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New Trend in Wastewater Treatment: Photovoltaic Wastewater Treatment

New Trend in Wastewater Treatment: Photovoltaic Wastewater Treatment

Introduction As one of the national strategic emerging industries, China's photovoltaic industry has achieved a transition from catching up to surpassing, and has gradually become a global leader in the photovoltaic industry. It is foreseeable that, driven by both policy support and technological innovation, the photovoltaic industry will steadily advance in its gradual maturity. On August 3, the Ministry of Industry and Information Technology (MIIT) released the operating status of China's photovoltaic industry in the first half of 2023. In the first half of 2023, China's photovoltaic industry maintained a generally stable and positive development trend, with the output of major links in the industrial chain achieving rapid growth. According to the information of photovoltaic industry standard announcement enterprises and industry association calculations, the national output of polysilicon, silicon wafers, cells, and components hit new highs, with year-on-year growth exceeding 65%. The total export value of photovoltaic products reached US$28.92 billion, a year-on-year increase of 11.6%. Specifically, in the polysilicon segment, the national output in the first six months exceeded 606,000 tons, a year-on-year increase of 66.1%; in the silicon wafer segment, the national output in the first six months exceeded 253.4 GW, a year-on-year increase of 65.8%; in the cell segment, the national output of crystalline silicon cells in the first six months exceeded 224.5 GW, a year-on-year increase of 65.7%; in the component segment, the national output of crystalline silicon components in the first six months exceeded 204 GW, a year-on-year increase of 65%; exports reached 108 GW, a year-on-year increase of 37.3%. The photovoltaic industry is developing steadily, and the development of the industry must also adapt to people's new expectations and requirements for environmental protection. Therefore, the photovoltaic industry will also become a new outlet for industrial wastewater treatment. 01 Development History of China's Photovoltaic Industry At present, China's photovoltaic industry ranks among the top in the world in terms of manufacturing scale, industrialization technology level, application market expansion, and industrial system construction. It has formed a complete industrial chain ranging from high-purity silicon materials, silicon ingots/silicon rods/silicon wafers, cells/components, photovoltaic auxiliary materials, photovoltaic production equipment to system integration and photovoltaic product applications, and has a solid foundation for advancing towards intelligent photovoltaics. 02 Competitive Landscape of the Photovoltaic Industry 2.1 Regional Competition Data on the cumulative installed capacity of photovoltaics in various provinces and cities in the first half of 2021 released by the National Energy Administration shows that, as of the first half of 2021, Shandong, Hebei, and Jiangsu ranked among the top three in the country in terms of cumulative installed capacity of photovoltaics, with Shandong Province ranking first with a cumulative installed capacity of 26.06 GW. 2.2 Enterprise Competition Longi Green Energy ranked first in the 2020 list of top 10 photovoltaic companies in China. The 2020 list of top 10 photovoltaic companies in China compiled by 365PV has been released. Among them, Longi Green Energy Technology Co., Ltd. ranked first, with an operating income of 32.897 billion yuan in 2019. In addition, GCL-Poly Energy Holdings Limited, JinkoSolar Holding Co., Ltd., Trina Solar Co., Ltd., and Canadian Solar Inc. ranked second, third, fourth, and fifth, respectively. 03 Development Prospects and Trend Predictions of the Photovoltaic Industry 3.1 China's Energy Transformation Strategy The "14th Five-Year Plan" outline proposes to build a clean, low-carbon, safe, and efficient modern energy system. During the "14th Five-Year Plan" period, the proportion of non-fossil energy in total energy consumption will increase to about 20%. Policies continue to promote industry development. 3.2 Distributed Photovoltaic Development Goals Distributed photovoltaic power generation is an inevitable trend in the development and promotion of the photovoltaic power generation industry. Provinces such as Zhejiang, Shandong, Jilin, and Guangdong have included the development of distributed photovoltaics as an important part of promoting energy transformation in their "14th Five-Year Plans". 