What are the reasons for c0d exceeding the standard in sewage treatment?

Chemical oxygen demand (COD) refers to the amount of oxidation consumed when treating water samples with a strong oxidant under certain conditions. It is an index indicating the quantity of reducing substances in water. Reducing substances in water include various organic substances, nitrite, sulfide, ferrous salt and so on. But mainly organic matter. Therefore, chemical oxygen demand (COD) is often used as an index to measure the organic matter content in water. The greater the chemical oxygen demand, the more serious the water body is polluted by organic matter. Doctor da environmental protection, answer for you, thank you.