Question

When sewage enters a flowing river; the rapid depletion of dissolved oxygen is due to:

1. The respiratory activity of aquatic plants in the river
2. The suspended particles in the river and the waste
3. The change in temperature of the river water
4. The microbial activity

Answer 1

Correct Answer - Option 4 : The microbial activity

Explanation:

• Sewage is another source of water pollution. If untreated sewage gets into rivers, microorganisms decompose it. They use oxygen from the water for aerobic respiration. As a result, there is a depletion of oxygen dissolved in water.
• Due to less dissolved oxygen in water, aquatic organisms such as fish and insects may be unable to survive.
• In sewage treatment works, lots of oxygen is provided by stirring the waste or by injecting jets of compressed air. This allows the microorganisms to break down the waste completely before the treated water is discharged into a river or the sea.