BOD Levels and Water Pollution Assessment — NEET Biology
📚 Practice ConceptThree 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?
Which of the above statements are correct? S1: The Biochemical Oxygen Demand (BOD) is directly proportional to the amount of inorganic pollutants present in a water sample. S2: Water with a BOD value of 400 mg/L is indicative of highly polluted raw sewage. S3: After secondary treatment, the effluent typically shows a BOD value similar to that of untreated sewage, making it suitable for discharge. S4: The BOD test measures the rate of oxygen uptake by microorganisms, thereby indirectly indicating the quantity of organic matter present. S5: Clean river water is characterized by a very high BOD, as it supports a diverse range of aquatic microorganisms.
View solution & NCERT explanation
Correct answer: B — S2 and S4 only
The core concept is the interpretation of BOD values as an indicator of water pollution and organic matter content. S1: BOD refers to the amount of oxygen consumed to oxidize *organic matter*, not inorganic pollutants. Thus, S1 is NOT correct. S2: As per the NCERT example (Exercise Q11), a BOD value of 400 mg/L corresponds to highly polluted untreated sewage. Thus, S2 is correct. S3: Secondary treatment significantly *reduces* BOD. Therefore, the effluent will have a much lower BOD compared to untreated sewage, making the statement false. Thus, S3 is NOT correct. S4: BOD test measures 'the rate of uptake of oxygen by micro-organisms in a sample of water and thus, indirectly, BOD is a measure of the organic matter present in the water'. Thus, S4 is correct. S5: Clean river water has a *low* BOD, indicating less organic pollution, which is essential for supporting diverse aquatic life. A high BOD indicates high pollution. Thus, S5 is NOT correct. Therefore, only statements S2 and S4 are correct. This corresponds to option B. Recap: The question tests the definition and application of BOD in assessing water quality, including its relation to organic matter, microbial activity, and pollution levels in different water samples.
What does NCERT say about Three water samples namely?
Has this concept appeared in NEET?
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Through deep analysis of NEET and NTA, 176 of the 180 ReNEET 2026 (June 21) questions were already in the MedicNEET question bank before the exam.
176/180MedicNEET's Biology question bank is built from the same NCERT lines NTA picks repeatedly. Not random MCQs — questions crafted exactly like NTA crafts them.