In the process of sewage treatment, there are hundreds of organic substances. It is time-consuming and medicine-consuming to analyze these organic substances one by one. After research, it has been found that all organic substances have two commonalities. 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 harmless with oxygen. Carbon dioxide and water. Organic matter in sewage, whether in the process of chemical oxidation or in the process of biological oxidation to consume oxygen, the more organic matter in wastewater, the more oxygen consumption, there is a positive proportional relationship between the two. Therefore, the amount of oxygen consumed by the chemical oxidation of sewage is called COD (chemical oxygen demand), and the amount of oxygen consumed by the microbial oxidation of sewage is called BOD (gas oxygen demand).

Because COD (chemical oxygen demand) and BOD (gas oxygen demand) can comprehensively reflect the amount of all organic matter in the water, there are many such detection instruments, the detection method is simple, and the detection results can be obtained in a short time. Therefore, it is widely used in water quality detection and analysis, becoming an important indicator of water quality monitoring and an important basis for environmental monitoring of water bodies. We all hear more in sewage treatment.

In fact, COD (chemical oxygen demand) not only reacts 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 are ferrous ions in the biochemical treatment effluent, and the effluent COD (chemical oxygen demand) may exceed the standard.

Some organic substances in sewage can be biologically oxidized (such as glucose and ethanol), some can only be partially degraded by biological oxidation (such as methanol), and some organic substances cannot be degraded by biological oxidation, and there are certain Toxic (some surfactants). In this way, the organic matter in the 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 (gas oxygen demand) is biodegradable organic matter in sewage, so the difference between COD and BOD can represent non-biodegradable organic matter in sewage.