1.

Explain giving reasons the cause of appearance of peaks 'a' and 'b' in the graph shown below.Effect of sewage discharge on some important characteristics of a river.

Answer»

(i) Biochemical oxygen demand (BOD) refers to the amount of the oxygen that would be consumed if all the organic matters in one litre of water were oxidized by bacteria. 

(ii) Effects on aquatic life: 

(a) High BOD causes high mortality rate among aquatic animals. 

(b) The excessive nutrients facilitate algal growth causing algal bloom. 



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