100 Whys of Wastewater Treatment -- A Beginner’s Guide, A Reader for Wastewater Treatment Operators

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100 Whys of Wastewater Treatment Table of Contents I. Introduction 1. Why is wastewater treatment necessary? 2. What is ISO14000 (Environmental Management Standard)? 3. How to implement scientific environmental management? 4. What is “Environmental Protection 110”? 5. What tasks are included in clean production management? 6. What are the various types of wastewater at ***********? What is the quality and quantity of the water? 7. What are the wastewater treatment standards for ***********? 8. What is the wastewater treatment process of *********** company? 9. What are the operating costs of the wastewater treatment plant at *********** Company? 10. Why are COD and BOD, these two pollution indicators, frequently used in wastewater analysis? 11. What is COD (Chemical Oxygen Demand)? 12. What is BOD5 (Biochemical Oxygen Demand)? 13. What is the relationship between COD and BOD5? 14. What is B/C? What does B/C mean? 15. What is pH? 16. Why is the concentration unit of milligrams per liter (mg/L) often used in wastewater analysis? II. Pretreatment of wastewater 17. What is the pretreatment of wastewater? What are the objectives of this pretreatment? 18. What is the purpose of a wastewater collection tank? 19. Why do colloidal particles in wastewater not settle naturally? 20. How to precipitate colloidal particles? 21. What is cohesion? 22. What is flocculation? 23. Why is polyferric compound used for flocculation and adsorption pre-treatment of wastewater? 24. What is coagulation? 25. What is adsorption? 26. What is the iron-carbon treatment method? 27. Why is lime powder used for neutralization treatment of the water coming out of the iron-carbon process? 28. How to estimate the amount of chemical sludge generated? III. Biological treatment of wastewater? 29. What is the biochemical treatment of wastewater? 30. How do microorganisms break down and remove organic pollutants from wastewater? 31. What factors are related to microorganisms? 32. What temperature range is most suitable for microorganisms to grow and reproduce? 33. What should be the optimal pH range for microorganisms? 34. What is dissolved oxygen? What is the relationship between dissolved oxygen and microorganisms? 35. Why do high-concentration saline wastewater have a particularly significant impact on microorganisms? 36. What is aerobic biological treatment? What is aerobic biochemical treatment? What is the difference between the two? 37. What are the applications of biological treatment in wastewater treatment projects? 38. What are the similarities and differences between the biofilm process and the activated sludge process? 39. What is activated sludge? 40. How to evaluate the activated sludge in the activated sludge process and the biofilm process? 41. When observing biological phases under a microscope, which type of microorganism directly indicates that the biochemical treatment has been effective? 42. What is Mixed Liquor Suspended Solids (MLSS)? 43. What are Mixed Liquor Volatile Suspended Solids (MLVSS)? 44. Sludge settling ratio (SV)? 45. Sludge Volume Index (SVI)? 46. What does dissolved oxygen (DO) represent? 47. What factors are related to the dissolved oxygen content in wastewater? 48. Who provides the oxygen that microorganisms need in biochemical processes? 49. Why is it necessary to frequently replenish nutrients in wastewater during biochemical processes? 50. What is the ratio among the various nutritional elements required by microorganisms in wastewater? 51. Why is excess sludge generated? 52. How to estimate the amount of excess sludge generated? 53. What is the biochar method (PACT method)? IV. Operation and Control of Treatment Processes 54. How many stages are there in the entire wastewater treatment process? 55. What are the main technical parameters for the preprocessing process? 56. What are the main technical indicators for the biochemical treatment process? 57. What are the main technical indicators of a sludge dewatering system? 58. What chemical agents are required for wastewater treatment at Shanghai Xinyi Bailuda Pharmaceutical Co., Ltd.? 59. How to prepare dilute sulfuric acid? 60. Why must the pH of the effluent from the neutralization sedimentation tank be adjusted to above 9? 61. How is sludge removed from a neutralization sedimentation tank? 62. What kind of activated sludge should be added to the biochemical tank? 63. How much sludge should be added to the biochemical tank for the first time? 64. How to add sludge to a biochemical tank? How to apply a film? 65. How to cultivate and acclimate sludge? 66. Why should the COD concentration of wastewater in the regulating tank be kept below 700 mg/L? 67. How much urea should be added to the biochemical tank daily? 68. How much potassium dihydrogen phosphate should be added to the biochemical tank? 69. What level should the dissolved oxygen in the effluent from the biochemical tank be maintained at? 70. How to transfer the excess sludge from the SBR biological tank to the sludge tank? 71. How is the sludge in the sludge tank dewatered? 72. How does the biochemical tank operate in winter? 73. How should the biological treatment tank operate when there is no production wastewater due to holidays or temporary shutdowns? 74. What measures should be taken when the biochemical tank is subjected to load shocks and the microorganisms are damaged? 75. What should be done when a large number of microorganisms die? V. Treatment Facilities and Equipment 76. What treatment facilities and equipment does the wastewater treatment plant of *********** Company have? 77. What are the specifications, models, and structural types of the wastewater treatment facilities and equipment at *********** Company? VI. Management, operation, and maintenance of power equipment 78. How can we ensure that power equipment remains in good working condition at all times? 79. What special equipment is generally used in wastewater treatment systems? 80. What are the common specialized electrical equipment in wastewater treatment systems? 81. What are the common auxiliary equipment used in wastewater treatment systems? 82. What instruments and equipment are commonly used in wastewater treatment systems? 83. What are the key points in the management of equipment for wastewater treatment systems? 84. What are the standards for the proper condition of equipment in a sewage treatment system? 85. What is the typical maintenance cycle for equipment in wastewater treatment systems? 86. How are equipment management records typically organized in wastewater treatment systems? 87. What do the data and symbols represent for the pump of model 40FYS20 685A? 88. What are the key points for the operation and management of sewage pumps? 89. What precautions should generally be taken when a water pump is in operation? 90. What is the six-diligence approach to pump station management that the workers at Shanghai’s pump stations developed through practical experience? 91. What are the routine maintenance tasks for a sewage treatment system? 92. What is commonly known as “pei geng”? 93. What is commonly known as “butter”? 94. Why use “butter”? 95. How should one pay attention to the dripping at the packing while the water pump is in operation? How to adjust it? 96. How to fill the shaft seal packing of a water pump? 97. What could be the possible reasons for insufficient flow rate in a water pump? How to solve it? 98. What could be the reasons for insufficient head of a water pump? How to solve it? 99. What could be the possible reasons for overheating of the water pump bearings? How to solve it? 100. What could be the possible reasons for excessive pump power overload? How to solve it? 101. What could be the possible reasons for vibration or noise in a water pump? How to solve it? 102. What could be the possible reasons for low liquid output or no output from the water pump? How to solve it? 103. What is a Roots blower? 104. What are the operating condition requirements for Roots blowers? 105. What preparations are needed before starting a Roots blower? 106. How do rotary vane blowers operate under no-load conditions and under normal load conditions? 107. To what level should the oil in the oil tank of the Roots blower unit be filled? 108. How to start a Roots blower? 109. How to turn off a Roots blower? 110. How to maintain and service a Roots blower? 111. Blower failures and troubleshooting methods? 112. How to maintain pipeline valves? I. Introduction 1. Why is wastewater treatment necessary? ***********The company is located at ********************** and mainly produces *********************************. The wastewater originates from the ginkgo leaf extraction process, during which ethanol is used as an extractant; this wastewater contains various pollutants in concentrations that far exceed the allowed limits, and this has long been one of the challenges that affect and hinder the company’s production and development. If these wastewater are discharged without treatment, they will severely pollute the environment. Generally speaking, when the environment and resources are damaged and the ecological balance is disrupted, it takes more than a decade or two decades, or even hundreds of years to recover, and sometimes it is impossible to recover at all. Therefore, any enterprise that attempts to achieve short-term economic growth at the expense of the environment and resources is not only prohibited by national law and rejected by the public, but its very existence and development will also inevitably be restricted. 2. What is ISO14000 (Environmental Management Standard)? The ISO14000 series of standards are environmental management standards developed by the Environmental Management Technical Committee of the International Organization for Standardization. Their guiding principles are \"comprehensive management, pollution prevention, and continuous improvement,\" representing an innovation in approaches and methods for environmental management. ISO14000 has very strict standards and regulations; there are corresponding inspection criteria for every production step and management aspect, from the procurement of raw materials to the delivery of the finished products. It prevents the generation of pollutants during the production process in a systematic manner and ensures their effective treatment. Wastewater treatment is just one part of the ISO14000 series of standards. At present, the ISO14000 series of standards are being piloted and implemented in some large cities and large enterprises in China. The ISO14000 environmental quality certification is known as a \"green passport\" recognized in the international market; those who obtain this certification undoubtedly gain an \"international pass\". Many have announced that goods and products without environmental management certification will face restrictions on quantity and price upon import. Therefore, as companies gradually integrate with international markets, the ISO14000 environmental quality certification is being widely adopted by all enterprises in the country, just like ISO9000 (the quality management standard). Therefore, from the perspective of environmental management standards, we must not only strive to carry out effective wastewater treatment at the source of pollution and implement scientific environmental protection measures to ensure that the treated wastewater meets the required discharge standards ； Greater efforts should be made to focus on clean production management at the source of pollution, in order to prevent and reduce pollution. 3. How to implement scientific environmental management? Protecting the environment has become a fundamental national policy for the sustainable development of China’s economy; therefore, wastewater treatment must comply with the environmental protection laws and policies established in the country. In environmentally friendly planning and design, it is necessary to integrate production concepts with ecological principles and environmental protection, as well as to combine wastewater treatment with process improvements and the adoption of clean production methods. Through systematic analysis and verification, seek a relatively reasonable governance solution. The main principles of environmental management can be summarized as follows: (1) Phasing out unreasonable products – For those traditional, low-value products that pose great challenges in terms of wastewater treatment, it is necessary to replace them with high-value products that require advanced technology. If the annual profit of a certain product is not even sufficient to cover the annual costs associated with wastewater treatment, then such production should be discontinued in favor of products that cause less pollution and are easier to treat to meet regulatory standards. (2) Strengthen management and reduce pollution – managing enterprises is also an important factor in preventing pollution. Such as leaks, spills, drips, and seeps from the equipment ； The generation of large amounts of high-concentration wastewater resulting from production accidents or product scrap caused by failure to follow operating procedures ； Rinsing equipment and floors with large amounts of water increases the amount of wastewater generated ； The failure to separate cooling water from production wastewater results in an increase in the volume of wastewater and makes its treatment more difficult. (3) Establishing regional small-scale wastewater treatment plants in areas where factories are concentrated means that it is not necessary to apply the principle of \"whoever pollutes, whoever cleans up.\" Instead, it is important to strengthen cooperation among various enterprises and consider comprehensive strategies for dealing with pollution. If necessary and feasible, the wastewater from all factories can be treated together in a centralized wastewater treatment plant, adopting a system where \"whoever pollutes, pays\" for the cleanup. Since the quality of wastewater varies from factory to factory depending on the products produced, some factories have acidic wastewater while others have alkaline wastewater. Treating them together can reduce the costs associated with using neutralizing agents ； Some factories discharge wastewater with high salt content and low COD, while the wastewater from other factories is highly concentrated and biodegradable. If treated separately, both types of wastewater present significant challenges for treatment; however, when treated together through biochemical processes, the improved water quality not only reduces the difficulty of treating the wastewater but also increases the efficiency of the treatment process. (4) Improving the recycling rate of water: To reduce the amount of wastewater, efforts should first be made at the source where wastewater is generated. Options such as considering the recycling of water or its reuse multiple times can be explored to increase the rate of water recycling and minimize the amount of wastewater discharged. Abroad, some advanced companies have achieved a water recycling rate of over 96%, while the water recycling rate in manufacturing enterprises in Shanghai remains at a relatively low level of 20-30%; there is still significant potential for improvement. Increasing the recycling rate of water used in production not only helps to reduce environmental pollution but also decreases the need for fresh water, thereby alleviating the increasingly severe water resource shortage to some extent. In wastewater treatment, consideration should also be given to the recycling of the treated water as much as possible. (5) The recycling and comprehensive utilization of pollutants in wastewater involve raw materials, semi-finished products, finished products, and reaction media (such as solvents) that enter the water during the production process. Especially in the production of fine chemicals, some chemical reactions are often not entirely safe, and the separation of products cannot be thorough enough; as a result, wastewater, particularly the reaction mother liquor, often contains a certain amount of useful substances. Releasing these pollutants contaminates the environment and causes harm. However, if it is recycled or utilized comprehensively, waste can be turned into treasure, and harm can be turned into benefit ； Or by using waste to address other wastes, leveraging strengths to compensate for weaknesses, and adopting comprehensive management approaches, it is possible to save on water treatment costs. 4. What is “Environmental Protection 110”? To address the current situation where environmental administration enforcement does not match environmental management and public complaints, Shanghai has established an environmental emergency hotline at 62863110, commonly known as the “Environmental 110”. In the future, the phone number will be simplified to 63110 (a homophone for “Green 110”). This is the first “Environmental Protection 110” in the national environmental protection system. With increased efforts to protect the environment, environmental emergency hotlines will be introduced across the country one after another. 　　The responsibilities of the environmental emergency hotline include: receiving and handling major pollution incidents that occur throughout the city, as well as addressing reports of illegal pollution discharge by enterprises, such as secret discharges or direct discharges into the environment ； Receiving and handling incidents caused by environmental issues that may lead to social instability ； Assist relevant departments in handling major incidents that may have an impact on the environment ； For other environmental pollution issues that do not require on-site handling, the environmental emergency hotline is available 24/7 to accept complaints from residents across the city falling within the aforementioned scope. 　　