When domestic sewage mixes with river water

When domestic sewage mixes with river water
  1. small animals like rats will die after drinking river water
  2. the increased microbial activity releases micronutrients such as iron
  3. the increased microbial activity uses up dissolved oxygen
  4. the river water is still suitable for drinking as impurities are only about $0.1 \%$

Solution

Domestic sewage consists a lot of degradable organic matter. When this sewage is released into river water, it will result in an increase of Biochemical Oxygen Demand (BOD) and decrease in the amount of dissolved oxygen due to decomposing activity of microbes.

Asked in: NEET 2010 (Mains)

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