What is BOD? Meaning, Measurement Methods, and Treatment in Wastewater Standards [Basic Knowledge]
BOD, which represents the amount of oxygen used by microorganisms to decompose organic matter, indicates water pollution. This explains the mechanism of measuring it over five days and how it is treated in wastewater standards.
BOD (Biochemical Oxygen Demand) is one of the indicators that represent the pollution of water by organic substances. It is derived from the initials of the English term "Biochemical Oxygen Demand," and is expressed in units of mg/L. ■ What amount of oxygen? It refers to the amount of oxygen consumed by aerobic microorganisms when they decompose organic matter in water, in the presence of sufficient dissolved oxygen (DO). It is characterized by representing pollution not by measuring the weight of the organic matter itself, but by the amount of oxygen used in the decomposition. ■ Rough indicator of organic matter BOD serves as a rough indicator of organic pollution. The larger the value, the more organic matter is being decomposed by microorganisms, indicating that the water is polluted. However, it cannot measure organic substances that are difficult for microorganisms to decompose or pollution caused by toxic substances. ■ What happens if BOD is high? Water with high BOD is prone to a deficiency of DO, and when BOD exceeds 10 mg/L, it is likely to lead to the occurrence of unpleasant odors. This is because the oxygen in the water is consumed when microorganisms decompose the organic matter that has flowed into the water. Please feel free to consult us first.
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■5 days at 20°C! BOD is determined by culturing the sample in a dark place at 20°C for 5 days and measuring the amount of oxygen consumed during that time. The analysis flow based on JIS K 0102-1 is as follows: 1. Dilute the sample in several stages with dilution water or seed dilution water. 2. Measure the dissolved oxygen (D1) before culturing. 3. Measure the dissolved oxygen (D2) after culturing for 5 days at 20°C. 4. Calculate from the results of the dilution stage that meets the conditions. Seed dilution water is dilution water to which a seed solution containing microorganisms has been added. ■Why dilute for measurement? Since the dissolved oxygen in the bottle is limited, culturing a sample with high pollution concentration as is may lead to a shortage of oxygen during the process. Therefore, the sample is diluted in several stages, and values that fall within a consumption range of 3.5 to 6.2 mg/L and a consumption rate of 40 to 70% are adopted. ■Why are values prone to fluctuation? BOD is a parameter that depends on microbial activity, making it difficult to achieve complete precision control. Additionally, if ammonia or nitrite is present, it can be oxidized by nitrifying bacteria, which may result in higher values.
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■ How is it handled under drainage standards? According to the uniform drainage standards of the Water Pollution Control Law, the BOD is 160 mg/L (with a daily average of 120 mg/L). The BOD standards apply to wastewater discharged into public water areas other than marine and lake areas, while the COD standards apply to wastewater discharged into marine and lake areas. This applies to factories and businesses with an average daily discharge of 50 cubic meters or more. ■ Some regions have stricter standards than the national ones For water areas where it is difficult to maintain environmental standards with just the uniform standards, prefectures can establish stricter additional standards through ordinances. Since fiscal year 1975, all prefectures have set additional standards. It is important to check the standards for the discharge destination of your wastewater in advance. ■ Does it take 5 days to get results? Culturing takes 5 days, during which the temperature must be maintained at 20±1°C. Therefore, the BOD results will be available more than 5 days after sampling. ■ Get an approximate value first Before measurement, it is important to obtain an approximate value of BOD using COD, TOC, or pack tests, and to determine the dilution conditions. Please feel free to consult with us first.
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