3.3 Cumulative Installed Capacity of Photovoltaics May Exceed 700 GW in 2026 Under the favorable conditions of policy promotion and the decline in the cost of photovoltaic power generation, the installed capacity of photovoltaics will continue to climb. According to the prediction of the China Photovoltaic Industry Association, during the "14th Five-Year Plan" period, China's annual new installed capacity of photovoltaics may be between 70 and 90 GW. To achieve carbon peaking by 2030 and carbon neutrality by 2060, the photovoltaic industry will become one of the new energy industries that will remain in high-speed development for a long time. Based on this prediction, the cumulative installed capacity of China's photovoltaic power generation industry in 2026 may be between 673 and 793 GW. 04 Overview of the Photovoltaic Industry Chain 05 Photovoltaic Industry Production Process 5.1 Silicon Material → Silicon Wafer 5.2 Silicon Wafer → Cell 1. Texturing: Make the surface of the silicon wafer rough to reduce reflectivity 2. Diffusion: Apply a layer of phosphorus to form a PN junction 3. Etching & Edge Isolation: Remove the PN junction on the side surface to prevent short circuits 4. Back Surface Passivation: Reduce light transmission and improve the photoelectric conversion rate. 5. Coating: Silicon nitride film, reduce reflection, protect the battery from corrosion, etc. 6. Screen Printing: Print metal electrodes 7. Sintering: Sinter electrodes and silicon wafers 8. Sorting: Sort cells with different efficiencies 5.3 Cell → Component 06 Wastewater Treatment in the Photovoltaic Industry With the increasing emphasis on resource utilization and the popularization of solar energy applications, the photovoltaic industry has gradually occupied the market due to the development of solar energy and solar cells. The photovoltaic industry has become a new outlet for industrial wastewater treatment. Wastewater in the photovoltaic industry mainly includes acid washing wastewater, alkali washing wastewater, fluorine-containing wastewater, and organic wastewater. 6.1 Acid & Alkali Washing Wastewater Wastewater Source: The texturing process uses alkali to corrode the surface of the silicon wafer to form a pyramid-shaped morphology, and the process uses strong oxidizing solutions such as chromic acid, nitric acid, hydrofluoric acid, and sulfuric acid for cleaning. 6.2 Fluorine-Containing Wastewater Wastewater Source: Mainly includes hydrofluoric acid and silicon-containing fluorine-containing rinsing wastewater 6.3 Organic Wastewater Wastewater Source: The main organic pollutants come from isopropyl alcohol used in the texturing tank. Water Quality Characteristics: High concentration of organic matter, difficult to degrade. A large amount of wastewater will cause serious pollution to the ecological environment, and it also restricts the continuous and rapid development of industry enterprises. The characteristics of photovoltaic industrial wastewater are strong acid-base corrosiveness and high content of solid suspended matter in the water. The pH value can be less than 2, showing high acidity; high can be greater than 12, showing high alkalinity; and the water contains a large amount of solid suspended matter, the content of solid suspended matter in the water, i.e., SS value, is as high as 6000 mg/L or more. At the same time, due to the high content of high molecular polymer polyethylene glycol in the industrial wastewater, the chemical oxygen demand (COD) of the wastewater is high, and the COD value in the water is as high as 10,000-20,000 mg/L, indicating that the water body is highly polluted by organic matter. Various wastewaters are classified and pre-treated and pH adjusted according to their characteristics. Through treatment, solid suspended matter in industrial wastewater is removed, acid-base corrosiveness is eliminated, and COD content is reduced, so that the pH value, solid suspended matter content, and COD value of the wastewater meet the requirements of subsequent biological treatment systems. Considering the comprehensive water quality characteristics, pretreatment is carried out on acidic wastewater, alkaline wastewater, etc., and a pretreatment process combining natural sedimentation and plate-and-frame filter pressing separation is adopted. Wastewater mixing pH adjustment technology is used to purify the water body and adjust the pH value of the water body to eliminate the influence of wastewater quality differences on wastewater treatment. Different types of wastewater are mixed evenly as needed to improve the water quality of the wastewater, making the wastewater quality stable and reducing the acid-base corrosiveness of the wastewater. Because the wastewater has high salt content and high organic matter content, and considering the difficulty of degrading high polymers in the wastewater, aeration flotation and flocculation sedimentation treatment processes are used for pretreatment before the wastewater enters the biological system. After the wastewater is mixed with other wastewater, physical and chemical flocculation sedimentation is used as a pretreatment measure before entering the biological system. After pretreatment and flocculation sedimentation, the SS value and salt content are greatly reduced, but the concentration of organic matter in the wastewater is still very high, and high polymers are difficult to degrade. In addition, the wastewater is nutritionally unbalanced. Anaerobic bacteria and aerobic bacteria in activated sludge are used to degrade and remove organic matter in wastewater. A multi-stage biological treatment process is used, and the pretreatment combined evaporation and concentration process of photovoltaic wastewater can better reflect the reliability and economy of the technology.

