Question

When domestic sewage mixes with river water, __________

• A. Small animals like rats will die after drinking river water
• B. The increased microbial activity released 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 $0.1\%$

Answer 1

Answer: (C)

The increased microbial activity uses up dissolved oxygen

Domestic sewage contains large amounts of organic matter. When domestic sewage mixes with river water, organic matter will content will increase in the river. This will lead to increase in BOD or Biochemical Oxygen Demand.

BOD is the amount of oxygen required by bacteria to decompose organic matter in one litre of polluted water.

If the water is more polluted, more will be the BOD because more will be the organic matter present in it and hence, more oxygen will be required to decompose it.

Hence, the dissolved oxygen will be used up by the bacteria to decompose the organic matter.

So, the correct answer is 'The increased microbial activity uses up dissolved oxygen'.