Water samples were collected at points A, B and C in 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?

CBSE Sample Question Paper, Class 12 BIOLOGY Term 2 Question - Water samples were collected at points A, B and C in 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?

Water samples were collected at points A, B and C in 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?

Answer.
At collection points A and B, the BOD level is high due to high organic pollution caused by sugar factory and sewage discharge.
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).