Due to the time-consuming nature of the BOD5 test, another index called COD has been defined to measure the content of organic substances in wastewater, and the concentration of organic compounds in wastewater is evaluated based on the amount of oxygen required for the oxidation of organic substances in wastewater.

The COD test is the measurement of the oxygen required for the oxidation of that part of organic substances that are susceptible to oxidation by a strong oxidizing agent such as permanganate or potassium dichromate, and is one of the most important and fastest methods in the study and control of purification systems. It is considered water and sewage.