CHEMICAL OXYGEN DEMAND
The BOD test provides a measure of the biodegradable organic material in water, i.e., of the substance that microbe can readily use for food. There might also be non biodegradable or slowly biodegradable substance that will not detected by the convectional BOD test.
The Chemical Oxygen Demand, COD is another parameter of water quality which measures all organics, including the non biodegradable substances. It is a chemical test using a strong oxidizing agent (Potassium Dichromate), sulphuric acid and heat. The result of the COD test can be available in just 2hours, a definite advantage over the 5d required for the standard BOD test.
COD values are always higher than BOD values for the same sample, but there is generally no consistent correlation between the two tests for different wastewater. In other word, it is not feasible to simply measure the COD and then predict the BOD. Because most waste water treatment plant are biological in their mode of operation, the BOD is more representative of the treatment process and remains a more commonly used parameter than the COD.