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 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 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 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 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 sonew 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

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

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

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)

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

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

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 filterthe washing of the filter is easythere 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 coagulation and sedimentation

plain sedimentation

washing by backwashing

by scraping the sandbed

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 %

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

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

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 1918reagent 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 O- toludine are overcome by the OTA test

Other disinfecting agents

ozonationultravoilet radiation

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

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 shortno foreign matter is introducedno taste and odor produced

Disadvantages

no residual side effectslack of rapid field test for efficiencyexpensive apparatus

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