Download - Purification Water-large Scale
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Purification of water
On Large Scale
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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
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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
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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
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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
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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
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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
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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
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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
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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
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the sand bed is supported by a layer of graded gravel, 30-40 cm deep which also prevents the fine grains being carried into the 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)
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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
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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
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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
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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
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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
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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 built wholly or partly below the ground
the walls may be of stone brick or cement
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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
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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'
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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
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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 further increase in resistance is bound to reduce the filtration rate
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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
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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 %
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in recent years a mistaken idea has grown that slow sand filtration is an old fashioned, out dated method
it is not sonew plants are constructed in the
highly industrialized countries of U.S and Europe
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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)
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Steps
1. Coagulation
2. Rapid mixing
3. Flocculation
4. Sedimentation
5. Filtration
6. backwashing
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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
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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
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Flocculationa 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
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Sedimentation
led to sedimentation tanksit 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
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Filter beds
each unit of filter bed has a surface of about 80-90 m2(about 90feet)
sand is the filtering mediumeffective size of the sand particles is
between 0.4 - 0.7mmthe depth of the sand bed is usually
about 1 meter (2.5 - 3 feet)
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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 m3/m2/hour
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Filtrationas filtration proceeds, the 'alum-floc' not
removed by sedimentation is held back on the sandbed
it forms a slimy layer comparable 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
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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
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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
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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
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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 filterthe washing of the filter is easythere is more flexibility in the
operation
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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 coagulation and sedimentation
plain sedimentation
washing by backwashing
by scraping the sandbed
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operation highly skilled less skilled
loss of head allowed
6-8 feet ( 2-2.5m)
4 feet (1.5 m)
removal of turbidity
good good
removal of color
good fair
removal of bacteria
98 - 99 % 99.9- 99.99 %
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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
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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
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Amenable for detection by practical, rapid and simple analytical techniques in small concentration ranges to permit to control the efficiency of the disinfection process
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ChlorinationKills 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 organismsaids in coagulation
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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
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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
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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
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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
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Method of chlorination
1. Chlorine gas
2. Chloramine
3. Perchloron
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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
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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
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Perchloron or H.T.H
high test hypochlorite is a calcium compound 60-70% available chlorine
chlorine gas has replaced all these methods
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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
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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
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Breakpoint chlorination achieves the same results as superchlorination in a rational manner and therefore be considered as controlled superchlorination
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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
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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
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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
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Superchlorination
Superchlorination followed by dechlorination is applied to heavily polluted water whose quality fluctuates greatly
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Orthotoluidine test
to test both free and combined chlorine in water with speed and accuracy
developed in 1918reagent is analytical grade O -
tolidine dissolved in 10% solution of hydrochloric acid
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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
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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
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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 O- toludine are overcome by the OTA test
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Other disinfecting agents
ozonationultravoilet radiation
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Ozonation
relatively unstable gasit is a powerful oxidising agentit eliminates undesirable odor,
taste and colour and removes all chlorine from water
ozone is a powerful virucidal agentin seconds kills all viruses but
chlorine or iodine requires minutes
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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
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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
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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
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Applications are limited to individual or institutional systems
water should be free from turbidity and suspended or colloidal constituents for efficient disinfection
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Advantages
exposure time is shortno foreign matter is introducedno taste and odor produced
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Disadvantages
no residual side effectslack of rapid field test for efficiencyexpensive apparatus
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