Key points for water quality testing operations in sewage treatment part one
1. What are the main physical characteristics indicators of wastewater?
⑴Temperature: The temperature of wastewater has a great influence on the wastewater treatment process. The temperature directly affects the activity of microorganisms. Generally, the water temperature in urban sewage treatment plants is between 10 and 25 degrees Celsius. The temperature of industrial wastewater is related to the production process of discharging wastewater.
⑵ Color: The color of wastewater depends on the content of dissolved substances, suspended solids or colloidal substances in the water. Fresh urban sewage is generally dark gray. If it is in an anaerobic state, the color will become darker and dark brown. The colors of industrial wastewater vary. Papermaking wastewater is generally black, distiller's grain wastewater is yellow-brown, and electroplating wastewater is blue-green.
⑶ Odor: The odor of wastewater is caused by pollutants in domestic sewage or industrial wastewater. The approximate composition of wastewater can be directly determined by smelling the odor. Fresh urban sewage has a musty smell. If the smell of rotten eggs appears, it often indicates that the sewage has been anaerobically fermented to produce hydrogen sulfide gas. Operators should strictly abide by anti-virus regulations when operating.
⑷ Turbidity: Turbidity is an indicator that describes the number of suspended particles in wastewater. It can generally be detected by a Turbidity Meter, but turbidity cannot directly replace the concentration of suspended solids because color interferes with the detection of turbidity.
⑸ Conductivity: The conductivity in wastewater generally indicates the number of inorganic ions in the water, which is closely related to the concentration of dissolved inorganic substances in the incoming water. If the conductivity rises sharply, it is often a sign of abnormal industrial wastewater discharge.
⑹Solid matter: The form (SS, DS, etc.) and concentration of solid matter in wastewater reflect the nature of wastewater and are also very useful for controlling the treatment process.
⑺ Precipitability: Impurities in wastewater can be divided into four types: dissolved, colloidal, free and precipitable. The first three are non-precipitable. Precipitable impurities generally represent substances that precipitate within 30 minutes or 1 hour.
2. What are the chemical characteristics indicators of wastewater?
There are many chemical indicators of wastewater, which can be divided into four categories: ① General Water Quality indicators, such as pH value, hardness, alkalinity, residual chlorine, various anions and cations, etc.; ② Organic matter content indicators, biochemical oxygen demand BOD5, Chemical oxygen demand CODCr, total oxygen demand TOD and total organic carbon TOC, etc.; ③ Plant nutrient content indicators, such as ammonia nitrogen, nitrate nitrogen, nitrite nitrogen, phosphate, etc.; ④ Toxic substance indicators, such as petroleum , heavy metals, cyanides, sulfides, polycyclic aromatic hydrocarbons, various chlorinated organic compounds and various pesticides, etc.
In different sewage treatment plants, analysis projects suitable for the respective Water Quality characteristics should be determined based on the different types and quantities of pollutants in the incoming water.
3. What are the main chemical indicators that need to be analyzed in general sewage treatment plants?
The main chemical indicators that need to be analyzed in general sewage treatment plants are as follows:
⑴ pH value: pH value can be determined by measuring the hydrogen ion concentration in water. The pH value has a great influence on the biological treatment of wastewater, and the nitrification reaction is more sensitive to the pH value. The pH value of urban sewage is generally between 6 and 8. If it exceeds this range, it often indicates that a large amount of industrial wastewater is discharged. For industrial wastewater containing acidic or alkaline substances, neutralization treatment is required before entering the biological treatment system.
⑵Alkalinity: Alkalinity can reflect the acid buffering ability of wastewater during the treatment process. If the wastewater has a relatively high alkalinity, it can buffer the changes in pH value and make the pH value relatively stable. Alkalinity represents the content of substances in a water sample that combine with hydrogen ions in strong acids. The size of the alkalinity can be measured by the amount of strong acid consumed by the water sample during the titration process.
⑶CODCr: CODCr is the amount of organic matter in wastewater that can be oxidized by the strong oxidant potassium dichromate, measured in mg/L of oxygen.
