About the Overview of High Salt Wastewater Treatment Technology!

Pay more attention to the public account: environmental protection water treatment

High-salt wastewater treatment is a major environmental protection problem faced by 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 to achieve significant economic, environmental and social benefits. This paper discusses the current situation and research progress of high-salt wastewater treatment.

At this stage, the large-scale treatment of high-salt wastewater still has the characteristics of low treatment efficiency and high operating cost, and there are still many key technical problems that need to be broken through and solved. For example, when the forward osmosis method is used to treat high-salt wastewater, the core problems such as the forward osmosis membrane and the absorption liquid are still not well solved, and how to increase the amount of water treated by reverse osmosis, how to extend the service life of the membrane, and how to effectively prevent membrane fouling still need to be solved.

01

Introduction to high-salt wastewater

High-salt wastewater refers to the discharge wastewater from domestic sewage and industrial wastewater with a total salt content of more than 1%, containing high inorganic ions such as Cl-, SO42-, Na+, Ca2+, etc., as well as organic matter such as glycerol and medium and low carbon chains. Due to its complex and diverse composition, high salinity, and strong inhibitory effect on microbial growth, the difficulty of this wastewater treatment technology is much greater than that of ordinary sewage treatment. The amount of high-salt wastewater generated in mainland China accounts for 5% of the total wastewater, and it is still increasing at a rate of 2% per year. Therefore, high-salt wastewater treatment plays an important role in sewage treatment, which is the focus of wastewater treatment research, and it is also a difficult point. At present, the research and commonly used methods of high-salt wastewater include evaporation method, electrolysis method, membrane separation method, incineration method and biological method. High-saline wastewater refers to wastewater with a mass fraction of 1% or more of total salt calculated based on NaCl content. In addition to organic pollutants, this type of wastewater also contains a large number of soluble inorganic salt ions such as calcium, magnesium, sodium, chlorine and sulfate, and even radioactive substances.

The main sources of high-salt wastewater are as follows: (1) Seawater: It usually comes from the drainage or cooling circulating water in the process of industrial water use in coastal cities. (2) Industrial production: The main source of high-salt wastewater is the drainage generated in the production process of printing and dyeing, refining, oil production, pharmaceuticals and salt production. (3) Saline domestic sewage: mainly from the use of seawater, seawater is used in urban life for fire fighting, flushing roads, toilets and other domestic miscellaneous water that is not in direct contact with the human body. (4) Groundwater with high salinity: The groundwater in some areas has a high salt content and a large content of total dissolved solids, such as brackish water and brackish water in some shallow groundwater in some areas of the Hetao of Inner Mongolia and some areas of the Hebei Plain.

02

Analysis of the application status and advantages and disadvantages of high-salt wastewater treatment technology

1. High-efficiency evaporation technology

The high-efficiency evaporation technology of high-brine water is generally for high-salt wastewater with a salt content of more than 40,000 mg/L, and for low-concentration high-brine with a salt content of 1%~4%, the high-efficiency evaporation technology mainly includes: multi-effect evaporation technology and mechanical vapor recompression technology. Multi-effect evaporation technology refers to the use of multiple series evaporation at the same time, the hot steam passes through several evaporations in turn, and the hot steam of the previous evaporation enters the latter evaporation, evaporation step by step, and the effective use of heat source to achieve the purpose of desalination of high-salt wastewater.

Mechanical vapor recompression technology, referred to as MVR technology, is a process that uses steam compressors to effectively utilize heat sources, obtains power through steam recompression, and continuously reciprocates to improve the heat utilization efficiency of steam. The technology of efficient evaporation can successfully separate the salt and water in the wastewater, and then treat them separately, which is a more thorough method for treating high-salt wastewater, so at present, this technology is widely used in the coal chemical industry, medicine and pesticide industries. However, for brine with too high content of organic pollutants in the brine, it is very easy to produce foam during the evaporation process, which may also affect the quality of the salt, resulting in excessive organic matter entrainment in the salt output, and it needs to be further treated.

