What do you mean by Biological Oxygen Demand (BOD) ? 

Explanation

The microbes that decompose organic wastes in water bodies consume a lot of oxygen.
This results into sharp decline in dissolved oxygen downstream from the point of sewage discharge.

This causes mortally of fish and other aquatic creatures.
BOD refer to the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidized by bacteria.
The BOD test measures the rate of uptake of oxygen by micro-organisms in a sample of water.
Indirectly BOD is a measure of the organic matter present in the water.
The greater the BOD of waste water, more is its polluting potential.