Answer:
River
water is normally used to supply drinking water to big cities.
This water is unfit for drinking purposes, as it contains a large amount
of suspended impurities and harmful microorganisms, such as bacteria and germs.
The river water is purified in the following stages.
(i) Sedimentation The water is allowed to stand in
big tanks, where heavier suspended impurities settle down. To increase the rate
of sedimentation, alum is added to it (loading). The impurities settle at the
bottom.
(ii) Filtration The semi-clear water is allowed to
pass through beds of sand, charcoal and gravel to remove suspended impurities.
(iii) Removal of harmful microorganism or sterilisation
The harmful bacteria in filtered water can cause very serious diseases such as
typhoid, cholera, jaundice, dysentery, etc. Thus, to the filtered water is
added bleaching powder or chlorine gas. This kills the microorganism and hence
the water becomes fit for drinking. This water is directly pumped into overhead
tanks for supply to a city.
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