Consider the following statements:
A. BOD is the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria.
B. Sewage water is treated till its BOD is reduced.
C. The greater the BOD of waste water, less is its polluting potential.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → A and B only
A. BOD is the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria. — True. This is the book’s definition of BOD (biochemical oxygen demand). It is an indirect measure of the organic matter in water.
B. Sewage water is treated till its BOD is reduced. — True. In secondary treatment, aerobic microbes in the aeration tank use up the organic matter, so the BOD of the effluent falls. Treatment goes on until the BOD is reduced significantly.
C. The greater the BOD of waste water, less is its polluting potential. — False. A high BOD means the water holds a lot of organic matter. Such water is more polluting, not less. The statement reverses the book’s line.
NCERT Extract:
Microbes in Sewage Treatment – “… BOD refers to the amount of the oxygen that would be consumed if all the organic matter in one liter of water were oxidised by bacteria. The sewage water is treated till the BOD is reduced. The BOD test measures the rate of uptake of oxygen by micro-organisms in a sample of water and thus, indirectly, BOD is a measure of the organic matter present in the water. The greater the BOD of waste water, more is its polluting potential.”