1. Sources of high-salt wastewater in chemical production

　　Generally, for wastewater biotreatment, high-salt wastewater refers to wastewater containing organic matter and a total dissolved solids (TDS) mass fraction greater than 3.5%. This is because, in addition to organic pollutants, this type of wastewater also contains a large amount of soluble inorganic salts, such as Cl-, Na+, SO42-, Ca2+, etc. Therefore, this type of wastewater is generally the limit of biotreatment. Besides seawater desalination, other main sources include: ① Chemical production, incomplete chemical reactions or by-products of chemical reactions, especially the large amount of high-COD, high-salt, toxic wastewater produced during the production of dyes, pesticides, and other chemical products; ② Wastewater treatment, the mineralization caused by the addition of water treatment agents and acids and alkalis during wastewater treatment, as well as the concentrated liquid produced by the recovery of most "fresh" water, will increase the concentration of soluble salts, forming so-called "high-salinity wastewater" that is difficult to biotreat. It is evident that this type of saline wastewater poses a greater environmental pollution risk than ordinary wastewater.
　　In this introduction, high-salt wastewater refers to the concentrated brine produced after most of the "fresh water" is recovered from the discharge water using reverse osmosis technology, and then treated by evaporation or other desalination technologies to obtain a total dissolved solids (TDS) mass fraction greater than 8%, which is difficult to biotreat; or wastewater directly produced during chemical production with high COD content, a total dissolved solids (TDS) mass fraction greater than 15%, and which cannot be biotreated. To completely eliminate the pollution of this type of high-salt wastewater, it is necessary not only to reduce its COD content but also, more importantly, to achieve the complete separation of soluble salt substances from the wastewater. Only in this way can it truly be achieved High-salt wastewater treatment Goal.
　　 1.1 High-salt wastewater from chemical production processes
　　Since the 1990s, with the rapid development of China's textile industry, the scale of the dyeing and finishing industry has rapidly expanded, and the production and use of dyes have increased. As a result, a large amount of high-COD, high-chromaticity, high-toxicity, high-salinity, and low B/C dye wastewater has been produced. According to statistics, the total amount of dye wastewater produced by the dyeing and finishing industry in 2009 reached 2.43 billion tons, accounting for more than 80% of the total wastewater discharge from the textile industry. This type of dye wastewater has the characteristics of "four highs and one low," and is related to the types of dyes used. At the same time, in dye production, the enrichment of salts in the discharged wastewater is mainly caused by the production process and the addition of process aids. For example, in the comprehensive wastewater of a certain dye factory in Jiangsu Province, the mass fraction of chloride alone is as high as 60 g/L. It can be seen that how to efficiently treat high-salinity and high-pollution dyeing wastewater, achieve the separation of chloride from the discharge water, and meet the requirements of freshwater resource recycling has become a difficult problem in dyeing wastewater treatment.
　　In chemical production, the pesticide production process also produces a large amount of high-salt wastewater. According to statistics, there are about 1600 pesticide production plants nationwide, with an annual output of 476,000 tons of pesticides. Among them, the production of organophosphorus pesticides accounts for more than 50% of the pesticide industry. The characteristics of this type of pesticide wastewater are: high organic matter concentration, complex pollutant components, high toxicity, difficult degradation, and unstable water quality. For example, in the production process of the herbicide glyphosate, the concentration process will produce high-concentration phosphate and sodium chloride wastewater, with a COD of about 50000 mg/L and a salt content of up to 150 g/L. For this type of high-COD, high-salt pesticide wastewater, effective treatment measures must be adopted. Otherwise, it will cause serious environmental pollution.
