The BOD test measures the rate of uptake of oxygen by microbes in water bodies. What does a greater BOD of a water sample indicate?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → It is highly polluted.
BOD (biochemical oxygen demand) is the amount of oxygen that would be used up if all the organic matter in one litre of water were oxidised by bacteria. More organic matter means more microbes using more oxygen. So BOD is an indirect measure of the organic matter in the water, and a greater BOD means greater pollution.
Option (1) It is not polluted. — Incorrect. Clean water has little organic matter, so its BOD is low, not high.
Option (2) It is moderately polluted. — Incorrect. A greater BOD points to a large load of organic matter. The book links a greater BOD with a greater polluting potential, not a moderate one.
Option (3) It is highly polluted. — Correct. A greater BOD shows that microbes are using a lot of oxygen to break down a lot of organic matter. So the water is highly polluted.
NCERT Extract:
Microbes in Sewage Treatment – “… 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.”