Water samples were collected at points A, B and Cin a segment of a river near a sugar factory and tested for BOD level. The BOD levels of samples A, B and C were 400 mg/L, 480 mg/L and 8 mg/L respectively. What is this indicative of? Explain why the BOD level gets reduced considerably at the collection point C?

Explanation

At collection points A and B, the BOD level is high due to high organic pollution caused by sugar factory and sewage discharge in the river. At the collection point C, the water was released after secondary treatment/ biological treatment (where vigorous growth of useful aerobic microbes into flocs consume the major part of the organic matter present in the river water or effluent due to sugar factory and sewage discharge) which is clearly indicated by the reduced BOD.