For pollution-emitting entities, the establishment of Environmental Protection 110 represents both pressure and motivation; only by earnestly carrying out pollution management and control efforts can they withstand the oversight of environmental enforcement agencies and the public. 5. What tasks are included in clean production management? Wastewater and the pollutants contained in it are products of the production process; therefore, reforming the production processes and adopting clean production methods are the fundamental measures to eliminate or reduce the hazards caused by wastewater. Through reforms in processes and equipment, wastewater can be eliminated at the production stage, which not only improves the utilization rate of raw materials and auxiliary materials but also reduces the costs associated with wastewater treatment. This work should be carried out in collaboration between process engineering engineers and environmental engineers. It should be understood that protecting the environment is not solely the responsibility of environmental engineers; control must be exerted at the source of pollution in order to truly manage wastewater effectively. Therefore, environmental pollution issues that may arise in the future must be taken into account during process design and product prototyping. When selecting a synthetic route, it is advisable to use production processes that generate little or no pollution; the route with the highest raw material utilization efficiency should be chosen. Biological substances that are difficult to degrade or toxic and harmful substances, including raw materials, auxiliary materials, and solvents, should be used as little as possible in the production process. Additionally, efforts should be made to recycle and make comprehensive use of solvents and by-products. The specific methods are roughly as follows: (1) Adopting new processes, new technologies, and new routes. Adopting new processes, new technologies, and new routes. First, it is possible to verify the ingredient ratios in the production process; those raw materials that cause significant pollution and exceed the theoretical ratios should be reduced, in order to improve the utilization rate of these raw materials as well as the treatability of the wastewater. In chemical production, new approaches are sometimes adopted, which can not only improve production levels but also address wastewater treatment issues. For example, in the past, isonicotinic acid, a raw material for anti-tuberculosis drugs, had to be produced through electrolytic oxidation using sulfuric acid as the electrolyte; the acidic wastewater generated during this process was large in volume and difficult to treat. Currently, a new air catalytic oxidation technology is employed; the reaction takes place in a fluidized bed. The volume of wastewater generated is relatively small, and the pollution problems are easier to resolve. (2) Replacing raw materials and auxiliary materials is a common method; for example, using non-toxic or low-toxic materials in place of highly toxic ones, and using biodegradable substances in place of those that are difficult to biodegrade. Furthermore, it is necessary to avoid using, or use as little as possible, the substances whose use is restricted by emission standards, especially those with strict requirements, thereby reducing the burden on wastewater treatment. For example, there are now strict requirements regarding the ammonia nitrogen concentration in wastewater, which means that it is necessary to use as little ammonia water or liquid ammonia as possible in production. For example, in the past when adjusting the pH of wastewater, some treatment processes used ammonia water for this purpose; as a result, the ammonia nitrogen level in the treated water would exceed the allowed limits, which also increased the difficulty of biological treatment of the wastewater. By the same principle, we should use potassium dichromate less as an oxidizing agent, and use nitrocompounds and chlorinated hydrocarbons less as solvents. When selecting a solvent, in addition to meeting the requirements of the production process, its biodegradability and toxicity must also be considered. Based on the above requirements, the priority order for selection is shown in the table below. Solvent selection priority table: Prefer to use | Can be used | Avoid using. Methanol, ethanol, isopropanol, propanol, acetic acid, ethyl acetate, glycerin, ethylene glycol; Benzene, toluene, dimethylformamide (DMF), formamide, xylene; tert-Butanol, dimethyl sulfoxide (DMSO), triethylamine, di**amine, chloroform*, carbon tetrachloride*, chlorobenzene, ****, pyridine*, morpholine, **furan. Note: * indicates toxicity or inhibitory effects on microorganisms. (3) Adopt new post-treatment processes to reduce or eliminate pollution during the production process. This method is highly useful for technicians working in the field of chemical and industrial production. For example, in the organic synthesis industry, the method of diluting the reaction mixture with water (water precipitation) is commonly used to precipitate the reaction products from the organic solvents used in the reaction. The mother liquor resulting from this process contains a large amount of water, making it difficult to recover the organic solvents present therein, such as methanol and ethanol, which are water-soluble; these solvents end up in wastewater streams and cause pollution. If most of the solvent is first recovered by distillation before dilution, and then diluted with water, the organic content in the wastewater can be significantly reduced. To ensure a high quality of the resulting product, the reaction products or intermediate products often need to be washed in order to remove impurities entrained within them. Whether the washing operation is proper has a significant impact on the degree of wastewater pollution. However, if new post-treatment technologies are adopted, it is possible to eliminate all washing wastewater during the process operations, achieving zero discharge. Excess salt content in wastewater can inhibit the growth and reproduction of microorganisms, affecting the efficiency of biochemical treatment. We can also adopt new post-treatment processes to address this challenge in wastewater treatment. For example, a certain factory will react p-nitrochlorobenzene with sodium hydroxide in a methanol solvent to produce p-*-*methyl ether. The original post-treatment process involved washing with water to remove the NaCl salts from the reaction mixture; as a result, large amounts of wastewater were generated, and the salt concentration in this wastewater was high, which made subsequent biochemical treatment difficult. Later, the plant improved the post-treatment process by first filtering out NaCl from the reaction mixture (the organic phase), followed by washing with water to precipitate p-*N*-methyl ether. This improved process not only reduced the volume of wastewater by 50%, but also enabled the recovery of 97.4% of the salts present in the wastewater; it also reduced the organic load in the wastewater by 58.7%, resulting in a significant improvement in the biodegradability of the wastewater. (4) Strengthening solvent recovery: In most chemical raw material production plants, solvents account for a significant proportion of the materials used in the production process. It can be said that the organic load in many wastewater streams originates primarily from solvents. Therefore, paying attention to and improving solvent recovery is not only an important measure for preventing and reducing pollution, but it is also a key way to cut costs, increase efficiency, and boost profits, offering dual benefits in terms of both the environment and the economy. For example, a hormone-producing pharmaceutical factory in Shanghai has a daily total discharge of 8 tons of organic load (COD), making it a major source of pollution in the area. The plant’s environmental protection efforts began with solvent recovery: waste water containing the same solvents was collected and recycled. As a result, the daily total organic load discharged in the wastewater decreased from 8 tons to 3 tons. The revenue generated from solvent recovery exceeded the operating costs of the wastewater treatment plant. 6. What types of wastewater does *********** company have? What are the quality and quantity of the wastewater? ********************** The company currently has one stream of concentrated wastewater. This wastewater originates from the ****** extraction process; the daily volume of this wastewater is approximately 20 m³. In addition, there is also domestic sewage and other lightly polluted wastewater from the company’s production area that needs to be treated. The quality and quantity of each type of wastewater are detailed in the table below: Table showing the quality and quantity of various types of wastewater. Name of wastewater, Water volume (m³/day), pH, COD (mg/L), BOD5 (mg/L), B/C ratio, NH4-N (mg/L). Concentrated production wastewater: 20, 7, 20,000, 8,000, 0.4, 4. Domestic sewage and other lightly polluted wastewater: 100, 7–8, 400, 200, 0.4, 40. Total: 120, 7. What are the wastewater treatment standards at ********************** Company? According to the regulations set by the Environmental Protection Bureau, the treated wastewater discharged by *********** Company must meet the second-level standards for comprehensive sewage discharge in Shanghai (DB31/199-1997), namely pH=6–9, COD≤100mg/L, BOD≤30mg/L, NH4-N≤15mg/L, and SS≤70mg/L. 8. What is the wastewater treatment process of *********** company? The processing procedure is detailed in the block diagram, and the steps are briefly described as follows: (1) Preparation of 60% waste sulfuric acid: First, add water to the waste acid preparation tank, then slowly pour in 98% concentrated sulfuric acid until a 50%-60% dilute sulfuric acid solution is obtained. (2) Pretreatment of process wastewater: First, the concentrated process wastewater is placed in a pH adjustment tank, followed by the addition of dilute sulfuric acid to adjust the pH of the wastewater to 2–3; thereafter, it is slowly pumped into an iron-carbon treatment tank. The iron-carbon tank is a forced-flow tank; as acidic wastewater is pumped in, the water that has been treated with iron and carbon also flows into the iron-carbon effluent tank. After pumping the iron-carbon treated effluent into the neutralization sedimentation tank, lime powder is added, and a mixer is activated to carry out the neutralization process. Following sedimentation, the chemical sludge in the neutralization sedimentation tank is first discharged into the sludge thickening tank using a hydrostatic pressure method. Subsequently, the clarified liquid is transferred to the equalization tank, where it is mixed with other lightly polluted wastewater to become the influent for biological treatment. (3) Biochemical treatment: The wastewater from the equalization tank, after being homogenized and proportioned, is pumped into the biological contact oxidation tank for primary biochemical treatment. The treated water then flows by gravity into the SBR biochemical tank for secondary biochemical treatment. The effluent from biochemical treatment meets the discharge standards, and the excess sludge from the SBR biochemical tank is sent to the sludge thickening tank. (4) Sludge treatment: The excess sludge from the SBR biological tank and the chemical sludge generated during the pretreatment process are concentrated in a sludge thickening tank, and then lifted by screw pumps to a plate and frame filter press for dewatering and drying; the dried sludge is transported away for disposal. The wastewater treatment process is as follows: Acid preparation tank, 98% sulfuric acid; pH adjustment; Fe/C outlet tank; Fe/C tank; process wastewater; adjustment tank; neutralization and sedimentation; other types of wastewater; contact oxidation; SBR. The treated wastewater then goes through plate and frame filtration, followed by a sludge tank; the sludge is transported for disposal outside. 9. What are the operating costs of *********** company’s wastewater treatment plant? ***********The operating costs for the company’s wastewater treatment consist of three components: chemical costs, energy costs, and labor costs. Among these, the cost of chemicals is about 20 yuan per day, the cost of electricity is 150 yuan per day, and the labor cost is 53 yuan per day; in total, it amounts to 223 yuan per day. 10. Why are COD and BOD commonly used as pollution indicators in wastewater analysis? Wastewater contains many organic substances; it is common to find wastewater with a dozen, dozens of, or even hundreds of such organic substances. Conducting qualitative and quantitative analyses of each organic substance in the wastewater is both time-consuming and requires a large amount of reagents. So, is it possible to use just one pollution indicator to represent all the organic substances in wastewater and their quantities? Through research, environmental scientists have found that all organic substances share two common characteristics: first, they are all composed, at minimum, of carbon and hydrogen ； Second, the vast majority of organic substances can be chemically oxidized or oxidized by microorganisms; their carbon and hydrogen combine with oxygen to form non-toxic and harmless carbon dioxide and water. Organic matter in wastewater consumes oxygen, whether during chemical oxidation or biological oxidation. The more organic matter there is in the wastewater, the more oxygen is consumed, and there is a direct proportional relationship between the two. Therefore, environmental scientists refer to the amount of oxygen consumed in the oxidation of wastewater using chemical agents as chemical oxygen demand, or COD ； The amount of oxygen consumed by microorganisms in the oxidation of wastewater is called Biological Oxygen Demand, or BOD. Since COD and BOD can comprehensively reflect the amount of all organic substances in wastewater and are relatively easy to analyze, they are widely used in wastewater analysis and environmental engineering. In fact, COD does not refer only to organic substances in water; it can also indicate inorganic substances with reducing properties in water, such as sulfides, ferrous ions, sodium sulfite, and even chloride ions. For example, if the ferrous ions in the water exiting the iron-carbon tank are not completely removed in the neutralization tank, the COD level of the water after biological treatment may exceed the allowed limits due to the presence of these ferrous ions. 11. What is COD (Chemical Oxygen Demand)? Chemical oxygen demand (COD) refers to the amount of oxygen required to oxidize the substances in wastewater that can be oxidized by chemical oxidants, measured in milligrams of oxygen per liter. It is currently one of the most commonly used methods for determining the organic content in wastewater. Common oxidants used in COD analysis include potassium permanganate (manganese method CODMn) and potassium dichromate (chromium method CODCr); currently, the potassium dichromate method is more commonly used. Wastewater is used to oxidize organic substances under conditions of strong acid, heating, boiling, and reflux; the use of silver sulfate as a catalyst can increase the oxidation rate of most organic substances to 85-95%. If the wastewater contains a high concentration of chloride ions, mercury sulfate should be used to shield these ions in order to reduce interference with the COD measurement. 12. What is BOD5 (Biochemical Oxygen Demand)? Biochemical oxygen demand can also be used to indicate the degree of organic pollution in wastewater. The most commonly used measure is the five-day biochemical oxygen demand, denoted as BOD5; it represents the amount of oxygen required for the biochemical degradation of wastewater over a period of five days in the presence of microorganisms. In the future, we will use the five-day biochemical oxygen demand frequently. 13. What is the relationship between COD and BOD5? Some organic substances can be biologically oxidized and degraded (such as glucose and ethanol), some can only be partially biologically oxidized and degraded (such as methanol), while others cannot be biologically oxidized or degraded and are also toxic (such as ginkgolides, ginkgic acid, and certain surfactants). Therefore, we can divide organic matter in water into two categories: biodegradable organic matter and non-biodegradable organic matter. It is generally believed that COD can essentially represent all organic matter in water. BOD refers to the biodegradable organic matter in water; therefore, the difference between COD and BOD represents the organic matter in wastewater that is not biodegradable. 14. What is B/C? What does B/C mean? B/C is an abbreviation for the ratio of BOD5 to COD, and this ratio can indicate the biodegradability of wastewater. If CODNB represents the biodegradable portion of COD, then the proportion of organic matter in wastewater that cannot be biodegraded by microorganisms can be expressed as CODNB/COD. There is a relationship between BOD5/COD and CODNB/COD as shown in the table below: CODNB/COD: 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8; BOD5/COD: 0.52, 0.46, 0.41, 0.35, 0.29, 0.23, 0.17, 0.12. When BOD5/COD ≥ 0.45, biodegradable organic matter accounts for less than 20% of the total organic matter, whereas when BOD5/COD ≤ 0.2, biodegradable organic matter makes up more than 60% of the total organic matter. Therefore, the BOD5/COD ratio is often used as an indicator to evaluate the biodegradability of organic matter. BOD5/COD　　　　0.45　　　　Easily biodegradable BOD5/COD　　　　0.30　　　　Biodegradable BOD5/COD　　　　0.30　　　　Slightly difficult to biodegrade BOD5/COD　　　　0.20　　　　Very difficult to biodegrade. The B/C ratio holds great practical significance in environmental engineering. 