2. Biological desalination

This process mainly uses microorganisms to oxidize and decompose organic matter. Microorganisms can treat and adsorb harmful organic pollutants, high-salt wastewater can be converted into inorganic matter after its degradation, and wastewater is re-applied to the industrial field through purification, this process method has the advantages of other physical and chemical treatment methods, environmental protection and safety are stronger. There are various types of microorganisms, and the environment of various polluted wastewater can have strong adaptability through mutation, and has good metabolic ability, and can produce specific degradation 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 volumetric loading, and can maintain the age of sludge, so it is very commonly used as a biological desalination technology. Shorter hydraulic retention time than conventional activated sludge treatment methods.

For 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 use anaerobic bacteria, nitrifying bacteria, halophiles and other microorganisms to the special environmental adaptability of high-salt wastewater to achieve the effect of reducing salinity, 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 mud age is about 18 days, the halophilic bacteria can achieve 95% COD treatment rate in the SBR reaction vessel, which is higher than the ammonia nitrogen treatment rate of 61%. However, at present, the process technology of this method in mainland China is not perfect, and the technical proficiency is not high, but the environmental protection and economy of biological desalination will have a good prospect in the future high-salt wastewater treatment.

3. Membrane treatment technology

Membrane distillation is a new type of water treatment technology, which is characterized by no need for heating and pressurization, only needs to be treated under the conditions of room temperature and pressure, and its filter material is a hydrophobic microporous membrane. When membrane distillation technology is used for water treatment, the gas formed by the volatilization of volatile substances contained in the treated liquid is used to form a pressure difference on both sides of the treatment membrane, and through the treatment membrane, and finally the screening and separation is realized.

Compared with traditional recovery methods, this method is simple to operate, has less one-time investment, and is very efficient in recovering brine water. Sun Xiangcheng's research shows that the membrane distillation technology is stable and the desalination rate is as high as 99%. Nie Yingying et al. selected medium-pressure reverse osmosis, high-pressure reverse osmosis and ultra-high-pressure reverse osmosis as the core processes of high-concentration brine treatment, and determined the structure and membrane element types of medium-pressure reverse osmosis, high-pressure reverse osmosis and ultra-high-pressure reverse osmosis units calculated by Dow ROSA software in the United States. Finally, the treatment process of "conditioning tank + high-efficiency sedimentation tank + steam-water backwash filter + ultrafiltration + high-pressure reverse osmosis + DTRO + evaporation crystallization" was determined. After using this system, the high-concentration brine can finally be converted into reuse water, sludge and salt sludge, achieving zero discharge of the system, and the treatment cost of the system per ton of water is 23.243 yuan. Columbia University in the United States developed the use of "reverse osmosis + membrane distillation (MD)" technology to treat concentrated brine for salt recycling, the scheme is now in the experimental research stage, respectively, NaCl solution, synthetic seawater, high salt water through the process combination, showing good stability, compared with the traditional technology, the salt quality is very good, the recovery rate of water can reach more than 90%. Marian Turek et al. in Poland used the technology of "electrodialysis (ED) + evaporation crystallization", the combined process compared with a single evaporation, concentration and crystallization, the power consumption of crystallization of one ton of salt was reduced from 970kW·h to 500kW·h, and the energy-saving effect was obvious.

03

Prospect of future treatment technology for high-salt organic wastewater

The treatment of high-salt organic wastewater is mainly constrained by factors such as high cost of physicochemical treatment and large area of biological method, especially the salinity of high-salt wastewater with high salt content seriously affects the application of biological method in high-salinity wastewater treatment. Therefore, the future research on high-salt organic wastewater treatment process mainly focuses on the bioreactor of high-salt organic wastewater treatment with high efficiency and rapid and the combination of multiple methods. The mechanism research mainly focuses on the salt reduction mechanism and process conditions of halophilic bacteria.

With the growth of population and socio-economic development, the demand for water has shown a sharp upward trend, however, the increasing pollution of water bodies and the deterioration of water quality have made the 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, so finding more economical and effective sewage treatment technologies has become an urgent problem to be solved for the sustainable and healthy development of society.