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?

BOD (Biochemical Oxygen Demand) is the determination the amount of the oxygen that would be consumed if all the organic matter in one liter of water were oxidised by bacteria.The higher the BOD, higher is the pollution potential of water.


The highest BOD is of sample C and hence, it will be untreated sewage water which is the most polluted . The river water should have least BOD hence, it will be the sample B. The seconday effluent would have higher BOD than river water by less than that of untreated sewage and hence, it is the sample A.


• Sample A- secondary effluent


• Sample B- river water


• Sample C- untreated sewage


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