15. What is pH? pH is actually a way of expressing the acidity or alkalinity of an aqueous solution. Normally, we are accustomed to using percentage concentrations to indicate the acidity or alkalinity of aqueous solutions, such as 1% sulfuric acid solution or 1% alkaline solution. However, when the acidity or alkalinity of an aqueous solution is very low, it becomes cumbersome to use percentage concentrations; in such cases, pH can be used to express it. The range of pH values is between 0 and 14; water is neutral when pH = 7 ； Water is acidic when pH is less than 7; the lower the pH, the more acidic the water ； When pH > 7, water is alkaline; the higher the pH, the more alkaline the water. All living organisms in the world depend on water, but the pH range suitable for their survival is often very narrow; therefore, the Environmental Protection Agency strictly regulates the pH level of treated water to be between 6 and 9. The detection of pH value in water often relies on pH test strips, but instruments such as pH meters are also used for this purpose. 16. Why is the concentration unit of milligrams per liter (mg/L) frequently used in wastewater analysis? Generally speaking, the amounts of organic and inorganic substances in wastewater are very small. It is too cumbersome and inconvenient to express these amounts in terms of percentage concentrations or other similar units. For example, there are usually only a few grams, dozens of grams, hundreds of grams, or even a few kilograms of pollutants in one ton of wastewater; the unit used for this is grams per ton (g/T). If tons are converted to liters, the unit becomes milligrams per liter (mg/L). For calculations, you can refer to the conversion table below: 1 milligram per liter = one millionth; 1000 milligrams per liter = one thousandth; 10000 milligrams per liter = one percent. II. Pretreatment of wastewater 17. What is the pretreatment of wastewater? What are the objectives of preprocessing? The treatment performed prior to biochemical treatment is generally customarily referred to as pretreatment. Due to the relatively low treatment costs and stable operation of biochemical methods, these methods are used to treat industrial wastewater in general. ********************** The treatment of wastewater at this company also relies on biochemical methods as the primary approach. However, the wastewater from *********** Company contains certain organic substances that inhibit or are toxic to microorganisms. Therefore, necessary pretreatment must be carried out on the wastewater before it enters the biochemical tank. The purpose of this pretreatment is to minimize or eliminate those substances that have an inhibitory or toxic effect on microorganisms, thereby ensuring the proper functioning of the microorganisms in the biochemical tank. There are two purposes for pretreatment: one is to minimize and remove, or convert into substances that are harmless or beneficial to microorganisms, those substances in the wastewater that have an inhibitory or toxic effect on them, so as to ensure the normal functioning of the microorganisms in the biochemical tank ； The second is to reduce the COD load during the pretreatment process in order to alleviate the operating burden on the biochemical tank. **********************The company’s pretreatment processes involve iron-carbon microelectrolysis and the Fe2+/Fe3+ reduction-oxidation method. The numerous tiny iron-carbon galvanic cells formed facilitate the occurrence of redox reactions, enabling the destruction and removal of toxic and harmful substances in wastewater. During the neutralization and precipitation process, active flocs formed by ferrous and ferric ions under alkaline conditions can adsorb organic substances in the wastewater, thereby reducing the COD load and ensuring the proper operation of subsequent biochemical treatment systems. 18. What is the purpose of a wastewater collection tank? The function of a wastewater collection tank is to gather, store, and balance the quality and quantity of wastewater. The amount and quality of wastewater generated by various production workshops are generally uneven; wastewater is produced during manufacturing processes, but not when there is no production. There can even be significant variations within a single day or between different shifts. This is especially true for wastewater from the fine chemical industry. If wastewater with high and low pollution levels is not separated, then the quality and quantity of the concentrated wastewater and that with lower pollution levels vary greatly. Such variations are detrimental to the proper operation of wastewater treatment facilities and equipment, as well as to the effectiveness of the treatment process. Therefore, before wastewater enters the main sewage treatment system, a wastewater collection tank with a certain capacity must be installed to store the wastewater and homogenize it, thereby ensuring the proper operation of the wastewater treatment equipment and facilities. 19. Why do colloidal particles in wastewater not settle naturally? Many impurities in wastewater with a density greater than 1, as well as large particles and suspended substances that settle easily, can be removed through methods such as natural sedimentation or centrifugation. However, tiny suspended particles with a density of less than 1, which are even invisible to the naked eye, find it difficult to settle naturally. For example, colloidal particles are particles sized between 10-4 and 10-6 mm; they are very stable in water, and their settling speed is extremely slow – it takes 200 years for them to settle 1 meter. There are two reasons for the slow sedimentation: (1) Generally, colloidal particles carry a negative charge, and due to the repulsion between like charges, this prevents the colloidal particles from coming into contact with each other and sticking together, allowing them to remain suspended in water. (2) The surface of the colloidal particles is also tightly surrounded by a layer of molecules; this hydration layer prevents and isolates contact between the colloidal particles, stopping them from sticking together and allowing them to remain suspended in water. 20. How can colloidal particles be precipitated? To precipitate colloidal particles, it is necessary to cause them to come into contact with each other, forming larger particles; in other words, they need to aggregate so that their specific gravity exceeds 1 and they can thus precipitate. There are many methods available, and the commonly used techniques in engineering include coagulation, flocculation, and coacervation. 21. What is cohesion? Coagulant agents carrying positive ions are added to wastewater; the presence of a large number of positive ions between colloidal particles eliminates the electrostatic repulsion between them, thereby causing the particles to aggregate. This process of causing colloidal particles to aggregate through the addition of positive ion electrolytes is known as coagulation. Common coagulants include aluminum sulfate, ferrous sulfate, alum, ferric chloride, etc. 22. What is flocculation? Flocculation involves adding polymeric coagulants to wastewater; once these polymers dissolve, they form polymeric compounds. The structure of this polymer is linear; one end of the chain is attached to one tiny particle, while the other end is attached to another tiny particle. It acts as a bonding bridge between these two relatively distant particles, causing them to gradually grow in size until they form large flocs (commonly known as “alum flowers”), thereby accelerating the settling of the particles. Common flocculants include polyacrylamide (PAM), polyferric sulfate (PF), etc. 23. Why is polyferric compound used for flocculation and adsorption pre-treatment of wastewater? Ferric polymers form ferric hydroxide flocs during the coagulation process, and these flocs have a strong ability to adsorb organic substances in wastewater. Experimental data show that after treating wastewater with ferric polymer flocs for adsorption, approximately 10%-20% of the COD in the wastewater can be removed. This helps to reduce the operational load on biological treatment tanks, thereby facilitating the proper treatment of wastewater before it is discharged. Furthermore, coagulation pretreatment using polyferric salts can remove trace substances in wastewater that are toxic and inhibitory to microorganisms, thereby ensuring the proper functioning of the microorganisms in the biological treatment tank. Among various coagulants, polyferric salts are relatively inexpensive (25–300 yuan per ton), resulting in low treatment costs; hence they are suitable for the pretreatment of process wastewater. Polyferric acid is an acidic substance with strong corrosivity; therefore, the treatment equipment must be properly protected against corrosion. 24. What is coagulation? The process of combining coagulation and flocculation is known as coagulation-flocculation. Coagulation is frequently used in experiments and engineering applications: first, chemicals such as ferrous sulfate are added to water to eliminate the electrostatic repulsion between colloidal particles; then polyacrylamide (PAM) is added, causing the particles to grow larger and form flocs visible to the naked eye, which eventually settle. 25. What is adsorption? Adsorption treatment involves using porous solids (such as activated carbon) or floc materials (such as polyferric compounds) to adsorb toxic and harmful substances in wastewater onto the surface or within the micropores of these solids or flocs, thereby purifying the water quality. The substances to be adsorbed can be either insoluble solid substances or soluble substances. Adsorption treatment has high efficiency and produces water of good quality, which is why it is often used for the advanced treatment of wastewater. Adsorption treatment can also be introduced into the biochemical treatment unit to improve its efficiency (the PACT method is one such example). 26. What is the iron-carbon treatment method? The iron-carbon treatment method, also known as iron-carbon microelectrolysis or iron-carbon internal electrolysis, is a form of technology for treating wastewater using metallic iron. It exhibits unique effectiveness when employed as a pretreatment technique for dealing with toxic and harmful wastewater with high COD levels. The treatment mechanism of the iron-carbon method is not yet fully understood. One widely accepted explanation is that under acidic conditions, numerous micro-current reaction cells are formed between iron and carbon, and organic substances are reduced or oxidized under the action of these micro-currents. The water exiting the iron-carbon process is then neutralized with lime or lime milk; the resulting Fe(OH)2 colloidal flocs possess a strong flocculating and adsorbing capacity for organic substances. Therefore, the iron-carbon method makes comprehensive use of the reductive properties of iron, the electrochemical properties of iron-carbon, and the flocculation and adsorption effects of iron ions. It is precisely the combined action of these three properties that enables the iron-carbon method to achieve excellent treatment results. The disadvantages of the iron-carbon method are: (1) After being immersed in an acidic medium for a long time, iron filings tend to clump together, causing blockages and channeling, which makes operation difficult and reduces the treatment efficiency ； (2) More iron is dissolved under acidic conditions, resulting in a larger amount of sludge upon neutralization with alkali. 27. Why is lime powder used for neutralization treatment of the water coming out of the iron-carbon process? After wastewater with a pH of 2, adjusted using sulfuric acid, undergoes iron-carbon treatment, the sulfuric acid is converted into ferrous sulfate, and the pH of the wastewater rises from 2 to 5–6. Then, why is it still necessary to neutralize this treated water with lime powder? Or could we use less lime powder during the neutralization process? The effluent from the iron-carbon process contains large amounts of ferrous sulfate; if this is not removed, it will affect the growth and reproduction of microorganisms in the subsequent biochemical tanks. Therefore, we must use lime to raise the pH value of the wastewater from 5-6 to above 9, thereby converting soluble ferrous sulfate into insoluble ferric hydroxide and calcium sulfate. These substances are then precipitated through coagulation and sedimentation, ensuring that the wastewater entering the biochemical tanks does not contain ferrous sulfate. Can we add less lime powder during neutralization? We can conduct a comparative experiment in the laboratory. The same amount of iron-carbon influent (with a pH of around 2) and iron-carbon effluent (with a pH of 5–6) were placed into two beakers respectively. Then, lime powder was added in measured amounts to each beaker for neutralization and coagulation. When the pH values of the wastewater in both beakers reached 9, it was found that the amounts of lime powder added to the two beakers were identical. This is because iron is not a neutralizing agent; ferrous sulfate, which is formed from sulfuric acid, remains an acidic substance. Lime powder is absolutely essential for the conversion of ferrous sulfate into ferrous oxide and calcium sulfate during the neutralization process. Therefore, lime powder must be added when treating the effluent from iron-carbon processes. 28. How to estimate the amount of chemical sludge generated? Sludge generated through chemical reactions (such as neutralization) and physicochemical treatments (such as coagulation with chemicals) is generally referred to as chemical sludge. The sludge formed after neutralization and coagulation treatment of the iron-carbon effluent is mainly composed of ferrous hydroxide and calcium sulfate. The amount of sludge generated can be calculated based on the amounts of sulfuric acid and lime powder added. In engineering, estimates can also be made using experience. Generally, if the pH of the iron-carbon feedwater is around 2, the amount of chemical sludge produced per ton of wastewater after neutralization and coagulation (with a moisture content of 80%) is approximately 50 kilograms. III. Biological Treatment of Wastewater 29. What is biological treatment of wastewater? Biological chemical treatment of wastewater is one of the most important processes in wastewater treatment systems, commonly referred to as biochemical treatment. Biochemical treatment utilizes the life processes of microorganisms to effectively remove soluble organic substances as well as some insoluble organic substances from wastewater, thereby purifying the water. In fact, we are not unfamiliar with biochemical treatment. In natural water bodies, there exists a food chain: larger fish eat smaller fish, smaller fish eat shrimp, shrimp eat small insects, small insects eat microorganisms, and microorganisms consume sewage. Without this food chain, nature would fall into chaos. In natural rivers, there are large numbers of microorganisms that rely on organic matter for survival. These microorganisms oxidize or reduce the organic substances that people discharge into the rivers day and night – such as industrial wastewater, pesticides, fertilizers, feces, and other organic materials – turning them into inorganic substances. Without these microorganisms, the rivers around us would become foul-smelling within a few months at the earliest, or even one to two years at the latest. It’s just that these microorganisms are so tiny and scattered that they cannot be seen with the naked eye. The biochemical treatment of wastewater is an enhancement of this process under artificial conditions. People concentrate countless microorganisms in one tank, creating an environment highly suitable for their reproduction and growth (such as temperature, pH level, oxygen, and nutrients like nitrogen and phosphorus), thereby allowing the microorganisms to multiply in large numbers and increasing the speed and efficiency with which they can break down organic matter. Then, wastewater is pumped into the tank, where the organic substances in it are oxidized and degraded as a result of the metabolic activities of microorganisms, thereby purifying and treating the wastewater. Compared with other treatment methods, the biochemical method features low energy consumption, no need for chemicals, good treatment efficiency, and low treatment costs. 30. How do microorganisms break down and remove organic pollutants from wastewater? Due to the presence of organic substances such as carbohydrates, fats, and proteins in wastewater, these inorganic organic compounds serve as food for microorganisms. Some of them are degraded and synthesized into cellular materials (anabolic metabolites), while others are degraded and oxidized into water, carbon dioxide, and other substances (catabolic metabolites). In this process, the organic pollutants in the wastewater are removed through degradation by microorganisms. 31. What factors are related to microorganisms? In addition to nutrients, microorganisms also require appropriate environmental factors such as temperature, pH value, dissolved oxygen, and osmotic pressure in order to survive. If environmental conditions are abnormal, it will affect the metabolic activities of microorganisms, and they may even mutate or die. 32. What temperature range is most suitable for microorganisms to grow and reproduce? In biological wastewater treatment, the optimal temperature range for microorganisms is generally 16–30°C, with the maximum temperature being 37–43°C; when the temperature drops below 10°C, microorganisms cease to grow. Within the appropriate temperature range, for every 10°C increase in temperature, the metabolic rate of microorganisms increases accordingly, and the COD removal rate also rises by about 10% ； Conversely, for every 10°C decrease in temperature, the COD removal rate decreases by 10%; therefore, during winter, the biochemical removal rate of COD is significantly lower than in other seasons. 