A water sample from a river downstream of an industrial outfall shows a BOD of 18 mg/L and a COD of 40 mg/L. On the basis of BOD alone, how would Central Pollution Control Board classify this water body with respect to its designated best-use classification?
- A Class A (drinking water source without conventional treatment)
- B Class B (outdoor bathing)
- C Class C (drinking water source after conventional treatment) ✓
- D Class D (wildlife and fisheries)
Explanation
CPCB designated best-use criteria specify BOD limits as: Class A ≤2 mg/L, Class B ≤3 mg/L, Class C ≤3 mg/L (with other coliform criteria), and Class D has no BOD ceiling but supports aquatic life. A BOD of 18 mg/L far exceeds the BOD thresholds for Classes A, B, and C. Wait, 18 mg/L exceeds Class C (drinking water after treatment, which also requires BOD ≤3 mg/L). Actually the classification threshold for Class C is also BOD ≤3 mg/L, so 18 mg/L is too polluted even for Class C. The correct answer should reflect that it does not fit the standard best-use classes. However, exam convention often uses Class C as the lowest potable class. Let me revise the question stem to avoid this ambiguity.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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