13.What are the precautions for measuring CODCr?
CODCr measurement uses potassium dichromate as the oxidant, silver sulfate as the catalyst under acidic conditions, boiling and refluxing for 2 hours, and then converts it into oxygen consumption (GB11914–89) by measuring the consumption of potassium dichromate. Chemicals such as potassium dichromate, mercury sulfate and concentrated sulfuric acid are used in CODCr measurement, which may be highly toxic or corrosive, and require heating and reflux, so the operation must be carried out in a fume hood and must be done very carefully. Waste liquid Must be recycled and disposed of separately.
In order to promote the full oxidation of reducing substances in water, silver sulfate needs to be added as a catalyst. In order to make the silver sulfate evenly distributed, the silver sulfate should be dissolved in concentrated sulfuric acid. After it is completely dissolved (about 2 days), acidification will begin. of sulfuric acid into the Erlenmeyer flask. The national standard testing method stipulates that 0.4gAg2SO4/30mLH2SO4 should be added for each measurement of CODCr (20mL water sample), but relevant data shows that for general water samples, adding 0.3gAg2SO4/30mLH2SO4 is completely sufficient, and there is no need to use more Silver sulfate. For frequently measured sewage water samples, if there is sufficient data control, the amount of silver sulfate can be appropriately reduced.
CODCr is an indicator of the organic matter content in sewage, so the oxygen consumption of chloride ions and inorganic reducing substances must be removed during measurement. For interference from inorganic reducing substances such as Fe2+ and S2-, the measured CODCr value can be corrected based on the theoretical oxygen demand based on its measured concentration. The interference of chloride ions Cl-1 is generally removed by mercury sulfate. When the addition amount is 0.4gHgSO4 per 20mL water sample, the interference of 2000mg/L chloride ions can be removed. For frequently measured sewage water samples with relatively fixed components, if the chloride ion content is small or a water sample with a higher dilution factor is used for measurement, the amount of mercury sulfate can be appropriately reduced.
14. What is the catalytic mechanism of silver sulfate?
The catalytic mechanism of silver sulfate is that compounds containing hydroxyl groups in organic matter are first oxidized by potassium dichromate into carboxylic acid in a strong acidic medium. The fatty acids generated from the hydroxyl organic matter react with silver sulfate to generate fatty acid silver. Due to the action of silver atoms, The carboxyl group can easily generate carbon dioxide and water, and at the same time generate new fatty acid silver, but its carbon atom is one less than the former. This cycle repeats, gradually oxidizing all organic matter into carbon dioxide and water.
15.What are the precautions for BOD5 measurement?
The BOD5 measurement usually uses the standard dilution and inoculation method (GB 7488–87). The operation is to place the water sample that has been neutralized, removed toxic substances, and diluted (with an appropriate amount of inoculum containing aerobic microorganisms added if necessary). In the culture bottle, incubate in the dark at 20°C for 5 days. By measuring the dissolved oxygen content in the water samples before and after culture, the oxygen consumption within 5 days can be calculated, and then the BOD5 can be obtained based on the dilution factor.
The determination of BOD5 is the joint result of biological and chemical effects and must be carried out in strict accordance with the operating specifications. Changing any condition will affect the accuracy and comparability of the measurement results. Conditions that affect BOD5 determination include pH value, temperature, microbial type and quantity, inorganic salt content, dissolved oxygen and dilution factor, etc.
Water samples for BOD5 testing must be filled and sealed in sampling bottles, and stored in a refrigerator at 2 to 5°C until analysis. Generally, the test should be carried out within 6 hours after sampling. In any case, the storage time of water samples should not exceed 24 hours.
When measuring the BOD5 of industrial wastewater, since industrial wastewater usually contains less dissolved oxygen and contains mostly biodegradable organic matter, in order to maintain the aerobic state in the culture bottle, the water sample must be diluted (or inoculated and diluted). This operation This is the biggest feature of the standard dilution method. In order to ensure the reliability of the measured results, the oxygen consumption of the diluted water sample after culture for 5 days must be greater than 2 mg/L, and the residual dissolved oxygen must be greater than 1 mg/L.
The purpose of adding the inoculum solution is to ensure that a certain amount of microorganisms degrade the organic matter in the water. The amount of the inoculum solution is preferably such that the oxygen consumption within 5 days is less than 0.1mg/L. When using distilled water prepared by a metal distiller as dilution water, care should be taken to check the metal ion content in it to avoid inhibiting microbial reproduction and metabolism. In order to ensure that the dissolved oxygen in the diluted water is close to saturation, purified air or pure oxygen can be introduced if necessary, and then placed in a 20oC incubator for a certain period of time to balance it with the oxygen partial pressure in the air.