33. What should be the optimal pH range for microorganisms? The life activities and metabolic processes of microorganisms are closely related to pH value. Most microorganisms can adapt to a pH range of 4.5–9, with the optimal pH range being 6.5–7.5. When the pH is below 6.5, fungi begin to compete with bacteria; at a pH of 4.5, fungi gain complete dominance in the biochemical tank, which severely affects the sedimentation of sludge ； When the pH exceeds 9, the metabolic rate of microorganisms is hindered. Different microorganisms have different requirements for the pH range to which they can adapt. In aerobic biological treatment, the pH can vary between 6.5 and 8.5 ； In anaerobic biological treatment, microorganisms have relatively strict requirements regarding pH, which should be between 6.7 and 7.4. 34. What is dissolved oxygen? What is the relationship between dissolved oxygen and microorganisms? Oxygen dissolved in water is called dissolved oxygen. The organisms in water bodies and aerobic microorganisms rely on dissolved oxygen to survive. Different microorganisms have different requirements for dissolved oxygen. Aerobic microorganisms require an adequate supply of dissolved oxygen; generally, it is advisable to maintain the dissolved oxygen level at 3 mg/L, with a minimum level of not less than 2 mg/L ； Facultative anaerobic microorganisms require a dissolved oxygen level between 0.2–2.0 mg/L ； Anaerobic microorganisms require a dissolved oxygen level of below 0.2 mg/L. 35. Why do high-concentration saline wastewater have a particularly significant impact on microorganisms? Let’s first describe an experiment on osmotic pressure: Two salt solutions of different concentrations are separated by a semipermeable membrane. Water molecules from the solution with the lower concentration will pass through the semipermeable membrane into the solution with the higher concentration, while water molecules from the solution with the higher concentration will also pass through the membrane into the solution with the lower concentration, but in smaller quantities. As a result, the liquid level on the side of the solution with the higher concentration rises. Osmosis stops when the pressure generated by the difference in liquid levels between the two sides becomes sufficient to prevent further water flow; this pressure is what is known as osmotic pressure. Generally, the higher the salt concentration, the greater the osmotic pressure. The situation of microorganisms in saline solutions is similar to the osmotic pressure experiment. The basic structural unit of microorganisms is the cell; the cell wall functions as a semipermeable membrane. When the chloride ion concentration is 2000 mg/L or less, the osmotic pressure that the cell wall can withstand is 0.5–1.0 atmospheres. Even considering the certain toughness and elasticity of both the cell wall and the cytoplasmic membrane, the osmotic pressure that the cell wall can tolerate does not exceed 5–6 atmospheres. However, when the chloride ion concentration in the aqueous solution exceeds 5000 mg/L, the osmotic pressure rises to approximately 10–30 atmospheres. Under such high osmotic pressure, water molecules within the microorganisms penetrate into the external solution in large quantities, causing the cells to lose water and undergo plasmolysis; in severe cases, the microorganisms die. In daily life, people use table salt (sodium chloride) to pickle vegetables and fish in order to sterilize and preserve food, applying this very principle. Engineering experience data shows that when the chloride ion concentration in wastewater exceeds 2000 mg/L, microbial activity is suppressed, and the COD removal rate decreases significantly ； When the chloride ion concentration in wastewater exceeds 8000 mg/L, it causes the sludge volume to expand, large amounts of foam to form on the surface, and the microorganisms die one after another. However, through long-term domestication, microorganisms gradually adapt to growing and reproducing in high-concentration saltwater. People have already domesticated microorganisms capable of adapting to chloride or sulfate concentrations of over 10,000 mg/L. However, the principles of osmotic pressure tell us that microorganisms adapted to growing and reproducing in high-concentration saline solutions have cell fluids with a very high salt concentration. Once the salt concentration in the wastewater is low or extremely low, water molecules from the wastewater will flood into the microorganisms, causing their cells to swell; in severe cases, this leads to cell rupture and death. Therefore, microorganisms that have undergone long-term domestication and can gradually adapt to growing and reproducing in high-concentration saline solutions require a salt concentration in the biochemical feedwater to remain at a fairly high level; it cannot fluctuate greatly, otherwise a large number of these microorganisms will die. 36. What is aerobic biological treatment? What is aerobic biochemical treatment? What is the difference between the two? Based on the different oxygen requirements for microbial growth, biochemical treatment can be divided into two main categories: aerobic biochemical treatment and anaerobic biochemical treatment. Anaerobic biochemical treatment can further be divided into facultative anaerobic biochemical treatment and anaerobic biochemical treatment. During aerobic biological treatment, aerobic microorganisms must grow and reproduce in the presence of large amounts of oxygen, thereby reducing the organic matter in wastewater ； During the anaerobic-oxidizing biological treatment process, anaerobically oxidizing microorganisms require only a small amount of oxygen to grow and reproduce as well as to degrade the organic substances in wastewater. If there is too much oxygen in the water, these microorganisms cannot grow properly, which in turn affects their efficiency in treating organic substances. Aerobic microorganisms can adapt to wastewater with high COD concentrations; the incoming water’s COD level can reach over 2000 mg/L, with a COD removal rate of generally 50-80% ； Aerobic microorganisms can only thrive in wastewater with low COD concentrations; the COD level in the incoming water is generally kept below 1000–1500 mg/L. The COD removal rate is usually between 50% and 80%. Neither anaerobic nor aerobic biological treatment processes take very long, typically ranging from 12 to 24 hours. Taking advantage of the differences as well as the similarities between anaerobic and aerobic biological treatment processes, these two methods are combined. Wastewater with a high COD concentration is first treated using anaerobic biological treatment, and the effluent from this anaerobic tank is then used as the feedwater for the aerobic tank. This combined treatment approach allows for a reduction in the volume of the biological treatment tanks, thereby saving on environmental protection investments as well as reducing daily operating costs. The principles and functions of anaerobic biological treatment are the same as those of facultative aerobic biological treatment. The difference between anaerobic biological treatment and aerobic biological treatment is that anaerobic microorganisms do not require any oxygen for their reproduction and growth, nor for the degradation of organic matter; moreover, they can cope with wastewater containing higher COD concentrations (4000–10000 mg/L). The disadvantage of anaerobic biological treatment is that the biological treatment process takes a long time; the retention time of wastewater in the anaerobic biological tank generally requires more than 40 hours. 37. What are the applications of biological treatment in wastewater treatment projects? There are two main categories of biological treatment techniques that are most widely used and practical in wastewater treatment projects: one is called the activated sludge process, and the other is called the biofilm process. The activated sludge process is a form of aerobic wastewater treatment that utilizes the biochemical metabolism of suspended biological communities. During their growth and reproduction, microorganisms can form colony aggregates with a large surface area. These aggregates are capable of fusing together and absorbing large amounts of suspended colloidal or dissolved pollutants in wastewater, taking them into the cells. With the presence of oxygen, these substances are completely oxidized, releasing energy, CO2, and H2O. The sludge concentration in the activated sludge process is generally 4 g/L. In the biofilm process, microorganisms attach to the surface of the filler, forming a gelatinous, interconnected biofilm. Biofilms generally have a fluffy flocculent structure, with numerous micropores and a large surface area, which gives them strong adsorption capabilities; this facilitates microorganisms in further decomposing and utilizing the adsorbed organic substances. During the treatment process, the flow of water and the agitation of air keep the surface of the biofilm in constant contact with water. Organic pollutants and dissolved oxygen present in the wastewater are absorbed by the biofilm, and the microorganisms on the biofilm continuously break down these organic substances. As these organic substances are oxidized and decomposed, the biofilm itself also undergoes continuous metabolism; the aged biofilm then falls off and is carried away by the treated water out of the biological treatment facility, where it separates from the water in a sedimentation tank. The sludge concentration in the biofilm process is generally 6-8 g/L. To increase the sludge concentration and thereby improve treatment efficiency, the activated sludge process can be combined with the biofilm process by adding fillers to the activated sludge tank. Such a bioreactor, which contains both microorganisms attached to the fillers and suspended microorganisms, is known as a composite bioreactor; it features a very high sludge concentration, typically around 14 g/L. 38. What are the similarities and differences between the biofilm process and the activated sludge process? The biofilm process and the activated sludge process are different reactor configurations for biochemical treatment; their main visual difference lies in the fact that in the former, the microorganisms do not require a packing medium, as the biological sludge remains suspended, whereas in the latter, the microorganisms are attached to the packing. However, their mechanisms for treating wastewater and purifying water quality are the same. Furthermore, the biological sludge of both is aerobic activated sludge, and the composition of the sludge also shows certain similarities. Furthermore, in the biofilm process, the microorganisms being fixed on the packing materials can form a relatively stable ecosystem. Their energy requirements for survival and metabolism are not as high as those of the microorganisms in the activated sludge process; as a result, the amount of excess sludge generated in the biofilm process is less than that in the activated sludge process. The contact oxidation tank of Shanghai Xinyi Bailuda Pharmaceutical Co., Ltd. uses the biofilm process, while the SBR biochemical tank employs the activated sludge process. 39. What is activated sludge? From a microbial perspective, the sludge in a biochemical tank is a biological community composed of various microorganisms with biological activity. If the sludge particles are observed under a microscope, it is possible to see various microorganisms within them—bacteria, molds, protozoa, and metazoans (such as rotifers, insect larvae, and worms). These organisms form a food chain; bacteria and molds can break down complex organic compounds to obtain the energy needed for their activities and to build themselves. Protozoa feed on bacteria and molds, and they in turn are consumed by metazoans, which can also live off bacteria directly. These microbial-rich, floc-like sediment particles with the ability to degrade organic matter are called activated sludge. In addition to being composed of microorganisms, activated sludge also contains some inorganic substances and organic matter that is adsorbed on the activated sludge and cannot be biodegraded any further (i.e., metabolic residues of microorganisms). The moisture content of activated sludge is generally 98-99%. Activated sludge, resembling flocs, has a large surface area, which gives it strong adsorption capacity and the ability to oxidize and decompose organic matter. 40. How to evaluate the activated sludge in the activated sludge process and the biofilm process? The identification and evaluation of activated sludge growth in the activated sludge process and the biofilm process are different. In the biofilm process, the evaluation of activated sludge growth is primarily based on direct microscopic observation of the biological community. In the activated sludge process, in addition to direct microscopic observation of the biological community to assess the growth of the activated sludge, common evaluation indicators include: mixed liquor suspended solids (MLSS), mixed liquor volatile suspended solids (MLVSS), sludge settling ratio (SV), and sludge settling index (SVI). 41. When observing biological phases under a microscope, which type of microorganism directly indicates that the biochemical treatment has been effective? The presence of microzoa (such as rotifers and nematodes) indicates that the microbial community is thriving and that the ecosystem of the activated sludge is relatively stable; under such conditions, the biochemical treatment efficiency is at its best. It’s similar to a river where large fish can often be caught, allowing small fish and shrimp to thrive as well. 42. What is Mixed Liquor Suspended Solids (MLSS)? Mixed liquor suspended solids (MLSS), also known as sludge concentration, refers to the weight of dry sludge per unit volume of the mixed liquor in the biological reactor; it is expressed in milligrams per liter and is used to indicate the concentration of activated sludge. It consists of organic and inorganic substances. Generally, it is appropriate to keep the MLSS level in the SBR biological tank at around 2000–4000 mg/L. 43. What are Mixed Liquor Volatile Suspended Solids (MLVSS)? Mixed liquor volatile suspended solids (MLVSS) refer to the weight of volatile substances in the dry sludge contained per unit volume of the biochemical tank mixture, also expressed in milligrams per liter. Since it does not include the inorganic substances in the activated sludge, it can more accurately represent the number of microorganisms in the activated sludge. 44. Sludge settling ratio (SV)? The sludge settling ratio (SV) refers to the volume percentage (%) of settled sludge to the volume of the mixed liquid in a 100-milliliter cylinder, after the mixture has been allowed to settle at rest for 30 minutes; therefore, it is sometimes denoted as SV30. Generally, the SV in the biochemical tank ranges from 20% to 40%. Determining the sludge settling ratio is relatively simple; it is one of the important indicators for evaluating activated sludge. It is often used to control the discharge of excess sludge and to address abnormal conditions such as sludge bulking in a timely manner. Obviously, SV is also related to the sludge concentration. 45. Sludge Volume Index (SVI)? SV*10, the Sludge Volume Index, is the volume in milliliters occupied by 1 gram of dry sludge in its wet state. The calculation formula is as follows: SVI = MLSS / … The SVI takes into account factors other than sludge concentration, and thus provides a better indication of the coagulation and sedimentation properties of activated sludge. It is generally believed that when 60 < SVI < 100, the sludge has good sedimentation properties; when 100 < SVI < 200, the sedimentation properties are average; when 200 < SVI < 300, the sludge shows a tendency to expand; and when SVI > 300, the sludge has already expanded. 46. What does dissolved oxygen (DO) represent? Dissolved oxygen (DO) refers to the amount of oxygen dissolved in water, and it is expressed in mg/L. Different biochemical treatment methods have varying requirements for dissolved oxygen. During anaerobic-biological processes, the dissolved oxygen level in water is generally between 0.2–2.0 mg/L, whereas in SBR aerobic biological processes, it is usually between 2.0–8.0 mg/L. Therefore, during the operation of the aerotank, the aeration volume should be low and the aeration time should be short ； During the operation of the SBR aerobic tank, the aeration volume and aeration time are much greater and longer; whereas we use contact oxidation, with the dissolved oxygen level controlled at 2.0–4.0 mg/L. 47. What factors are related to the dissolved oxygen content in wastewater? The concentration of dissolved oxygen in water can be expressed by Henry’s law: when dissolution equilibrium is reached, C = KH * P, where C represents the solubility of oxygen in water at that equilibrium state ； P is the partial pressure of oxygen in the gas phase ； KH is the Henry coefficient, which is dependent on temperature ； Increased aeration efforts are made to achieve oxygen dissolution equilibrium, while activated sludge also consumes oxygen in the water. Therefore, the actual dissolved oxygen level in wastewater is related to factors such as water temperature, effective water depth (which affects pressure), aeration volume, sludge concentration, and salinity. 48. Who provides the oxygen required by microorganisms in biochemical processes? The oxygen required by microorganisms in biochemical processes is mainly supplied by rotary blowers. 