Bacteria and other micro organisms are responsible for decomposing organic waste. When organic matterials like dead plants, leaves, manure, sewage or food waste are present in water supply, bacteria started to break them. By that time, much of the available Dissolved Oxygen (D.O.) is consumed by bacteria from other aquatic organisms of the oxygen they need to live.

Biological Oxygen Demand (B.O.D.) is one of the most common measures of the oxygen used by micro organisms to decompose the waste. If  there  is a large amount of organic waste is present in the water supply, there will also be a lot of bacteria present which decompose the waste. In this case, the demand for oxygen will be high. As a result, Biological Oxygen Demand (B.O.D.) level will be high.

A high B.O.D. value indicates pollution, i.e; water containing higher level of organic wastes that consumed the Dissolved oxygen (D.O.)  and is thus unsafe for human consumption.

Tulip Das