[COD] chemical oxygen demand. It is the amount of oxidant consumed when a certain strong oxidant is used to treat a water sample under certain conditions. It is an indicator of the amount of reducing substances in water. The reducing substances in water include various organic substances, nitrites, sulfides, ferrous salts, etc. But the main thing is organics. Therefore, chemical oxygen demand (COD) is often used as an indicator to measure the amount of organic matter in water.

[Influent COD concentration] refers to the COD concentration of the water inlet of the sewage treatment plant, and is an indicator to measure the amount of reducing substances in the water at the water inlet. If the sewage with low COD concentration enters the sewage treatment plant for treatment, it will not only increase the operational burden of the sewage treatment plant, but also destroy the sludge activity and affect the operation efficiency of the sewage biological treatment system. These phenomena should be given full attention and effective measures should be taken for rectification and governance.