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About the Method Principle of Determination of BOD by Dilution Inoculation Method

by:BOQU     2023-03-09
Biochemical oxygen demand refers to the amount of dissolved oxygen consumed by microorganisms in the biochemical process of decomposing certain oxidizable substances in water, especially organic matter, under specified conditions. The whole process of biooxidation takes a long time. For example, when cultured at 20°C, it takes more than 100 days to complete this process. At present, it is generally stipulated at home and abroad to culture at 20 ℃ ± 1 ℃ for 5 days, and measure the dissolved oxygen of the samples before and after the culture respectively. The difference between the two is the BOD5 value, expressed in mg/L of oxygen. For some surface water and most industrial wastewater, because they contain more organic matter, they need to be diluted and then cultivated for measurement to reduce their concentration and ensure sufficient dissolved oxygen. The degree of dilution should be such that the dissolved oxygen consumed in the culture is greater than 2 mg/L, while the remaining dissolved oxygen is above 1 mg/L. In order to ensure sufficient dissolved oxygen after water sample dilution, the dilution water is usually aerated with air (or oxygen) to make the dissolved oxygen in the dilution water close to saturation. A certain amount of inorganic nutrient salts and buffer substances (calcium phosphate, magnesium and iron salts, etc.) should also be added to the dilution water to ensure the growth of microorganisms. For industrial wastewater containing no or little microorganisms, including acidic wastewater, alkaline wastewater, high-temperature wastewater or chlorinated wastewater, inoculation should be carried out when measuring BOD to introduce microorganisms that can decompose organic matter in wastewater. When there are organic substances in the wastewater that are difficult to be degraded by microorganisms in general domestic sewage at a normal speed or contain highly toxic substances, the domesticated microorganisms should be introduced into the water sample for inoculation.
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