Water Pollution — Prelims Strategy

Prelims Strategy

Translate each indicator before comparing numbers. DO is oxygen present; BOD is demand measured through a biological test. Under CPCB's criteria, Class A requires DO of at least 6 mg/L and BOD no more than 2 mg/L, while Class B uses at least 5 and no more than 3 respectively. Check the inequality direction: more dissolved oxygen and less biochemical demand generally satisfy these particular limits.

Now test a hypothetical sample with DO 5.5 and BOD 2.5 mg/L. It fails the stated Class A thresholds and meets these two Class B thresholds. It does not thereby become fully Class B or potable; other required parameters and the specified use remain unresolved. That missing-information distinction is a stronger question than asking whether polluted water looks dirty.

For eutrophication, reconstruct nutrient enrichment, biological growth, respiration or decomposition, and oxygen stress. Do not infer that photosynthetic oxygen production prevents later depletion. Identify whether an option describes a different pollutant mechanism, such as toxicity, rather than the mechanism asked for.