Question

When domestic sewage mixes with river water

• A. Small animals like rats will die after drinking river water
• B. The increased microbial activity releases micronutrients such as iron
• C. The increased microbial activity uses up dissolved oxygen
• D. The river water is still suitable for drinking as impurities are only about 0$\cdot$1%

Answer 1

Answer: (C)

The increased microbial activity uses up dissolved oxygen

Answer 2

When sewage mixes with water body, micro- organisms present in it biodegrade organic matter of sewage using oxygen. This results into a sharp decline in dissolved oxygen which may cause mortality of aquatic creatures. Gradually, however, dissolved oxygen increases in concentration with the completion of biodegradation of sewage matter.