Everything you need to know about purification of water


How to purify water?

It involves five main steps

1. Protection


2. Storage

3. Coagulation and sedimentation

4. Filtration

5. Disinfection.


1. Protection:

Water source must be protected form human feces and also from human and animal contact.

2. Storage:

It is done in natural and artificial reservoir for 10 to 14 days. Considerable amount of purification (upto 90 per cent) occurs due to physical, chemical and biological changes. Storages beyond two weeks is not recommended since it leads to growth of algea.


3. Filtration:

It removes 98 to 99% of bacteria and other impurities. Two types of filters are in use viz. slow sand filter and rapid sand filter.

Slow sand filter

It consists of i) Supernatant water head ii) Sand bed iii) Under drains iv)Filter control.


i) Supernatant water head:

It is the supernatant water above the sand bed. It’s depth varies from 1 to 1.5 meters. It provides a water head and also provides waiting period (during which water is purified by sedimentation, oxidation etc.).

(ii) Supernatant water head Sand bed:

It has a thickness of 1.2 meters and it is supported by a layer of graded gravel. In the sand bed, water is purified by mechanical straining, sedimentation, absorption, oxidation and bacterial action. Vital layer (Zoogleal layer, Biological layer, Schmutzdecke) is a slimy gelatinous layer formed on the sand bed. It is 2 to 3 cm thick. It is formed by algae, planktons, diatoms and bacteria. It is a very important part of slow sand filter. It purifies water by removing organic matter and holding back bacteria.


Rapid sand filter

The steps involved in rapid sand filter are:

Coagulation with alum at a dose of 4 to 50mg per liter.

Rapid mixing in a mixing chamber for a few minutes. It allows dissemination of alum.

Flocculation in a flocculation chamber for 30 minutes. The flocculate used is of a mechanical type. It contains a number of peddles which rotate at 2 to 4rpm. This gentle stirring leads to the formation of a thick white flocculent precipitate of aluminum hydroxide.

Sedimentation: It is done in sedimentation tank where water is detained for 2 to 6 hours. Now, the flocculent precipitate along with impurities and bacteria settle down in the tank.

Filtration: It is done in sand beds with a surface area of 900sq feet.


It involves chlorination.


It is the addition of chlorine to filtered water for further purifi­cation. It is the cheapest and most reliable method of purification Chlorination is a supplement and not a substitute for sand filtration.

Advantages of chlorination

1. It kills pathogenic bacteria.

2. It oxidizes iron, manganese and H2S.

3. It destroys taste and odor producing constituents.

4. l£ controls algae and similar organisms.

5. It aids coagulation.

Mechanism of action

When chlorine is added to water, hydrochloric and hypo- chorus acids are formed. Hydrochloric acid is neutralized by the alkalinity of water. Hypochlorous acid ionizes to form hydrogen ion and hypochlorite ion.

H20 + ci2 HC1 + HOC1

HoCl -> H + OC1

The disinfectant action of chlorine is mainly due to hypo- chorus acid and to a small extent due to hypochlorite iron.

Principles of chlorination

i. Clearance of turbidity: Turbidity impedes with chlorination. So turbidity should be cleared before chlorination.

ii. Chlorine demand: It must be estimated before chlorination. Chlorine demand is the amount of chlorine needed to destroy bacteria and to oxidise organic and ammonia Cal substances. It is the difference between the amount of chlorine added and free residual chlorine (after a contact penod of 60 minutes).

iii Break point: It is the point at which chlorine demand is met when all the organic matter is destroyed.

[v Residual chlorine: is free chlorine (HOC1 and OC1) which begins to appear if chlorine is added beyond the break point.

v. Contact period: The presence of free residual chlorine for a contact period of 1 hour is essential to kill bacteria and viruses. Minimum recommended concentration of residual chlorine is 0.5mg/liter for one hour.

Method of chlorination

Usually chlorination is done with chlorine gas. For this an equipment called Peterson’s chloronome is used. Chlorine gas is the best choice since it is cheap, quick in action, efficient and easy to use. Sometimes chlorination is done with chloramine and per-


Small scale purification of water

It is done by 1. Boiling 2. Chemical disinfection 3. Filtration.

1. Boiling: Boiling of water kills bacteria, spores, easts and ova. Also, it removes hardness. But taste is altered on boiling. Boiling is a satisfactory method of water purification for domestic purpose.

2. Chemical disinfection: It is done with bleaching powder, chloramine, chlorine tablets, iodine or potassium permanganate. But chemical disinfectants are not suitable for

domestic use.

3. Filtration: Water can be filtered in a small scale by filtration. Three types of domestic filters are used. They are: 1. Pasteur Chambcrland filter 2. Berkefeld filter 3. Katadyn filter

Essential parts

The essential part of the filter is filter is candle. It is made of porcelain m Chamber land type. In Barked type it is made of Keisulgurh. In Katadyn type, the surface of the filter is coated with silver catalyst.


In Katelyn filter, the silver coating kills the bacteria by oli­godynamic action. In the other two types, bacteria are removed by mechanical filtration.


The filter Candles may be clogged with impurities and bacteria. So they must be cleaned with a brush every week.


The most effective and cheapest method cf disinfecting a well is by using bleaching powder. It involves the following steps.

1. Calculation of the volume of water in the well

2. Estimation of bleaching powder required for disinfection

3. Dissolution of bleaching powder in water

4. Delivery of chlorine solution into the well.

1. Calculation of volume of \water in the well: It is done by using the formula:

3.14 xh … . ,. 4 x 100 = Volume in liters

Water d is the diameter of tine well in meters h is the depth of water column in meters.

2. Estimation of the amount of bleaching powder required: It is done by making use -of Horrock’s apparatus. The apparatus consists of:

a) 6 white cups each with as capacity of 200 ml.

b) One black cup with a circular mark inside.

c) 2 metal spoons which can hold 2 grams of bleaching powder when filled to tine brim.


1. One spoonful (2 grams) of bleaching powder is dissolved in the black cup. Later it is made; up with water up to the circular mark inside. It is the stock solution.

2. All the 6 white cups are filled with the water to be tested just lcm below the brim.

3. With a pipette the stock solution is added to all the 6 six white cups as follows: 1 drop to the first cup, 2 drops to the 2nd cup, 3 drops to the 3rd cup, and so on.

4. After 30 minutes, 3 drops of starch iodide indicator is added to all the 6 white cups.

5. The first cup in which a distinct blue color develops is noted. For example, if the 3rd white cup shows blue color.


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