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2023

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08

Salt resource utilization in saline wastewater treatment

Salt resource utilization in saline wastewater treatment

Abstract For many years, China's environmental protection market has focused more on water treatment than solid waste treatment. According to incomplete statistics, the disposal rate of solid waste in China is less than 40% (the accuracy of the data needs further verification). Regarding the destination of solid substances produced during wastewater treatment, the current approach mainly focuses on reduction and harmlessness, resulting in a low resource utilization rate of solid waste. However, resource utilization of solid waste is the ultimate development direction of the solid waste treatment field. For many years, China's environmental protection market has focused more on water treatment than solid waste treatment. According to incomplete statistics, the disposal rate of solid waste in China is less than 40% (the accuracy of the data needs further verification). Regarding the destination of solid substances produced during wastewater treatment, the current approach mainly focuses on reduction and harmlessness, resulting in a low resource utilization rate of solid waste. However, resource utilization of solid waste is the ultimate development direction of the solid waste treatment field. In recent years, many zero-discharge wastewater projects have been implemented across various regions and industries. A major concern arising from these projects is the disposal of the resulting salt. Sending it to solid waste disposal sites is not only expensive (>3000 yuan/ton) but also carries the risk of secondary environmental pollution. Different industries produce different types of salt. This article briefly introduces the resource utilization of sodium chloride and sodium sulfate generated during wastewater treatment. Based on the consulted materials and personal understanding, the following summary is made: 1) Resource utilization direction of sodium sulfate 2) Resource utilization direction of sodium chloride For zero-discharge projects that have already been implemented and produce byproduct salts according to the standards of "Industrial Sodium Chloride from Coal Chemical Industry" (T/CCT002-2019) or "Industrial Salt" (GB/T5462-2015), and "Industrial Sodium Sulfate from Coal Chemical Industry" (T/CCT001-2019), the resource utilization direction can be determined according to the table above. For future new zero-discharge wastewater projects, the focus will be on achieving salt resource utilization during wastewater treatment, as shown in the figure below: Process route for salt resource utilization during zero-discharge wastewater treatment 1) For sodium chloride solution produced after nanofiltration salt separation, with a concentration in the range of 3%-5%, a sodium hypochlorite generator can be directly used to produce sodium hypochlorite (concentration 0.5%-0.8%). The scale of the sodium hypochlorite generator can be calculated based on the amount of sodium hypochlorite used in the entire zero-discharge wastewater system (salt consumption 4.5 kg salt/kg sodium hypochlorite), achieving self-sufficiency of sodium hypochlorite in the system; 2) For sodium chloride and sodium sulfate solution produced by nanofiltration salt separation (total divalent and above divalent cations <1 ppm, which is more difficult), after concentration to a salt solution concentration of 10%-15%, bipolar membranes are used to produce acid and alkali. The concentration of the produced acid is approximately 10%-15%. The treatment scale of the bipolar membrane can be determined according to the consumption of acid and alkali in the entire water treatment system. The energy consumption per ton of water is between 120-180 kWh, and the investment per ton of water is estimated to be between 3-5 million yuan. 3) The above two methods are difficult to scale up to completely achieve resource utilization of all the salt produced by the system. The way to achieve complete resource utilization of all the salt produced by the system is to produce sodium carbonate and ammonium sulfate based on the principles of the Solvay process and the combined soda ash process. The market price of sodium carbonate is above 3500 yuan/ton, and that of ammonium sulfate is around 1000 yuan/ton. There are already operating cases of this method in China, which greatly increases the added value of byproduct salt. However, this method is based on the availability of carbon dioxide resources around the project, such as flue gas from industrial kilns (carbon dioxide concentration between 15%-35%). Otherwise, the source of carbon dioxide will significantly affect the investment and operating costs of the project. The significance of resource utilization of industrial waste salt is significant, mainly in two aspects: it can solve the problem of land occupation caused by landfill and avoid secondary pollution; and it can achieve resource utilization of salt, reducing the mining of mineral resources. With the continuous improvement and perfection of resource utilization technology and equipment for industrial waste salt, the resource utilization of industrial waste salt will become increasingly mature and stable in the future.

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2023

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07

Another Breakthrough | Hozon Think Wins Approval for Grade 2 Environmental Engineering Professional Contracting Qualification

Another Breakthrough | Hozon Think Wins Approval for Grade 2 Environmental Engineering Professional Contracting Qualification

Recently, with the approval of the Beijing Municipal Commission of Housing and Urban-Rural Development, Hezhongsi (Beijing) Environmental Engineering Co., Ltd. (hereinafter referred to as "Hezhongsi") successfully obtained the Grade 2 Environmental Protection Engineering Professional Contracting "Construction Enterprise Qualification Certificate". Hezhongsi closely follows national policies, actively responds to the national advocacy of licensed operations, strictly prohibits illegal operations, strictly implements the regulations on special personnel holding certificates, abides by national construction, environmental protection, production and design industry system standards, constructs high-quality projects, continuously introduces high-level professional talents, improves talent quality, optimizes talent structure, improves enterprise management plans and system standards, strictly adheres to the safety bottom line, and contributes to the standardized and sustainable development of the country's environmental protection cause. "Hezhongsi" Construction Enterprise Qualification Certificate (Grade 2 Environmental Protection Engineering Professional Contracting) Enterprise qualifications are intangible assets and a reflection of comprehensive strength. The acquisition of this certificate marks a further expansion of Hezhongsi's qualification scope. This is a recognition of Hezhongsi's technical level, project management capabilities, and talent team building, laying the foundation for further expanding its business scope and enhancing its core market competitiveness. In the future, Hezhongsi will always adhere to the corporate vision of "being a world-class supplier and operator of zero-discharge and resource utilization solutions for wastewater", taking green, environmentally friendly and energy-saving development as its mission, continuously integrating the upstream and downstream of the environmental protection industry, keeping pace with the times, seizing opportunities, accelerating development, and making unremitting efforts for the national blue sky protection war.

