In the process of sewage treatment, in order to make the treated water meet the standard discharge, water quality monitoring equipment will be used to test the water quality in every link of sewage treatment. According to the data measured by the water quality monitoring equipment, corresponding treatment methods will be adopted to make this link The water quality index meets the requirements, and then enters the next treatment link. Among these water quality monitoring indicators, the two most important indicators that everyone hears are COD and BOD
. So what is the difference and connection between these two? Let me introduce to you why COD and BOD are commonly used in water pollution indicators, as well as the differences and connections between COD and BOD. What are CODs? COD (Chemical Oxygen Demand): It is the amount of oxidant consumed when a certain strong oxidant is used to treat water samples under certain conditions. It reflects the degree of material pollution in water, the greater the chemical oxygen demand, the more serious the pollution of organic matter in water. COD is expressed in mg/L. According to the COD value detected by the water quality monitoring instrument, the water quality can be divided into five categories. Among them, the COD of the first and second categories is less than 15mg/L, which can basically meet the drinking water standard, and the water with a value greater than the second category If it cannot be used as drinking water, the three types of COD ≤ 20mg/L, the fourth type of COD ≤ 30mg/L, and the fifth type of COD ≤ 40mg/L belong to polluted water quality. The higher the COD value, the more serious the pollution. What is BOD? BOD (Biochemical Oxygen Demand): refers to the mass concentration of dissolved oxygen required for the biochemical process of microbial decomposition of organic matter in water under aerobic conditions. In order to make the BOD detection values comparable, a period of time is generally specified, and the consumption of dissolved oxygen in water is determined. Generally, five days are used, which is called five-day biochemical oxygen demand, recorded as BOD5, and five-day biochemical oxygen demand is often used. . The larger the BOD value, the more organic matter is contained in the water, so the pollution is more serious. BOD is an environmental monitoring index used to monitor the pollution of organic matter in water. Organic matter can be decomposed by microorganisms. Oxygen is consumed during this process. If the dissolved oxygen in water is not enough to supply the needs of microorganisms, the water body will be treated for pollution. What is the relationship between COD and BOD? 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. 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 (gas oxygen demand). Since COD (chemical oxygen demand) and BOD (gas 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. Therefore, 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. We all hear more about it in sewage treatment. 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, 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 indicate that sewage is not biodegradable of organic matter.
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