Chemical oxygen demand (COD) refers to the amount of oxidizing agent consumed when treating water samples with strong oxidizing agents under certain conditions, 

expressed in mg/l of oxygen. Chemical oxygen demand reflects the degree of pollution of water by reducing substances.

 Reducing substances in water include organic matter, nitrite, ferrite, sulfide and so on. It is common for water to be polluted by organic matter,

 so chemical oxygen demand is also one of the indicators of the relative content of organic matter.

The chemical oxygen demand of the water sample may vary depending on the type and concentration of the oxidant added, 

the acidity of the reaction solution, the reaction temperature and time, and the presence or absence of a catalyst. 

Therefore, the chemical oxygen demand is also a conditional indicator, which must be carried out in strict accordance with the operational procedures. 

For industrial wastewater, our country stipulates that potassium dichromate method is used, and the measured value is called chemical oxygen demand.