Question

Which of the following does not occur when the sewage is discharged into water?

• A. Increase in $O_2$
• B. Cyanophycean blooms occur
• C. Depletion of $O_2$ layers
• D. Eutrophication

Answer 1

The correct option is A Increase in $O_2$

Decomposition of organic matter by microbes requires oxygen. Degree of impurity of water due to organic matter is measured in terms of Biochemical oxygen demand or (BOD).

BOD is the oxygen required by microorganisms to completely metabolise organic wastes present in water.

When untreated sewage is directly discharged into water bodies, it is called organic loading. Organic loading is one source for increased nutrient concentration in water bodies (or eutrophication). It results in increase in BOD and explosive multiplication of blue green alga, called as algal bloom. Algal bloom and increased consumption of oxygen by sewage metabolising microorganisms results in decrease in level of dissolved oxygen in water.