Why are COD and BOD commonly used pollution indicators?

Author: Hongjie Water

In the process of water treatment, in order to ensure that the treated water meets the discharge standards, water quality monitoring equipment is used to detect water quality in each stage of water treatment. Based on the data measured by the water quality monitoring equipment, corresponding treatment methods are adopted to ensure that the water quality indicators in this stage meet the requirements before proceeding to the next treatment stage. Among these water quality monitoring indicators, the two most common and important ones are COD and BOD. So what are the differences and connections between these two? Han High School Alliance will introduce to you why COD and BOD are commonly used in water quality pollution indicators, as well as the differences and connections between COD and BOD.

What is COD?

COD (Chemical oxygen demand): refers to the amount of oxidation consumed when a certain strong oxidant is used to treat water samples under certain conditions. It reflects the degree of pollution of water by substances. The greater the Chemical oxygen demand, the more serious the pollution of water by organic substances. COD is expressed in mg/L, and the COD values detected by water quality monitoring instruments can be divided into five categories. The first and second categories of COD ≤ 15mg/L can basically meet the drinking water standards, while water with values greater than the second category cannot be used as drinking water. The third category of COD ≤ 20mg/L, the fourth category of COD ≤ 30mg/L, and the fifth category of COD ≤ 40mg/L belong to polluted water quality. The higher the COD value, the more serious the pollution is

What is BOD?

BOD (Biochemical oxygen demand): refers to the mass concentration of dissolved oxygen required in the biological Chemical process of microbial decomposition of organic matter in water under aerobic conditions. In order to make the BOD detection values comparable, a time period is generally specified and the consumption of dissolved oxygen in water is measured. Generally, five days is used, which is called five-day Biochemical oxygen demand, recorded as BOD5, and five-day Biochemical oxygen demand is often used. The higher the BOD value, the more organic matter is contained in the water, and therefore the pollution is also more severe.

BOD is an environmental monitoring indicator used to monitor the pollution of organic matter in water. Organic matter can be decomposed by microorganisms, and oxygen needs to be consumed during this process. If the dissolved oxygen in the water is insufficient to meet 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 over a hundred types of organic substances. Analyzing these organic substances one by one consumes both time and medicine. After research, it has been found that all organic substances have two commonalities. Firstly, they are composed of hydrocarbons. Secondly, the vast majority of organic substances can be chemically oxidized or oxidized by microorganisms. Their carbon and hydrogen form non-toxic and harmless carbon dioxide and water with oxygen, respectively. The organic matter in sewage, whether in the process of chemical oxidation or biological oxidation, consumes oxygen. The more organic matter in wastewater, the more oxygen is consumed, and there is a positive proportional relationship between the two. Therefore, the oxygen consumed by chemical oxidation of sewage is called COD (Chemical oxygen demand), and the oxygen consumed by microbial oxidation of sewage is called BOD (oxygen demand of gas).

Since COD (Chemical oxygen demand) and BOD (gas oxygen demand) can comprehensively reflect the amount of all organic substances 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, they are widely used in water quality detection and analysis, becoming an important indicator of water quality monitoring, and also an important basis for environmental monitoring of water bodies. We all hear more about them in sewage treatment.

In fact, COD (Chemical oxygen demand) not only reacts with organic matters 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 ion in the sewage is not completely removed in the neutralization tank, the effluent COD (Chemical oxygen demand) may exceed the standard due to the presence of ferrous ion in the effluent of biochemical treatment.

Some organic substances in sewage can be biooxidized (such as glucose and ethanol), some can only be partially biodegraded (such as methanol), and some organic substances cannot be biodegraded and have certain toxicity (certain surfactants). In this way, the organic matter in sewage can be divided into two parts, biodegradable and non biodegradable organic matter. Traditionally, COD (Chemical oxygen demand) basically represents all organics in sewage, and BOD (gas oxygen demand) is the biodegradable organics in sewage, so the difference between COD and BOD can represent the non biodegradable organics in sewage.