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2023

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07

Common influencing factors in the commissioning of evaporative crystallization systems

Common influencing factors in the commissioning of evaporative crystallization systems

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2023

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Common wastewater treatment technology knowledge points!

Common wastewater treatment technology knowledge points!

I. Chemical Water Treatment 1. Surface water: Refers to water existing on the earth's surface and exposed to the atmosphere. It is the general term for rivers, glaciers, lakes, and marshes, also known as "land water". 2. Groundwater: Water stored in the spaces below the vadose zone (the vadose zone refers to the geological medium below the earth's surface and above the water table), including rock pores, fissures, and caves. Groundwater exists in the cracks of the earth's crust or soil gaps. 3. Raw water: Refers to water collected from nature, including but not limited to groundwater and reservoir water, which is visible in nature and has not undergone any artificial purification treatment. 4. PH: A numerical value indicating the acidity or alkalinity of a solution. pH = -lg[H+], which is the negative value of the common logarithm of the hydrogen ion concentration. 5. Total alkalinity: The total amount of substances in water that can neutralize strong acids. These substances include strong bases, weak bases, and strong base weak acid salts. 6. Phenolphthalein alkalinity: The alkalinity measured using phenolphthalein as an indicator (titration endpoint pH = 8.2～8.4). 7. Methyl orange alkalinity: The alkalinity measured using methyl orange as an indicator (titration endpoint pH = 3.1～4.4). 8. Total acidity: Acidity refers to the total amount of substances in water that can neutralize strong bases, including inorganic acids, organic acids, and strong acid weak base salts. 9. Total hardness: In general natural water, it is mainly Ca2+ and Mg2+. The content of other ions is very low. Usually, the total content of Ca2+ and Mg2+ in water is called the total hardness of water. 10. Temporary hardness: Hardness formed due to the presence of Ca(HCO3)2 and Mg(HCO3)2 in water. The hardness can be removed after boiling. This hardness is called carbonate hardness, also known as temporary hardness. 11. Permanent hardness: Hardness formed due to the presence of salts such as CaSO4 (CaCl2) and MgSO4 (MgCl2) in water. It cannot be removed after boiling. This hardness is called non-carbonate hardness, also known as permanent hardness. 12. Dissolved substances: Exist in the form of simple molecules or ions in water (or other solvents). The particle size is usually only a few tenths to several nanometers, invisible to the naked eye, and there is no Tyndall phenomenon. It cannot be seen with an optical microscope. 13. Colloids: Particle groups formed by the combination of several molecules or ions. The size is usually tens of nanometers to tens of micrometers, invisible to the naked eye, but the Tyndall phenomenon will occur. Small colloidal particles cannot be seen with an optical microscope, while large ones can be seen. 14. Suspended solids: Small particles visible to the naked eye formed by the combination of a large number of molecules or ions. The size is usually tens of micrometers or more. It can be clearly seen with an optical microscope. Suspended solids can settle after a long time of standing. 15. Total dissolved solids: The total amount of ions in water is called total dissolved solids. It is obtained by adding the amounts of all cations and anions obtained from water quality analysis. The unit is mg/L (formerly also PPM). 16. Turbidity: Also known as turbidity. In a technical sense, turbidity is a water quality surrogate parameter used to reflect the content of suspended solids in water. The main suspended solids in water are generally soil. 1 mg of silicon dioxide in 1 L of distilled water is used as the unit of standard turbidity, expressed as 1 PPM. 17. Total dissolved solids: TDS, also known as the total amount of dissolved solids, is measured in milligrams per liter (mg/L), indicating how many milligrams of dissolved solids are dissolved in 1 liter of water. 18. Resistance: According to Ohm's law, under a certain water temperature, the size of the water resistance value R is inversely proportional to the vertical cross-sectional area F of the electrode and directly proportional to the distance L between the electrodes. 19. Conductivity: The strength of the water's ability to conduct electricity is called conductivity S (or conductance). 20. Conductivity: The conductivity of water, which is the reciprocal of the resistance of water, is usually used to indicate the purity of water. 21. Resistivity: The resistivity of water refers to the resistance between the opposite sides of a 1 cm cube of water at a certain temperature. The unit is ohm*centimeter (Ω*CM), which is generally a parameter indicating the quality of high-purity water. 22. Softened water: Refers to water in which the hardness (mainly calcium and magnesium ions) has been removed or reduced to a certain extent. During the softening process, only the hardness is reduced, while the total dissolved solids remain unchanged. 23. Desalted water: Refers to water in which salts (mainly strong electrolytes dissolved in water) have been removed or reduced to a certain extent. Its conductivity is generally 1.0-10.0 μs/cm, resistivity (25℃) 0.1-1000000 Ω.cm, and salt content is 1.5 mg/L. 24. Pure water: Refers to water in which strong electrolytes and weak electrolytes (such as SiO2, CO2, etc.) have been removed or reduced to a certain extent. Its conductivity is generally 1.0-0.1 μs/cm, resistivity 1.0-1000000 Ω.cm. Salt content < 1 mg/L. 25. Ultrapure water: Refers to water in which conductive media have been almost completely removed, and non-dissociated gases, colloids, and organic substances (including bacteria, etc.) have also been removed to a very low extent. Its conductivity is generally 0.1-0.055 μs/cm, resistivity (25℃) > 10 × 1000000 Ω.cm, and salt content < 0.1 mg/L. Ideal pure water (theoretically) has a conductivity of 0.05 μs/cm and a resistivity (25℃) of 18.3 × 1000000 μs/cm. 26. Deoxygenated water: Also known as deoxygenated water, it removes dissolved oxygen from water, generally used for boiler water. 