Overview of High-salt Wastewater Treatment Technology
The treatment of high-salt wastewater is a major environmental issue facing industrial development at this stage. Comprehensive utilization is an important way to solve the bottleneck of high-salt wastewater. The application of high-salt wastewater reuse technology is an important guarantee for achieving significant economic, environmental and social benefits. This article is based on the status quo and research progress of high-salt wastewater treatment.
At this stage, large-scale treatment of high-salt wastewater still has the characteristics of low treatment efficiency and high operating costs, and there are still many key technical problems that need to be broken through and resolved. For example, when the forward osmosis method is used to treat high-salt wastewater, the core problems such as forward osmosis membranes and draw fluids are still not well solved; how to increase the water volume of reverse osmosis treatment, how to extend the service life of membranes, and how to effectively prevent membrane pollution The letter is still to be resolved.
Introduction to high-salt wastewater
High-salt wastewater refers to discharged wastewater with a total salt content greater than 1% from domestic sewage and industrial wastewater, containing high inorganic ions such as Cl-, SO42-, Na+, Ca2+, and also containing glycerin, medium and low carbon chains Of organic matter. Because of its complex and diverse components, high salt content, and strong inhibition of microbial growth, the technical difficulty of the wastewater treatment is much greater than that of ordinary wastewater treatment. The amount of high-salt wastewater produced in my country is 5% of the total wastewater, and it is still increasing at a rate of 2% every year. Therefore, the treatment of high-salt wastewater has an important position in wastewater treatment, and it is the focus and difficulty of wastewater treatment research. Currently researched and commonly used methods for high-salt wastewater include evaporation, electrolysis, membrane separation, incineration, and biological methods. High-salt wastewater refers to wastewater whose total salt mass fraction calculated by NaCl content is greater than or equal to 1%. In addition to organic pollutants, this type of wastewater also contains a large amount of soluble inorganic salt ions such as calcium, magnesium, sodium, chlorine and sulfate, and even contains radioactive substances.
The main sources of high-salt wastewater are as follows: (1) Seawater: usually comes from the drainage or cooling circulating water in the industrial water process of coastal cities. (2) Industrial production: The main source of high-salt wastewater is the drainage produced in the production process of printing and dyeing, refining, oil extraction, pharmacy and salt making enterprises. (3) Salty domestic sewage: Mainly comes from the utilization of seawater, which is used for domestic miscellaneous water that does not directly contact the human body, such as fire fighting, washing roads, and toilet flushing in urban life. (4) Groundwater with high salinity: In some areas, the groundwater has high salt content and high total dissolved solid content. For example, some areas of the Inner Mongolia Hetao and some shallow groundwaters in the Hebei Plain have brackish water and saline water.
Application status and advantages and disadvantages of high-salt wastewater treatment technology
Efficient evaporation technology
The high-efficiency evaporation technology of high salt water is generally aimed at high-salt wastewater with a salt content of more than 40,000 mg/L. For low-concentration high-salt water with a salt content of 1% to 4%, the high-efficiency evaporation technology mainly includes: Effective evaporation technology, mechanical vapor recompression technology. Multi-effect evaporation technology refers to the simultaneous use of multiple evaporations in series, the hot steam passes through several evaporations in turn, the hot steam from the previous evaporation enters the latter evaporation, and the evaporation is step by step, effectively using the heat source to achieve the desalination of high-salt wastewater the goal.
Mechanical vapor recompression technology is abbreviated as MVR technology. It is a process that uses a vapor compressor to effectively utilize the heat source. It obtains power through the recompression of steam and continuously reciprocates to improve the heat utilization efficiency of steam. Efficient evaporation technology can successfully separate the salt and water in wastewater, and then treat them separately. It is a relatively thorough method to treat high-salt wastewater. Therefore, this technology is currently widely used in the coal chemical industry, medicine, and pesticide industries. application. However, for the brine with too high content of organic pollutants in the brine, it is very easy to produce foam during the evaporation process to cause flushing. At the same time, it may also affect the quality of the salt, resulting in excessive organic matter in the salt, which needs to be processed continuously.
