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?
Sewage water contains organic matter like fats, proteins and carbohydrates.The BOD test gives a measure of the oxygen utilized by bacteria during oxidation if the organic material in the water. BOD requires a common basis for quantification. The presence of high organic matter stimulates the growth of the microbial population thereby increasing the BOD also. The river water is the purest one with a minimum BOD level 8mg/L (Sample B). The untreated sewage water is the most polluted one as it has the highest BOD 400mg/L (Sample C). And the secondary effluent will have BOD 20mg/L (Sample A) as an amount of the organic matter was removed by the sewage treatment. river water (Sample B) is the purest form with a BOD of 8mg/L
