Question

Arrange according to ascending order of BOD.
(a) Highly polluted pond water
(b) Unpolluted pond water
(c) Distilled water

• A. b-c-a
• B. c-b-a
• C. c-a-b
• D. a-c-b

Answer 1

Answer: (B)

c-b-a

The concentration of the organic waste and its subsequent decomposition determines the BOD of the water. If the organic concentration is high, there will be high BOD because more dissolved oxygen will be used to breakdown the organic mass by the bacteria. So, high BOD marks high pollution. Distilled water has no content of the organic substances as it has been treated and filtered. Unpolluted water pond will not have polluted source of organic mass but the existing organisms die and their dead bodies contribute to a little raised BOD. Highly polluted water will show a very high value of BOD as it will take a lot of dissolved oxygen to breakdown the organic mass in water body. So, distilled water is least polluted, followed by unpolluted pond water and then highly polluted pond water.

Hence, the correct answer is 'c - b - a'.