Civil Engineering

# Biochemical Oxygen Demand (B.O.D)

The amount of oxygen required to carryout the biological decomposition of dissolved solids in sewage under aerobic conditions at standard temperature is known as biochemical oxygen demand.

It indicates the amount of biologically active organic matter present in the sewage. It is generally less than C.O.D.
The B.O.D of water during 5 days at 20 ° C is generally taken as the standard demand and it is about 68% of the total demand.

## B.O.D Test

To determine the standard 5 day B.O.D in the laboratory, a known volume of sewage sample is mixed with a known volume of aerated pure water. The dissolved oxygen (D.O) of the diluted sample is then determined.

The diluted sample is then incubated for 5 days at 20 ° C. The D.O of the sample after this period is again determined. The difference between the initial D.O and the final D.O gives the oxygen consumed by the sample in 5 days. The B.O.D is then calculated by using the following equation.

B.O.D OR B.O.D5 = ( D.O consumed by the test sample Х Dilution factor )
Where, Dilution factor = Volume of diluted sample/Volume of undiluted sewage sample