The difference between COD and BOD

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COD represents the amount of oxygen required for potassium dichromate to oxidize the organic matter in one liter of wastewater under strongly acidic conditions, and it can roughly indicate the amount of organic matter present in the wastewater. BOD5 is the amount of oxygen required by microorganisms to biodegrade the organic matter in one liter of wastewater over a period of five days (under cultivation at 20 degrees Celsius). Since about 70% of the carbonization process of organic matter is completed during this five-day cultivation period, BOD5 can indirectly reflect the amount of organic matter in wastewater that can be degraded by microorganisms. 　　 　　COD stands for chemical oxygen demand, and it is of course related to the oxidant used (the measurement data should indicate what oxidant was used). BOD5 is biological oxygen demand, and it is closely related to factors such as water temperature, water quality, and whether the water is toxic or not (microbial activity varies under different conditions). 　　 　　COD is greater than BOD. COD – BOD is approximately equal to the amount of organic matter that cannot be biodegraded. This can be said roughly, but it’s not exact, as COD = COD(B) + COD(NB), where the former represents the portion that can be biodegraded and the latter represents the portion that cannot. Microorganisms require about 20 days to complete the carbonization process at 20 degrees Celsius (i.e., BOD20, which is roughly equal to CODNB); therefore, COD–BOD20 is approximately equal to the biologically unavailable organic matter (ignoring the influence of reducing inorganic substances). 　　 　　CODcr, or chemical oxygen demand, has the advantage of being able to accurately indicate the amount of organic matter in wastewater, with a short measurement time and no restrictions based on water quality. However, its drawback is that it cannot show the amount of oxygen consumed, as is the case with BOD measurements. The amount of organic matter oxidized by microorganisms, along with many inorganic substances that are also oxidized, all represent the organic matter content. 　　 　　BOD5, or biochemical oxygen demand, refers to the amount of oxygen required under aerobic conditions, over a specified temperature and time period, for microorganisms (mainly bacteria) to degrade organic matter in water. It generally takes 100 days for organic matter to be completely degraded. In practice, BOD20 at 20°C over 20 days is used as a representative value. Often, 20 days of production application is too long; it does not facilitate the guidance of production processes for urban wastewater. Its BOD5 is approximately 70% to 80% of BOD20. 　　 　　In urban wastewater, COD is greater than BOD. The difference between the two is roughly the amount of biodegradably difficult organic matter. In municipal wastewater, the BOD/COD ratio is used as an indicator of biodegradability. When BOD/COD ≥ 0.3, the biodegradability is good, making it suitable for biochemical treatment processes. 　　 　　In industrial wastewater, the BOD/COD ratio is usually below 0.03, resulting in poor biodegradability; therefore, the values need to be adjusted before biological treatment can be carried out. BOD: The amount of oxygen required for aerobic microorganisms to decompose organic pollutants in water is known as biochemical oxygen demand. 　　 　　COD: The amount of oxidizing agent consumed in the oxidation of organic pollutants in water is referred to as chemical oxygen demand. Both COD and BOD are indicators used to measure the amount of organic matter in wastewater. BOD represents the amount of oxygen required for the aerobic decomposition of organic matter in wastewater. It is generally believed that BOD indicates the organic matter that can be biodegraded. However, there are some misunderstandings here: since the conditions under which BOD is measured are completely different from those in actual operation, it is unreasonable to use the difference between COD and BOD to indicate organic matter that cannot be degraded. 　　 　　Furthermore, the removal of organic matter in actual systems involves many processes, not just biological degradation. 　　 　　In practice, BOD/COD is used to indicate the biodegradability of wastewater, based on practical experience; it is not appropriate to apply the concepts of BOD and COD in a metaphysical manner to real-world situations.