⑷BOD5: BOD5 is the amount of oxygen required for the biodegradation of organic matter in wastewater, and is an indicator of the biodegradability of wastewater.
⑸Nitrogen: In sewage treatment plants, the changes and content distribution of nitrogen provide parameters for the process. The content of organic nitrogen and ammonia nitrogen in the incoming water of sewage treatment plants is generally high, while the content of nitrate nitrogen and nitrite nitrogen is generally low. The increase in ammonia nitrogen in the primary sedimentation tank generally indicates that the settled sludge has become anaerobic, while the increase in nitrate nitrogen and nitrite nitrogen in the secondary sedimentation tank indicates that nitrification has occurred. The nitrogen content in domestic sewage is generally 20 to 80 mg/L, of which organic nitrogen is 8 to 35 mg/L, ammonia nitrogen is 12 to 50 mg/L, and the contents of nitrate nitrogen and nitrite nitrogen are very low. The contents of organic nitrogen, ammonia nitrogen, nitrate nitrogen and nitrite nitrogen in industrial wastewater vary from water to water. The nitrogen content in some industrial wastewater is extremely low. When biological treatment is used, nitrogen fertilizer needs to be added to supplement the nitrogen content required by microorganisms. , and when the nitrogen content in the effluent is too high, denitrification treatment is required to prevent eutrophication in the receiving water body.
⑹ Phosphorus: The phosphorus content in biological sewage is generally 2 to 20 mg/L, of which organic phosphorus is 1 to 5 mg/L and inorganic phosphorus is 1 to 15 mg/L. The phosphorus content in industrial wastewater varies greatly. Some industrial wastewater has extremely low phosphorus content. When biological treatment is used, phosphate fertilizer needs to be added to supplement the phosphorus content required by microorganisms. When the phosphorus content in the effluent is too high, , and phosphorus removal treatment is required to prevent eutrophication in the receiving water body.
⑺Petroleum: Most of the oil in wastewater is insoluble in water and floats on the water. The oil in the incoming water will affect the oxygenation effect and reduce the microbial activity in the activated sludge. The oil concentration of the mixed sewage entering the biological treatment structure should usually not be greater than 30 to 50 mg/L.
⑻Heavy metals: Heavy metals in wastewater mainly come from industrial wastewater and are very toxic. Sewage treatment plants usually do not have better treatment methods. They usually need to be treated on-site in the discharge workshop to meet national discharge standards before entering the drainage system. If the heavy metal content in the effluent from the sewage treatment plant increases, it often indicates that there is a problem with the pretreatment.
⑼ Sulfide: When the sulfide in water exceeds 0.5mg/L, it will have a disgusting smell of rotten eggs and is corrosive, sometimes even causing hydrogen sulfide poisoning.
⑽Residual chlorine: When using chlorine for disinfection, in order to ensure the reproduction of microorganisms during the transportation process, the residual chlorine in the effluent (including free residual chlorine and combined residual chlorine) is the control indicator of the disinfection process, which generally does not exceed 0.3mg/L.
4. What are the microbial characteristics indicators of wastewater?
The biological indicators of wastewater include the total number of bacteria, the number of coliform bacteria, various pathogenic microorganisms and viruses, etc. Wastewater from hospitals, joint meat processing enterprises, etc. must be disinfected before being discharged. The relevant national wastewater discharge standards have stipulated this. Sewage treatment plants generally do not detect and control biological indicators in the incoming water, but disinfection is required before the treated sewage is discharged to control the pollution of the receiving water bodies by the treated sewage. If the secondary biological treatment effluent is further treated and reused, it is even more necessary to disinfect it before reuse.
⑴ Total number of bacteria: The total number of bacteria can be used as an indicator to evaluate the cleanliness of water quality and assess the effect of water purification. An increase in the total number of bacteria indicates that the disinfection effect of the water is poor, but it cannot directly indicate how harmful it is to the human body. It must be combined with the number of fecal coliforms to determine how safe the water quality is for the human body.
