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BOD: Biological Oxygen Demand; COD: Chemical Oxygen Demand. Biochemical Oxygen Demand (BOD) is an environmental monitoring indicator primarily used to assess the level of organic pollution in water bodies. Most organic substances can be broken down by microorganisms, but these microorganisms require oxygen to decompose organic compounds in water. If the dissolved oxygen in the water is not sufficient to meet their needs, the water body becomes polluted. To ensure comparability of testing data, a time period is generally specified during which microorganisms are cultured in water samples at a certain temperature, and the consumption of dissolved oxygen in the water is measured. Typically, a period of five days is used, which is referred to as the five-day biochemical oxygen demand, denoted as BOD5. The higher the value, the more organic matter is present in the water, and thus the greater the level of pollution. Chemical oxygen demand (COD) is the amount of oxidizing agent consumed when a water sample is treated with a certain strong oxidant under specific conditions. It is an indicator of the amount of reducing substances in water. Reducing substances in water include various organic compounds, nitrites, sulfides, ferrous salts, etc. But the main component is organic matter. Therefore, Chemical Oxygen Demand (COD) is often used as an indicator to measure the amount of organic matter in water. The higher the chemical oxygen demand, the more severe the pollution of the water body by organic matter. The determination of Chemical Oxygen Demand (COD) varies depending on the reductive substances present in the water sample being tested, as well as the testing method used. The most commonly used methods at present are the potassium permanganate oxidation in acidic medium and the potassium dichromate oxidation method. The potassium permanganate (KmnO4) method has a low oxidation rate, but it is simple to use; it can be employed for determining relative values of organic matter content in water samples. The potassium dichromate (K2Cr2O7) method features a high oxidation rate and good reproducibility, and is suitable for determining the total amount of organic matter in water samples. Organic substances pose a great threat to industrial water systems. Water containing a large amount of organic matter contaminates the ion exchange resins when passing through the desalination system, particularly affecting anion exchange resins and reducing their exchange capacity. Organic matter can be reduced by about 50% during pretreatment processes such as coagulation, clarification, and filtration; however, it cannot be removed in desalination systems. As a result, it is often introduced into the boiler through make-up water, thereby lowering the pH value of the boiler water. Sometimes organic substances can also enter the steam system and condensate water, lowering the pH and causing corrosion in the system. A high organic content in circulating water systems promotes microbial growth. Therefore, for desalination, boiler water, or circulating water systems, the lower the COD, the better, although there is no unified limit value. In a circulating cooling water system, when COD (DmnO4 method) is >5mg/L, the water quality begins to deteriorate.