Biological Oxygen Demand (BOD)
BOD is used by regulatory agencies for monitoring wastewater treatment facilities and monitoring surface water quality. BOD is the biochemical oxygen demand of the water and it is related to the concentration of the bacterial facilitated decomposable organic material in the water.
A laboratory measurement of wastewater that is one of the main indicators of the quantity of pollutants present; a parameter used to measure the amount of oxygen that will be consumed by microorganisms during the biological reaction of oxygen with organic material. The total milligrams of oxygen required over a 5-day test period to biologically assimilate the organic contaminants in 1 litre of wastewater maintained at 20°C, is termed as BOD. Now-a-days BOD tests are performed for 3 days at 27oC. The BOD of a wastewater is widely used as an indicator of the fraction of organic matter that may be degraded by microbial action in a given time period at a temperature of 20°C. BOD is a measure of the pollution strength of a wastewater and the test is related to the oxygen that would be required to stabilize the waste after discharge to a receiving body of water. The BOD test has been widely used by regulatory agencies to gauge overall treatment plant performance. The classical measurement of BOD of the plant effluent, primary tank effluent and final effluent gives the most common measure of treatment plant efficiency. The drop in BOD from raw effluent to final effluent is usually used in calculating the solids growth rate in the aeration tank. This test is too slow to gives timely data to the operator for control purposes. It could, therefore, provide the operator with the historic results of previous operating decisions. Tests for BOD are to be made on composite samples daily. The BOD tests run for at least 20 days should also be made on the effluent periodically to determine the oxygen requirements of the nitrogen compounds present in the effluent.
COD measurements are preferred for a mixed domestic-industrial wastewater or where a more rapid determination of the load is desired. The COD test would record the oxygen demand for certain industrial wastes which cannot be used readily as food through the treatment plant organisms. The COD test might be run in various hours, giving the operator a more timely measurement of what is entering the plant and how the plant is performing.