Research Progress of High-Salinity Wastewater Treatment Technology

Abstract

With the continuous expansion of industrial enterprises, a large amount of high-salt wastewater with complex components is produced. Direct discharge will cause great harm to the ecosystem and waste a large amount of potential salt resources. This paper summarizes the source, water quality characteristics, and environmental impact of high-salinity wastewater, and introduces the desalination and treatment technologies of high-salinity wastewater. The desalination technology of high-salinity wastewater mainly includes two processes: concentration and crystallization, obtaining concentrated solution through membrane concentration or thermal concentration and then carrying out crystallization treatment on the concentrated solution, thereby realizing the recovery of salt. The advanced treatment technologies of high-salinity wastewater were analyzed, including physicochemical treatment, biological treatment, and coupling treatment. Catalytic ozonation is one of the most widely used physicochemical technologies for the advanced treatment of high-salinity wastewater. Biological treatment processes operating in the presence of halotolerant bacteria show excellent performance at high salinity. High salinity has a negative impact on the performance of various physicochemical processes and biological treatment technologies. However, high salinity has little effect on the performance of a coupled system designed to treat high-salinity wastewater. In this review, the effect of salinity on the scaling and corrosion of equipment is also illustrated. It is suggested that the research direction of high-salinity wastewater should be to develop new membrane materials and catalysts, develop salt-tolerant microorganisms, explore high-efficiency and energy-saving physico–chemical–biochemical combination processes, improve the treatment efficiency of high-salinity organic wastewater, and reduce treatment costs.

1. Introduction

In many industrial processes such as the chemical coal industry, the processing of agricultural by-products, pharmacology, and papermaking, large amounts of high-salinity wastewater with complex components and large amounts of difficult-to-degrade pollutants are generated. If discharged directly, it not only wastes a large amount of potential salt resources, but it also causes environmental problems such as water mineralization, soil salinization, and water eutrophication [1].

Salt-rich wastewater has a variety of adverse effects on the ecological environment. First, the inorganic salts in high-saline sewage lead to freshwater mineralization, in which the organic matter also leads to eutrophication, and the heavy metal elements accumulate within the food chain, which directly affects the safety of drinking water. Second, salt infiltration into the soil will alter soil structure and permeability, reducing soil water permeability and increasing the likelihood of flooding [2]. In addition, the use of salt water for irrigation leads to irreversible soil salination, retards crop growth and development, significantly reduces agricultural productivity, and has a negative impact on social and economic development.

Wastewater with a high salt content is more difficult to treat than other industrial wastewater. Due to the complex and varied composition of saline wastewater, it is often necessary to choose a suitable pre-treatment solution according to the actual situation. Excessive inorganic salts in the water can be toxic to microorganisms and seriously impair the treatment efficiency, making it difficult to treat high-salinity wastewater with conventional biological technology [3]. Traditional physical and chemical treatment methods are also difficult to generalize due to excessive chemical consumption, equipment corrosion and fouling, and high operating costs. In addition, it is still very difficult to purify the waste salt after desalination. With the zero discharge of highly saline wastewater policy and legislation proposed or tested in recent years, zero discharge is quite hot in the field of industrial wastewater treatment; however, the reality is that companies often find it difficult to bear the high costs and there are difficulties in developing efficient and stable reuse technology. In addition, the problem difficulty of treating and discharging high-saline wastewater has become increasingly evident in recent years, and environmental authorities continue to support the requirements for high-saline wastewater treatment when approving the environmental impact assessments (EIAs) of relevant construction projects. High-salinity wastewater is complicated, and the national and local discharge standards are increasingly strict, which makes it more and more difficult to treat high-salinity wastewater.

This paper provides an overview of the water quality characteristics of saline wastewater discharged from different industrial sectors, summarizes the process characteristics and practical applications of existing technologies for treating saline wastewater, and discusses the performance of different physicochemical, biological, and combined systems in the treatment of flood saline wastewater and explains the effects of increasing salinity on various processes. Finally, suggestions for the future development of high-saline wastewater treatment technologies are presented to assist researchers in this field.

2. Water Quality Characteristics of High-Salinity Wastewater

The chemical composition and concentration of high-salinity wastewater are determined by the discharge source; in particular, the organic matter contained in high-salinity wastewater generated by various industrial processes varies significantly. The inorganic salt components contained therein consist mainly of sulfates and chlorides of metal ions such as calcium, magnesium, potassium, sodium, etc. Table 1 below lists the salinity and main salts of effluents discharged from some industries.

Effluent from tanneries contains high concentrations of total dissolved solids (TDS), suspended solids (SS), ammonia, chromium, organic nitrogen, sulfides, and acid ions [4]. The printing and dyeing industry generates a large amount of saline effluent during dyeing, mercerizing, bleaching, and sizing, which contains a variety of pollutants such as chromium, suspended solids, chlorides, nitrogen, heavy metals, sulfates, and many harmful substances that are difficult to biodegrade [5]. Wastewater from the petroleum industry contains a large number of toxic organic pollutants such as halocarbons, polycyclic aromatic hydrocarbons, and aromatic amines, as well as inorganic pollutants such as mercury and lead [6].

It can be seen from the above that the generation and discharge of high-salt industrial wastewater are large. The unit operation of the reaction process in many industrial departments is complex, and the nature of wastewater is complex. The high content of organic matter, especially aromatic compounds and their derivatives, is manifested by the high concentration of chemical oxygen demand (COD) and biochemical oxygen demand (BOD) in the wastewater. In order to strengthen environmental risk prevention, a series of discharge standards were formulated according to the water quality characteristics of different high-salt industrial wastewater. Table 1 also lists the water pollutant discharge standards for some industrial sectors in China. However, in the implementation of the standards, due to the sewage treatment mode and other reasons, the treatment effect of high-salt wastewater is not ideal, and it is difficult to reach the discharge standard of organic characteristic pollutants in wastewater. It is also a research hotspot to optimize the existing high-salinity wastewater treatment process to make it meet the current discharge standards.