27. Ion exchange: A method of separation using the different ion exchange capacities of exchangeable groups in ion exchangers and various ions in solution. 28. Cation resin: Has acidic groups. In aqueous solution, acidic groups can ionize to generate H+, which can exchange ions with cations in water. 29. Anion resin: Contains basic groups. They ionize in aqueous solution and exchange ions with anions. 30. Inert resin: Has no active groups and no ion exchange effect. The relative density is generally controlled between anion and cation resins. It is used to separate anion and cation resins to avoid cross-contamination during regeneration of anion and cation resins, making regeneration more complete. 31. Microfiltration: MF, also known as microporous filtration, belongs to precision filtration. Microfiltration can filter out micrometer or nanometer-level particles and bacteria in the solution. 32. Ultrafiltration: UF, one of the membrane separation technologies driven by pressure. It aims to separate macromolecules from small molecules. The pore size of the membrane is between 20-1000 Å. 33. Nanofiltration: NF, a pressure-driven membrane separation process between reverse osmosis and ultrafiltration. The pore size of the nanofiltration membrane is around several nanometers. 34. Osmosis: Osmosis is the phenomenon of water molecules diffusing through a semipermeable membrane. It penetrates from a high water molecule area (i.e., a low-concentration solution) into a low water molecule area (i.e., a high-concentration solution). 35. Osmotic pressure: For a semipermeable membrane with different concentrations of aqueous solutions on both sides, the minimum additional pressure applied to the high-concentration side to prevent water from penetrating from the low-concentration side to the high-concentration side is called osmotic pressure. 36. Reverse osmosis: RO, reverse osmosis is to artificially pressurize water from a concentrated solution to a low-concentration solution. The pore size of the RO reverse osmosis membrane is as small as the nanometer level. Under a certain pressure, water molecules can pass through the RO membrane, while inorganic salts, heavy metal ions, organic matter, colloids, bacteria, viruses, and other impurities in the source water cannot pass through the RO membrane. 36. Dialysis: Also known as dialysis. A membrane separation operation driven by concentration difference, using the selective permeability of the membrane to different substances to achieve the separation of substances with different properties. 37. Electrodialysis: ED, when dialysis is carried out under the action of an electric field, the phenomenon that charged solute particles (such as ions) in the solution migrate through the membrane is called electrodialysis. 38. EDI: Also known as continuous electro-deionization technology, it is a pure water production technology that combines ion exchange technology, ion exchange membrane technology, and ion electromigration technology. 39. Recovery rate: The percentage of feed water converted into product water or permeate in the membrane system. 40. Desalination rate: The percentage of the total soluble impurity concentration removed from the system feed water through the reverse osmosis membrane, or the percentage of specific components such as divalent ions or organic matter removed through the nanofiltration membrane. 41. Salt passage rate: The opposite of the desalination rate. It is the percentage of soluble impurities in the feed water that pass through the membrane. Permeate: Purified product water produced by the membrane system. 42. Flux: The flow rate of permeate per unit membrane area, usually expressed in liters per square meter per hour (l/m2h) or gallons per square foot per day (gfd). 43. Product water: The purified aqueous solution, which is the product water of the reverse osmosis or nanofiltration system. 44. Concentrate: The part of the solution that passes through the membrane, such as the concentrated water of the reverse osmosis or nanofiltration system. II. Recirculating Water Treatment 45. Recirculating water: A system that uses water to cool process media is called a cooling water system. 46. Once-through cooling water system: Cooling water only passes through the heat exchange equipment once, and the water is discharged after use. 47. Open recirculating water: Water is used to cool and remove the heat dissipated by the process media or heat exchange equipment. Then, part of the hot water is evaporated by direct contact between the hot water and air, and most of the hot water is cooled and reused. 48. Closed recirculating water system: Also known as a closed-loop recirculating cooling water system. In this system, the cooling water is not immediately discharged after use but is recovered and reused. 49. Cooling tower: A device that uses water as a circulating coolant to absorb heat from a system and discharge it into the atmosphere to reduce the water temperature. It is divided into natural ventilation and mechanical ventilation cooling methods. 50. Water distributor: The return water is evenly distributed to the packing through the water distributor. 51. Packing: The return water passes through the packing to form a water film, increasing the contact area with the air. 52. Water collector: Collects the liquid water carried in part of the evaporated water vapor. 53. Circulation water volume: Refers to the total circulation water volume of the cooling towers in the circulation water system. n50 Water retention volume: The total volume of all water in the circulation water system, equal to the sum of the volume of the water pool and the volume of water in the pipes and water-cooled equipment. 54. Makeup water volume: The water needed to make up for the loss of water in the circulation water system due to evaporation/blowdown/splashing. 55. Side filtration water volume: The amount of water diverted from the circulating cooling water system, treated as required, and then returned to the system. 56. Evaporation water volume: The amount of water lost due to evaporation in the circulating cooling water system during operation. 