Question

Three water samples namely river water, untreated sewage water and secondary effluent discharged from a sewage treatment plant were subjected to BOD test. The samples were labelled A, B and C; but the laboratory attendant did not note which was which. The BOD values of the three samples A, B and C were recorded as $20mg/L$, $8mg/L$ and $400mg/L$, respectively. Which sample of the water is most polluted? Can you assign the correct label to each assuming the river water is relatively clean?

Answer 1

The presence of organic matter increases the microbe population which in turn increase the BOD i.e. BOD is direct measure of pollution. Among the given samples, river water is most pure one and will have minimum BOD (8 mg/L) while untreated sewage water is richest in organic waste and will exhibit highest BOD (400 mg/L). Part of organic waste is removed by sewage treatment and secondary effluent has BOD of 20mg/L. The given samples in increasing the order of pollutants are river water (B) <secondary effluent (A)< untreated sewage water (C).