Table 1. Wastewater quality from different industrial processes.

Types of Wastewater	Salt Content (mg/L)	Characteristics of Wastewater Quality	Chinese Discharge Standard of Water Pollutants	References
Tannery wastewater	37,813 ± 32,041	Chromium, salt, organic nitrogen, sulfide, phosphorus, and ammonium	GB 30486-2013 COD ≤ 100 mg/L NH4-N ≤ 25 mg/L	[7,8,9]
Printing and dyeing wastewater	7256 ± 4489	Chromium, SS, chlorides, nitrogen, heavy metals, sulfates, and organic pollutants	GB 4287-2012 COD ≤ 80 mg/L NH4-N ≤ 10 mg/L	[7,10,11]
Petrochemical wastewater	35,026 ± 28,397	Halogenated hydrocarbons, polycyclic aromatic hydrocarbons, aromatic amines, mercury, and lead	GB 31571-2015 COD ≤ 60 mg/L NH4-N ≤ 8 mg/L	[7,9,12]
Pharmaceutical wastewater	35,499 ± 16,478	Ammonia nitrogen, suspended solids, drug residues, drug intermediates, and waste solvents	GB 21904-2008 COD ≤ 120 mg/L NH4-N ≤ 25 mg/L	[7,13,14]

Table 1. Wastewater quality from different industrial processes.

Types of Wastewater	Salt Content (mg/L)	Characteristics of Wastewater Quality	Chinese Discharge Standard of Water Pollutants	References
Tannery wastewater	37,813 ± 32,041	Chromium, salt, organic nitrogen, sulfide, phosphorus, and ammonium	GB 30486-2013 COD ≤ 100 mg/L NH4-N ≤ 25 mg/L	[7,8,9]
Printing and dyeing wastewater	7256 ± 4489	Chromium, SS, chlorides, nitrogen, heavy metals, sulfates, and organic pollutants	GB 4287-2012 COD ≤ 80 mg/L NH4-N ≤ 10 mg/L	[7,10,11]
Petrochemical wastewater	35,026 ± 28,397	Halogenated hydrocarbons, polycyclic aromatic hydrocarbons, aromatic amines, mercury, and lead	GB 31571-2015 COD ≤ 60 mg/L NH4-N ≤ 8 mg/L	[7,9,12]
Pharmaceutical wastewater	35,499 ± 16,478	Ammonia nitrogen, suspended solids, drug residues, drug intermediates, and waste solvents	GB 21904-2008 COD ≤ 120 mg/L NH4-N ≤ 25 mg/L	[7,13,14]

3. Desalination Technology for High-Salinity Wastewater

As one of the most important ways to achieve zero discharge of wastewater, the basic idea of high-salt wastewater desalination technology is to separate salt and water in a cost-effective way and recycle them separately. At present, the widely used desalination technologies for zero discharge and resource utilization of high-salinity wastewater are mainly membrane desalination technology and the thermal concentration method.

3.1. Membrane Technology

Membrane technology uses pressure difference, concentration difference, or potential difference as the driving force and uses the selective permeability of the membrane to trap a large amount of salt in highly saline wastewater [15]. Since it does not require a large amount of thermal energy, it is suitable for the desalination of large-, medium-, and small-sized wastewater with high-salinity. After treatment by membrane technology, the freshwater produced can be reused directly, and the concentrated brine can be vaporized and crystallized to achieve zero discharge. Desalination membrane technologies currently in common use in the industry mainly include nanofiltration (NF), reverse osmosis (RO), forward osmosis (FO), electrodialysis (ED), and membrane distillation (MD). Table 2 lists the research examples of membrane technology in the field of high-salinity wastewater treatment.

Nanofiltration (NF) is a pressure-driven membrane separation technology between ultrafiltration and reverse osmosis [16]. Due to the Donnan effect, the selectivity of the nanofiltration membrane for ions of different sizes and charge densities is obviously different, and the charged or uncharged pollutants can be effectively removed, with the characteristics of a low operating pressure, a large capacity, and a stable treatment effect. Much research has been carried out on the application of nanofiltration membranes in the desalination of highly saline wastewater, particularly in sodium chloride and sodium sulfate systems. Researchers are currently using NF membranes in the advanced treatment of printing and dyeing effluents, textile effluents, and other high-salinity effluents [17,18], which can largely remove salt and organic pollutants from the water.

Reverse osmosis (RO) is a highly efficient membrane separation process driven by pressure difference to separate the solvent from the solution [19]. Reverse osmosis membranes can trap various inorganic ions, colloidal substances, and macromolecular solutes in water, so it is a relatively mature desalination technology at present. The study shows that the reverse osmosis membrane has good performance in terms of its desalination rate, water flow, and organic matter trapping, and the system runs stably, which is conducive to the circulation and reuse of water generated by the system, saving more water resources for businesses [20]. In addition, some studies have shown that the new reverse osmosis membrane has a certain potential in effectively separating oily and saline wastewater [21].