49. Why is it necessary to frequently replenish nutrients in wastewater during biochemical processes? The method of removing pollutants using biochemical processes relies primarily on the metabolic activities of microorganisms; whereas life processes such as cellular synthesis in microorganisms require an adequate amount and variety of nutrients, including trace elements. For chemical industrial wastewater, due to the uniformity of the products manufactured, the composition of the wastewater is also relatively uniform; it lacks the necessary nutrients for microorganisms. For example, the production wastewater from *********** company contains only carbon and nitrogen, with no phosphorus; such wastewater cannot meet the metabolic needs of microorganisms. Therefore, phosphorus must be added to this wastewater in order to facilitate the metabolic processes of these microorganisms and promote the synthesis of their cells. It’s like how people need to consume enough vitamins while eating rice and flour. 50. What is the ratio among the various nutritional elements required by microorganisms in wastewater? Like animals and plants, microorganisms also require essential nutrients in order to grow and reproduce. The main nutrients needed by microorganisms are carbon (C), nitrogen (N), and phosphorus (P). There are specific requirements regarding the proportion of these nutrients in wastewater; for aerobic biological treatment, the ratio is generally C:N:P = 100:5:1 (by weight). 51. Why is excess sludge generated? During the biochemical treatment process, the microorganisms in the activated sludge continuously consume the organic substances in the wastewater. Among the organic matter that is consumed, part of it is oxidized to provide the energy required for the metabolic activities of microorganisms, while another portion is used by these microorganisms to synthesize new cytoplasm, thereby enabling them to reproduce. As microorganisms carry out their metabolism, some of them die, which results in the formation of excess sludge. 52. How to estimate the amount of excess sludge generated? During the metabolism of microorganisms, some organic substances (BOD) are utilized by the microorganisms to synthesize new cytoplasm, thereby replacing the dead microorganisms. Therefore, the amount of excess sludge produced is related to the amount of BOD that has been decomposed; there is a correlation between the two. In engineering design, it is generally assumed that for every kilogram of BOD5 treated, 0.6–0.8 kilograms of excess sludge is generated (100%); this corresponds to 3–4 kilograms of dry sludge with a moisture content of 80%. 53. What is the biochar method (PACT method)? For certain pharmaceutical wastewater that is difficult to biodegrade, it is quite challenging to keep the COD level in the effluent from biological treatment below the **first-class discharge standard (100 mg/L)**. Therefore, it is essential to employ granular activated carbon adsorption technology on the biologically treated effluent to ensure that it meets the discharge standards. However, the granular activated carbon adsorption treatment method has one fatal drawback: its treatment cost is excessively high. The fundamental reason for this is that the dynamic adsorption capacity of granular activated carbon for COD is only about 10% (by weight); in other words, one ton of activated carbon can adsorb and treat merely around 100 kilograms of COD in wastewater. Due to the difficulty in regenerating granular activated carbon and the high treatment costs, the application of granular activated carbon treatment technology is not yet widespread in China. So, is it possible to develop a new technology that can significantly increase the dynamic adsorption capacity of activated carbon and effectively reduce the cost of wastewater treatment? The Powdered Activated Carbon Treatment Process, first developed by DuPont, is one of the representatives of such new technologies. The biochar method, also known as the “PACT method” or “PACSBR biochemical method”, is regarded abroad as the most promising new biochemical wastewater treatment process. Its process flow is shown in the figure below: powdered activated carbon → aeration tank → secondary sedimentation tank → influent for biochemical treatment; treated effluent, carbon-containing return sludge, and sludge thickening tank; excess sludge. Schematic diagram of the PACT process: Powdered activated carbon is added to the influent for biochemical treatment (or directly into the aeration tank), where it mixes with the carbon-containing return sludge. The excess sludge discharged from the sludge thickening tank is then sent to a sludge dewatering unit. In the aeration tank, activated sludge adheres to the surface of powdered activated carbon. Due to the large specific surface area and strong adsorption capacity of powdered activated carbon, the adsorption capacity of the sludge is enhanced; in particular, the concentrations of dissolved oxygen and degradation substrates at the interface between the activated sludge and powdered activated carbon increase significantly, which in turn improves the rate of COD degradation and removal. Generally, in PACT systems, the dynamic adsorption capacity of activated carbon for removing COD is between 100–350% (on a weight percentage basis); that is, one kilogram of powdered activated carbon can remove 1.0–3.5 kilograms of COD. Moreover, the PACT method can handle toxic and harmful organic pollutants that are difficult for organisms to degrade. Based on our experience in engineering commissioning, regularly adding powdered activated carbon in fixed amounts directly into the SBR aerobic biological tank every 15–30 days can achieve excellent treatment results. In fact, the adsorption mechanism for both powdered activated carbon and granular activated carbon is the same; however, using powdered activated carbon in SBR biological tanks offers the following advantages: reduced investment costs ； Flexible and convenient to operate ； High utilization rate of activated carbon ； It overcomes the drawback of granular activated carbon, where biofilms tend to form, leading to clogging and affecting the effluent flow rate. In the powder activated carbon–activated sludge system, the activated sludge adheres to the surface of the powder activated carbon. Thanks to the large specific surface area of the powder activated carbon and its strong adsorption capacity, the concentrations of dissolved oxygen and degradation substrates at the interface between the activated sludge and the powder activated carbon increase significantly, thereby enhancing the rate of COD degradation and removal. Generally, COD removal (depending on the type of wastewater) can be increased by 10-40% ； Since the toxic and harmful organic substances in the wastewater are adsorbed by powdered activated carbon, the concentration of such substances in the wastewater can be maintained at a low level, thereby ensuring the proper operation of the biochemical treatment system ； It is highly effective in preventing the rebound of ammonia nitrogen levels and ensuring that the ammonia nitrogen levels in the effluent meet the specified standards. We once used the PAC-SBR method to treat the wastewater generated by ************************** factory, and the results showed that this method achieved a fairly significant treatment effect, with the effluent from biochemical treatment meeting the **first-class discharge standards. For the wastewater treatment system of ************ Company, if the effluent from the SBR biological treatment process does not meet the discharge standards, we can also add a small amount of powdered activated carbon to the SBR tank in order to improve the efficiency of biological treatment and ensure that the effluent meets the specified discharge standards. IV. Operation and Control of Treatment Processes 54. How many stages are there in the entire wastewater treatment process? The entire wastewater treatment process is divided into two stages: the pretreatment stage (iron-carbon microelectrolysis–neutralization coagulation) and the biological treatment stage (biological contact oxidation tank–SBR biological treatment). 55. What are the main technical parameters for the preprocessing process? (1) For the iron-carbon electrolytic cell process, the volume of concentrated wastewater is 20 tons per day. The pH of the inlet water is 2–3, while the pH of the outlet water is 5–6. The reaction time is >8 hours. (2) For the neutralization and precipitation tank process, the volume of concentrated wastewater is also 20 tons per day. The pH of the inlet water is 5–6, and the pH of the outlet water is >9. The amount of lime powder added is 0.8 kilograms per ton of wastewater, and the retention time is 2 hours. 56. What are the main technical parameters for the biochemical treatment process? (1) Maximum water storage capacity of the regulating tank: 120 tons. pH: 6–8. COD range: 700 mg/L. BOD5 range: 250 mg/L.
(2) Operation mode of the biochemical contact oxidation tank: Continuous flow operation. Maximum treatment capacity: 120 tons per day. Hydraulic retention time: 20 hours. Aeration time: ≥16 hours. COD in the effluent: ≤300 mg/L.
(3) Operation mode of the SBR biochemical tank: Batch operation. Maximum treatment capacity: 120 tons per day. Hydraulic retention time: 20 hours. Inlet time: 6 hours. Aeration time: 4 hours. Outlet time: 2 hours. COD in the effluent: ≤100 mg/L. BOD5 in the effluent: ≤30 mg/L.
57. What are the main technical parameters of the sludge dewatering system? Sludge treatment capacity: 8 tons per day. Sludge concentration before dewatering: 3%. Sludge concentration after dewatering: 20%. 58. What chemicals and materials are required for wastewater treatment at ************ Company? ************List of chemical materials required at the treatment station (for reference)
Name of chemical | Specification | Required amount | Market price
98% sulfuric acid, industrial grade | 20 Kg/day | 0.5 yuan/Kg
Calcium hydroxide, CaO >93%, packaged in 25 Kg units | 12.5 Kg/day | 0.75 yuan/Kg
Potassium dihydrogen phosphate | 2-3 Kg/day | 5 yuan/Kg
Granular activated carbon, 17# grade, Φ3-4, L=4-8 mm | 5 tons per 1-2 years | 2500 yuan/Kg
Powdered activated carbon, type 670, packaged in 20 Kg units | 100 Kg | 6000 yuan/Kg
Cast iron scrap | 25 tons per 1-2 years | 1100 yuan/Kg
Biochemical sludge, with 81% moisture content | 10 tons (one-time use) | 150 yuan/Kg
59. How to prepare dilute sulfuric acid? First, add clean water to the waste acid preparation tank, then slowly pour in 98% concentrated sulfuric acid until a dilute sulfuric acid solution with a concentration of 50%-60% is obtained. When preparing dilute sulfuric acid, the following three points should be kept in mind: Both 98% concentrated sulfuric acid and the dilute acid that is prepared afterward are highly corrosive. 98% sulfuric acid is also highly hygroscopic and can cause skin burns. Therefore, protective equipment must be worn at all times during operation. Concentrated sulfuric acid generates a large amount of heat during dilution; therefore, it is absolutely forbidden to pour water into concentrated sulfuric acid – instead, the concentrated sulfuric acid must be poured into water. When carrying out this operation, it is necessary to add the concentrated sulfuric acid to the water slowly and gradually. Since the dilution of concentrated sulfuric acid is a highly exothermic process, the plastic components in the mixing tank (such as water pumps and pipes) should be removed in advance to prevent them from deforming due to heat and getting damaged. 60. Why must the pH of the effluent from the neutralization sedimentation tank be adjusted to above 9? The effluent from the iron-carbon process contains large amounts of ferrous sulfate; if this is not removed, it will affect the growth and reproduction of microorganisms in the subsequent biochemical tanks. Therefore, we must use lime to raise the pH value of the wastewater from 5-6 to above 9, thereby converting soluble ferrous sulfate into insoluble ferric hydroxide and calcium sulfate. These substances are then precipitated through coagulation and sedimentation, ensuring that the wastewater entering the biochemical tanks does not contain ferrous sulfate. Whether ferrous hydroxide precipitates depends mainly on the pH value of the wastewater. When the pH of the wastewater reaches 6.5, some ferrous hydroxide begins to precipitate; however, for complete precipitation of ferrous hydroxide in the wastewater, its pH must reach 9.7. Therefore, it is essential to adjust the pH of the wastewater to above 9 during neutralization, so as to keep the ferrous ions in the wastewater entering the biological treatment tank at a very low level. 61. How is sludge removed from a neutralization sedimentation tank? After the wastewater in the neutralization sedimentation tank has been completely coagulated and settled by adding lime, the sludge and water are clearly separated, with the chemical sludge settling at the bottom of the reaction tank. When discharging sludge, first open the sludge pipeline valves at the bottom of the sedimentation tank as well as those in the sludge tank itself; use the pressure from the water level to push the sludge out of the reaction tank and into the sludge tank. After the sludge has been discharged, close the sludge pipeline valves in both tanks. Then open the sewage valve to pour the clear liquid into the regulating tank. The neutralization sedimentation tank is equipped with a decanter; its design consists of a rubber ring beneath which a hose is fixed, the other end of which is connected to the sewage outlet pipe at the bottom of the tank. It works by having a rubber ring float on the water’s surface, rising and falling with it. Since the separation of sludge from water always starts at the water surface, as clear liquid forms on the surface, it flows out of the tank through the hose; therefore, drainage and sludge separation occur simultaneously, eliminating the need to wait until the sludge and water are completely separated before draining, which saves time in operation. However, when operating, be careful to lift the decanter above the water surface while mixing the slurry to prevent mud from entering the hose. 62. What kind of activated sludge should be added to the biochemical tank? The so-called cultivation of activated sludge involves providing certain growth conditions for the microorganisms that constitute activated sludge. Under these conditions, over time, activated sludge is formed, its quantity gradually increases, and it eventually reaches the concentration required for wastewater treatment. The microbial cultivation process for domestic sewage treatment plants is relatively simple, whereas it is more challenging to cultivate microorganisms in toxic and harmful industrial wastewater; the time required for sludge acclimatization is also longer. Generally, for industrial wastewater, the dry sludge cultivation method is used – that is, dried sludge obtained from a properly operating sewage treatment plant (with a moisture content of around 80%), without the addition of any chemicals during dehydration, is used as a source of microorganisms for cultivation. To enable the microorganisms to adapt to toxic and harmful industrial wastewater as quickly as possible, it is best to use dry sludge of the same type, or from the same type of wastewater treatment plant after dewatering, as a source of microorganisms. 63. How much sludge should be added to the biochemical tank for the first time? If the dry sludge cultivation method is used, we must maintain a sludge concentration in the biological tank of around 3 g/L, or 3 Kg/m3. Since the moisture content of dry sludge is 80%, at least 15 Kg/m3 of dry sludge should be added to the aeration tank; in other words, for a tank with a volume of 100 m3, about 1.5 tons of dry sludge should be added. 64. How to add sludge to a biochemical tank? How to apply a film? If the dry sludge cultivation method is used, first fill the aeration tank with clean water or river water and aerate it, while gradually adding the prepared dry sludge into the aeration tank. After all the components have been added, aeration continues for 2–4 hours. After aeration is completed, the mixture is left to stand for 2 hours before the supernatant is removed. This process can be repeated 2–3 times until the supernatant becomes clear and transparent, without any turbidity. This process is known as sludge washing, sludge activation, or sludge biofilm formation. After the sludge is activated, acclimatization is initiated using nutrient-rich water or wastewater at low concentrations. 