⑵Number of coliforms: The number of coliforms in water can indirectly indicate the possibility that the water contains intestinal bacteria (such as typhoid, dysentery, cholera, etc.), and therefore serves as a hygienic indicator to ensure human health. When sewage is reused as miscellaneous water or landscape water, it may come into contact with the human body. At this time, the number of fecal coliforms must be detected.
⑶ Various pathogenic microorganisms and viruses: Many viral diseases can be transmitted through water. For example, viruses that cause hepatitis, polio and other diseases exist in the human intestines, enter the domestic sewage system through the patient's feces, and then be discharged into the sewage treatment plant. . The sewage treatment process has limited ability to remove these viruses. When the treated sewage is discharged, if the use value of the receiving water body has special requirements for these pathogenic microorganisms and viruses, disinfection and testing are required.
5. What are the common indicators that reflect the content of organic matter in water?
After organic matter enters the water body, it will be oxidized and decomposed under the action of microorganisms, gradually reducing the dissolved oxygen in the water. When oxidation proceeds too fast and the water body cannot absorb enough oxygen from the atmosphere in time to replenish the consumed oxygen, the dissolved oxygen in the water may drop very low (such as less than 3~4mg/L), which will affect aquatic organisms. required for normal growth. When the dissolved oxygen in the water is exhausted, organic matter begins anaerobic digestion, producing odor and affecting environmental hygiene.
Since the organic matter contained in sewage is often an extremely complex mixture of multiple components, it is difficult to determine the quantitative values of each component one by one. In fact, some comprehensive indicators are commonly used to indirectly represent the content of organic matter in water. There are two types of comprehensive indicators indicating the content of organic matter in water. One is an indicator expressed in oxygen demand (O2) equivalent to the amount of organic matter in water, such as biochemical oxygen demand (BOD), chemical oxygen demand (COD), and total oxygen demand (TOD). ; The other type is the indicator expressed in carbon (C), such as total organic carbon TOC. For the same kind of sewage, the values of these indicators are generally different. The order of numerical values is TOD>CODCr>BOD5>TOC
6. What is total organic carbon?
Total organic carbon TOC (abbreviation for Total Organic Carbon in English) is a comprehensive indicator that indirectly expresses the content of organic matter in water. The data it displays is the total carbon content of organic matter in sewage, and the unit is expressed in mg/L of carbon (C). . The principle of measuring TOC is to first acidify the water sample, use nitrogen to blow off the carbonate in the water sample to eliminate interference, then inject a certain amount of water sample into the oxygen flow with a known oxygen content, and send it into a platinum steel pipe. It is burned in a quartz combustion tube as a catalyst at a high temperature of 900oC to 950oC. A non-dispersive infrared gas analyzer is used to measure the amount of CO2 generated during the combustion process, and then the carbon content is calculated, which is the total organic carbon TOC (for details, see GB13193--91). The measurement time only takes a few minutes.
The TOC of general urban sewage can reach 200mg/L. The TOC of industrial wastewater has a wide range, with the highest reaching tens of thousands of mg/L. The TOC of sewage after secondary biological treatment is generally <50mg/L, and the TOC of cleaner river water is average. <10mg/L. In the study of sewage treatment, TOC is used as an indicator of sewage organic matter, but this indicator is generally not analyzed in conventional sewage treatment operations.
7. What is total oxygen demand?
Total oxygen demand TOD (abbreviation for Total Oxygen Demand in English) refers to the amount of oxygen required when reducing substances (mainly organic matter) in water are burned at high temperatures and become stable oxides. The result is measured in mg/L. The TOD value can reflect the oxygen consumed when almost all organic matter in the water (including carbon C, hydrogen H, oxygen O, nitrogen N, phosphorus P, sulfur S, etc.) is burned into CO2, H2O, NOx, SO2, etc. quantity. It can be seen that the TOD value is generally greater than the CODCr value. At present, TOD has not been included in water quality standards in my country, but is only used in theoretical research on sewage treatment.
