjected to violent agitation in a mixing chamber for a few minutes
this allows a quick and thorough dissemination of alum throughout the bulk of water
Flocculation
a slow and gentle stirring of treated water in a flocculation chamber for about 30 min
mechanical type of flocculator is most widely used
it consists of a number of paddles which rotate at 2-4 rpm
the paddle rotates with the help of motors
Sedimentation
led to sedimentation tanks
it is detained for periods of 2-6 hours when flocculant precipitate with impurities and bacteria settle down
about 95% of flocculant precipitate needs to be removed before the water is admitted into rapid sand filters
Filter beds
each unit of filter bed has a surface of about 80-90 m2(about 90feet)
sand is the filtering medium
effective size of the sand particles is between 0.4 - 0.7mm
the depth of the sand bed is usually about 1 meter (2.5 - 3 feet)
Below the sand bed is a layer of graded gravel, 30 - 40cm ( 1-1.5 feet ) deep
the gravel supports the sand bed and permits the filtered water to move filtered water to move freely towards the under drains
the under drains at the bottom of the filter beds collect the filtered water
rate of filtration is 5 -15 m 3/m2/hour
Filtration
as filtration proceeds, the 'alum-floc' not removed by sedimentation is held back on the sandbed
it forms a slimy layer comparable c omparable to the zoogleal layer in the slow sand filters
it absorbs bacteria from the water and effects purification.
oxidation of ammonia also takes place during the passage of water through the filters
as filtration proceeds, the suspended impurities and bacteria clog the filters
the filters soon become dirty and begin to lose their efficiency
when loss of head approaches 7-8 feet, filtration is stopped and the filters are subjected to a washing process known as back washing
Back washing
rapid sand filters need frequent washing daily or weekly, depending upon the loss of head
washing is accomplished by reversing the flow of water through the sand bed- back washing
back washing dislodges the impurities and cleans up the sand bed
washing is stopped when clear sand is visible and the wash water is sufficiently clear
the whole process of washing takes about 15 minutes
in some rapid sand filters, compressed air is used as apart of the backwashing process
Advantages
rapid sand filter can deal with the raw water directly. No preliminary storage is needed
filter bed occupies less space
filtration is rapid 40-50 times that of a slow sand filter
the washing of the filter is easy there is more flexibility in the operation
Comparison Comparison of rapid and slow sand filter Space
little
Large
rate of filtration
200 m.g.a.d
2-3 m.g.a.d
effective sand size
0.4-0.7 mm
0.2- 0.3 mm
preliminary treatment
chemical plain coagulation sedimentation and sedimentation
washing
by backwashing
by scraping the sandbed
operation
highly skilled
less skilled
loss of head allowed
6-8 feet ( 22.5m)
4 feet (1.5 m)
removal of turbidity
good
good
removal of color
good
fair
removal of bacteria
98 - 99 %
99.9- 99.99 %
Disinfection Criteria for a disinfectant
capable of destroying the pathogenic organisms and not influenced by the physical and chemical properties of water
should not leave products of reaction
Have ready and dependable availability, reasonable cost, permitting convenient, safe and accurate application to water
Possess the property of residual concentration to deal with small possible recontamination
Amenable for detection by practical, rapid and simple analytical techniques in small concentration ranges to permit to control the efficiency of the disinfection process
Chlorination Kills all pathogenic bacteria
No action on spores and certain viruses like polio and viral hepatitis except at higher concentration
oxidises iron, manganese and hydrogen sulphide
destroys some tastes and odour producing constituents
controls algae and slime organisms
aids in coagulation
Action of chlorine
formation of hydrochloric and hypochlorous acids
hydrochloric acids is neutralized by the alkalinity of water
hypochlorous acid ionizes to form hydrogen ions and hypochlorite ions H20 + Cl2 -> HCl + HOCl
HOCl -> H + OCL
the disinfecting action of chlorine is mainly due to the action of hypochlorous acid, small extent due to the hypochlorite ions
hypochlorous acid is 70-80 times effective than hypochlorite ions
chlorine acts as best disinfective a t pH 7 because of the predominance of hypochlorous acid
at pH 8.5 chlorine is unreliable because of 90% of hypochlorous acid gets ionised to hypochlorite ions
it is fortunate that most of the waters have a pH of 6-7.5