65. How to cultivate and acclimate sludge? The cycle of biochemical fermentation depends on the water temperature and quality of the wastewater. When the water temperature is above 15°C, the process of culturing bacteria proceeds more rapidly; whereas when the temperature is below 15°C, it takes longer to acclimate the sludge. Therefore, the cultivation and acclimatization of sludge should be carried out preferably between May and November (in the Yangtze River basin). In terms of wastewater quality, for wastewater that is non-toxic, harmless, and easily biodegradable, the biochemical cultivation time is generally 10–20 days. On the other hand, toxic, harmful, and difficult-to-biodegrade wastewater requires a longer period, around 30–60 days, or even more. After the commissioning in clean water, for wastewater with good biodegradability, microorganisms can be directly acclimated using the wastewater itself ； For chemical wastewater or wastewater with poor biodegradability, a stepwise bacterial cultivation method should be employed, with the specific steps as follows: (1) Rapid proliferation. The purpose of rapid proliferation is to enable the sludge to quickly grow on the filler. Generally, during the dewatering or transportation of the purchased sludge, the microorganisms suffer varying degrees of damage. They need a time to recover and grow in the new environment, and therefore require a favorable living condition. If acclimatization is carried out directly using chemical wastewater at this time, it will inevitably lead to the massive death of microorganisms. Therefore, in the first phase, domestic wastewater, glucose, or cooked paste made from dry flour can be used to cultivate microorganisms (during the initial 3–5 days, 5–10 kilograms of dry flour can be added per 100 m3 of bioreactor volume). Aeration is carried out twice a day, for 8 hours each time in the aerobic tank, to enable rapid recovery and growth of the microorganisms; this method is known as the rapid proliferation method. During the rapid proliferation period, the wastewater in the biochemical tank can be discharged through the sludge acclimation pipe; stop aeration before releasing the water, and wait for 4–8 hours for the sludge to settle before doing so. The rapid proliferation period is generally 7-10 days. During the operation of the biochemical tank, when microorganisms are subjected to a load shock and the COD removal rate or SV drops suddenly, the rapid proliferation method can also be employed to help the microorganisms recover and grow. (3) Wastewater acclimatization. After the sludge grows on the filler, the amount of dry flour added daily to the 100 m3 biochemical tank can be increased to 20–30 kg, while biochemical influent or wastewater is pumped into the tank at the same time. The initial amount of wastewater to be fed in can be pumped in at a rate of 1-2% of the volume of each 100 m3 of the biological treatment tank. Thereafter, the amount of wastewater pumped in is increased gradually by 2% every two days, until the designed wastewater inflow rate is reached. As the amount of wastewater pumped in gradually increases, the amount of glucose or dry flour added, as well as the amount of domestic sewage pumped in, should be reduced accordingly until addition is stopped; alternatively, waste alcohol can be added in proportion (1 kilogram of waste alcohol corresponds to 1.5 kilograms of COD). During the culture and acclimation period, COD must be measured daily; if a sudden drop in the COD removal rate or SV is observed, the flow rate of wastewater fed into the system should be stopped immediately until the COD removal rate rises above 50% and the SV stops declining. When wastewater is properly fed into the aerobic tank, the COD removal rate can be maintained above 80%, and if the COD concentration of the treated effluent is below 200 mg/L, it can be assumed that the biological treatment tank is functioning properly. During the sludge acclimatization period, it is essential to avoid sudden increases in load (such as high water volumes or high concentrations); once the microbial culture has been established, normal operation can proceed. 66. Why should the COD concentration of wastewater in the regulating tank be kept below 700 mg/L? The wastewater from the regulating tank serves as the biochemical feedwater, and its COD concentration is typically determined based on experimental values and design parameters. For wastewater that is easy to treat biologically, the COD level of the wastewater in the regulating tank can generally be kept around 1000 mg/L. In the case of industrial wastewater, especially that which is difficult to biodegrade, the COD level of the wastewater fed into the biological treatment process is usually maintained within the range of 500–800 mg/L. Otherwise, it is difficult to ensure the stable operation of the biochemical system, as well as to guarantee that the wastewater treated by this system meets the specified discharge standards. The COD concentration of the biochemical wastewater from Shanghai Xinyi Bailuda Pharmaceutical Co., Ltd. (700 mg/L) was determined based on experimental values. 67. How much urea should be added to the biochemical tank daily? The appropriate nutritional ratio is: carbon:nitrogen:phosphorus = 100:5:1, calculated based on a carbon-to-nitrogen ratio of 100:5 (by weight); strictly speaking, the carbon referred to here is BOD5. Therefore, if the inflow to the biological treatment tank is 240 tons per day and the BOD5 concentration is 250 mg/L, the daily amount of BOD5 in the influent water will be 240 tons × 0.25 kg/ton = 60 kg. The daily nitrogen requirement is 60 ÷ 100 × 5 = 3 kg. The amount of urea that needs to be added is: 3 × 44 ÷ 14 = 9.4 kg/day. For ease of calculation, we can use the following simplified formula. W = BOD5 × Q × 0.157 ÷ 1000; W = COD × B/C × Q × 0.157 ÷ 1000. Where: COD refers to the COD content in the biochemical wastewater, with the unit being mg/L ； BOD5—refers to the BOD5 in the biochemical influent, with the unit being mg/L ； B/C—is dimensionless ； Q—Is the amount of biochemical wastewater input, in tons per day ； W— represents the daily dosage of urea, in kilograms per day ； Since the wastewater from ************ Company already contains a certain amount of nitrogen, it is not necessary to add urea during operation. 68. How much potassium dihydrogen phosphate should be added to the biochemical tank? Converted at a ratio of 100:1 for carbon to phosphorus (by weight); strictly speaking, the carbon referred to here is BOD5. Therefore, if the inflow to the biological treatment tank is 240 tons per day and the BOD5 concentration is 250 mg/L, the daily amount of BOD5 in the influent water will be 240×0.25 kg/ton = 60 kg. The daily phosphorus requirement is 60÷100 = 0.6 kg. The amount of potassium dihydrogen phosphate that needs to be added is calculated as follows: 0.6×136÷31 = 2.6 kg/day. For ease of calculation, we can use the following simplified formula. W = BOD5 × Q × 0.044 ÷ 1000
W = COD × B/C × Q × 0.044 ÷ 1000
Where: COD refers to the COD in the influent water, measured in mg/L ； BOD5—refers to the BOD5 in the biochemical influent, with the unit being mg/L ； B/C—is dimensionless ； Q—Is the amount of biochemical wastewater input, in tons per day ； W—is the daily dosage of potassium dihydrogen phosphate, expressed in kilograms per day ； 69. What level should the dissolved oxygen in the effluent from the biochemical tank be maintained at? Activated sludge is a method that utilizes the metabolic activities of aerobic microorganisms under nitrogen-rich conditions to oxidize and decompose organic matter in wastewater into inorganic substances. Therefore, the level of dissolved oxygen directly affects the metabolic activity of such microorganisms. To meet the dissolved oxygen requirements of aerobic microorganisms and improve the efficiency of the treatment system, it is necessary to supply oxygen to the system. Although for aerobic microorganisms, a higher level of dissolved oxygen in water is more conducive to their growth and reproduction, excessively high levels of dissolved oxygen not only increase energy consumption, but also cause intense agitation within the tank due to high-speed airflow; this can break up biological flocs and facilitate sludge aging. Generally speaking, a dissolved oxygen level of over 3 mg/L in the aeration tank is sufficient to meet the growth and reproduction needs of microorganisms as well as the requirements of biological treatment. It is advisable to maintain the dissolved oxygen level at around 2 mg/L at the outlet of the aeration tank. The reasons are as follows: If the biochemical process employs the activated sludge method, then the dissolved oxygen level inside the activated sludge flocs should be maintained at 2.0 mg/L or higher. Too low dissolved oxygen can affect the metabolic rate of microorganisms within the flocs, thereby impacting the efficiency of biochemical treatment. If the biochemical process employs the contact oxidation method, then the dissolved oxygen within the biofilm must not be too low, as this could affect the treatment efficiency. 70. How to transfer the excess sludge from the SBR biological tank to the sludge tank? The excess sludge in the SBR biological tank should be regularly discharged into the sludge tank; otherwise, it will affect the proper operation of the SBR biological tank and the quality of the biological treatment effluent. During sludge discharge, first open the pipeline valve between the SBR biological tank and the sludge tank, and use the pressure of the water level in the SBR tank to push the remaining sludge into the sludge tank. After sludge discharge is completed, the valve on the sludge pipe between the SBR tank and the sludge tank should be closed. 71. How is the sludge in the sludge tank dewatered? The main methods for sludge dewatering include vacuum filtration, pressure filtration, centrifugation, and natural drying. Shanghai Xinyi Bailuda Pharmaceutical Co., Ltd. uses the pressure filtration method, employing specialized equipment—plate and frame filter presses—to carry out pressurized filtration of the chemical sludge and excess sludge generated by the system; after dehydration, the moisture content of the sludge typically reaches 80-85%. 72. How do biochemical treatment tanks function in winter? As we know, the optimal temperature range for microorganisms to grow and reproduce is 16–30°C. When the temperature drops below 10°C, the efficiency of wastewater purification significantly decreases. Generally, for every 10°C drop in temperature, the removal rate of COD decreases by 10%. So, how do the biochemical tanks function in winter? One method is to introduce steam into the equalization tank to raise the temperature of the influent for biochemical treatment ； Another approach is to add biological sludge to the biochemical tank in order to increase the sludge concentration and reduce the sludge load; as long as the water temperature can be maintained at 6–7°C, the activated sludge can still perform its purification functions effectively. 73. How should the biological treatment tank operate when there is no production wastewater due to holidays or temporary shutdowns? In ************ Company, it is common to encounter situations where no production wastewater is generated due to holidays or temporary shutdowns; in such cases, we can add domestic sewage to the biological treatment tank or pump in river water, and add a paste made from dry flour cooked to heat, in order to maintain the growth and reproduction of microorganisms. In the biochemical tank, 5–10 kilograms of dry flour can be added per 100 cubic meters of volume, or waste alcohol can be added in proportion; aeration should take place for 4–8 hours per day. 74. What measures should be taken when the biochemical tank is subjected to load shocks and the microorganisms are damaged? During the operation of the biochemical tank, once the microorganisms are subjected to stress due to changes in water volume or concentration, the COD removal efficiency drops sharply; in severe cases, sludge detaches from the biological fillers, causing the effluent to become turbid. At this point, the water supply should be stopped immediately, and powdered activated carbon should be added to the biological treatment tank in order to reduce the sludge load. The dosage of powdered activated carbon is 10 kilograms per 100 m3 of the tank’s volume. Once the sedimentation properties of the sludge have improved, the rapid proliferation method for sludge acclimatization can be employed: domestic wastewater can be added to the biochemical tank, or waste alcohol or wet paste made from cooked dry flour can be used. The addition rate is 5–10 kilograms of dry flour per 100 m3 of the biochemical tank’s volume. Water is introduced after 2–3 days, with the amount of water added increasing day by day until the microorganisms return to normal activity. 75. What should be done when a large number of microorganisms die? When microorganisms are severely damaged, die in large numbers, and rescue efforts prove ineffective, it is necessary to report the situation to the local environmental protection authorities immediately and replace the activated sludge right away. Then identify the cause to prevent similar accidents from occurring again. As long as the declaration is made in a timely manner, wastewater discharged during the updating and acclimatization of sludge can be exempt from fines for pollution discharge. V. Treatment Facilities and Equipment 76. What treatment facilities and equipment does ************ Company’s wastewater treatment plant have? Pre-treatment section: waste acid tank (1), wastewater collection tank (1), pH adjustment tank (1), iron-carbon micro-electrolysis cell (1 set), iron-carbon effluent tank (1), neutralization and sedimentation tank (1). Biochemical section: equalization tanks (2), biological contact oxidation tanks (1), SBR biochemical tanks (2), discharge tank (1), Roots blowers (5). Sludge dewatering section: sludge tank (1), plate and frame filter press (1 set). Auxiliary facilities and equipment: fan room (1), control room (1), laboratory (1), water pumps (9). 77. What are the specifications, models, and structural types of the wastewater treatment facilities and equipment at ************ Company? List of Sewage Treatment Structures
Name, Specifications (Inner Diameter), Structural Type, Quantity, Effective Volume
Acid Waste Tank: 1800*1750*2080, Reinforced Concrete, 1 unit, 5 m³
Wastewater Collection Tank: 1 unit, 20 m³
pH Adjustment Tank: 4200*1750*3200, 1 unit, 20 m³
Iron-Carbon Electrolysis Cell: 4500*3700*2080, 1 set, 20 m³
Iron-Carbon Effluent Tank: 4200*1750*3200, 1 unit, 20 m³
Neutralization and Sedimentation Tank: 1200*3000*3300, 1 unit, 10 m³
Adjustment Tanks: 5250*3700*3200, 2 units, 120 m³ each
Biological Contact Oxidation Tank: 6700*3200*5500, 1 unit, 100 m³
SBR Biological Reactor: 3500*3500*5500, 2 units, 120 m³ each
Discharge Tank: 700*3700*3200, 1 unit, 8 m³
Sludge Tank: 1725*3700*3200, 1 unit, 20 m³
Ventilation Room: 8000*3000, Brick-Concrete, 1 room, 24 m²
Control Room: 2000*3000, 1 room, 6 m²
Laboratory: 2000*3000, 1 room, 6 m²

List of Main Equipment and Materials for Sewage Treatment
Name, Model/Specifications, Material, Quantity, Remarks
Roots Blower SSR80: Standard equipment, 2 units; Power: 5.5 KW, Air volume: 3.34 m³/min
Roots Blower SSR80: Standard equipment, 1 unit; Power: 4.0 KW, Air volume: 3.0 m³/min
Roots Blower SSR50: Standard equipment, 2 units; Power: 3.0 KW, Air volume: 1.14 m³/min
Plate and Frame Filter Press XAY20-650: Standard equipment, 1 unit; Filtration area: 20 m²
Screw Pump G30-1: Standard equipment, 1 unit; Flow rate: 5 m³/hr, Power: 2.2 KW
lift Pumps (for pH adjustment tank and iron-carbon effluent tank): 40FPZ-20A, Standard equipment, 4 units; Flow rate: 11 m³/hr, Power: 2.2 KW
Lift Pumps (for adjustment tanks): WQ7-15-1.1, Standard equipment, 2 units; Flow rate: 7 m³/hr, Power: 1.1 KW
Acid Pump: 40FPZ-20A, Standard equipment, 1 unit; Flow rate: 11 m³/hr, Power: 2.2 KW
Alkali Pump NL50-8: Standard equipment, 2 units; Flow rate: 25 m³/hr, Power: 1.5 KW
Composite Fiber Packing ZV-150-80: Acetified vinylon, 70 m³

VI. Management, Operation, and Maintenance of Power Equipment
78. How to ensure that power equipment remains in good working condition at all times? To achieve good treatment results in a wastewater treatment system, it is necessary to keep all types of equipment in good working condition and maintain their proper technical performance. Proper operation, maintenance, and repair of the equipment are prerequisites for the normal functioning of the wastewater treatment system. With the development of wastewater treatment, the degree of mechanization and automation in wastewater treatment systems is also increasing continuously. More and more equipment is used in these systems, making them increasingly complex. Wastewater treatment systems use not only many devices specific to wastewater treatment but also many general-purpose devices, all of which should be used properly, maintained well, and repaired as needed. All these devices have their own rules regarding operation, handling, maintenance, and repair. Only by operating and maintaining them properly in accordance with the specified conditions and operational rules can the devices be kept in good technical condition. At the same time, during long-term operation, mechanical equipment inevitably experiences wear and tear of its components, misalignment, and a gradual deterioration of its technical condition as a result of friction, high temperatures, moisture, and various chemical effects. This leads to a decline in performance. Therefore, it is necessary to carry out repairs in a precise, timely, rapid, and high-quality manner in order to restore the equipment’s performance and keep it in good working condition. 79. What special equipment is generally used in wastewater treatment systems? Specialized equipment: various types of sewage pumps, sludge pumps, water storage pumps, metering pumps, screw pumps, air compressors, Roots blowers, centrifugal blowers, surface aeration devices, automatic water sampling machines, grille cleaning machines, sand scraping machines, sludge scraping machines, sludge suction machines, sludge concentration and scraping equipment, sludge mixing equipment for digester tanks, biogas boilers, heat exchangers, chemical solution mixing machines, and sludge dewatering machines, etc. 80. What are the common specialized electrical equipment in wastewater treatment systems? Electrical equipment: AC and DC motors, variable-speed motors, starting switchgear, lighting equipment, lightning protection equipment, power transformation and distribution equipment (including cables, indoor wiring and overhead lines, isolating switches, load switches, fuses, a small number of oil-filled switches, voltage transformers, current transformers, power capacitors, circuit breakers, protectors, automatic devices, and grounding devices, etc.). 