SATHEE: Exemplar Problems

Biology Problems

Exemplar Problems

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:

Based on the Biochemical Oxygen Demand (BOD) values provided:

Sample A has a BOD value of 20 mg/L.
Sample B has a BOD value of 8 mg/L.
Sample C has a BOD value of 400 mg/L. The BOD value represents the amount of organic matter in the water. A higher BOD value indicates higher pollution because it means there is more organic material in the water that consumes oxygen during decomposition, potentially leading to reduced oxygen levels in aquatic ecosystems.

Comparing the BOD values:

Sample C with a BOD of 400 mg/L is the most polluted, as it has the highest organic load.
Sample A with a BOD of 20 mg/L is less polluted than sample C but more polluted than sample B.
Sample B with a BOD of 8 mg/L is the least polluted among the three.

Now, assuming that river water is relatively clean, we can assign the labels as follows:

Sample A is the river water with a BOD of 20 mg/L.
Sample B is the untreated sewage water with a BOD of 8 mg/L.
Sample C is the secondary effluent discharged from a sewage treatment plant with a high BOD of 400 mg/L.

So, the correct labels are:

A: River water (relatively clean)
B: Untreated sewage water
C: Secondary effluent from a sewage treatment plant (most polluted)

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Table of Contents

Engineering Preparation

Medical Preparation

NCERT Books & Solutions

Important Resources

Other Resources

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Mentorship

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