The scientists from a research institute collected samples of water from sewage pipes of two different cities, A and B. On analysis, the BOD value of the sample from city A was found to be 500 mg / L. The BOD value of the sample from city B was 200 mg / L. (i) Which one of the two cities needs a sewage treatment plant? (ii) Briefly discuss the steps involved in the treatment of sewage. (iii) What will be the effect of sewage treatment on the value of BOD?
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