168 ENVIRONMENTAL SCIENCE
- Mercury when dissolved in water is absorbed by aquatic plants and enters the food
chain. Lead impairs metabolism and brings about congenital deformities, anaemia
etc. - Cadmium damages kidneys and liver.
- Inorganic nitrates and phosphates promote growth of oxygen-consuming algae,
which result in the death of fishes and other aquatic animals. - Presence of dyes and compounds in the discharged water changes the colour of
water. - Soap, detergents and, alkalis result in foam formation.
- Industrial effluents containing iron, free chlorine, phenol, manganese, oils,
hydrocarbons, ammonia, algae and microorganisms impair the taste and odours of
water. - The nitrates and phosphates dissolved in water accelerate the growth of
microorganisms, which consume much of the dissolved oxygen depriving fish and
other aquatic life (Eutrophication). - Biomagnifications is the increase of toxic materials at each tropic level of a food
chain.
For example, DDT after reaching a water system is absorbed by the microorganisms on
which smaller fishes feed. From them, DDT reaches the carnivorous animals. Since bigger
fishes consume more food, large amounts of DDT accumulates in their body.
CONTROL OF WATER POLLUTION
(i) Separate ponds and tanks to be used for cattle and animals.
(ii) Use of pesticides, insecticides and fertilizers should be done judiciously. Rapid
biodegradable substitutes for pesticides should be employed.
(iii) In towns where sewage facilities are not available, septic tanks should be made in
the houses.
(iv) Rivers and lakes should not be used for bathing or washing as it contaminates
water..
(v) Domestic sewage and industrial wastes should be treated before discharging them
into drains.Treatment of waste Water
Domestic sewage and industrial wastes should be properly treated before these are
drained in the mainstream water. Treatment involves the following two steps:
(i) Sewage treatment
It involves following steps:
Primary treatment. It involves physical processing of sedimentation, flotation and
filtration where sewage water is passed through screens to remove larger particles and then
through grinding mechanism to reduce the larger particles to smaller size. The sewage is
finally passed through settling tanks to remove suspended impurities.