Why COD and BOD are commonly used pollution indicators？

In the process of sewage treatment, in order to achieve standard discharge of treated water, water quality monitoring equipment is used to detect water quality in each stage of sewage 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, and then proceed to the next treatment stage. Among these water quality monitoring indicators, the two most commonly heard and important are COD and BOD. So what are the differences and connections between these two? Shuibo Network will 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 is COD?

COD (Chemical Oxygen Demand): refers to the amount of oxidation consumed when using a certain strong oxidant to treat water samples under certain conditions. It reflects the degree of material pollution in water, and the higher the chemical oxygen demand, the more severe the pollution of organic matter in water. COD is expressed in mg/L. According to the COD values detected by water quality monitoring instruments, water quality can be divided into five categories. Among them, Class I and Class II COD ≤ 15mg/L can basically meet the drinking water standards. Water with values greater than Class II cannot be used as drinking water. Class III COD ≤ 20mg/L, Class IV COD ≤ 30mg/L, and Class V COD ≤ 40mg/L belong to polluted water quality. The higher the COD value, the more severe 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 ensure comparability of BOD detection values, a time period is generally specified and the consumption of dissolved oxygen in water is measured. A period of five days is generally used, which is called the five day biochemical oxygen demand and recorded as BOD5. The 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 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, which requires the consumption of oxygen. If the dissolved oxygen in the water is insufficient to meet the needs of microorganisms, the water 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. Analyzing each of 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 wastewater, whether in chemical oxidation or biological oxidation processes, consumes oxygen. The more organic matter in wastewater, the more oxygen is consumed, and there is a positive proportional relationship between the two. So, the amount of oxygen consumed by chemical oxidation of wastewater is called COD (Chemical Oxygen Demand), and the amount of oxygen consumed by microbial oxidation in wastewater is called BOD (Oxygen Demand for Gas).

Due to the comprehensive reflection of the quantity of all organic substances in water by COD (Chemical Oxygen Demand) and BOD (Gas Oxygen Demand), there are also many detection instruments with simple detection methods that can obtain detection results in a short period of time. Therefore, they are widely used in water quality detection and analysis, becoming important indicators for water quality monitoring and an important basis for environmental monitoring of water bodies. We have heard a lot about them 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 sulfides, ferrous ions, sodium sulfite, etc. For example, if the ferrous ions in sewage are not completely removed in the neutralization tank, there may be ferrous ions present in the biochemical treatment effluent, and the COD (chemical oxygen demand) of the effluent may exceed the standard.

Some organic substances in wastewater 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 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 wastewater, while BOD (Oxygen Demand for Gas) is the biodegradable organic matter in wastewater. Therefore, the difference between COD and BOD can represent the non biodegradable organic matter in wastewater.