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Precautions when testing water quality BOD

by:BOQU     2023-03-08
Problems that need attention in water sample collection and processing 1. This method can still be used for water samples with BOD5 greater than 6000mg/L, but due to dilution will cause errors, it is necessary to carefully explain the measurement results. 2. The results of this test may be interfered by certain substances present in the water. Those substances that are toxic to microorganisms, such as fungicides, toxic metals, or free chlorine, inhibit biochemical action. Algae or nitrifying microorganisms in the water can also cause falsely high results. 3. The copper content in the water is not higher than 0.01mg/L, and does not contain chlorine, chloramine, caustic, organic matter and acid. 4. When the BOD5 value is low, it is best to use a glass container for sampling. Oxygenation should not be used when sampling. The sampling tube should be completely submerged in water to avoid inhalation of gas and allow the water sample to slowly flow into the sampling bottle. And seal with water. 5. If there are free acids and alkalis in the water samples, they should be neutralized and then diluted for culture. Thymol blue can be used as an indicator, which can be neutralized with hydrochloric acid solution (1mol/L) or sodium carbonate solution. Matters needing attention during trial operation 1. Glassware should be cleaned. First soak and clean with detergent, then soak with dilute hydrochloric acid, and finally wash with tap water and third-grade reagent water in turn. 2. It is recommended to use a compressed air bottle or compressor (diaphragm pump compressor) to inhale the air without contacting any lubricating oil with the aeration equipment. The air should be filtered and cleaned before use. 3. If compressed air cylinders are used, heat, impact, sunlight and baking should be avoided during transportation and use of the cylinders. Do not roll on the ground during transportation. Compressed oxygen cylinder valves must not come into contact with oil. 4. For samples with two or three dilution ratios, the dissolved oxygen consumption is greater than 2mg/L, and the remaining dissolved oxygen is greater than 1mg/L. The average value should be taken when calculating the results. If the remaining dissolved oxygen is less than 1mg/L or even zero, the dilution factor should be increased. If the dissolved oxygen consumption is less than 2mg/L, there are two possibilities. One is that the dilution factor is too large; at this time, it may appear at several dilution factors, and the greater the dilution factor, the more dissolved oxygen consumption. 5. When the diluted water sample exceeds 100 times, it should be diluted step by step to reach the predetermined dilution factor. 6. The dilution ratio is generally within the range of 40%-70% of the dissolved oxygen consumption. The more classifications in this range, the more accurate. 7. The temperature of the dilution water should be around 20°C. It should be preheated if it is lower than 20°C in winter, and it should be cooled if it is higher than 20°C in summer. 8. Regarding inoculation, for some organic industrial wastewater that is not easily decomposed by general microorganisms, microbial domestication is required. This domesticated microbial population is best obtained from the water body receiving the wastewater. To this end, water samples can be taken 3-8km below the outlet, and then inoculated into the diluted water by cultivation; it can also be artificially domesticated, that is, a certain amount of domestic sewage is used, and a certain amount of wastewater to be tested is added every day for continuous aeration. nourish. Until the microbial population suitable for decomposing organic matter in wastewater is cultivated. Use the same method to inoculate the cultured bacteria into the dilution water. In general, domestic sewage can be placed at 20°C for 24-36 hours, the supernatant is taken as the inoculum, and 1-3mL of the above inoculum can be added to 1L of water. If inoculation is not possible, try not to inoculate. The method is simple and avoids the oxygen consumption caused by inoculation. 9. Use the CODcr value to determine the dilution ratio, and multiply the CODcr value by 0.075, 0.15, and 0.225 to obtain three dilution ratios. If you don't know the nature of the wastewater, you need to choose more than three dilution ratios, namely COD/3, COD/4, and COD/5, which have certain reference value for most wastewater. 10. When cultivating in the incubator, pay attention to adding sealed water, and the temperature is strictly controlled at (20±1) °C. After 5 days of culture, if air bubbles are generated in the culture flask, an accurate BOD value cannot be obtained. The reason for the bubbles is that the temperature of the diluted water is lower than 20°C, or a large amount of low-temperature river water is taken as a water sample. In addition, water samples containing algae that were grown without complete protection from light also produced air bubbles.
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