The principle of measuring TOD is to inject a certain amount of water sample into the oxygen flow with known oxygen content, and send it into a quartz combustion tube with platinum steel as a catalyst, and burn it instantly at a high temperature of 900oC. The organic matter in the water sample That is, it is oxidized and consumes the oxygen in the oxygen flow. The original amount of oxygen in the oxygen flow minus the remaining oxygen is the total oxygen demand TOD. The amount of oxygen in the oxygen flow can be measured using electrodes, so the measurement of TOD only takes a few minutes.
8. What is biochemical oxygen demand?
The full name of biochemical oxygen demand is biochemical oxygen demand, which is Biochemical Oxygen Demand in English and abbreviated as BOD. It means that at a temperature of 20oC and under aerobic conditions, it is consumed in the biochemical oxidation process of aerobic microorganisms decomposing organic matter in water. The amount of dissolved oxygen is the amount of oxygen required to stabilize biodegradable organic matter in the water. The unit is mg/L. BOD not only includes the amount of oxygen consumed by the growth, reproduction or respiration of aerobic microorganisms in the water, but also includes the amount of oxygen consumed by reducing inorganic substances such as sulfide and ferrous iron, but the proportion of this part is usually very small. Therefore, the larger the BOD value, the greater the organic content in the water.
Under aerobic conditions, microorganisms decompose organic matter into two processes: the oxidation stage of carbon-containing organic matter and the nitrification stage of nitrogen-containing organic matter. Under natural conditions of 20oC, the time required for organic matter to oxidize to the nitrification stage, that is, to achieve complete decomposition and stability, is more than 100 days. However, in fact, the biochemical oxygen demand BOD20 of 20 days at 20oC approximately represents the complete biochemical oxygen demand. In production applications, 20 days is still considered too long, and the biochemical oxygen demand (BOD5) of 5 days at 20°C is generally used as an indicator to measure the organic content of sewage. Experience shows that the BOD5 of domestic sewage and various production sewage is about 70~80% of the complete biochemical oxygen demand BOD20.
BOD5 is an important parameter for determining the load of sewage treatment plants. The BOD5 value can be used to calculate the amount of oxygen required for the oxidation of organic matter in wastewater. The amount of oxygen required for the stabilization of carbon-containing organic matter can be called carbon BOD5. If further oxidized, nitrification reaction can occur. The amount of oxygen required by nitrifying bacteria to convert ammonia nitrogen into nitrate nitrogen and nitrite nitrogen can be called nitrification. BOD5. General secondary sewage treatment plants can only remove carbon BOD5, but not nitrification BOD5. Since the nitrification reaction inevitably occurs during the biological treatment process of removing carbon BOD5, the measured value of BOD5 is higher than the actual oxygen consumption of organic matter.
BOD measurement takes a long time, and the commonly used BOD5 measurement requires 5 days. Therefore, it can generally only be used for process effect evaluation and long-term process control. For a specific sewage treatment site, the correlation between BOD5 and CODCr can be established, and CODCr can be used to roughly estimate the BOD5 value to guide the adjustment of the treatment process.
9. What is chemical oxygen demand?
Chemical oxygen demand in English is Chemical Oxygen Demand. It refers to the amount of oxidant consumed by the interaction between organic matter in water and strong oxidants (such as potassium dichromate, potassium permanganate, etc.) under certain conditions, converted into oxygen. in mg/L.
When potassium dichromate is used as the oxidant, almost all (90%~95%) of the organic matter in the water can be oxidized. The amount of oxidant consumed at this time converted into oxygen is what is commonly called chemical oxygen demand, often abbreviated as CODCr (see GB 11914--89 for specific analysis methods). The CODCr value of sewage not only includes the oxygen consumption for the oxidation of almost all organic matter in the water, but also includes the oxygen consumption for the oxidation of reducing inorganic substances such as nitrite, ferrous salts, and sulfides in the water.
10. What is potassium permanganate index (oxygen consumption)?
The chemical oxygen demand measured using potassium permanganate as the oxidant is called the potassium permanganate index (see GB 11892--89 for specific analysis methods) or oxygen consumption, the English abbreviation is CODMn or OC, and the unit is mg/L .