This process mainly uses microorganisms to oxidize and decompose organic matter. Microorganisms can treat and adsorb harmful organic pollutants. High-salt wastewater can convert a large amount of organic matter into inorganic matter through its degradation. Wastewater is reused in industrial fields through purification. This process has different advantages from other physical and chemical treatment methods. Environmentally friendly and safer. There are many kinds of microorganisms, and the environment that faces various polluted wastewater can have strong adaptability through mutation, and has good metabolism ability. It can produce specific degrading enzymes to treat all kinds of high-salt wastewater, which has great potential. For example, the biological contact oxidation process has the advantages of anti-toxicity, impact resistance, relatively stable microorganisms, strong volume loadability, and ability to maintain the age of sludge. It is very commonly used as a biological desalination technology. The hydraulic retention time is shorter than that of the conventional activated sludge treatment method.
Example: The two-stage contact oxidation process can reduce the concentration of inorganic salts in wastewater to less than 2.5*104mg/L, and can achieve a COD removal rate of 95%. Anaerobic technology and its improvement process utilize the special environmental adaptability of anaerobic bacteria, nitrifying bacteria, halophiles and other microorganisms to the high-salt wastewater to reduce the salt content. They can maintain low water activity in the body in the high-salt water environment. So as to achieve the purpose of reducing COD of high-salt wastewater. According to the data, if the sludge age is about 18 days, the halophilic bacteria can achieve 95% COD treatment rate in the SBR reaction vessel, which is higher than 61% ammonia nitrogen treatment rate. However, the current technology of this method in our country is not perfect, and the technical proficiency is not high. However, the environmental protection and economy of biological desalination will have a good prospect in the future high-salt wastewater treatment.
Membrane treatment technology
Membrane distillation is a new type of water treatment technology. Its characteristic is that it does not require heating and pressure, but only needs to be processed under normal temperature and pressure conditions. Its filter material is a hydrophobic microporous membrane. When the membrane distillation technology is used for water treatment, the gas formed by the volatilization of the volatile substances contained in the liquid to be treated is used to form a pressure difference on both sides of the treatment membrane, and pass through the treatment membrane to finally achieve screening and separation.
Compared with the traditional recovery method, this method is simple to operate, less one-time investment, and has a very high recovery efficiency of concentrated water. Sun Xiangcheng's research shows that membrane distillation technology is stable, and the salt rejection rate is as high as 99%. Nie Yingying and others selected medium-pressure reverse osmosis, high-pressure reverse osmosis, and ultra-high-pressure reverse osmosis as the core processes for high-concentration brine treatment, and determined the medium-pressure reverse osmosis, high-pressure reverse osmosis, and ultra-high-pressure reverse osmosis units through calculations with Dow ROSA software. The structure and membrane element type. Finally, the treatment process of 'regulating tank + high-efficiency sedimentation tank + soda water recoil filter + ultrafiltration + high pressure reverse osmosis + DTRO + evaporation crystallization' is determined. After the system is used for treatment, the high-concentration brine can finally be converted into reuse water, sludge and salt sludge, and zero discharge of the system is realized. The treatment cost of the system is 23.243 yuan per ton of water. Columbia University in the United States developed the use of 'reverse osmosis + membrane distillation (MD)' technology to process concentrated brine for salt recovery. This program is currently in the experimental research stage, and NaCl solution, synthetic seawater, and high salt water are combined through the process. , Showing good stability, compared with traditional technology, the salt quality is very good, and the water recovery rate can reach more than 90%. Polish Marian Turek et al. adopted the 'Electrodialysis (ED) + Evaporative Crystallization' technology. Compared with a single evaporative concentration and crystallization, this combined process reduces the electricity consumption of one ton of salt from 970kW·h to 500kW·h, which is energy-saving. Obviously, this treatment system has been pre-treated before the ED film and evaporative crystallization, adding calcium hydroxide to remove part of the hardness and silicon, in order to facilitate the better work of the ED film.
The high-salt organic wastewater treatment is mainly restricted by the high cost of physical and chemical methods and the large area of the biological method. In particular, the salinity of high-salt wastewater with excessive salinity seriously affects the biological method in the treatment of high-salinity wastewater. application. Therefore, the future research on high-salt organic wastewater treatment technology will mainly focus on the efficient and fast bioreactor for high-salt organic wastewater treatment and its combination of multiple methods. The mechanism research mainly focuses on the salt-reducing mechanism and technological conditions of halophilic bacteria.
With population growth and social and economic development, the demand for water has shown a sharp upward trend. However, the increasingly serious water pollution and deteriorating water quality have made available water resources increasingly scarce. The state has implemented a series of laws and regulations to protect water resources and strictly control the discharge of sewage. Therefore, the search for more economical and effective sewage treatment technology has become an urgent problem for the sustainable and healthy development of society.