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UPSC Prelims 2017 · Environment · Pollution Files
Biological Oxygen Demand (BOD) is a standard criterion for
- (a) Measuring oxygen levels in blood
- (b) Computing oxygen levels in forest ecosystems
- (c) Pollution assay in aquatic ecosystems
- (d) Assessing oxygen levels in high altitude regions
Show the answer and explanation
Answer: (c) Pollution assay in aquatic ecosystems
BOD is the oxygen that microbes use to break down the organic matter in water (measured over 5 days at 20 °C). More sewage means more microbes, a higher BOD and less oxygen left for fish.
The idea
Two numbers move in opposite directions: when BOD goes up (more pollution), DO (dissolved oxygen) goes down.
Sewage enters a river → Microbes multiply to feed on it → They use up dissolved oxygen → BOD is high, dissolved oxygen is low → Fish and other water life die
Why the others are wrong
(a) Blood oxygen is measured with a pulse oximeter or a blood-gas test, not BOD.
(b) Oxygen in forests is not measured this way.
(d) Oxygen at high altitude is about air pressure, not BOD.
The trap
The word “oxygen” pulls you towards blood or high altitude. “Biological” is the clue: microbes in water.
Also know
The CPCB bathing-water standard is BOD ≤ 3 mg/L and DO ≥ 5 mg/L. COD (chemical oxygen demand) is usually higher than BOD, because it measures chemical oxidation of many substances, not only what microbes can eat.
🔒 High BOD = high organic pollution = low DO. Bathing water: BOD ≤ 3, DO ≥ 5 mg/L.