81. What are the common auxiliary equipment used in wastewater treatment systems? General equipment: electric hoists, centrifuges, incubators, ovens, refrigerators, various manual and electric gate valves, butterfly valves, gate operators, check valves, sprayers for landscaping chemicals, manual and electric lawnmowers, winches, lathes, planers, milling machines, bridge cranes, transport vehicles, etc. 82. What instruments and equipment are commonly used in wastewater treatment systems? Instrumentation and equipment: various balances, common analytical instruments used in laboratories, electromagnetic flowmeters, level gauges, air flowmeters, and dissolved oxygen meters, etc. 83. What are the key points in the management of equipment for wastewater treatment systems? For wastewater treatment systems, there are four key points regarding equipment management: (1) Proper use of equipment – all types of equipment must come with operating procedures that specify the steps to be followed. The equipment operation procedures are primarily developed by combining the manufacturer’s instructions with the actual conditions on site. Workers must follow the operating procedures strictly. Condition records should be kept during the use of the equipment. (2) Proper maintenance of equipment: Maintenance procedures should be established for all types of equipment. These procedures are developed based on the manufacturer’s instructions and the actual conditions at the site; they can also be included along with the operating instructions. The maintenance regulations include tasks such as cleaning, adjustment, tightening, lubrication, and corrosion prevention. Maintenance work should also be recorded. Maintenance work can be divided into: routine maintenance – which refers to inspections and maintenance during operation. Regular inspection and maintenance -- periodic shutdown for inspection and maintenance. Parking maintenance refers to the maintenance of standby generators or equipment that is not in use. Seasonal maintenance refers to the maintenance tasks required by equipment during seasonal changes such as the arrival of summer, winter, or the plum rain season, including measures such as protection from sunlight, cold, moisture, and heat. (3) After repairing the equipment, establish maintenance standards for the main equipment, and restore its technical performance through maintenance. For some equipment, it is necessary to clarify the boundaries between major, medium, and minor repairs, as well as to define the responsibilities for each type of repair. The maintenance schedule for key equipment must be clearly defined, and regular maintenance should be carried out; one should not wait until the equipment is severely damaged before thinking about repairing it. For routine repairs, standard rates for labor and materials should be established to reduce repair costs, and detailed records should be kept for each repair. (4) Managing equipment: The term “management” here refers to the overall oversight of the equipment throughout its life cycle, from purchase and installation, through commissioning and acceptance, to use, maintenance, repair, disposal, and replacement. This includes the financial management of equipment (such as major repair costs and depreciation expenses), with regulations to be established for each stage. 84. What are the standards for the proper condition of equipment in a sewage treatment system? The following criteria can be used as standards for good condition: (1) The equipment operates well, and all its key technical specifications meet the requirements of the original design or the minimum standards necessary for the sewage treatment process. (2) The safety system for operational control is fully equipped, with sensitive and reliable operation. (3) It operates stably, with no abnormal vibrations or noise. (4) The insulation level of electrical equipment and its safety protection devices shall comply with the electrical installation regulations. 　　(5) The equipment is equipped with complete ventilation, heat dissipation and cooling, as well as sound insulation systems that function effectively, keeping the temperature rise within the specified range. (6) The equipment is clean inside and outside, properly lubricated, with no leaks (oil leakage, air leakage, wind leakage, water leakage). (7) Complete operation records and technical documentation. 85. What is the typical maintenance cycle for equipment in wastewater treatment systems? After being in use for a certain period of time, equipment must undergo minor, medium, or major repairs. Some equipment manufacturers specify the intervals for these minor and major repairs ； For devices for which no specific regulations exist, the maintenance interval must be determined based on the complexity of the device, the durability of its vulnerable components, and the maintenance conditions available at the factory. The repair cycle refers to the time interval between two repairs of a piece of equipment; the major repair cycles for various components in the wastewater treatment system are shown in the table. (For reference only) Table: Equipment Name, Major Overhaul (hours), Scheduled Maintenance 1. Centrifugal sewage pump <600 r/min: 40,000 hours, 500 hours 2. Centrifugal sewage pump <800 r/min: 30,000 hours, 500 hours 3. Centrifugal sewage pump <1000 r/min: 20,000 hours, 500 hours 4. Centrifugal sewage pump >1000 r/min: 10,000 hours, 500 hours 5. Sludge pump (>1000 r/min): 8,000 hours, 500 hours 6. Sludge pump (<1000 r/min): 10,000 hours, 500 hours 7. Aeration pump (air lift): 8 years, 1 year 8. Screw pump: 20,000 hours, 500 hours 9. Centrifugal fan: 15,000 hours, 500 hours 10. Sand scraper: 10,000 hours, 500 hours 11. Roots blower: 15,000 hours, 500 hours Question 86: How is equipment management documentation typically organized in a sewage treatment system? Records should be kept of the operation and maintenance of the equipment, as well as of any repairs carried out. Operation and maintenance records are generally kept in the shift work diary, and should include the following details: date of operation, name of the operator, and shift ； Routine maintenance tasks for each shift, as well as the equipment’s daily and cumulative operating time ； Records of lubrication, oiling, and inspection ； Actual technical parameters of the unit ； Contents of regular inspection and maintenance, parking maintenance, and seasonal maintenance ； Fault handling, etc. Records of major, medium, and minor repairs to the equipment should also be kept, and these records should include: unit number, model, rated technical parameters, manufacturer, date of manufacture, and serial number ； Total operating hours before repair ； Repair details ； Repair date and technician ； Equipment medium inspection status ； Spare parts repair and replacement records ； Equipment assembly, clearance adjustment, and legend ； Test run conditions and operating parameters ； Acceptance comments, evaluations by the user unit, repair unit, and supervising department, etc. The above information can be briefly filled in on the equipment registration form. Most equipment is classified as fixed assets. The equipment management department should maintain registration cards for fixed assets in the form of mechanical and electrical equipment, and establish a ledger for such fixed assets. 87. What do the data and symbols represent for the pump with model number 40FYS20 685A? 40 represents the pump inlet diameter (mm) ； F denotes a cantilevered corrosion-resistant centrifugal pump ； Y represents a submersible pump ； S indicates that the components of the pump’s flow-through section are made of reinforced polypropylene ； 20 represents the pump’s design point head (m) ； 685 represents the submersion depth (mm) ； A denotes the main shaft processing classification code; A refers to corrosion-resistant stainless steel ； 88. What are the key points for the operation and management of sewage pumps? 　　A water pump is a mechanical device used to transport water and increase its energy. In wastewater treatment systems, centrifugal pumps are the most commonly used type of water pump, although screw pumps and gear pumps are also employed. (1) The following checks should be carefully performed before starting the pump (especially for pumps that have been newly installed or overhauled) ； Check the direction of rotation of the motor, the concentricity and clearance of the coupling, whether the screws in various parts are loose; rotate the coupling by hand to see if it moves smoothly. Listen for any noises inside the pump, ensure that there is enough lubricating oil in the bearings, and check that nothing is present around the pump and motor that could interfere with its operation. Also, verify whether there is water in the water tank. If there are small pipes for storing water at the bottom of the pumping station on the suction pipe, make sure that the stopcocks on those pipes are properly closed. (2) Close the outlet gate valve and open the inlet gate valve. The total number of operations and the direction of rotation for the gate valve should be known in advance. (3) Driving: When operating the machine, no one should stand beside it. After starting the machine, the water outlet valve should be opened immediately, and close attention should be paid to the sound of the pump as well as its vibration and other operational conditions; if anything abnormal is detected, the machine should be stopped right away for inspection. (4) Turn off the water outlet valve before stopping the machine; this helps to reduce vibration. (5) After parking, wipe the water and oil stains off the surface of the pump and motor. 89. What precautions should generally be taken when a water pump is in operation? (1) Check whether all instruments are functioning properly and stably; pay special attention to whether the ammeter reading exceeds the motor’s rated current. If the current is too high or too low, the pump should be stopped immediately for inspection. 　　(2) Check whether the flow rate of the water pump is normal. If a flow meter is installed, verify whether the flow rate indicated by it is normal; alternatively, the flow rate can be estimated based on the current reading of the ammeter, the flow condition of the water in the outlet pipe, and changes in the water level in the collection well. (4) Check whether the pump stuffing gland pressure plate is hot and whether the dripping is normal. (5) Pay attention to the noise and vibration of the unit. (6) Pay attention to the bearing temperature rise, which should generally not exceed 35°C above the ambient temperature. (7) Check the motor’s temperature rise; if it is too high, stop the machine for inspection. (8) Check whether the water pump and pipes are leaking. (9) Check whether the water level in the sump is too low, and whether the grille or inlet is blocked. 90. What is the six-diligence approach to pump station management that the workers at Shanghai’s pump stations developed through practical experience? (1) Regular inspection: All visual signs that can indicate whether mechanical and electrical equipment is in normal condition, as well as factors such as water level, weather, and environment, fall under the scope of inspection. 1) Check whether the voltage meter reading is within the specified range (±10℃) and whether the three phases are balanced. 2) Check whether the ammeter reading is within the rated range; if the current is too high or too low, identify the cause. 3) Whether the oil level in the oil switch meets the standards, whether there is any oil leakage, and whether the operating mechanism and support structures of the oil switch are in good condition. 4) Check whether the overcurrent relay trips. 5) Is the oil level in the water pump’s oil tank within the standard range, and are there any signs of oil leakage? 6) Whether there is severe leakage from the filler. 7) Is there a large amount of water accumulation on the ground floor of the pump station? 8) Whether the indicator lights are functioning properly. 9) Changes in the water level of the sump well. 10) Check whether there is any discoloration due to overheating at the connections of the casings of various devices. 11) Are the signal devices of the high and low voltage fuses in good condition? 12) Is the metal casing of electrical equipment properly grounded? (2) Listen carefully: Check whether the sounds emitted by the equipment during operation are normal. 1) Verify whether the \"humming\" sound produced by the transformer is normal and whether there are any unusual changes. 2) Is the rotating sound of the motor even and balanced? 3) Are there any abnormal noises from the bearings of the motor and water pump due to cracking or lack of oil? 4) Whether there are sounds of impact from impellers or other debris inside the water pump. 5) Are there any unusual noises from various electromagnetic magnets and pressure-free release coils? (3) Regular inspection for odors: Check whether there is any abnormal burnt smell during equipment operation. 1) Verify whether various types of transformers and their coils emit a burnt smell due to overload or faults. 2) Whether the wire is overheating and producing a burnt smell. 3) Is the water pump packing gland too tight, causing a burnt smell? (4) Regular inspection by touch: All areas and parts that can be touched by hand to determine whether the equipment is in normal condition without the need for special safety measures should be checked (be careful not to touch areas where it is not allowed, to prevent accidents such as electric shock). 1) Check whether the motor casing’s temperature has exceeded the rated value. 2) Is the temperature rise of the water pump oil tank casing normal? 3) Water drips from the pump gland, but it isn’t overheating. 4) Whether the temperature rise of the enclosures of various electrical devices is normal. 5) Motor and water pump bearings (including upper and lower seats ； Is the temperature rise of the intermediate bearing housing normal? 6) Is the temperature rise of the wire wrapped with an insulator normal? 7) Check whether there is excessive vibration or any unusual changes in the motor and transmission components, as well as the water pump. 8) Is the air temperature rise at the motor’s exhaust outlet too high? (5) Be proactive 1) Regularly carry out cleaning and maintenance of mechanical and electrical equipment, as well as cleaning of the surrounding area. 2) When signs of malfunctions are detected, a thorough inspection should be conducted to identify the cause and promptly rectify it, so that the equipment remains in good working condition at all times. (6) Regularly remove debris: Under the premise of taking safety measures, regularly remove debris. 1) Frequently clear debris trapped in the pool’s grates to ensure unobstructed water flow. 2) If debris such as wood is found in the water inlet tank, it should be removed promptly to prevent it from damaging the screen or clogging the pump. 3) Excessive accumulation of debris at the bottom of the water inlet pool will impede smooth water flow, and efforts should be made promptly to remove it. 91. What are the routine maintenance tasks for a sewage treatment system? (1) Clean the pump house and the surface of the units. (2) The water tank and impeller should be inspected regularly, and debris inside the pump should be removed. (3) Pay attention to the oil level of the bearing. Oil quality and temperature: The lubricating oil in bearings should be replaced regularly. The lubricant for the water pump should be replaced after 500 hours of operation, while the bearing grease should be replaced after 2000 hours of operation. (4) Check whether the screws of all components are loose. (5) Regularly inspect and replace the packing. 92. What is commonly known as “pei geng”? Commonly known as “matching filler,” it is the commonly used shaft seal packing. There are three types: (1) it is a filler made by weaving oil-soaked cotton yarns, commonly known as “butter filler”; it is soft, elastic, prone to wear, and has a short service life ； (2) It is a filler made of graphite, lead powder, and asbestos woven together; commonly referred to as \"black powder mixture.\" It has good formability, is wear-resistant and corrosion-resistant, and has a long service life. However, it is relatively expensive and requires stringent installation conditions ； (3) Filler with a cotton core wrapped in soft metal. It is coated with graphite lead powder, making it durable, heat-resistant, and corrosion-resistant; it offers excellent performance, but its price is high. 93. What is commonly known as “butter”? Commonly known as “butter,” it refers to grease, which is a colloidal lubricant synthesized from lubricating oil and thickeners. It is divided into four types: calcium-based greases, sodium-based greases, calcium-sodium based greases, and lithium-based greases. (1) Calcium-based greases have good water resistance but are not resistant to high temperatures; they are suitable for use as oil seals. (2) Sodium-based greases can withstand high temperatures, but they emulsify easily when exposed to water, forming foam; they are suitable for use in bearings. (3) Calcium-sodium-based greases improve water and high-temperature resistance. (4) Lithium-based greases are resistant to high temperatures as well as low temperatures, but they are expensive. 94. Why use “butter”? There are two functions of using butter (grease): the first is to reduce friction and wear on the working surface, thereby improving efficiency and extending its service life. The second is to prevent rust, dissipate heat, reduce noise, and improve sealing in certain situations. 95. How should one pay attention to the dripping at the packing while the water pump is in operation? How to adjust it? While the water pump is in operation, it is necessary to check whether the dripping at the packing area is normal. A small amount of dripping at the packing area is acceptable, but water flowing down from it is not; this can be achieved by adjusting the tightness of the packing gland. 