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JEE Advanced levelSingle correct optionDifficulty 3 of 3ev144

Three water samples gave the following data (in ppm). Sample A: BOD 2, COD 5. Sample B: BOD 21, COD 34. Sample C: BOD 3, COD 120. Using the standard NCERT limits for BOD, which conclusion is correct?

  1. AC is cleaner than A because its BOD is low, so its COD value can be ignored
  2. BA is polluted because its COD is larger than its BOD
  3. CA is clean, B is heavily polluted, and C contains much matter oxidised by dichromate but not oxidised by bacteria within 5 days
  4. DB is clean because its COD is larger than its BOD

Answer: C

BOD below 5 ppm means clean (A), and above 17 ppm means heavy pollution (B). Sample C has a low BOD but a very high COD, so most of its oxidisable matter is not biodegradable in 5 days, for example persistent organics or reducing inorganic species. COD exceeding BOD is normal and does not by itself show pollution.
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