The BOD test is used to judge how polluting a sample of waste water is. What does this test measure?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → The rate of uptake of oxygen by micro-organisms in the sample
BOD stands for biochemical oxygen demand. It is the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria. The BOD test does not weigh the organic matter directly; it measures how fast micro-organisms in the sample take up oxygen. Since more organic matter means more oxygen use, BOD indirectly shows the organic matter present, and the higher the BOD, the more polluting the water.
Option (1) The number of pathogenic microbes in the sample — Incorrect. Pathogenic microbes make sewage harmful, but the BOD test does not count them. It measures oxygen uptake by the micro-organisms present.
Option (2) The amount of solids that settle as primary sludge — Incorrect. Solids that settle in primary treatment form the primary sludge. Their amount is a matter of physical settling, not of oxygen demand.
Option (3) The amount of oxygen given out by algae in the sample — Incorrect. Oxygen given out by algae in light is photosynthesis. BOD is about oxygen used up, not oxygen released.
Option (4) The rate of uptake of oxygen by micro-organisms in the sample — Correct. The BOD test measures the rate of uptake of oxygen by micro-organisms in a sample of water. From this, the organic matter content and the polluting potential of the water are judged.
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.”