For the treatment of high-salinity wastewater, the main goal of membrane concentration technology is to continuously improve the desalination rate. The main limiting factor of the desalination rate of nanofiltration and reverse osmosis is osmotic pressure. Osmotic pressure is the function of the salinity or organic matter concentration in water. The higher the salinity, the higher the osmotic pressure will be. Under the condition of constant inlet water pressure, the net pressure will decrease and the water yield will decrease. The salt permeation rate is proportional to the salt concentration difference between the positive and negative sides of the reverse osmosis membrane. The higher the salt content of the influent, the greater the concentration difference, and the higher the salt permeation rate, resulting in a decrease in the desalination rate. Therefore, regarding the application of membrane concentration technology in the treatment of high-salinity wastewater, if the salinity is too high, this will cause a poor treatment effect. When the conductivity of the high-salinity wastewater is more than 25,000 us/cm, the membrane flux will decay rapidly, and the membrane scaling phenomenon is serious. Membrane concentration technology has higher requirements for the quality of influent water. In order to alleviate membrane fouling and improve the desalination effect, wastewater pretreatment is often needed before membrane concentration treatment.

Forward osmosis (FO) technology is a membrane separation process in which a forward osmosis membrane is used to separate the high-salinity water from the draw solution without applying external pressure, and the water in the high-salinity water permeates the membrane and spontaneously enters the draw solution by using the difference in osmotic pressure between the salt-containing water and the draw solution on both sides of the membrane, so as to realize the concentration of the high-salinity water. FO has a wide range of potential applications, including seawater desalination, wastewater recycling, industrial wastewater treatment, and power generation. Due to no or low hydraulic pressures, FO has a lower tendency for irreversible fouling and higher cleaning efficiency over RO. Therefore, FO can be used for the desalination of high-salinity wastewater. Fan et al. [22] studied a polyamide composite forward osmosis membrane and studied its continuous forward osmosis desalination performance. The salt rejection rate of the composite membrane was basically stable at about 97.5%, showing good operation reliability. Removal of heavy metals from high-salinity wastewater is also a challenging goal. Zhao et al. [23] studied the performance of a forward osmosis membrane in treating high-salinity wastewater containing heavy metal Ni^{2 +}, and the removal rate of Ni^{2 +} was more than 93%.

Electrodialysis (ED) is the product of the combination of electrochemical process and osmotic diffusion process, and the potential difference is the driving force of the membrane separation process [24]. The ability of electrodialysis to separate monovalent ions allows for its application in the recovery of organic/inorganic matter, heavy metals, and nutrients, which are receiving more and more attention in the resource utilization of saline industrial effluents. Electrodialysis is suitable for the treatment of high-salinity water with a salt content of 500~4000mg/L. Although the conventional electrodialysis method can achieve a better sewage effect in the treatment of highly saline sewage, the sewage quality gradually decreases as the flow rate increases [25]. In recent years, the electrodialysis-method-derived bipolar membrane electrodialysis (BMED) method has the advantages of low running costs, a low feed quality, and wide applicability to salt species. BMED can simultaneously realize the desalination and acid–alkali treatment of saline wastewater without introducing other components, which has more advantages than traditional electrodialysis methods in the resource treatment of industrial saline wastewater [26].

Membrane distillation (MD) is a thermally driven separation process that combines thermal distillation and membrane filtration driven by the vapor pressure difference between the membranes [27]. This vapor pressure difference is caused by the temperature difference between the inlet side and the permeation side of the MD. The utility model has the advantages of a low operating temperature, simple equipment, and a high desalination rate. The study shows that the desalination rate of membrane distillation technology for petrochemical wastewater, coal chemical wastewater, and other typical industrial wastewater is more than 95% [28,29]. Membrane distillation requires little pre-treatment and does not need a pressure difference as the driving force, so its stability is better than reverse osmosis technology. However, the industrialization of membrane distillation technology is limited to some extent due to the high energy consumption and cost of membrane distillation. At present, most of the research on membrane distillation for treating high-salinity wastewater is still at a laboratory scale, and there are few cases of industrial application. Concentration polarization and temperature polarization are also the limitations of the industrial application of membrane distillation. With the process of membrane distillation, the phenomenon of concentration polarization often appears. When the solute concentration on the membrane surface is high to a certain extent, the membrane will be wetted, and the porous hydrophobic membrane will be destroyed so that the membrane flux will be greatly reduced. In the process of membrane distillation, due to the existence of a thermal boundary layer, the temperature of the membrane surface at the feed side is lower than that of the main body of the feed liquid, and the temperature of the membrane surface at the permeate side is higher than that of the main body of the permeate, resulting in temperature polarization. Temperature polarization is an important factor affecting the thermal efficiency of membrane distillation processes, which makes the temperature difference between the two sides of the membrane not be used for the vaporization of the feed liquid, reduces the driving force of the transmembrane mass transfer, and reduces the flux.

Table 2. Performance and experimental conditions of membrane filtration techniques for the treatment of saline wastewater.

