Question

When does domestic sewage mix with river water?

• A. The increased microbial activity releases micronutrients such as iron
• B. The river water is still suitable for drinking as impurities are only about 0.1%
• C. The increased microbial activity uses up dissolved oxygen
• D. Small animals like rats will die after drinking river water

Answer 1

The correct option is C

The increased microbial activity uses up dissolved oxygen

The correct option is C.

Explanation of the correct option:

1. When domestic sewage mixes with river water, the microbes present in the water start to degrade it.
2. It increases microbial activity and increases the biological oxygen demand of the water.
3. As a result the percentage of dissolved oxygen decreases which leads to the death of aquatic life.

Explanation of the incorrect options:

Option A:

1. Sewage is harmful to drinking water or river water.
2. The increased microbial activity due to sewage water does not release micronutrients such as iron.

Option B:

1. The river water becomes 100% harmful if we add river water to sewage water.
2. The river water is completely destroyed by adding sewage water.

Option D:

1. Sewage is harmful for drinking because sewage contains harmful microbes.
2. As it is harmful so not only rats, all organisms that drink sewage water will die.

Final Answer: Hence, wdomestic sewage mix with river water when the increased microbial activity uses up dissolved oxygen.