Since the oxidizing ability of potassium permanganate is weaker than that of potassium dichromate, the specific value CODMn of the potassium permanganate index of the same water sample is generally lower than its CODCr value, that is, CODMn can only represent the organic matter or inorganic matter that is easily oxidized in the water. content. Therefore, my country, Europe and the United States and many other countries use CODCr as a comprehensive indicator to control organic matter pollution, and only use the potassium permanganate index CODMn as an indicator to evaluate and monitor the organic matter content of surface water bodies such as seawater, rivers, lakes, etc. or drinking water.
Since potassium permanganate has almost no oxidizing effect on organic matter such as benzene, cellulose, organic acids, and amino acids, while potassium dichromate can oxidize almost all of these organic matter, CODCr is used to indicate the degree of pollution of wastewater and to control sewage treatment. The parameters of the process are more appropriate. However, because the determination of the potassium permanganate index CODMn is simple and rapid, CODMn is still used to indicate the degree of pollution, that is, the amount of organic matter in relatively clean surface water, when evaluating the water quality.
11. How to determine the biodegradability of wastewater by analyzing the BOD5 and CODCr of wastewater?
When the water contains toxic organic matter, the BOD5 value in the wastewater generally cannot be accurately measured. The CODCr value can more accurately measure the content of organic matter in the water, but the CODCr value cannot distinguish between biodegradable and non-biodegradable substances. People are accustomed to measuring the BOD5/CODCr of sewage to judge its biodegradability. It is generally believed that if the BOD5/CODCr of sewage is greater than 0.3, it can be treated by biodegradation. If the BOD5/CODCr of sewage is lower than 0.2, it can only be considered. Use other methods to deal with it.
12.What is the relationship between BOD5 and CODCr?
Biochemical oxygen demand (BOD5) represents the amount of oxygen required during the biochemical decomposition of organic pollutants in sewage. It can directly explain the problem in a biochemical sense. Therefore, BOD5 is not only an important water quality indicator, but also an indicator of sewage biology. An extremely important control parameter during processing. However, BOD5 is also subject to certain limitations in use. First, the measurement time is long (5 days), which cannot reflect and guide the operation of sewage treatment equipment in a timely manner. Second, some production sewage does not have the conditions for microbial growth and reproduction (such as the presence of toxic organic matter). ), its BOD5 value cannot be determined.
Chemical oxygen demand CODCr reflects the content of almost all organic matter and reducing inorganic matter in sewage, but it cannot directly explain the problem in a biochemical sense like biochemical oxygen demand BOD5. In other words, testing the chemical oxygen demand CODCr value of sewage can more accurately determine the organic content in the water, but the chemical oxygen demand CODCr cannot distinguish between biodegradable organic matter and non-biodegradable organic matter.
The chemical oxygen demand CODCr value is generally higher than the biochemical oxygen demand BOD5 value, and the difference between them can roughly reflect the content of organic matter in the sewage that cannot be degraded by microorganisms. For sewage with relatively fixed pollutant components, CODCr and BOD5 generally have a certain proportional relationship and can be calculated from each other. In addition, the measurement of CODCr takes less time. According to the national standard method of reflux for 2 hours, it only takes 3 to 4 hours from sampling to the result, while measuring the BOD5 value takes 5 days. Therefore, in actual sewage treatment operation and management, CODCr is often used as a control indicator.
In order to guide production operations as quickly as possible, some sewage treatment plants have also formulated corporate standards for measuring CODCr in reflux for 5 minutes. Although the measured results have a certain error with the national standard method, because the error is a systematic error, the continuous monitoring results can correctly reflect the water quality. The actual changing trend of the sewage treatment system can be reduced to less than 1 hour, which provides a time guarantee for timely adjustment of sewage treatment operating parameters and preventing sudden changes in water quality from impacting the sewage treatment system. In other words, the quality of the effluent from the sewage treatment device is improved.
