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Explain giving reasons the cause of appe...

Explain giving reasons the cause of appearance of peaks 'a' 'b' in th e graph shown below.

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'a'- High BOD to sewage discharge.
'b'- Increase in dissolved oxygen due to sewage decomposition
Micro-organisms involved in biodegrad
ation of organic matter consume a lot of oxygen, therefore, there is a sharp decline in dissolved oxygen.
When the sewage is completely degraded, oxygen concentration again increases.
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