When domestic sewage mixes with river water
- The river water is still suitable for drinking as impurities are only about 0.1%
- Small animals like rats will die after drinking river water
- The increased microbial activity releases micro-nutrients such as iron
- The increased microbial activity uses up dissolved oxygen
Answer
(4) The increased microbial activity uses up dissolved oxygen
Solution
As per NCERT, domestic sewage is rich in biodegradable organic matter. Micro-organisms decompose this matter, and in doing so consume a large amount of the oxygen dissolved in the water (high BOD).
Dissolved oxygen falls sharply, causing the death of fish and other aquatic animals.
Answer: (4)