57. Blowdown water volume: Under the condition of a determined concentration factor, the amount of water that needs to be discharged from the circulating cooling water system. 58. Wind and leakage loss water volume: The amount of water lost due to wind and leakage in the circulating cooling water system during operation. 59. Makeup water volume: The amount of water supplemented to the circulating cooling water system during operation to compensate for the loss of water. 60. Concentration factor: The ratio of the salt concentration of the circulating cooling water to the salt concentration of the makeup water. 61. Heat exchange: The exchange of heat between objects is called heat exchange. There are three basic forms of circulating water heat exchange: heat exchange, convection heat exchange, radiation heat exchange, and evaporation heat exchange. 62. Heat conduction: The phenomenon of heat transfer between different parts of directly contacting objects is called heat conduction. 63. Convection heat exchange: In a fluid, the heat transfer between fluids is mainly due to the movement of the fluid, causing part of the heat in the hot flow to be transferred to the cold fluid. This heat transfer method is called convection heat exchange. 64. Radiation heat exchange: Part of the thermal energy of a high-temperature object is converted into radiant energy and emitted outward in the form of electromagnetic waves. After being received by the receiving object, the radiant energy is converted into thermal energy and absorbed. This method of electromagnetic wave heat transfer is called radiation heat exchange. 65. Evaporation heat exchange: A form of heat exchange through which water molecules evaporate and take away the latent heat of vaporization. 66. Inlet and outlet temperature difference of cooling water: The temperature difference between the inlet and outlet of the cooling tower. 67. Wet-bulb temperature: The air temperature when the water vapor in the air reaches saturation under the same enthalpy air state. 68. Dry-bulb temperature: The temperature measured by a thermometer in ordinary air, which is the temperature often mentioned in our general weather forecast. 69. Physical cleaning: Cleaning debris in the pipeline by the flow rate of water. 70. Chemical cleaning: Using chemical agents to keep the surface of the metal heat exchanger clean and activated, preparing for pre-filming. 71. Pre-filming: Also known as chemical conversion film, it is a type of protective layer on the surface of metal equipment and pipelines, especially for pipelines that have passed acid washing and passivation. The pre-filming method can be used for protection. 72. Corrosion inhibitor: The process of inhibiting or slowing down the corrosion of metals. 73. Scale inhibitor: The process of preventing the formation of deposits on the heating surface of heat exchange equipment using chemical or physical methods. 74. Oxidizing biocide: A biocide with strong oxidizing properties, usually a strong oxidizing agent, which has a strong bactericidal effect on microorganisms in water. 75. Non-oxidizing biocide: It does not kill microorganisms by oxidation but by poisoning specific parts of microorganisms. Therefore, it is not affected by reducing substances in water. 76. Available chlorine: Refers to the amount of chlorine with equivalent oxidizing capacity in chlorine-containing compounds (especially as disinfectants), which can quantitatively represent the disinfection effect. 77. Residual chlorine: Residual chlorine refers to the available chlorine remaining in the water after chlorination disinfection and contact for a certain period of time. 78. Combined chlorine: Refers to the compounds of chlorine and ammonia in water, including NH2Cl, NHCl2, and NHCl3. NHCl2 is more stable and has a good bactericidal effect, also called combined residual chlorine. 79. Free available chlorine: Refers to ClO-, HClO, Cl2, etc. in water. It has a fast bactericidal speed and strong bactericidal power, but it disappears quickly, also called free residual chlorine. 80. Orthophosphate: +5 valent phosphorus in phosphate. 81. Organophosphorus: A compound containing a carbon-phosphorus bond or a phosphorus acid derivative containing an organic group. 82. Total iron: Iron in various states of existence, including all iron elements. 83. Total zinc: Zinc in various states of existence, including all zinc elements. 84. Chemical retention time: The effective time of the chemical agent in the circulating cooling water system. 85. Scaling: Calcium and magnesium bicarbonate dissolved in water decompose when heated, precipitating white precipitates, gradually accumulating and adhering to the container, which is called scaling. 86. Corrosion: Refers to the process of loss and damage (including metals and non-metals) under the action of surrounding media (water, air, acids, bases, salts, solvents, etc.). 87. Biological slime: Viscous substances adhering to the surface of objects, formed by microorganisms and their produced mucus mixed with other organic and inorganic impurities. III. Wastewater Treatment 88. Domestic sewage: Mainly the discharge water produced by various kitchen water, washing water, and toilet water used in human life. It is mostly non-toxic inorganic salts. Domestic sewage contains a lot of nitrogen, phosphorus, and sulfur, and many pathogenic bacteria. 89. Municipal wastewater: The general term for wastewater discharged into the urban sewage system. In the combined sewer system, it also includes industrial wastewater and intercepted rainwater. Municipal wastewater mainly includes domestic sewage and industrial wastewater, which is collected by the urban drainage network and transported to the wastewater treatment plant for treatment. 90. Industrial wastewater: Refers to wastewater, sewage, and waste liquid produced during industrial production, which contains industrial production materials, intermediate products, and products lost with water flow, as well as pollutants produced during production. 91. COD: Chemical oxygen demand, the amount of oxidant consumed in the chemical oxidation process of oxidizable substances in water under specified conditions, expressed as the number of milligrams of oxygen consumed per liter of water sample, usually denoted as COD. 92. BOD: The amount of dissolved oxygen consumed in the process of microorganisms decomposing organic matter in surface water is called '

