The amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria is called:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → BOD
Sewage contains a lot of organic matter. Bacteria break this matter down and use oxygen while doing so. The amount of oxygen they would use to oxidise all the organic matter in one litre of water is the biochemical oxygen demand (BOD). So BOD is an indirect measure of how much organic matter the water holds.
Option (1) DO — Incorrect. DO stands for dissolved oxygen, the oxygen already present in the water. It is not the oxygen that bacteria would use up to oxidise the organic matter.
Option (2) COD — Incorrect. COD stands for chemical oxygen demand. It measures the oxygen needed when the matter is oxidised by a strong chemical, not by bacteria.
Option (3) BOD — Correct. BOD stands for biochemical oxygen demand. The more organic matter in the water, the higher its BOD and the more polluting it is. Sewage is treated until its BOD falls.
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.”