96. How to fill the shaft seal packing of a water pump (replacement)? Be careful not to press the filler plate in completely. When replacing the packing, it is necessary to measure the circumference of the water pump shaft first, then increase its length by about 25 mm; sharp edges should be cut at both ends to create slopes of around 45 degrees. When inserting the filler cylinder, its ends should overlap vertically to prevent leakage. At the same time, the joints of the upper and lower filler layers should be offset by 180 degrees from each other. Generally, 1 to 6 layers of packing are used; the pressure plate should be inserted into the packing barrel to a depth of about 1/3 of its total length. The gaps around the pressure plate and the pump shaft should be as even as possible. Insufficient water filling; the inlet valve is not open for water filling and air release – open the inlet gate. 97. What could be the possible reasons for insufficient water pump flow? How can it be resolved? (1) There is air inside the water pump. Solution: Release air. (2) The seal ring of the water pump is damaged as a separate part. Solution: Replace the seal ring component. (3) Air leakage at the filler. Solution: Apply some butter to the packing and tighten the packing gland. (4) The pump speed is too low; is the voltage too low? Solution: Try to increase the voltage. (5) Clogging of the impeller, inlet, and pipes. Solution: Remove debris. (6) Excessive wear of the impeller. Solution: Replace the impeller and check for bearing wear. (7) The motor rotation direction is incorrect. Solution: Adjust the phase. (8) The centerline of the impeller is not submerged in the liquid. Solution: Adjust the immersion height. 98. What could be the reasons for insufficient head of a water pump? How to solve it? (1) Impeller damage. Solution: Replace with another part. (2) Insufficient rotational speed. Solution: Check the voltage and whether the motor is functioning properly. (3) The infusion contains gas. Solution: Lower the liquid temperature to remove gas. 99. What could be the possible reasons for overheating of the water pump bearings? How to solve it? (1) The spindle and the motor shaft are not aligned. Solution: Adjust the concentricity. (2) The bearing cover is lacking oil or the oil has deteriorated. Solution: Refuel or change the oil. 100. What could be the possible reasons for excessive pump power overload? How to solve it? (1) The specific gravity of the medium is too high. Solution: Replace with a motor of higher power. (2) The flow rate exceeds the allowable range. Method: Work according to the scope of use. (3) Mechanical friction is generated. Solution: Inspect, adjust, or replace the worn components. 101. What could be the possible reasons for vibration or noise in a water pump? How to solve it? (1) The spindle and motor shaft are not concentric. Solution: Adjust the concentricity. (2) Rotor imbalance. Solution: Replace with another part. (3) The nut is loose. Solution: Tighten the nuts at all locations. (4) Operation with gas in the infusion or liquid depletion. Solution: Lower the liquid temperature, remove gases, and submerge the impeller in the liquid. (5) Excessive wear of the water bearing and shaft journal. Solution: Replace the water bearing and restore shaft stiffness. 102. What could be the possible reasons for low liquid output or no output from the water pump? How to solve it? (1) The motor rotation direction is incorrect. Solution: Adjust the phase. (2) The centerline of the impeller is not submerged in the liquid. Solution: Adjust the immersion height. 103. What is a Roots blower? A Roots blower is a type of positive-displacement rotary blower. Its main feature is that, when the pressure is adjusted within the allowable range, the flow rate changes very little; it has a wide range of pressure options and is capable of forcing air to flow. The medium contains no oil during transportation. It features a simple structure, easy maintenance, a long service life, and low vibration levels. Rotary vane blowers are used to transport clean air, clean kerosene, sulfur dioxide, and other inert gases. Thanks to these properties, they can be widely applied in the metallurgy, chemical engineering, fertilizer production, petrochemicals, instrumentation, and building materials industries. 104. What are the operating condition requirements for Roots blowers? (1) The temperature of the inlet gas should not exceed 35°C. (2) The content of solid particles in the gas shall not exceed 100 m3, and the maximum size of these particles shall be no greater than the minimum working clearance specified in Table 2. (3) The coal tar content index of city gas shall comply with the provisions of the TJ28-78 Code for Design of City Gas. (4) The maximum bearing temperature shall not exceed 85°C. (5) The maximum temperature of the lubricating oil shall not exceed 65°C. (6) The pressure indicated by the pressure gauge on the fan must not exceed the pressure range specified on the label; otherwise, the machine must be stopped to check for issues such as blockages in the air inlets of the system, whether all the inlet and outlet valves are closed, and any friction between the moving and stationary parts within the fan, and appropriate measures must be taken. 105. What preparations are needed before starting a Roots blower? (1) Check the installation quality of all fasteners and positioning pins. (2) Check the installation quality of the intake and exhaust pipes, valves, etc. (3) Check whether the assembly clearance of the blower meets the requirements. (4) Inspect the fan and motor. The coaxiality of the fan’s drive shaft must not exceed 0.1 mm; otherwise, it may cause vibration in the fan, oil leakage, and other issues that can affect the fan’s service life. (5) Check whether the base of the component is fully padded all around, and whether the foot bolts are tightened. (6) Before starting the fan, lubricating oil should be added to the main and auxiliary oil tanks until it reaches 3 mm/2 mm above the oil level mark. 68# mechanical oil should be used in summer, while 100# mechanical oil should be used in winter. The lubricating oil should be replaced every three months. (7) Due to the pressure inside the main and auxiliary fuel tanks, the upper and lower fuel plugs of both tanks must be tightened; otherwise, oil leakage will occur. (8) Feed water to the cooling section. The cooling water temperature should not exceed 25°C, and the flow rate of cooling water is 10 L/min. (9) Fully open the inlet and outlet valves of the blower, rotate the rotor, and pay attention to listening for any abnormal noises from various parts. (10) Check the motor’s rotation direction; it must be in accordance with the direction indicated on the direction sign. Otherwise, the fan will not be able to exhaust air properly, and blade collisions may occur. When aeration is carried out in water, water will flow back into the fan, and during pneumatic conveying, the material to be transported may be drawn into the fan, leading to accidents. 106. How do rotary vane blowers operate under no-load conditions and under normal load conditions? (1) No-load test run of the blower. 1) Fans that have been newly installed or overhauled should undergo no-load test runs. 2) Concept of running a Roots blower under no-load conditions: It is operated with all the inlet and outlet valves open. 3) During the trial run, it is necessary to observe whether the splashing of the lubricating oil is normal; if it is excessive or insufficient, the oil volume should be adjusted. 4) There are no abnormal odors or smoking, nor any sounds of collision or friction, and the radial amplitude at the bearing area is not greater than 0.08 mm. 5) Run under no-load conditions for about 30 minutes; if everything is normal, then start operating under load. If abnormal operation is detected, an inspection should be carried out immediately; even after the fault has been resolved, operation at no load is still required. (2) The blower operates under normal load. 1) Adjust it gradually and slowly as required, until the rated load is reached. It is not allowed to reach the rated load immediately. 2) The so-called rated load refers to the static pressure difference between the inlet and outlet indicated on the nameplate. When the exhaust port pressure is normal, attention must be paid to changes in the intake port pressure to avoid overload. 3) Due to the characteristics of the Roots blower, it is not allowed to allow the gas from the exhaust port to flow back directly into the inlet port of the blower for an extended period of time, as this could affect the safety of the machine. 4) During normal operation of the fan, it is strictly prohibited to completely close the intake and exhaust valves, nor should it be operated beyond its capacity. 5) When the blower is operating under its rated conditions, the surface temperature of each rolling bearing generally does not exceed 85°C; the temperature of the lubricating oil in the oil tank generally does not exceed 65°C; and the radial vibration near the bearings does not exceed 0.08 mm. 6) Pay regular attention to the splashing of lubricating oil and the level of oil. (3) The parking blower should not be stopped suddenly under full load; it must be shut down gradually after reducing the load to avoid damaging the machine. Users may establish specific rules regarding emergency shutdown procedures as appropriate. 107. To what level should the oil in the oil tank of the Roots blower unit be filled? It should be added to half of the center line of the fuel gauge. 108. How to start a Roots blower? (1) First, check that the fan’s operating environment is suitable and that the instrumentation and electrical equipment are in good condition (including proper three-phase power supply with no missing phases). (2) Check the fan lubricating oil to see if it is at half the level of the sight glass. (3) Open the vent valve and outlet valve of the fan. (4) Start the fan. (5) After the fan is operating normally, slowly close the vent valve; at this point, the fan’s pressure gauge will rise to 0.04–0.05 Mpa. 109. How to turn off a Roots blower? (1) When stopping aeration, first slowly open the vent valve. (2) Close the outlet valve. (3) Turn off the power. 110. How to maintain and service a Roots blower? The safe operation and service life of the blower depend on proper and regular maintenance, as well as attention to any signs of potential problems. In addition to following general maintenance procedures, special attention should be paid to the following points: (1) Check the tightness of all components and ensure that the positioning pins are not loose. (2) Check whether there is any leakage of water or oil inside the blower housing. (3) There should be no scaling, rusting, or peeling inside the blower casing. (4) Pay attention to whether the lubricating oil is cooling properly, check the quality of the lubricating oil, regularly listen for any abnormal noises during the operation of the blower, and ensure that the unit is not operating under conditions that do not meet the specified requirements. (5) The overload of the blower is not always apparent immediately; therefore, it is necessary to pay attention to the increasing trends in inlet and outlet pressures, bearing temperature, and motor current in order to determine whether the machine is operating properly. (6) Before disassembling the machine, measure all the fitting dimensions of the machine, make proper records, and mark them on other components to ensure proper assembly and maintain the original fitting requirements. (7) For new machines or blowers that have been overhauled, the oil tank should be cleaned, and the machine should be put into operation following the specified procedures; it is recommended to replace all the lubricating oil after 8 hours of operation. (8) Maintenance and repairs should involve the establishment of a reasonable maintenance schedule based on specific usage conditions; these tasks should be carried out on a regular basis with proper recording. Under normal conditions, it is necessary to replace the lubricating oil after the unit has operated for 1,000 hours, to conduct minor repairs or inspections after 4,000 hours, and to perform major repairs annually along with the replacement of relevant wear-prone components. (9) Faults and troubleshooting: The faults and causes associated with Roots blowers involve factors such as operating conditions and usage scenarios; it is difficult to explain these causes and troubleshooting methods in simple terms. Analysis based on the actual situation is required to resolve them. For more details, see the section on “Blower Faults and Troubleshooting Methods”. 111. Blower failures and troubleshooting methods? Fault symptoms, causes, and troubleshooting methods: Insufficient air flow 1. Increased clearance between the impeller and the casing due to wear 2. Changes in the fitting clearance 3. Leaks in the system 1. Replace the worn parts 2. Adjust as required 3. Identify and fix the leak after inspection. Motor overload 1. Changes in system pressure a. Blockage of the inlet filter, or other factors that increase resistance and create negative pressure. b. Increased pressure in the outlet system: 1.a. Resolved after inspection; 1.b. Resolved after inspection. 2. Abnormalities in other components cause: a. Friction between stationary and moving parts; b. Damaged gears; c. Damaged bearings. 2.a. Adjust the clearance; 2.b. Replace the component; 2.c. Replace it. Excessively high temperature: 1. In the machine body: a. Due to an increased pressure ratio of P_out/P_in; b. Due to an increase in the temperature of the inlet gas; c. Friction between stationary and moving parts. 1.a. Resolved after inspection; 1.b. Resolved after inspection; 1.c. Adjust the clearance. 2. Bearings: a. Damaged bearings; b. Insufficient or excessive lubricating oil; c. Excessively high temperature of the lubricating oil or poor quality of the oil. 2.a. Replace them; 2.b. Adjust the amount of oil; 2.c. Refer to point 3 regarding lubricating oil. 3. Lubricating oil: a. Disruption in the cooling water supply or insufficient water volume; b. Abnormal alignment or damage to the gears; c. Damaged bearings; d. Poor quality of the oil. 3.a. Resolve or adjust after inspection; 3.b. Adjust or replace after inspection; 3.c. Replace it; 3.d. Replace it. Friction between impellers: 1. Looseness of the fasteners connecting the gear ring to the gear housing, resulting in an excessive displacement δ1; 2. Wear on the gear surfaces, leading to an increase in the gear clearance and thus changes in the gap between impellers; 3. Loose keys connecting the gears to the impellers; 4. Excessive bending of the drive and driven shafts; 5. Presence of impurities inside the machine or formation of lumps due to the medium; 6. Wear of rolling bearings, resulting in an increased clearance; 7. Operation at pressures exceeding the specified limits. 1. Adjust the clearance, then secure it in place; 2. If the wear is severe, replace the component; if the wear is minor, follow the adjustments outlined in points 4, 3, 2, a; 3. Replace the keys; 4. Straighten the shafts or change the oil; 5. Remove impurities or lumps; 6. Replace the bearings; 7. Identify the cause of overpressure and resolve it. Radial friction between the impeller and the casing: 1. The clearance δ2 is outside the acceptable range; 2. Wear of rolling bearings, resulting in an increased clearance; 3. Excessive bending of the drive and driven shafts; 4. Operation at pressures exceeding the specified limits. 1. Adjust δ2 according to the methods outlined in points 4, 3, 2, b; 2. Replace the bearings; 3. Straighten or replace the shafts; 4. Identify the cause of overpressure and resolve it. Friction between the impeller and the wall panel: 1. The clearances δ3 and δ4 are above the allowable values; 2. Adhesion of impurities or lumps on the end faces of the impeller and the wall panel; 3. Wear of rolling bearings, resulting in an increased clearance. 1. Adjust δ2 and δ4 according to the methods outlined in points 4, 3, 2, c; 2. Remove impurities or lumps; 3. Replace the bearings. Excessive vibration: 1. Low balance accuracy of the rotor; 2. Distortion in the rotor’s balance; 3. Wear or damage to the bearings; 4. Damaged gears; 5. Looseness of foundation bolts or other fasteners. 1. Adjust according to G6.3 standards; 2. Resolve the issue after inspection; 3. Replace the component; 4. Replace it; 5. Tighten the fasteners after inspection. Damaged gears: 1. Operation under overload conditions or exposure to abnormal shocks; 2. Insufficient amount of lubricating oil or poor quality of the oil; 3. Wear of the gears such that the side clearance exceeds 1/3 of the gap between impellers. 1. Replace the gears; 2. Replace them; 3. Replace them. Damaged bearings: 1. Poor quality of the lubricating oil or insufficient supply of oil; 2. Contact with corrosive gases due to a failed gas seal, leading to rapid bearing damage; 3. Long-term operation under overload conditions; 4. Operation beyond the specified service life. 1. Replace the bearings; 2. Repair the gas seal; 3. Replace the bearings; 4. Replace them. Oil leakage at the shaft end of the fan’s drive shaft: 1. Excessive misalignment between the fan’s drive shaft and the motor shaft, leading to accelerated wear of the shaft sleeves and premature failure of the oil seals; 2. Failure of the oil seals; 3. Wear of the shaft sleeves. 1. Adjust it within the allowable range after inspection; 2. Replace it; 3. Replace it. 112. How should pipeline valves be maintained? The sewage treatment plant is equipped with various types of pipes: air pipes, sewage pipes, and sludge pipes. Some of these pipes are installed above the ground while others are buried underground. It is essential to keep these pipes unobstructed during operation; especially in cold weather, it is necessary to regularly inspect the pipes outside and drain the condensation water from the air pipes. For sewage pipes and sludge pipes that are not in use, the water accumulated inside them must also be drained. Ensure that the flowing water pipes are not blocked, that stagnant water pipes are emptied, and that air pipes have their water drained regularly. After installation, the sewage pipes and sludge pipes in the wastewater treatment system need to be insulated, with special attention being paid to insulating the valves to prevent them from being damaged by freezing in winter.