　　In addition, high-salt wastewater is also produced in other chemical production processes. For example, in the production of soda ash by the ammonia-soda process, the soluble salt content of the wastewater discharged from the system after ammonia stripping generally reaches 15% to 20%, most of which is CaCl2 and NaCl. In the coal chemical industry, after the thermal concentration process of saline wastewater, the discharged concentrated wastewater has a salt content of more than 20%. For high-salt wastewater produced in chemical processes, due to the different chemical products and production processes, the properties of high-salt wastewater also vary. Therefore, for various high-salt wastewaters directly produced in chemical production, it is necessary to classify and select treatment processes according to the different sources and properties of high-salt wastewater.
　　 1.2 High-salt wastewater from chemical wastewater treatment and freshwater recycling processes
　　In the chemical wastewater treatment process, the sources and compositions of wastewater are different, and there are many treatment methods, but they all aim to reduce the COD content of wastewater and recover some "fresh" water. Therefore, after the COD value of the wastewater reaches the standard, reverse osmosis and other technologies will be further used to recover some "fresh" water for reuse to save water resources. In the entire process, the pretreatment system, the addition of water treatment agents, and the reuse of water all lead to an increase in the salt content of the wastewater and the formation of high-salt water.
　　Many industrial wastewaters contain mixed organic/inorganic pollutants, and some wastewaters even contain pollutants that are not conducive to the survival of microorganisms or are difficult to biodegrade. In this way, it is necessary to improve the biodegradability of wastewater through physicochemical pretreatment. After the wastewater is pretreated, although the content of toxic and refractory substances in the wastewater will be reduced, the addition of various additives will increase the salt content in the wastewater, forming wastewater with higher salt content. At the same time, desalination pretreatment will also produce high-salt wastewater with higher salt content.
　　Generally, methods for reducing wastewater COD can be divided into physical and chemical methods and biological methods. Among them, biological methods have advantages such as low cost and are the preferred treatment method. For wastewater with poor biodegradability, the use of physical-chemical-biological coupled process technology has become a development trend in the treatment of difficult-to-biodegrade wastewater. In recent years, various salt-tolerant bacteria used in wastewater treatment have been deeply studied and utilized, which has enabled the salt content of treated wastewater to be improved to some extent. Although the salt content in wastewater should still be controlled and should not be too high, studies have found that when the salt mass fraction reaches 3.5%, the COD removal rate can reach 60%; at the same time, when the salt content in wastewater reaches 5%, biotreatment using salt-tolerant bacteria is also effective. It can be seen that with the development of wastewater treatment technologies and processes, especially the combined application of physical and chemical methods and biological methods and the research and development and practice of salt-tolerant strains, the soluble salt content in the discharged water will increase to some extent while the COD of the wastewater is treated to meet the standard, leading to the formation of saline water.
　　As is known to all, reverse osmosis membrane technology is a commonly used desalination technology. At present, reverse osmosis membranes suitable for industrial scale mainly include cellulose acetate and polyamide membranes, with a salt rejection rate of 99%. Wastewater is treated by physical, chemical, and biological methods to meet discharge standards. In order to recover and recycle some freshwater resources, reverse osmosis membrane technology is generally used to recover and recycle up to 70% of the water. Currently, in actual production, the water production rate of reverse osmosis membranes is generally 50% to 60%. Therefore, after the qualified discharge water is treated with reverse osmosis technology, and 50% to 60% of the freshwater is recovered and recycled, the salt concentration of the discharged wastewater will be more than doubled, thus producing high-salt wastewater.