Types of Wastewater	Wastewater Quality	Membrane	Operating Conditions	Removal Efficiency	References
Printing and dyeing wastewater	NaCl: 1000–16,000 mg/L Dye: 100–6000 mg/L	NF	Transmembrane Pressure (TMP) = 0.4 MPa T = 25 ℃	Dye: 91.4% NaCl: 95.3%	[17]
Textile wastewater	Electrical Conductivity (EC): 3275 μS/cm COD: 1771 mg/L NH3-N: 10.16 mg/L	NF	TMP = 12 bar pH = 8	EC: 30.90 μS/cm COD: 34.35 mg/L NH3-N: 0	[18]
Coal chemical wastewater	EC: 50,000–65,000 μS/cm Cl−: 12,000mg/L	Two-stage RO	TMP = 6.8–7.2 MPa T = 30 ℃ pH = 9–10	EC: 200 μS/cm	[20]
Oily wastewater	NaCl = 2000 ppm Drop size: 300 nm	UF+RO	TMP = 1.6 MPa T = 24 h	Oil retention rate: 98% NaCl: 89.3%	[21]
Printing and dyeing wastewater	EC: 6500–7500 μS/cm TDS: 3500–4500 mg/L Cl-: 1600–1750 mg/L COD: 130–160 mg/L	ED	U = 80 V Q = 5.2 L/h I = 25 A	TDS: 78.07% Cl-: 88.50%	[25]
Glyphosate wastewater	EC: 23 mS/cm COD: 2800 mg/L Hardness: 570 mg/L	BMED	U = 24 V; V = 2.5 L S = 45 cm2	EC: 1.5 mS/cm COD:364 mg/L Hardness: 0	[26]
Petrochemical wastewater	EC: 82 mS/cm TDS: 84,000 mg/L Total Organic Carbon (TOC): 41 mg/L	Direct contact membrane distillation (DCMD)	Timport = 70 ℃; TInfiltration = 40 ℃	TDS > 99.5% EC: 195 μS/cm TOC: 1.6 mg/L	[28]
Petrochemical wastewater	TOC: 127 ± 6 mg/L EC: 2400 μS/cm	DCMD	Timport = 60 ℃; TInfiltration = 20 ℃	EC: 10 μS/cm; TOC: 8 mg/L	[29]

Table 2. Performance and experimental conditions of membrane filtration techniques for the treatment of saline wastewater.

Types of Wastewater	Wastewater Quality	Membrane	Operating Conditions	Removal Efficiency	References
Printing and dyeing wastewater	NaCl: 1000–16,000 mg/L Dye: 100–6000 mg/L	NF	Transmembrane Pressure (TMP) = 0.4 MPa T = 25 ℃	Dye: 91.4% NaCl: 95.3%	[17]
Textile wastewater	Electrical Conductivity (EC): 3275 μS/cm COD: 1771 mg/L NH3-N: 10.16 mg/L	NF	TMP = 12 bar pH = 8	EC: 30.90 μS/cm COD: 34.35 mg/L NH3-N: 0	[18]
Coal chemical wastewater	EC: 50,000–65,000 μS/cm Cl−: 12,000mg/L	Two-stage RO	TMP = 6.8–7.2 MPa T = 30 ℃ pH = 9–10	EC: 200 μS/cm	[20]
Oily wastewater	NaCl = 2000 ppm Drop size: 300 nm	UF+RO	TMP = 1.6 MPa T = 24 h	Oil retention rate: 98% NaCl: 89.3%	[21]
Printing and dyeing wastewater	EC: 6500–7500 μS/cm TDS: 3500–4500 mg/L Cl-: 1600–1750 mg/L COD: 130–160 mg/L	ED	U = 80 V Q = 5.2 L/h I = 25 A	TDS: 78.07% Cl-: 88.50%	[25]
Glyphosate wastewater	EC: 23 mS/cm COD: 2800 mg/L Hardness: 570 mg/L	BMED	U = 24 V; V = 2.5 L S = 45 cm2	EC: 1.5 mS/cm COD:364 mg/L Hardness: 0	[26]
Petrochemical wastewater	EC: 82 mS/cm TDS: 84,000 mg/L Total Organic Carbon (TOC): 41 mg/L	Direct contact membrane distillation (DCMD)	Timport = 70 ℃; TInfiltration = 40 ℃	TDS > 99.5% EC: 195 μS/cm TOC: 1.6 mg/L	[28]
Petrochemical wastewater	TOC: 127 ± 6 mg/L EC: 2400 μS/cm	DCMD	Timport = 60 ℃; TInfiltration = 20 ℃	EC: 10 μS/cm; TOC: 8 mg/L	[29]

3.2. Thermal Concentration Method

Thermal zero discharge technology developed based on a thermal desalination system is one of the most commonly used desalination technologies for high-salinity wastewater. Thermal concentration technology is suitable for treating wastewater with high TDS and COD up to hundreds of grams per liter, and the ions in the high-salt wastewater are highly concentrated by heating. The thermal concentration process mainly includes multi-stage flash evaporation (MSF), multi-effect evaporation (MED), and mechanical vapor recompression evaporation (MVR) [30].

Multi-stage flash distillation (MSF) refers to the distillation desalination method of evaporating and condensing high-salinity wastewater through multiple distillation chambers in which the temperature and pressure are gradually reduced after heating [31]. Figure 1 shows the MSF process flow. The main disadvantages of this method are low thermodynamic efficiency, high energy consumption, and the phenomenon of the scaling and corrosion of equipment. Therefore, the focus of multi-stage flash research is currently mainly on reducing the energy consumption per unit. In addition, efforts should be made to solve the scaling problem of heat exchangers to further improve the heat transfer efficiency and better apply it to the treatment of highly saline wastewater.

Multi-effect vaporization (MED) is based on single-effect vaporization, where multiple vaporizers are connected in a series to reuse vapor many times [32]. Each vaporization and vaporization process is called an effect and there is a pressure difference between each effect, i.e., vaporization under different pressures. Compared to the single effect, this process can greatly reduce live steam consumption to improve efficiency and reduce operating costs. In theory, the higher the efficiency, the more obvious the energy-saving effect and the less new steam consumed, but the reduction decreases as the efficiency increases, the investment cost increases accordingly, and the economy also decreases. Figure 2 shows the MED process flow.

