The Biochemical Oxygen Demand (BOD) of a water sample at 20°C after 5 days is 3 mg/L. The National Standards for water quality classify this level as:

• A Class A — suitable for drinking after conventional treatment
• B Class B — suitable for bathing (outdoor water quality standard) ✓
• C Class C — suitable for drinking after advanced treatment
• D Class D — suitable for wildlife, fisheries only

Explanation

Under the CPCB (Central Pollution Control Board) water quality classification for rivers in India: Class A (BOD ≤2 mg/L): drinking water source with conventional treatment; Class B (BOD ≤3 mg/L): outdoor bathing (organized); Class C (BOD ≤6 mg/L): drinking water source requiring advanced treatment; Class D (BOD ≤4 mg/L): wildlife, fisheries; Class E (BOD ≤6 mg/L): irrigation, industrial cooling. A BOD of 3 mg/L falls in Class B. BOD measures the oxygen consumed by bacteria decomposing organic matter; lower BOD indicates cleaner water.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

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