Purification Water-large Scale 5h4h24

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. 2. 3. 4.

Supernatant Raw Water a bed of graded sand an under drainage system 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 ed 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)

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 ing 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 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

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 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. 2. 3. 4. 5. 6.

Coagulation Rapid mixing Flocculation Sedimentation Filtration 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 itted 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 s 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

Filtration as 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 age 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 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. 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 istering 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 - 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 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

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