The mechanical vapor recompression system (MVR) uses a vapor compressor to process secondary vapor to increase the pressure and temperature of the secondary vapor, and the heated vapor can be reused as a heat source for evaporation [33]. MVR can realize the recovery and use of inferior waste heat in the evaporation process and reduce the need for external energy. Compared to MED, the energy consumption is greatly reduced. MVR technology has attracted more and more attention and has been studied in the field of industrial wastewater reuse due to its low energy consumption, compact structure, and high thermal energy efficiency. Qu et al. [34] carried out a pilot-scale experimental study on simulated saline wastewater (EC = 5.27 × 10⁴ μs·cm⁻¹, TDS = 3.46 × 10⁴ mg·L⁻¹). The results showed that the reuse rate of system water reached 85.10%, the desalination rate reached 99.66%, and the concentration multiple reached 6.20. At present, the main shortcomings of MVR technology are that the investment cost is high and scaling easily occurs due to the precipitation of salt in the desalination process. Figure 3 shows the MVR process flow.

3.3. Treatment of High-Salinity Concentrates

A high-salinity concentrated solution will be produced after the desalination of high-salinity wastewater by either membrane technology or the thermal concentration method. The concentrated solution is generally not biodegradable, with a high COD and a large amount of metal ions, and the TDS is between 20,000 mg/L–6000 mg/L. For the concentrated solution produced by nanofiltration and reverse osmosis processes, the COD is usually above 5000 mg/L, the ammonia nitrogen concentration is 100–1000 mg/L, and the conductivity is 40,000–50,000 µs/cm.

At present, the disposal methods of high-salt concentrated solutions can be divided into three categories: one is transfer treatment, that is, the transfer of membrane concentrated solution, mainly landfill treatment; the second is reduction treatment, that is, to reduce the absolute content of membrane concentrate by physical and chemical means; third, harmless treatment shall be carried out, and incineration shall be the main treatment technology. Landfill treatment is common in the treatment of high-salt concentrated solutions. It is a way to bury the concentrate into the soil and adsorb, degrade, and filter out the toxic and harmful substances through the metabolism of soil microorganisms. However, heavy metals and high-salt substances brought by concentrate landfill will inhibit biological activity, reduce microbial degradation efficiency and threaten the soil ecological environment. Incineration is widely used in the treatment of radioactive waste liquid and high-concentration organic waste liquid, which is a kind of reduction and harmless treatment means. Incineration can completely remove pollutants, with high efficiency and less land occupation, but the investment in the early stage of incineration treatment is relatively large, the incineration treatment is complex, and the technical requirements for operators are high.

On the other hand, how to recover salt from the concentrated solution after desalination and the high-salt wastewater after advanced treatment and how to realize the wastewater resource treatment is also one of the key research directions. Crystallization technology is an important process for recovering salt from high-salt wastewater and is also a key technology for the zero discharge of wastewater. After desalination and advanced treatment, high-salinity wastewater is solidified by a crystallization process to realize the final solid–liquid separation. In recent years, quality-differentiated crystallization technology has been widely used. Compared with evaporation crystallization technology, it not only improves the reuse rate of water, but also separates mixed crystallization salt, and obtains industrial-grade salt products with resource utilization. In the process of practical application, it is necessary to analyze the advantages and disadvantages among many combinations of evaporation and crystallization processes and determine the most suitable treatment scheme in combination with the water quality characteristics of high-salinity wastewater, the scale technology of the desalination project, the safety of investment management, and the climatic and geographical conditions of the plant. China has adopted the process route of “tubular microfiltration-multi-stage reverse osmosis-multi-stage electrically driven ionic membrane-evaporation and crystallization of nitrate-evaporation and crystallization of salt” to treat the high-salinity wastewater from pulverized coal gasification. Through the pilot project, the mass fraction of Na₂SO₄ obtained is more than 96%, the mass fraction of NaCl is more than 98%, and the mixed salt only accounts for less than 5% of the total salt, thus successfully realizing the resource utilization of inorganic salt crystallization by quality [35].

4. Treatment Technology of High-Salinity Wastewater

The ideal treatment method for high-salinity wastewater is to remove organic matter first and then separate salt from the water, which cannot only realize the recycling of water, but also obtain solid salt with high purity, thus realizing zero emissions. The treatment technology of high-saline wastewater is mainly aimed at removing COD and other organic pollutants in water, and the main research content is how to reduce the organic pollutants in water in a salt-rich environment. At present, the high-saline wastewater treatment technologies commonly used in industry include physicochemical treatment and biological treatment. Physicochemical treatment absorbs or decomposes pollutants in wastewater mainly through physical and chemical reactions, which has the advantage of a rapid reaction. Among them, advanced oxidation technology has been widely studied in the field of high-salinity wastewater treatment in recent years because of its characteristics such as a rapid reaction, a thorough reaction, no secondary pollution, a wide application range, inducing chain reactions, and a simple reaction principle. Advanced oxidation technologies degrade and mineralize organic matter by generating hydroxyl radicals. Biological treatment decomposes pollutants in wastewater through microbial reproduction and metabolism, which has the advantages of economy, high efficiency, a reliable process, and no secondary pollution. However, too high a salt content inhibits the growth of microorganisms in the water, which is also one of the difficulties in the biotechnological treatment of highly saline wastewater. It is reported that the acclimation method of gradually increasing the salt concentration of the system is used for the salt tolerance acclimation of sludge, and the acclimated sludge can well degrade organic matter under high-salinity [36]. In addition, the addition of halophilic microorganisms can accelerate the succession of microbial community structure, thus shortening the process of sludge domestication [37]. Bioaugmentation by adding complex bacteria has become a hot spot in the treatment of high-salinity wastewater. Therefore, the basis of the biological treatment of saline wastewater lies in the cultivation and domestication of halophilic microorganisms [38]. At present, advanced oxidation technology and biological treatment technology are widely used in the treatment of high-salinity wastewater.

