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Source of high-salt wastewater in chemical production

Generally, for the biochemical treatment of wastewater, high-salt wastewater refers to wastewater containing organic matter and at least 3.5% by mass of total dissolved solids (TDS). Because in this type of wastewater, in addition to containing organic pollutants, it also contains a large amount of soluble inorganic salts, such as Cl-, Na+, SO42-, Ca2+ and the like. Therefore, such wastewater is generally the limit of biochemical treatment. It has been reported that a special domesticated salt-tolerant halophilic bacteria has been used in foreign countries to treat 15% of phenol-containing wastewater containing salt; in China, there are also reports on the use of halophilic bacteria to treat 5% wastewater containing salt. In addition to seawater desalination, this type of wastewater mainly comes from the following fields: 1 chemical production, incomplete chemical reactions or chemical reaction by-products, especially large quantities of high-COD, high-salt toxic wastewater produced during the production of chemical products such as dyes and pesticides. ; 2 wastewater treatment, in the process of wastewater treatment, the mineralization caused by the addition of water treatment agent and acid, alkali, and the concentrate produced by most of the "light" water recovery will increase the concentration of soluble salts, forming the so-called "High salinity wastewater" that is difficult to biochemically treat. It can be seen that such salt-containing wastewater has been more polluting to the environment than ordinary wastewater.
       In the introduction of this article, high-salt wastewater refers to the quality of total dissolved solids (TDS) after the standard discharge water is recovered by using reverse osmosis technology to recover most of the “fresh water”, and the concentrated brine is treated by evaporation or other desalination techniques. More than 8% of concentrated waste liquid that is difficult to biochemical treatment; or high COD content directly produced in the chemical production process, total dissolved solids (TDS) mass fraction greater than 15% and wastewater that cannot be biochemically treated. In order to completely cure the pollution of such high-salt wastewater, it is necessary not only to reduce the content of COD, but more importantly to achieve complete separation of soluble salts from wastewater. Only in this way can we truly achieve the treatment goals of high-salt wastewater.
1. High-salt wastewater from chemical production processes
       Since the 1990s, with the rapid development of China's textile industry, the scale of printing and dyeing industry has expanded rapidly, and the production and use of dyes has become larger and larger. As a result, a large amount of dye wastewater having high COD, high chroma, high toxicity, high salinity, and low B/C is produced. According to statistics, the total amount of dye wastewater produced by the printing and dyeing industry in 2009 has reached 2.43 billion tons, accounting for more than 80% of the total discharge of textile industry wastewater. The dye wastewater has the characteristics of "four highs and one low" and is related to the type of dye used. At the same time, in the production of dyes, the enrichment of salts in the discharged wastewater is mainly caused by the addition of production processes and process aids. For example, in the comprehensive wastewater of a dye factory in Jiangsu, the mass fraction of only the chloride salt is as high as 60g/L. It can be seen that how to effectively treat high-salt and high-pollution printing and dyeing wastewater, realize the separation of chlorine salt from the standard water, and meet the recycling requirements of fresh water resources has become a difficult problem in the treatment of printing and dyeing wastewater.
       In chemical production, a large amount of high-salt wastewater is also produced in the pesticide production process. According to statistics, the national pesticide production plant has reached 1,600, and the annual output of pesticides has reached 476,000 tons. Among them, the production of organophosphorus pesticides accounts for more than 50% of the pesticide industry. The characteristics of the pesticide wastewater are: high organic matter concentration, complex pollution components, high toxicity, difficult to degrade, and unstable water quality. For example, in the production process of the herbicide glyphosate, the concentration of the mother liquor will produce a high concentration of phosphate and sodium chloride wastewater, the COD is about 50,000 mg / L, and the salt content can reach 150 g / L. For such high COD and high salt pesticide wastewater, effective treatment measures must be taken for treatment. Otherwise, it will cause serious environmental pollution.
       In addition, in other chemical production processes, high-salt wastewater will also be produced. For example, in the production of soda ash by the ammonia-alkali process, the soluble salt content of the discharged wastewater of the system after the ammonia treatment is generally 15% to 20%, and most of them are CaCl2 and NaCl. In the coal chemical industry, after the salt-containing wastewater is subjected to a heat concentration process, the salt content of the discharged concentrated wastewater can reach more than 20%. For the high-salt wastewater generated in the chemical process, the properties of high-salt wastewater vary depending on the different chemical products and production processes. Therefore, for various high-salt wastewaters directly produced in chemical production, it is necessary to classify and select the optimal process according to the different sources and properties of high-salt wastewater.
2. High-salt wastewater from chemical wastewater treatment and freshwater recycling process
       In the process of chemical wastewater treatment, the source and composition of wastewater are different, and there are many treatment methods, but the purpose is to reduce the COD content of wastewater and finally recover some “light” water. Therefore, after the COD value of the wastewater treatment reaches the standard, reverse osmosis and other technologies will be further used to recover part of the “light” water for reuse, thereby saving water resources. Throughout the process, the addition of the pretreatment system, the water treatment agent, and the reuse of water all lead to an increase in the salt content of the wastewater and the formation of a high salt water.
       Many industrial wastewaters contain organic/inorganic mixed contaminants, and in some wastewaters even contain contaminants that are detrimental to microbial survival or difficult to biodegrade. Thus, it is necessary to improve the biodegradability of wastewater by physical 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 and form a wastewater with higher salt content. At the same time, desalination pretreatment also produces high-salt wastewater with high salt content.
       In general, methods for reducing wastewater COD can be classified into physicochemical methods and biological methods. Among them, biological methods have the advantages of low cost and are the preferred treatment methods. For the biochemically poor wastewater, the treatment with physicochemical-biochemical coupling technology has become the development trend of today's difficult biochemical wastewater treatment technology. In recent years, various salt-tolerant bacteria used in wastewater treatment have been deeply researched and utilized, so that the salt content of treated wastewater has been improved. Although the salt content in the wastewater should be controlled and should not be too high, the study found that when the salt mass fraction reaches 3.5%, the COD removal rate can reach 60%; meanwhile, when the highest salt content in the wastewater reaches 5%, Biochemical treatment with salt-tolerant bacteria is also effective. It can be seen that with the development of wastewater treatment technologies and processes, especially the combination of physicochemical and biological processes and the development and practice of salt-tolerant strains, the soluble salt content of the discharged water in the wastewater is treated at the same time as the COD meets the standard treatment. A certain degree of improvement has led to the formation of saline.
       It is well known that reverse osmosis membrane technology is a commonly used desalination technique. At present, it is suitable for industrial scale reverse osmosis membranes, mainly including cellulose acetate and polyamide membranes, and has a salt rejection of 99%. The wastewater reaches the discharge standard through physicochemical, biological and other methods. In order to recycle some fresh water resources, reverse osmosis membrane technology is generally used to recover and recycle up to 70% of water. At present, in the actual production process, the water production rate of the reverse osmosis membrane is generally 50% to 60%. Therefore, the qualified discharge water is treated by reverse osmosis technology, and after recycling and recycling 50% to 60% of fresh water, the concentration of discharged wastewater will be more than doubled, thereby producing high-salt wastewater.