Concepts and units of CODcr in wastewater

COD is the abbreviation for Chemical Oxygen Demand; the methods for determining it vary depending on the type of oxidant used, such as the chromium Cr method and the manganese Mn method； The unit for general emission standards is mg/L, while the unit t/a may refer to the total amount of emissions.

         Chemical Oxygen Demand (COD) refers to the amount of oxidant consumed when a water sample is treated with potassium dichromate as an oxidizing agent under strong acidic conditions and at elevated temperatures, and it is expressed in mg/L of oxygen. Chemical oxygen demand indicates the degree of contamination by reducing substances in water; such reducing substances include organic compounds, nitrites, ferrous compounds, sulfides, etc. Contamination of water by organic compounds is quite common, and therefore chemical oxygen demand is also used as one of the indicators for the relative content of organic matter. However, it can only reflect contamination caused by organic compounds that can be oxidized, and it does not indicate levels of pollution from polycyclic aromatic hydrocarbons, PCBs, dioxins, etc. CODcr is one of the indicators used in China for implementing total emission controls. For wastewater, China specifies the use of the potassium dichromate method, and the value obtained through this method is referred to as chemical oxygen demand. An emission limit of 1.2118 tons/year means the maximum allowable total amount of pollutants that can be emitted each year; this value is calculated based on CODcr

COD stands for Chemical Oxygen Demand, measured in mg/L (it is also expressed in ppm). Nowadays, rapid reagent tests are used for this purpose, as the national standard method is too complicated!