4.1. Advanced Oxidation Processes

Advanced oxidation features high oxidation efficiency, a fast reaction speed, and thorough oxidation of organic compounds, so it has been widely studied in the field of high-salinity wastewater treatment in recent years. Advanced oxidation uses the strong oxidizability of OH· to break down organic compounds. According to different types and conditions of OH· production in the reaction process, it can be divided into Fenton, photocatalysis, electrochemical oxidation, ozonation, etc. [39]. Table 3 lists the research examples of advanced oxidation process in the field of high-salinity wastewater treatment.

Fenton oxidation was first discovered by H. J. H. Fenton in 1894. The reaction mechanism is that H₂O₂ reacts with Fe²⁺ under acidic conditions to form strongly oxidizing hydroxyl radicals as shown in Equation (1) [40]. Hydroxyl radicals can quickly and non-selectively degrade most stubborn organic pollutants into carbon dioxide and water with a simple operation and fast reaction. Yi et al. [41] use activated carbon adsorption Fenton coupled oxidation processes to treat chemical wastewater. Under optimal conditions, the maximum removal rate of COD for 30 min can reach 84.4%. However, since the optimum ratio of hydrogen peroxide to ferrous sulfate in the Fenton process is influenced by several factors and the conditions of use are limited to acidic conditions, a large amount of ferrous sludge is generated, which is difficult to handle in the large-scale application of Fenton.

Fe²⁺ + H₂O₂ + H⁺→Fe³⁺ + H₂O + ∙OH·

(1)

Electrochemical oxidation technology directly decomposes organic matter through anodic reactions or generates hydroxyl radicals through anodic reactions. The results show that high-salinity wastewater has high electrical conductivity because it contains a large number of anions and cations that can promote the removal of organics by electrochemical oxidation [42]. Darvishmotevalli et al. [43] used response surface methodology (RSM) to determine the best parameters for the removal of COD and TOC in saline effluents by electrochemical oxidation. Under optimal conditions, the removal rates of COD and TOC in the effluent were 91.78% and 68.49%, respectively. However, due to plate loss and other problems, the electrochemical oxidation method has poor treatment stability and high energy consumption, and there is still some distance to realizing industrialization.

Ozonation mainly uses catalysts to catalyze ozone depletion, resulting in large numbers of highly oxidizing OH· and non-selective degradation of organic pollutants in water [44]. Catalytic ozonation is mainly divided into homogeneous ozonation and heterogeneous ozonation. Compared with homogeneous ozonation, heterogeneous ozonation has the advantages of strong mineralization, a good treatment effect, low cost, low energy consumption, easy operation, a wide scope of application, and no secondary pollution. It is often used to break down pollutants in water. Many metal oxides have been applied in the heterogeneous catalytic ozonation process, which are catalytically active in the ozonation system, including manganese oxides, iron oxides/oxyhydroxide, aluminum oxides, and bimetallic/polymetallic oxides. Ahmadi et al. [45] prepared an iron-oxide-impregnated carbon (PAC@Fe₃O₄) magnetic catalyst and carried out a laboratory-scale study on its catalytic ozonation ability for high-salinity petrochemical wastewater. After 120 min of reaction under optimal operating conditions, the removal rates of COD and TOC reached 75.3% and 50.3%, respectively. In addition, one of the key parameters for the application of heterogeneous catalytic ozonation systems in wastewater treatment is the recyclability and stability of the catalyst. In general, after repeated use of the catalyst, the catalytic effect will be reduced due to the consumption of surface reaction sites, metal leaching, and the reduction in mechanical strength. Therefore, developing new materials as catalysts and improving the mechanical strength and stability of catalysts are one of the research directions of catalytic ozonation technology in the future.

Photocatalysis is a new advanced oxidation technology of recent years that uses semiconductor catalysts such as TiO₂ to generate hydroxyl radicals with high reaction activity under light conditions to decompose organic compounds in water. It is an environmentally friendly, economical, and efficient technology, widely used in solar cells, water decomposition, pollution removal, and other fields. Wen et al. [46] studied the performance of photocatalytic oxidation and its improved technology in the treatment of high-salinity pesticide effluents. Under optimal conditions, the removal efficiencies of COD and ammoniacal nitrogen in atrazine effluent by activated carbon (AC) catalytic UV/O₃ were 70.9% and 87.7%, respectively. At present, the related research is still in the laboratory stage due to the shortcomings of the photocatalytic process, such as high catalyst cost, low light-use efficiency, more toxic intermediates, and difficulties in catalyst recovery.

A large number of studies have found that high-salt wastewater contains a large number of inorganic anions, which can quench hydroxyl radicals and other active oxygen species and generate some by-products to reduce the treatment effect. Therefore, further research is needed to understand the effect of salinity on the quenching and formation of oxidative free radicals in order to provide a new direction for the treatment of highly saline wastewater.

Table 3. Performance and experimental conditions of the advanced oxidation process for the treatment of saline wastewater.

