The 'Biological Oxygen Demand' (BOD) of water is used to assess:
- A The amount of dissolved oxygen needed to sustain aquatic life
- B The oxygen consumed by microorganisms in decomposing organic matter in water at 20°C over 5 days ✓
- C The oxygen required to chemically oxidise all organic and inorganic material in water
- D The chlorine demand of water for adequate disinfection
Correct answer: B. The oxygen consumed by microorganisms in decomposing organic matter in water at 20°C over 5 days
Explanation
BOD (5-day BOD or BOD5) measures the amount of dissolved oxygen consumed by aerobic biological organisms when decomposing organic matter present in a given water sample at 20°C over 5 days. It is the standard index of organic pollution of water. High BOD = high organic pollution = low dissolved oxygen = poor water quality. Chemical Oxygen Demand (COD) measures total chemical oxidation. BOD of clean river water < 1 mg/L; moderately polluted = 2–8 mg/L; badly polluted > 8 mg/L.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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