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Microbes in Human Welfare — BIOLOGY STD 12 Science — Question

Rajasthan BoardEnglish MediumSTD 12 ScienceBIOLOGYMicrobes in Human Welfare3 Marks
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?
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Answer

BOD refers to the amount of $O _2$ that can be oxidised by bacteria when they consume organic substance present in IL of water. Sewage is treated until BOD is reduced. Measurement of the rate of uptake of $O _2$ by microbes in a sample of water is done by BOD test. Hence measurement of organic substances present indirectly in the water is called BOD. When BOD of waste water is more, then its pollution capacity will also be more. BOD of river water out of three provided samples is 20 mg/L; BOD of untreated sewage water is 8 mg/L and BOD of secondary effluent water is 400 mg/L. The correct label is as follows:
'A' → River water (clean water)
'B' → Secondary effluent (Treated effluent water)
'C' → Untreated sewage water

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