Biochemical Oxygen Demand (BOD) in a river water :
- Gives a measure of Salmonella in the water
- Has no relationship with concentration of oxygen in the water
- Increases when sewage gets mixed with river water
- Remains unchanged when algal bloom occurs
Answer
(3) Increases when sewage gets mixed with river water
Solution
As per NCERT, BOD refers to the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria.
Sewage contains a large amount of biodegradable organic matter.
⇒ When sewage mixes with river water, microbial decomposition increases and so the BOD increases.
Answer: (3)