What is the relationship between COD and BOD in sewage treatment

In the process of sewage treatment, there are hundreds of organic substances, and it is time-consuming and expensive to analyze these organic substances one by one. After research, it is found that all organic substances have two things in common. One is that they are all composed of hydrocarbons, and the other is that most organic substances can be chemically oxidized or oxidized by microorganisms. Their carbon and hydrogen form non-toxic substances with oxygen respectively. Harmless carbon dioxide and water. The organic matter in the sewage consumes oxygen no matter in the process of chemical oxidation or biological oxidation. The more organic matter in the waste water, the more oxygen will be consumed, and there is a direct proportional relationship between the two. Therefore, the amount of oxygen consumed by chemical oxidation of sewage is called COD (chemical oxygen demand), and the amount of oxygen consumed by microbial oxidation in sewage is called BOD (biochemical oxygen demand). Since COD (Chemical Oxygen Demand) and BOD (Biochemical Oxygen Demand) can comprehensively reflect the amount of all organic matter in water, there are many such detection instruments, the detection method is simple, and the detection results can be obtained in a relatively short period of time, so It is widely used in water quality testing and analysis, becoming an important indicator of water quality monitoring and an important basis for environmental monitoring of water bodies. In fact, COD (chemical oxygen demand) not only reflects organic matter in water, but also represents inorganic substances with reducing properties in water, such as: sulfide, ferrous ion, sodium sulfite, etc. For example, if the ferrous ions in the sewage are not completely removed in the neutralization tank, there will be ferrous ions in the biochemical treatment effluent, and the COD (chemical oxygen demand) of the effluent may exceed the standard. Some organic substances in sewage can be biooxidized (such as glucose and ethanol), some can only be partially biooxidized and degraded (such as methanol), and some organic substances cannot be biooxidized and degraded. Toxicity (certain surfactants). In this way, organic matter in sewage can be divided into two parts, biodegradable and non-biodegradable organic matter. Traditionally, COD (chemical oxygen demand) basically represents all organic matter in sewage, and BOD (biochemical oxygen demand) is biodegradable organic matter in sewage, so the difference between COD and BOD can indicate that sewage is not biodegradable of organic matter.
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