COD, which represents the pollution of water using oxidizing agents. This will explain the differences between Japan's official methods and the standard methods in Europe and the United States, the differences from BOD, and how it is treated in total quantity regulations.
COD (Chemical Oxygen Demand) is an abbreviation that represents one of the indicators of water pollution. It is the amount of oxidizing agent consumed when oxidizing organic matter in water, expressed in terms of the equivalent amount of oxygen. The unit is represented in mg/L. ■ Are we only measuring organic matter? While COD is used as a rough estimate of organic matter, the presence of divalent iron or nitrite can also be included in the measurement. It is important to note that it does not directly measure the amount of organic matter itself. ■ Can’t compare the same COD? There are methods for COD that use potassium permanganate as the oxidizing agent and methods that use potassium dichromate (dichromate). The oxidation rates differ based on the type of oxidizing agent, temperature, reaction time, etc. Therefore, the values of CODMn (using potassium permanganate) and CODCr (using potassium dichromate) cannot be directly compared. ■ Verify the measurement method When comparing with data or literature from overseas, it is necessary to confirm which method was used for the measurements. Please feel free to consult us first.
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■Japan's official method for CODMn According to the Environmental Agency notification that establishes the testing method for wastewater standards, COD is measured using the potassium permanganate method from the JIS factory wastewater testing method (JIS K 0102). The general flow of the measurement is as follows: 1. Add sulfuric acid and silver nitrate solution to the sample. 2. Add potassium permanganate solution and heat in a boiling water bath for 30 minutes. 3. Add sodium oxalate solution. 4. Titrate with potassium permanganate solution. ■Why is it important to adhere to temperature and time? It is important to maintain the water bath at 95-100°C (lower temperatures result in insufficient reactions) and strictly adhere to the 30-minute heating time. This is because COD is the value obtained when oxidation occurs under specified conditions. ■Why add silver nitrate? Due to the significant influence of chloride ions, in samples with high chloride ion content, a greater amount of silver salt than the equivalent is added. ■Standard CODCr in Europe and the United States In Europe and the United States, the standard method for COD is using potassium dichromate, which has a stronger oxidizing power. In one of the American methods, heating is done at 150°C for 2 hours in the presence of dichromate.
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■What is the difference from BOD? 1. BOD: The amount of oxygen consumed by aerobic microorganisms to decompose organic matter over five days. 2. COD: The amount of oxidizing agent consumed during oxidation, converted to oxygen equivalent. BOD takes five days to yield results, while CODMn heating takes 30 minutes. According to national environmental standards, rivers are regulated by BOD, while lakes and marine areas are regulated by COD. ■Total quantity regulation focuses on COD! In Tokyo Bay, Ise Bay, and Seto Inland Sea, total water quality reduction targets COD, nitrogen, and phosphorus. Specific facilities within designated areas that discharge an average of 50 cubic meters or more per day are subject to total quantity regulation standards based on the pollution load calculated by multiplying concentration by discharge volume. ■Is automatic measurement standard for over 400 cubic meters? According to the Environmental Agency notification, facilities within designated areas that discharge an average of 400 cubic meters or more per day are primarily required to calculate COD using a conversion formula derived from the relationship with standard methods based on measurements from automatic measuring devices. Measuring devices include COD automatic measuring instruments, TOC automatic measuring instruments, and UV absorbance automatic measuring instruments. The conversion formula will be reviewed if the characteristics of the wastewater change. Please feel free to consult us first.
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