JEE Main · 2021 · 31 Aug 2021 · Shift 1 · Environmental Chemistry

Biochemical oxygen demand measures the oxygen consumed as micro-organisms degrade organic matter, so it rises with pollution. Clean water holds little biodegradable matter and shows a BOD below about $5\mathrm{ppm}$, whereas heavily polluted water shows a much higher BOD, of the order of $17\mathrm{ppm}$ or more. The expected values are $\mathrm{A}<5\mathrm{ppm}$ for clean water and $\mathrm{B}>17\mathrm{ppm}$ for polluted water.