Attention should be paid to the dilution method to detect BOD in water

Problems to be paid attention to when collecting and processing water samples 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 require careful explanation of 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, etc., will inhibit the biochemical action. Algae or nitrifying microorganisms in the water may also cause falsely high results. 3. The copper content in the water should not be higher than 0.01mg/L, and there should be no chlorine, chloramine, caustic, organic matter and acids. 4. When the BOD5 value is low, it is best to use a glass container for sampling. Do not sample with oxygen when sampling. The sampling tube should be completely submerged in water to avoid inhaling gas, so that the water sample will slowly flow into the sampling bottle. The container must be filled and overflow about 1/2 of the bottle volume, leaving no space , and seal with water. 5. If there are free acids and bases in the water samples, they should be neutralized and then diluted for culture. Thymol blue can be used as an indicator and neutralized with hydrochloric acid solution (1mol/L) or sodium carbonate solution. Problems to be paid attention to during detection operation 1. Glassware should be washed. 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 a compressor (diaphragm pump compressor) whose inhaled air does not come into contact with any lubricating oil for the aeration equipment. Air should be filtered and washed before use. 3. If compressed air cylinders are used, the gas cylinders should avoid heat, impact, sunlight and baking during transportation and use. Do not roll on the ground during transportation. Compressed oxygen cylinder valves must not come into contact with oil. 4. In samples with two or three dilution ratios, if the consumed dissolved oxygen 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 ratio 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; the other is that the microbial strains are not suitable, the activity is poor, or the concentration of toxic substances is too high. At this time, it may appear that in several dilution ratios, the dissolved oxygen consumption with the larger dilution ratio is more. 5. When the diluted water sample exceeds 100 times, it should be diluted in multiple steps to reach the predetermined dilution factor. 6. The dilution ratio is generally in the range of 40%-70% of the dissolved oxygen consumption, and the more grades in this range, the more accurate. 7. The temperature of the dilution water is about 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 the inoculation problem, for some organic industrial wastewater that is not easy to be decomposed by general microorganisms, microbial domestication is required. The acclimatized microbial population is preferably obtained from the water body receiving the wastewater. To this end, water samples can be obtained at 3-8km below the outfall, and inoculated into diluted water after 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, and continuous aeration culture is carried out. Until the microbial population suitable for decomposing organic matter in wastewater is cultivated. The cultured bacteria solution was inoculated into the dilution water by the same method. Under normal circumstances, domestic sewage can be placed at 20°C for 24-36 hours, then the supernatant can be used as the inoculation solution, and 1-3mL of the above inoculation solution can be added to 1L of water. If you can avoid inoculation, 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 factor, and multiply the CODcr value by 0.075, 0.15, and 0.225 to obtain three dilution factors. If you don't know the nature of 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 an incubator, attention should be paid to adding sealing water, and the temperature should be strictly controlled at (20±1)°C. After culturing for 5 days, if air bubbles are produced in the culture flask, the accurate BOD value cannot be obtained. The reason for the bubbles is that the temperature of the dilution water is lower than 20°C, or that a large amount of low-temperature river water is taken as a water sample. In addition, water samples containing algae, cultured without complete protection from light, can also produce air bubbles.
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