Types of Wastewater	Technology	Wastewater Quality	Operating Conditions	Removal Efficiency	References
Chemical wastewater	Activated carbon adsorption—Fenton	TDS: 20% COD: 13,650 mg/L	pH = 6 FeSO4 = 3.0 g/L H2O2 = 20 mL/L T: 30 min	COD: 84.4%	[41]
Synthetic wastewater	Electrochemical oxidation	TOC: 2000 mg/L COD: 3500 mg/L TDS: 30.94 g/L	pH = 7.69 T = 30 min U = 7.41 V	COD: 91.78% TOC: 68.49%	[43]
Petrochemical wastewater	Catalytic ozonation	COD = 362 ± 36 mg/L TOC: 42 mg/L EC: 59.9 ms/cm	Catalyst dosing: 0.45g/L pH = 7.2 Ozone dosing: 0.3 g/h	COD: 75.3% TOC: 50.3%	[45]
Pesticide wastewater	AC catalytic—photolysis of ozone	COD:15,000 mg/L Cl−: 15,000 mg/L NH3-N: 40–60 mg/L	AC dosing:40 g/L P: 14w Aeration volume: 800 L/h	COD: 70.9% NH3-N: 87.7%	[46]

Table 3. Performance and experimental conditions of the advanced oxidation process for the treatment of saline wastewater.

Types of Wastewater	Technology	Wastewater Quality	Operating Conditions	Removal Efficiency	References
Chemical wastewater	Activated carbon adsorption—Fenton	TDS: 20% COD: 13,650 mg/L	pH = 6 FeSO4 = 3.0 g/L H2O2 = 20 mL/L T: 30 min	COD: 84.4%	[41]
Synthetic wastewater	Electrochemical oxidation	TOC: 2000 mg/L COD: 3500 mg/L TDS: 30.94 g/L	pH = 7.69 T = 30 min U = 7.41 V	COD: 91.78% TOC: 68.49%	[43]
Petrochemical wastewater	Catalytic ozonation	COD = 362 ± 36 mg/L TOC: 42 mg/L EC: 59.9 ms/cm	Catalyst dosing: 0.45g/L pH = 7.2 Ozone dosing: 0.3 g/h	COD: 75.3% TOC: 50.3%	[45]
Pesticide wastewater	AC catalytic—photolysis of ozone	COD:15,000 mg/L Cl−: 15,000 mg/L NH3-N: 40–60 mg/L	AC dosing:40 g/L P: 14w Aeration volume: 800 L/h	COD: 70.9% NH3-N: 87.7%	[46]

4.2. Membrane Bio-Reactor

A membrane Bioreactor (MBR) combines membrane technology with an activated sludge process and uses membrane separation instead of natural precipitation separation. Compared with the traditional activated sludge process, it has a larger solid-liquid separation capacity, higher volume loading, a higher biomass capture rate, and lower sludge production, which basically solves many open problems existing in traditional activated sludge processes [47,48]. The gel layer formed on the membrane surface plays an irreplaceable role in intercepting small molecular substances, improving the removal rate of organic substances and stabilizing the effluent quality of the system. With the deepening and expanding of related research and application, MBR is being gradually applied to the treatment of high-saline wastewater. Table 4 lists the research examples of the MBR process in the field of high-salinity wastewater treatment.

Pendashteh et al. [49] used sequencing batch MBR to treat high-salinity oily wastewater, and the removal efficiencies of COD and TOC were 97.5% and 97.2%, respectively. However, as salinity increases, the removal rates of COD and TOC drop sharply. In recent years, a number of new membrane bioreactors have been developed in the field of wastewater treatment and water resource utilization, which may be more suitable for the treatment of high-salinity wastewater. Juang et al. [50] inoculated Pseudomonas aeruginosa as halophilic bacteria in MBR to treat synthetic wastewater with a salinity of 100 g/L NaCl, and the removal rate of phenol in the system reached more than 95%, suggesting that the inoculation technology is suitable for the treatment of highly saline effluents.

The researchers found that the membrane bioreactor creates good conditions for salt-tolerant bacteria to adapt and grow, allowing them to better adapt to the salt-rich environment.

An anaerobic membrane bioreactor (AnMBR) combines anaerobic biological treatment with the membrane bioreactor and has been rapidly developed in recent years. On the basis of retaining many of the advantages of anaerobic biological treatment technology, such as low investment, low energy consumption, recyclable biogas energy, high load, less sludge production, impact load resistance, etc., AnMBR technology introduces a membrane module, which also brings a series of advantages, such as a good biochemical effect, good and stable water quality, etc. The AnMBR process is divided into two stages of anaerobic digestion and membrane separation, wherein the anaerobic digestion stage is no different from the traditional anaerobic treatment process, and the main difference between the membrane separation stage and the aerobic MBR is that aeration is not adopted to sweep the membrane surface. AnMBR is more economical than MBR because it can recycle biogas energy and does not need an aeration process. Compared with the traditional anaerobic process, the AnMBR system can operate in a high microbial environment and long solid residence time, so it has a higher practical application value [51]. Li et al. [52] studied the COD treatment capacity of an anaerobic PTFE hollow fiber membrane bioreactor under different salinity conditions, and the system maintained a high removal capacity with increasing salinity. For the practical engineering application of high-salt wastewater treatment, AnMBR also has good treatment performance. Qi et al. [51] used AnMBR inoculated with salt-tolerant bacteria to treat the high-salt wastewater produced by the mustard industry in Yuyao City. After the AnMBR system was stable, the removal rate of COD was always more than 80%.

The MBR process works well in the treatment of wastewater with high-salinity and organic matter. However, the process is also confronted with the general problem of membrane technology—membrane fouling. Membrane fouling is the result of the accumulation of inorganic and organic pollutants in the membrane pores and on the membrane surface [53]. Due to the high cost of membrane materials and the high energy requirements associated with preventing pollution, the efficiency of the MBR system is limited by membrane fouling, resulting in an increase in operation and maintenance costs. Therefore, membrane fouling is a major and ongoing challenge in the practical application of MBR technology.

Table 4. Performance and experimental conditions of MBR for the treatment of saline wastewater.

