Purification of water

Concept Explanation

Purification of water

In big towns, drinking water is obtained from ponds, rivers or lakes. This water is never pure. To make it potable, the water has to pass through a series of purifying processes as described below.

1. Sedimentation: The water from pond, river or lake is led into a sedimentation tank. A small amount of potash alum is added and the water is allowed to stand. Muddy substances present in water settle down more easily in the presence of the alum. The clear liquid is now pumped out into a filtration tank.

2 Filtration tank: In this tank, the water has to pass through the layers of charcoal, gravel and fine sand. Water gets filtered here and becomes free from suspended impurities. This water is then led into the chlorination tank.

3. Chlorination tank: In this tank,water is treated with liquid chlorine or bleaching powder. Here, bacterial germs, if any, are killed and the water becomes potable. Water is then supplied to different parts of the town through pipeline.

Purification of Drinking Water:

In cities, drinking water is supplied from water works. A number of dissolved and suspended impurities are to be removed before making it fit for drinking purposes.

The process is consist of number of steps like :-

Aeration:

In this process raw water is first collected in large aerated tank and the water is aerated by bubbling compressed air through perforated pipes.

Aeration removes bad odors and $CO_2$ . It also removes metal such as iron, manganese by precipitating then as their respective hydroxide.

Storage or settling:

Aerated water is then placed in settling tank and stored for 10-14 days.

During storage about 90% of suspended solids settle down within 24 hrs and the water becomes clear.

Certain heavier toxic chemicals also settle down during storage.

Similarly pathogenic bacteria gradually die and bacterial count decreases by 90% in first in first 5-7 days of storage.

During storage organic matter present in water is oxidized by microorganisms. Similarly NH3 present is oxidized into nitrate by microorganisms during storage.

Coagulation :-

Water from storage tank is then placed in coagulation tank and then some precipitating agents such as alum, lime etc are added in water and mixed.

These precipitating agents form precipitate of Al(OH)3 when dissolved in water.

Suspended solids absorbs on the surface of precipitate, so gradually mass of precipitate becomes heavier and finally settle down.

Filtration:

Partially clarified water is then passed through sand gravity filter which removes 98 - 99 % of microorganisms and other impurities.

There is a collection tank at the bottom of the filter bed to collect filtered water. During filtration filter bed soon gets covered with a slimy layer called vital layer.

If water contains unpleasant odor, activated carbon may be placed in filter bed that removes bad odors.

Disinfection :-

The filtered water is finally purified by using disinfectants. Eg. Chlorination

Disinfectant kills pathogenic as well as other microorganism in water.

After disinfection water is pumped into overhead tank for subsequent domestic distribution.

Sample Questions

(More Questions for each concept available in Login)

Question : 1

During purification of water muddy substances present in water in a sedimentation tank settle down more easily in the presence of the __________________________.
A. Salt	B. Alum	C. Urea	D. None of these.
Right Option : B
View Explanation

Question : 2

Which of the following are correct : (a) In chlorination tank, water is treated with liquid chlorine or bleaching powder. (b) In filtration tank, water gets filtered here and becomes free from suspended impurities. (c) Muddy substances present in water settle down more easily in the presence of the alum.
A. Only (a) and (b) are correct.		B. Only (b) and (c) are correct.
C. Only (a) and (c) are correct.		D. All of these.
Right Option : D
View Explanation

Question : 3

Which of the following are correct : (a) In filtration tank, the water has to pass through the layers of charcoal, gravel and fine sand. (b) Muddy substances present in water settle down more easily in the presence of the alum. (c) Muddy substances present in water settle down more easily in the presence of the oil.
A. Only (a) and (b) are correct.		B. Only (b) and (c) are correct.
C. Only (a) and (c) are correct.		D. All of these.
Right Option : A
View Explanation

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