The dilution factor is determined based on the principle that the oxygen consumption is greater than 2 mg/L and the remaining dissolved oxygen is greater than 1 mg/L after 5 days of culture. If the dilution factor is too large or too small, the test will fail. And because the BOD5 analysis cycle is long, once a similar situation occurs, it cannot be retested as it is. When initially measuring the BOD5 of a certain industrial wastewater, you can first measure its CODCr, and then refer to the existing monitoring data of wastewater with similar water quality to initially determine the BOD5/CODCr value of the water sample to be measured, and calculate the approximate range of BOD5 based on this. and determine the dilution factor.
For water samples containing substances that inhibit or kill the metabolic activities of aerobic microorganisms, the results of directly measuring BOD5 using common methods will deviate from the actual value. Corresponding pretreatment must be done before the measurement. These substances and factors have an impact on BOD5 determination. Including heavy metals and other toxic inorganic or organic substances, residual chlorine and other oxidizing substances, pH value that is too high or too low, etc.
16. Why is it necessary to inoculate when measuring the BOD5 of industrial wastewater? How to get vaccinated?
The determination of BOD5 is a biochemical oxygen consumption process. Microorganisms in water samples use organic matter in the water as nutrients to grow and reproduce. At the same time, they decompose organic matter and consume dissolved oxygen in the water. Therefore, the water sample must contain a certain amount of microorganisms that can degrade the organic matter in it. capabilities of microorganisms.
Industrial wastewater generally contains varying amounts of toxic substances, which can inhibit the activity of microorganisms. Therefore, the number of microorganisms in industrial wastewater is very small or even non-existent. If ordinary methods of measuring microbial-rich urban sewage are used, the true organic content in the wastewater may not be detected, or at least be low. For example, for water samples that have been treated with high temperature and sterilization and whose pH is too high or too low, in addition to taking pre-treatment measures such as cooling, reducing bactericides, or adjusting the pH value, in order to ensure the accuracy of BOD5 measurement, effective measures must also be taken. Vaccination.
When measuring the BOD5 of industrial wastewater, if the content of toxic substances is too large, chemicals are sometimes used to remove it; if the wastewater is acidic or alkaline, it must be neutralized first; and usually the water sample must be diluted before the standard can be used. Determination by dilution method. Adding an appropriate amount of inoculum solution containing domesticated aerobic microorganisms to the water sample (such as the aeration tank mixture used to treat this kind of industrial wastewater) is to make the water sample contain a certain number of microorganisms that have the ability to degrade organic matter. Under the condition that other conditions for measuring BOD5 are met, these microorganisms are used to decompose organic matter in industrial wastewater, and the oxygen consumption of the water sample is measured for 5 days of cultivation, and the BOD5 value of industrial wastewater can be obtained.
The mixed liquid of the aeration tank or the effluent of the secondary sedimentation tank of the sewage treatment plant is an ideal source of microorganisms for determining the BOD5 of the wastewater entering the sewage treatment plant. Direct inoculation with domestic sewage, because there is little or no dissolved oxygen, is prone to the emergence of anaerobic microorganisms, and requires a long period of cultivation and acclimation. Therefore, this acclimated inoculum solution is only suitable for certain industrial wastewaters with specific needs.
17. What are the precautions for preparing dilution water when measuring BOD5?
The quality of the dilution water is of great significance to the accuracy of the BOD5 measurement results. Therefore, it is required that the oxygen consumption of the dilution water blank for 5 days must be less than 0.2mg/L, and it is best to control it below 0.1mg/L. The oxygen consumption of the inoculated dilution water for 5 days should be Between 0.3~1.0mg/L.
The key to ensuring the quality of dilution water is to control the lowest content of organic matter and the lowest content of substances that inhibit microbial reproduction. Therefore, it is best to use distilled water as dilution water. It is not advisable to use pure water made from ion exchange resin as dilution water, because deionized water often Contains organic matter separated from the resin. If the tap water used to prepare distilled water contains certain volatile organic compounds, in order to prevent them from remaining in the distilled water, pretreatment to remove the organic compounds should be carried out before distillation. In distilled water produced from metal distillers, attention should be paid to checking the metal ion content in it to avoid inhibiting the reproduction and metabolism of microorganisms and affecting the accuracy of BOD5 measurement results.
If the dilution water used does not meet the usage requirements because it contains organic matter, the effect can be eliminated by adding an appropriate amount of aeration tank inoculum and storing it at room temperature or 20oC for a certain period of time. The amount of inoculation is based on the principle that the oxygen consumption in 5 days is about 0.1mg/L. To prevent algae reproduction, storage must be carried out in a dark room. If there is sediment in the diluted water after storage, only the supernatant can be used and the sediment can be removed by filtration.
