Question

Arrange according to the 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

Hint: The expanded form of BOD is biological oxygen demand. The value of BOD depends on the amount of the organic mass present in the water. It also accounts the amount of decomposition matter present in the water. If the organic matter in the water is high then its BOD is also high.

Complete Answer:
The biological oxygen demand is defined as the quantity of the oxygen needed for the decomposition of the dead organic matter by the microorganisms like bacteria, fungi etc. This decomposition process takes place only in the presence of oxygen. This value of BOD mainly depends on the decomposable or oxidizable substances that are present in the considered water surface.
When we consider the given options,
(a) Highly polluted pond water:
This water has a high amount of both organic and inorganic matters. Hence there requires more oxygen to decompose those organic matter present in it. Hence BOD of highly polluted water is maximum.

(b) Unpolluted pond water:
This water is clean when compared to polluted water. It does not contain any dead organic matter. But if the present living organic matter dies, the BOD of this water increases. Hence BOD of unpolluted water is moderate.

(c) Distilled water:
The distilled water is pure and it doesn’t contain any pollution of organic matter, and hence the biological oxygen demand of this water is very low or nearly zero.

Thus the option (C) is correct.

Note: This value of Biological Oxygen Demand is used in the wastewater treatment plants or sewage treatment plants in order to find the degree of the organic matter present in the water body. The change in the value of BOD is measured at a specific period of time and in a proper temperature.