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2023

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Design and application of zero-discharge technology for high-salt wastewater

Design and application of zero-discharge technology for high-salt wastewater

01 High-salinity wastewater water quality High-salinity wastewater refers to wastewater with significantly higher salinity compared to conventional domestic water and surface water. High-salinity wastewater is mostly wastewater discharged from industrial enterprises. A test of high-salinity wastewater discharged from a certain industrial enterprise found that the salt content in the wastewater liquid reached more than 1%. In addition to salt, the wastewater also contains a considerable amount of organic heavy metals, oils, and some substances with strong radioactivity and hazards. In addition, the TDS in high-salinity wastewater is high, including NaCl, Na2SO4, and both COD and chroma are relatively high, and it contains some impurity ions, such as Mg2+, Ca2+, and NH4+. 02 Process design for zero discharge of high-salinity wastewater 2.1 Salt mixing process technology The salt mixing process is also a commonly used treatment technology in the zero-discharge process design of high-salinity wastewater. This process technology uses ultrafiltration, evaporation crystallization, and salt mixing drying technologies to treat high-salinity wastewater and obtain mixed salts and condensate. First, ultrafiltration membranes are used to simply treat high-salinity wastewater. This process uses the pressure difference between the two sides of the ultrafiltration membrane as the driving force, using the principle of mechanical sieving to separate the solution and substances. Because the ultrafiltration membrane has a very small pore size, it can remove suspended solids and some macromolecules from the wastewater. The wastewater entering the ultrafiltration component, under the influence of the pressure on both sides of the ultrafiltration membrane, the liquid flows out of the ultrafiltration membrane to form ultrafiltration liquid, and another part of the liquid flows out as concentrated liquid. In the ultrafiltration process, a relatively stable and balanced state is mainly presented, which ensures the efficiency and quality of the ultrafiltration process. In addition, in the process of filtering high-salinity wastewater with ultrafiltration membranes, some substances with larger molecules will be removed, such as total silicon, suspended matter, etc.; the remaining small molecules and salts are introduced into the next evaporation crystallization process with the wastewater, thus achieving 95% recovery. Second, in the evaporation crystallization environment, the main task is to carry out mixed salt evaporation crystallization treatment. Because high-salinity wastewater has a relatively high COD content, the ultrafiltration membrane cannot remove all organic substances. During evaporation crystallization, bubbles are easily formed, and defoamers need to be added to ensure that the evaporation process continues. The organic substances affect the boiling point of the solution. If it is suppressed and maintained at the boiling point, the evaporation rate will be greatly reduced or even stop evaporation. Therefore, organic substances will have some adverse effects on the mixed salt evaporation crystallization treatment. After evaporation crystallization treatment, substances such as sodium sulfate and sodium chloride will be dried to form solid mixed salts. At this time, the enterprise needs to package these mixed salts for reuse. The application of this zero-discharge treatment technology for high-salinity wastewater improves the utilization rate of high-salinity wastewater and treats some beneficial substances, resulting in some solid mixed salt waste. However, this technology does not truly achieve zero discharge and needs to be innovatively developed. 2.2 Salt separation process design The salt separation process is based on the mixed salt process to fully separate the solid waste mixed salt to form single-substance crystalline salts, that is, inorganic salts that meet the purity standards, while the wastewater meets the water quality requirements of primary reclaimed water. In this way, both water and crystalline salts can be reused, truly achieving the zero-discharge goal of high-salinity wastewater. Taking specific coal chemical high-salinity wastewater as an example, the salt separation process design scheme is as follows: ultrafiltration-ozone oxidation-nanofiltration-evaporation crystallization-drying. (1) Ultrafiltration membrane pretreatment. After high-salinity wastewater passes through the ultrafiltration membrane, suspended matter, total silicon, and macromolecules are removed; the remaining small molecules and salts enter the evaporation crystallization system with the wastewater. In the ultrafiltration membrane process, the inlet water pressure reaches 1 MPa, the pH value can reach about 8.5～10, and the system is also equipped with an independent cleaning system, and the cleaning operation and ultrafiltration operation can be carried out simultaneously. The water production rate after the ultrafiltration membrane is