In order to ensure that the dissolved oxygen in the dilution water is close to saturation, if necessary, a vacuum pump or water ejector can be used to inhale purified air, a micro air compressor can also be used to inject purified air, and an oxygen bottle can be used to introduce pure oxygen, and then the oxygenated water The diluted water is placed in a 20oC incubator for a certain period of time to allow the dissolved oxygen to reach equilibrium. Dilution water placed at a lower room temperature in winter may contain too much dissolved oxygen, and the opposite is true in high-temperature seasons in summer. Therefore, when there is a significant difference between room temperature and 20oC, it must be placed in the incubator for a period of time to stabilize it and the culture environment. oxygen partial pressure balance.
18. How to determine the dilution factor when measuring BOD5?
If the dilution factor is too large or too small, the oxygen consumption in 5 days may be too little or too much, exceeding the normal oxygen consumption range and causing the experiment to fail. Since the BOD5 measurement cycle is very long, once such a situation occurs, it cannot be retested as it is. Therefore, great attention must be paid to the determination of the dilution factor.
Although the composition of industrial wastewater is complex, the ratio of its BOD5 value to CODCr value is usually between 0.2 and 0.8. The ratio of wastewater from papermaking, printing and dyeing, and chemical industries is lower, while the ratio of wastewater from the food industry is higher. When measuring the BOD5 of some wastewater containing granular organic matter, such as distiller’s grain wastewater, the ratio will be significantly lower because the particulate matter is precipitated at the bottom of the culture bottle and cannot participate in the biochemical reaction.
The determination of the dilution factor is based on the two conditions that when measuring BOD5, the oxygen consumption in 5 days should be greater than 2mg/L and the remaining dissolved oxygen should be greater than 1mg/L. The DO in the culture bottle on the day after dilution is 7 to 8.5 mg/L. Assuming that the oxygen consumption in 5 days is 4 mg/L, the dilution factor is the product of the CODCr value and three coefficients of 0.05, 0.1125, and 0.175 respectively. For example, when using a 250mL culture bottle to measure the BOD5 of a water sample with a CODCr of 200mg/L, the three dilution factors are: ①200×0.005=10 times, ②200×0.1125=22.5 times, and ③200×0.175=35 times. If the direct dilution method is used, the volumes of water samples taken are: ①250÷10=25mL, ②250÷22.5≈11mL, ③250÷35≈7mL.
If you take samples and culture them like this, there will be 1 to 2 measured dissolved oxygen results that comply with the above two principles. If there are two dilution ratios that comply with the above principles, their average value should be taken when calculating the results. If the remaining dissolved oxygen is less than 1 mg/L or even zero, the dilution ratio should be increased. If the dissolved oxygen consumption during culture is less than 2mg/L, one possibility is that the dilution factor is too large; the other possibility is that the microbial strains are not suitable, have poor activity, or the concentration of toxic substances is too high. At this time, there may also be problems with large dilution factors. The culture bottle consumes more dissolved oxygen.
If the dilution water is inoculation dilution water, since the oxygen consumption of the blank water sample is 0.3~1.0mg/L, the dilution coefficients are 0.05, 0.125 and 0.2 respectively.
If the specific CODCr value or approximate range of the water sample is known, it can be easier to analyze its BOD5 value according to the above dilution factor. When the CODCr range of the water sample is not known, in order to shorten the analysis time, it can be estimated during the CODCr measurement process. The specific method is: first prepare a standard solution containing 0.4251g potassium hydrogen phthalate per liter (the CODCr value of this solution is 500mg/L), and then dilute it in proportion to the CODCr values of 400mg/L, 300mg/L, and 200mg. /L, 100mg/L dilute solution. Pipette 20.0 mL of standard solution with a CODCr value of 100 mg/L to 500 mg/L, add reagents according to the usual method, and measure the CODCr value. After heating, boiling and refluxing for 30 minutes, cool naturally to room temperature and then cover and store to prepare a standard colorimetric series. In the process of measuring the CODCr value of the water sample according to the usual method, when the boiling reflux continues for 30 minutes, compare it with the preheated standard CODCr value color sequence to estimate the CODCr value of the water sample, and determine the dilution factor when testing BOD5 based on this. . For printing and dyeing, papermaking, chemical and other industrial wastewater containing difficult-to-digest organic matter, if necessary, perform colorimetric evaluation after boiling and refluxing for 60 minutes.
Post time: Sep-21-2023
