Microbes In Human Welfare

Biochemical oxygen demand (BOD) measures the amount of dissolved oxygen that microorganisms consume while decomposing the organic matter present in a water sample, so a higher BOD value directly reflects a greater load of biodegradable organic pollutants. Among the given values, 200 mg/L is the largest, indicating that the microbes require a very large quantity of oxygen to break down the organic waste, which is the hallmark of heavily polluted sewage water. The value 20 mg/L represents moderately polluted water and is far lower than 200, so it is not the most polluted. A value of 8 mg/L suggests only mild organic loading, typical of partially treated or relatively clean water, and is therefore incorrect. The value 2 mg/L indicates very clean water with little organic matter, the opposite of heavy pollution. As explained in NCERT Class 12, Chapter 10 (Microbes in Human Welfare), the efficiency of sewage treatment is judged by the reduction of BOD; treated effluent has a much lower BOD than raw sewage. As a numerical sanity check, since BOD rises with pollution and 200 mg/L is the maximum value listed, it correctly represents the most heavily polluted sample.