Chemical oxygen demand is that the oxygen consumption of the oxygenated parts of the waste water without the interactions of microorganisms. Many organic substances in wastewater are measured by means of potassium dichromate.

A any wastewater value of Chemical oxygen demand is higger than Biochemical oxygen demand. Reason is that many compounds that are not biologically oxidized can be chemically oxidized. Experiment of Chemical Oxygen demand can measure approximately in three hours whereas Biochemical oxygen demand (BOD5 nowadays used as only BOD) can measure in five days.

Between COD and BOD should establish an experimental ratio. After this ratio, measure of Chemical oxygen demand can measure Biochemical oxygen demand in wastewater too. According to studies of republic of Turkey, ratio of COD and BOD determine between 1,6 and 2,5. Discharge criteria are not provided in waste water which is higher than the determined rates. That's why, When Biological treatment plan designs, ratio of COD should consider. The components of the KOI parameter should be determined.

BOD and COD parameters should be controlled in the discharge of wastewater. Because higher value of COD and BOD can damage order of ecologically life, death of fish and level of oxygen. For this reason, high population density or in zone of industrial area, value of COD and BOD should control according to legislation of Country.