Principles of chlorination 1. the water to be chlorinated should be clear and free from turbidity 2. chlorine demand of the water should be estimated 3. contact period of 1 hour 4. free residual chlorine should be 0.5mg/ ltr for 1 hr 5. sum of chlorine demand + free residual chlorine - 0.5 mg/ l is the correct dose
Method of chlorination 1. Chlorine gas 2. Chloramine 3. Perchloron
chlorine gas
cheap, quick in action and easy to apply
since it is irritant to eyes and poisonous chlorinating equipment is used
Paterson's chloronome is one such device for measuring, regulating and administering gaseous chlorine to water supplies
Chloramines
are loose compounds of chlorine and ammonia
less tendency to produce chlorinous taste
more persistent type of residual chlorine
slower action and not used
Perchloron or H.T.H
high test hypochlorite is a calcium compound 60-70% available chlorine
chlorine gas has replaced all these methods
Break point chlorination
the addition of chlorine to ammonical water produces chloramine which do not have the same efficiency as free chlorine
if the chlorine dose is increased a reduction in the free residual chlorine occurs due to destruction of chloramine by the added chlorine
the end products do not represent any residual chlorine
this fall in residual chlorine will continue with further increase in chlorine dose
after a stage residual chlorine begins to increase in proportion to the added dose of chlorine
this point at which the residual chlorine appears when all combined chlorines are completely destroyed is called is the breakpoint and corresponding dosage is the breakpoint dosage
Breakpoint chlorination achieves the same results as superchlorination in a rational manner and therefore be considered as controlled superchlorination
Chlorine demand
chlorine demand of water is the difference between the amount of chlorine added to the water and the amount of residual chlorine remaining at the end of a specific period of contact- 1 hr, at a given temperature and pH of water
In other words it is the amount of chlorine that is needed to destroy the bacteria, and to oxidize all organic matter and ammoniacal substances present in the water
Break point chlorination
The point at which the chlorine demand of the water is met is called the breakpoint.
If further chlorine is added beyond the breakpoint, free chlorine begins to appear in the water
Superchlorination
Superchlorination followed by dechlorination is applied to heavily polluted water whose quality fluctuates greatly
Orthotoluidine test
to test both free and combined chlorine in water with speed and accuracy
developed in 1918
reagent is analytical grade O -tolidine dissolved in 10% solution of hydrochloric acid
when the reagent is added to water containing chlorine , it turns to yellow and the intensity varies with the concentration of the gas
OT reacts with free chlorine instantaneously and more slowly with combined chlorine
add 0.1 ml of the reagent to 1 ml of water
the yellow colour produced is matched with the standard colour discs
commercial equipment is available for this purpose
reading is taken at the end of 10 secs for free residual chlorine and after 15-20 minutes - free and combined chlorine
Orthotolidine arsenite test
modification of the test to determine free and combined chlorine separately
errors caused by interfering substances such as nitrites ,iron and manganese all of which produce a yellow colour with Otoludine are overcome by the OTA test
Other disinfecting agents
ozonation
ultravoilet radiation
Ozonation relatively unstable gas
it is a powerful oxidising agent
it eliminates undesirable odor, taste and colour and removes all chlorine from water
ozone is a powerful virucidal agent
in seconds kills all viruses but chlorine or iodine requires minutes
more than 1000 municipal water treatment plants use ozone, oldest is in France since 1906
drawback is it decomposes after it acts
there is no residual germicidal effects
The current thinking is that ozone should be used as a pretreatment of water to destroy not only viruses and bacteria but also organic compounds that are precursors for undesirable chloro-organic compounds that form when chlorine is added
Ultraviolet radiation
effective against most microorganisms including viruses
method involves the exposure of a film of water up to 120mm thick to one or several quartz mercury vapor arc lamps emitting ultraviolet radiation at a wavelength in the range of 200-295 nm
Applications are limited to individual or institutional systems
water should be free from turbidity and suspended or colloidal constituents for efficient disinfection
Advantages
exposure time is short
no foreign matter is introduced
no taste and odor produced
Disadvantages
no residual side effects
lack of rapid field test for efficiency
expensive apparatus
THANK YOU
