COD (chemical oxygen demand) is 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. The determination of chemical oxygen demand varies with the determination and determination method of reducing substances in water samples. At present, the most commonly used methods are acid potassium permanganate oxidation and potassium dichromate oxidation. Potassium permanganate (K2MnO4) method has a low oxidation rate, but it is relatively simple, and can be used to determine the relative comparative value of organic matter content in water samples. Potassium dichromate oxidation method is fast and reproducible, and is suitable for the determination of total organic matter in water samples. Organic matter is very harmful to industrial water system. Water containing a lot of organic matter will pollute ion exchange resin, especially anion exchange resin, and reduce the exchange capacity of resin when it passes through desalination system. Organic matter can be reduced by about 50% after pretreatment (coagulation, clarification and filtration), but it cannot be removed in the desalting system, so it is often brought into the boiler through make-up water, which reduces the pH value of boiler water. Sometimes organic matter may be brought into the steam system and condensate water, which will lower the pH value and cause system corrosion. High organic matter content in circulating water system will promote microbial reproduction. Therefore, no matter for desalting, boiler water or circulating water system, the lower the COD, the better, but there is no unified limit index. When COD(DmnO4 _ 4 method) in circulating cooling water system is more than 5mg/L, the water quality has begun to deteriorate.
COD emission reduction is to reduce the pollutants in water to no obvious harm to the environment and achieve the purpose of recycling water resources.