2. High-salt wastewater treatment technologies

　　 2.1 Disc tube reverse osmosis (DTRO) technology + evaporation crystallization technology
　　Disc tube reverse osmosis (DTRO) technology is a highly efficient reverse osmosis technology, originating in Germany. Compared to spiral wound reverse osmosis, its high-pressure resistance and anti-pollution characteristics are more pronounced. It can operate economically, effectively, and stably even under conditions of high turbidity, high SDI value, high salinity, and high COD, making it more adaptable High-salt wastewater treatment In China, it is mainly used in landfill leachate treatment, seawater desalination, and brackish water desalination projects.
　　The concentrated brine TDS content of disc tube reverse osmosis DTRO membrane is 100000～150000mg/L after concentration, recovering 70%～80% distilled water. Crystallization technology is used to crystallize the salt into a solid for recycling. The multi-effect evaporation process and steam mechanical recompression process generate secondary steam, which is compressed to increase pressure and temperature, increasing enthalpy. This is then sent to the heating chamber of the evaporator as heating steam, making full use of energy. The produced water undergoes suboptimal grading and is reused in the desalted water treatment and circulating water treatment systems. The DTRO salt rejection rate is 98%～99.8%, and the crystallized dried solids are recycled for resource utilization. It meets the requirements of zero liquid discharge.
　　 2.2 Incineration Technology
　　As mentioned earlier, for wastewater with high COD and high salinity, direct incineration can be used for treatment. Incineration for treating high-salinity wastewater began in the 1950s. This method involves spraying high-salinity wastewater in a mist-like form into a high-temperature combustion furnace, causing the water mist to completely vaporize, and allowing the organic matter in the wastewater to oxidize and decompose into carbon dioxide, water, and a small amount of inorganic ash.
　　Before incinerating high-salinity organic wastewater, suspended solids in the wastewater should be filtered out, or methods such as heating should be used to reduce the viscosity of the wastewater to prevent nozzle clogging and improve waste liquid atomization efficiency. For different types of industrial high-salinity wastewater, acid-base neutralization treatment may also be required to prevent acid corrosion of equipment and fouling due to excessive alkalinity. During the incineration stage, the incineration temperature needs to be determined based on the physical properties of the high-salinity wastewater, and factors such as incineration time and ventilation rate also need to be controlled to achieve better incineration results. In the flue gas treatment stage, since the waste liquid often contains elements such as N, S, and Cl, incineration usually produces polluting gases containing NOx, SOx, and HCl. Therefore, the generated flue gas needs to be purified before it can be discharged.
　　 2.3 Evaporation Concentration - Cooling Crystallization Technology
　　Evaporation concentration - cooling crystallization technology is a process technology that concentrates high-salinity wastewater through evaporation and then cools the concentrated liquid to crystallize and separate soluble salt substances in the high-salinity wastewater. This process can separate some salt substances to obtain crystalline salt compounds, while the mother liquor needs to be returned to the previous evaporation stage for recirculation and evaporation concentration.
　　This technology is suitable for high-salinity wastewater with relatively low COD and salts whose solubility is relatively sensitive to temperature changes. By controlling the crystallization temperature, relatively pure crystalline salts can be obtained. However, when the salts in the wastewater are relatively insensitive to temperature changes, such as when the main salts in the wastewater are chlorides, the efficiency of salt separation using cooling crystallization is very low. In addition, in the cooling crystallization process, a large amount of cooling mother liquor needs to be returned to the front-end process for reheating, evaporation, and concentration. This leads to a long process flow, high energy consumption, and low processing efficiency.
　　 2.4 Evaporation - Thermal Crystallization Technology
　　In the evaporation-thermal crystallization process, high-salinity wastewater is first evaporated and concentrated, and then a rotary thin-film evaporator is used to further heat the concentrated high-salinity wastewater to further evaporate and concentrate it, forming a supersaturated salt solution. By cooling, the temperature of the supersaturated salt solution is lowered to below 40℃, and salt sludge is obtained, thereby achieving the complete separation of soluble salt substances in the high-salinity wastewater. The key equipment is the rotary thin-film evaporator.
　　The innovation of the evaporation-thermal crystallization technology lies in: using a thin-film evaporation method to process the viscous concentrated liquid containing salt, its evaporation efficiency is high, and it is easy to make the concentrated liquid containing salt reach supersaturation, which is conducive to the continuous separation of salt substances from the viscous liquid, thus realizing the continuous separation of salt substances, and there is no mother liquor returning for reheating, resulting in lower energy consumption. Therefore, this technology has no special requirements for the salt substances contained in the high-salinity wastewater, and can achieve efficient and continuous treatment of all high-viscosity, high-salinity wastewater, and can achieve the separation of salt substances. Currently, this technology has been successfully used in the recovery and treatment of acidic high-salinity wastewater.

3. Conclusion

　　For some high-salinity, high-COD wastewater, when using direct incineration, it is necessary to strengthen the control of waste gas pollution. For low-COD, high-salinity wastewater where soluble salts are sensitive to temperature, the evaporation concentration-cooling crystallization technology can be used to separate some soluble salt substances.
　　In comparison, disc tube reverse osmosis technology + evaporation crystallization technology is suitable for treating high-COD, high-salinity wastewater. This technology has no special requirements for the type of soluble salts in high-salinity wastewater, and the higher the salt content, the higher the separation efficiency.
　　In order to fully recover and recycle water resources and reduce the "salinization" pollution of various high-salinity wastewaters on water resources and the salinization damage to soil, the use of highly efficient disc tube reverse osmosis technology + evaporation crystallization technology for the effective treatment of high-salinity wastewater, achieving efficient separation of salt and water to achieve resource recovery and zero discharge goals, is of great significance.