Question

When domestic sewage mixes with river water

• A. Small animals like rats will die after drinking river water.
• B. The increased microbial activity release micro nutrients, 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

The correct option is C The increased microbial activity uses up dissolved oxygen.

Domestic sewage primarily contains biodegradable organic matter, which readily decomposes by bacteria and other microorganisms, which can multiply using these organic substances as substrates and hence, utilise some of the components of sewage. It is possible to estimate the amount of biodegradable organic matter in sewage water by measuring biochemical oxygen demand (BOD). Microorganisms involved in biodegradation of organic matter in the receiving water body consume a lot of oxygen, and as a result there is a sharp decline in dissolved oxygen downstream from the point of sewage discharge. This causes mortality of fish and other aquatic creatures.