Question

A large amount of untreated sewage entered the river. Many fish died. Untreated sewage water contains organic matter and bacteria. Explain why many fish died.

Answer 1

If untreated sewage gets into rivers, microorganisms decompose it. They use oxygen from the water for aerobic respiration. As a result there is less oxygen dissolved in water, so aquatic organisms such as fish and insects may be unable to survive.