Water Science - Comprehensive Technical Knowledge on BOD, an Important Indicator for Sewage Treatment

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There are many kinds of organic substances in sewage. Some sewage contains dozens, even hundreds of kinds of organic substances. If we conduct qualitative and quantitative analysis of organic substances in wastewater one by one, it will be time-consuming and costly. So can we use only one pollution index to represent all organic substances and other quantities in sewage? That is BOD, biochemical oxygen demand.

BOD (abbreviation of Biochemical Oxygen Demand): Biochemical oxygen demand or biochemical oxygen consumption (generally refers to five-day biochemical oxygen demand), which is a comprehensive indication of the content of oxygen-demanding pollutants such as organic matter in water. It indicates the total amount of dissolved oxygen consumed in water when organic matter in water is oxidized and decomposed by the biochemical action of microorganisms to make it inorganic or gaseous.

In the deoxygenation bottle, it is cultured in the dark at 20 degrees C for 5 days, and the mass concentration of dissolved oxygen in the water sample before and after the culture is determined respectively. The amount of dissolved oxygen consumed per liter of sample is calculated by the difference in the mass concentration of dissolved oxygen before and after the culture, and expressed in the form of BOD5. Its unit is ppm or mg/L. The higher the value, the more organic pollutants there are in the water, and the more serious the pollution.

In order to make the test data comparable, a time period is generally specified. During this period, microorganisms are cultured with water samples at a certain temperature, and the consumption of dissolved oxygen in the water is determined. Generally, five days are used, which is called five-day biochemical oxygen demand, recorded as BOD5. The larger the value, the more organic matter is contained in the water, and therefore the more serious the pollution is.

BOD, biochemical oxygen demand (BOD) is an environmental monitoring indicator, mainly used to monitor the pollution status of organic matter in water bodies. Generally, organic matter can be decomposed by microorganisms, but microorganisms need to consume oxygen when decomposing organic compounds in water. If the dissolved oxygen in the water is not enough to supply the needs of microorganisms, the water body is in a polluted state. BOD is an indicator related to environmental protection.

The calculation method of biochemical oxygen demand is as follows:

BOD (mg/L) = (D1-D2) / P

D1: Initial dissolved oxygen of diluted water sample (mg/L)

D2: Dissolved oxygen of diluted water sample after 5 days of incubation at 20 constant temperature incubator (mg/L)

P = [water sample volume (mL)] / [final volume of diluted water sample (mL)]

The ratio of biochemical oxygen demand to chemical oxygen demand can indicate how much organic pollutants in water are difficult for microorganisms to decompose. Organic pollutants that are difficult for microorganisms to decompose cause greater harm to the environment.

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