about 95%, and further ozone catalytic oxidation treatment is required. (2) Ozone catalytic oxidation treatment. The main function of this step is to remove organic substances from high-salinity wastewater to avoid the problem of bubbles preventing evaporation during subsequent evaporation crystallization. Based on the heterogeneous ozone catalytic technology, undegraded organic substances are decomposed. In addition to using ozone as an oxidant for direct oxidation, this system mainly uses the way that ozone produces hydroxyl radicals on the surface of the solid catalyst to remove organic substances and achieve oxidation. In this process, the oxidation-reduction potential can reach more than 2.8 V, which can achieve good oxidation and thoroughly degrade refractory organic substances in wastewater. In the implementation of the ozone catalytic oxidation process, it is mainly divided into four processes: the first is the adjustment pool process, the second is the primary ozone catalytic oxidation process, the third is the secondary ozone catalytic oxidation process, and the fourth is the water release pool process. After this process, the COD concentration of the effluent can be reduced to less than 100 mg/L, the total COD removal rate reaches about 80%, and the chroma is less than 10 times. (3) Nanofiltration membrane system treatment. Commonly used salt separation processes at present include thermal salt separation technology and nanofiltration salt separation technology. Thermal salt separation technology uses the advantages of variable temperature crystallization for salt separation; nanofiltration salt separation technology uses the advantages of selective retention in nanofiltration membranes and the separation and extraction of inorganic salts for salt separation. For example, in some coal chemical wastewater, anions are usually dominated by chloride and sulfate ions, and monovalent cations are mainly sodium ions. After the ozone catalytic oxidation treatment, the effluent enters the nanofiltration membrane stage to treat the separated Na2SO4 and NaCl. In this process, the most important role is played by a translucent membrane, which can selectively retain divalent salts, so that divalent salts and monovalent salts can be separated. At the same time, the nanofiltration membrane is used to divide the water with high salt content into nanofiltration concentrate and effluent, with the effluent mainly composed of sodium chloride and the concentrate mainly composed of sodium sulfate. (4) Sodium chloride and sodium sulfate crystallization treatment. After nanofiltration membrane treatment, the next step is sodium chloride and sodium sulfate crystallization treatment to extract sodium chloride and sodium sulfate crystals. Because the amount of effluent is large, a three-effect evaporation method is needed to evaporate the solution to obtain crystals and form sodium chloride crystals. In this step, the mother liquor is discharged into the original water to improve the purity of sodium chloride crystals; the remaining condensate is used after ion detection. (5) Crystal salt drying treatment. After obtaining sodium sulfate and sodium chloride crystalline salts, they must be dried. After entering their respective dryers, a vacuum target dryer is used to treat sodium sulfate and sodium chloride crystals. The high-vacuum exhaust treatment and steam jacket indirect heating of the materials are used to obtain single-substance crystalline salts. In this process, the temperature must be controlled, and the temperature cannot be too high. If the temperature is too high, the oxidized materials are prone to powder. The crystalline salt is added from the top middle of the dryer, and the stirring action of the rake teeth and the indirect heating of steam are used to vaporize the water in the material, and then the vacuum pump removes the vaporized water to achieve the purpose of drying the crystalline salt. After the high-salinity wastewater zero-discharge process treatment, the water quality of the crystalline salt and condensate must meet industrial use standards. As shown in Table 2, various substances in sodium sulfate crystalline salt and sodium chloride crystalline salt must meet relevant indicators so that they can be put into industrial use. 03 Conclusion In summary, high-salinity wastewater is a type of industrial wastewater discharged by industrial enterprises, which has a great impact on the natural environment. Therefore, high-salinity wastewater needs to be treated to protect the natural environment. High-salinity wastewater zero-discharge process technology has been widely used in many industrial enterprises. Under the application of the zero-discharge concept, the zero-discharge treatment concept of high-salinity wastewater can better realize the recovery and utilization of crystalline salts, mainly using the salt separation process. This article analyzes and discusses the application of high-salinity wastewater zero-discharge process design, especially the implementation process of salt mixing process technology and salt separation process technology, in order to achieve zero-discharge treatment of high-salinity wastewater.

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