Question

Sewage water is purified for recycling by the action of

• A. Light
• B. Micro-organisms
• C. Aquatic plants
• D. Fishes

Answer 1

The correct option is B Micro-organisms

Domestic sewage primarily contains biodegradable organic matter, which readily decomposes by bacteria and other micro-organisms, 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.