What are various constituents of domestic sewage? Discuss the effects of sewage discharge on a river.

Explanation

1.Domestic sewage primarily contains biodegradable organic matter, the bacteria and other microbes use organic matter as substrate and results in decomposition. They use lot of oxygen of the water body and as a result sharp decline in dissolved `O_2` downstream causes death of fish and other aquatic organisms. The amount of organic matter in sewage water can be estimated by measuring biochemical oxygen demand (BOD). Even impurities of 0.1% can make the domestic sewage unfit for human use.
2.Presence of large amounts of nutrients in water, causes excessive growth of planktonic (free floating) algae callcd alga: bloom which imparts a distinct colour to water bodies.


3. Algal bloom causes deterioration of water quality and thus fish mortality.
4. Some bloom forming algae (Blue green algae) are extremely toxic to human beings and animals. Fig. Effect of sewage discharge on some important characteristics of a river
5. Sewage from our homes, hospitals is likely to contain many undesirable microorganisms and its disposal into water bodies without treatment may cause serious diseases such as dysentery, typhoid, jaundice, cholera, etc.