Types of Wastewater	Technology	Wastewater Quality	Operating Conditions	Removal Efficiency	References
Petrochemical wastewater	MSBR	TDS: 35,000 mg/L COD: 2250 mg/L	HRT = 48 h T = 30 °C Organic load = 1.124 kgCOD/(m3d)	COD: 97.5% TOC: 97.2%	[49]
Synthetic wastewater	two-phase MBR	NaCl: 100 g/L phenol: 2000 mg/L	T = 30 °C pH = 3 module area = 0.19 m2	phenol: 95%	[50]
Synthetic wastewater	AnMBR	COD: 20,000 ± 410 mg/L EC: 1100 ± 100 mS/cm	MLVSS/MLSS = 0.78 T = 35 ± 1 °C HRT = 5 d SRT = 226 d	COD: 89.9–95.5%	[52]
Mustard production wastewater	AnMBR	COD: 7500 mg/L EC: 54 mS/cm	Organic load: Phase 1: 0.5–1.0 kgCOD/(m3·d) Phase 2: 7.6 kgCOD/(m3·d)	COD: 80%	[51]

Table 4. Performance and experimental conditions of MBR for the treatment of saline wastewater.

Types of Wastewater	Technology	Wastewater Quality	Operating Conditions	Removal Efficiency	References
Petrochemical wastewater	MSBR	TDS: 35,000 mg/L COD: 2250 mg/L	HRT = 48 h T = 30 °C Organic load = 1.124 kgCOD/(m3d)	COD: 97.5% TOC: 97.2%	[49]
Synthetic wastewater	two-phase MBR	NaCl: 100 g/L phenol: 2000 mg/L	T = 30 °C pH = 3 module area = 0.19 m2	phenol: 95%	[50]
Synthetic wastewater	AnMBR	COD: 20,000 ± 410 mg/L EC: 1100 ± 100 mS/cm	MLVSS/MLSS = 0.78 T = 35 ± 1 °C HRT = 5 d SRT = 226 d	COD: 89.9–95.5%	[52]
Mustard production wastewater	AnMBR	COD: 7500 mg/L EC: 54 mS/cm	Organic load: Phase 1: 0.5–1.0 kgCOD/(m3·d) Phase 2: 7.6 kgCOD/(m3·d)	COD: 80%	[51]

4.3. Sequencing Batch Reactor

An SBR is a kind of activated sludge wastewater treatment technology operated by intermittent aeration, which consists of five stages: influent, reaction, precipitation, clarification, and standby. Fully aerate during the reaction, organic matter is oxidized and ammoniacal nitrogen in is nitrified wastewater. The SBR system combines aerobic and anaerobic stages in one unit. Simultaneous removal of nitrogen and phosphorus from wastewater can be realized by adjusting the actual operating cycle. The process has the advantages of low sludge output, a small footprint, low running costs, resistance to organic pollution, and flexible operation and is particularly suitable for the treatment of industrial effluents with small water volumes and intermittent discharge [54].

Table 5. Performance and experimental conditions of SBR for the treatment of saline wastewater.

Types of Wastewater	Technology	Wastewater Quality	Operating Conditions	Removal Efficiency	References
Textile wastewater	SBR	TDS: 1470 mg/L COD: 140.8 mg/L ABS: 1.387	T = 27 ± 2 °C HRT = 24 h	Effluent COD = 50 mg/L ABS = 0.5	[55]
Heparin sodium production wastewater	SBR	NaCl: 25,000–35,000 mg/L COD: 15,800–25,500 mg/L NH3-N: 1320–2650 mg/L	MLSS = 9000 mg/L DO = 3 mg/L T = 25 °C HRT = 12 h	COD = 80–85% NH3-N = 30–50%	[56]
domestic sewage	SBR	TOC: 133. 7 mg/L TDS: 471. 2 mg/L	HRT = 12 h T = 25–28 °C DO = 6–8 mg/L	TOC = 85%	[57]
Edible olive processing wastewater	SBR- membrane treatment	EC: 78.3 Ms/cm COD: 14.16 mg/L Phenolic compounds: 700–1500 mg TY/L	TMP = 15 bar HRT = 16.7 hPH = 8.2 T = 21.9 °C	COD: 80% Phenolic compounds:71%	[58]

lists the research examples of the SBR process in the field of high-salinity wastewater treatment.

However, the study found that as salinity increases, the efficiency of SBR in removing organic pollutants in wastewater is greatly reduced. It is difficult for microorganisms to adapt to the sudden increase in salinity, and the loss of metabolic activity leads to a reduction in removal efficiency. Mirbolooki et al. [55] Activated sludge microorganisms were used to treat saline textile effluent in SBR. It was found that increasing the salinity from 1 to 10 g/L resulted in a decrease in the COD removal rate from 80.71% to 14.92%. Wastewater with high-salinity has an obvious inhibiting effect on the ordinary batch-activated sludge process with sequencing.

To overcome the adverse effects of the SBR system in high-salinity environments, biological improvement of the sludge through inoculation of salt-tolerant composite bacteria has become a hotspot in saline wastewater treatment. Based on the successful domestication of highly salt-tolerant mud, Yang et al. [56] reported the performance of the SBR process in the treatment of high-salinity effluent from heparin sodium production. Under optimal operating conditions, the COD removal rate was stable at more than 85%, with it being almost unaffected by salinity.

A thorough investigation of the effect of introducing salt-tolerant bacteria on the succession of the microbial community in activated sludge is key to optimizing the SBR. Jiang et al. [57] used SBR to treat high-salt domestic wastewater, which can effectively remove TOC in the water, and the introduced salt-tolerant bacteria can stay in the small SBR for a long time and gradually become dominant bacteria. It can be seen that biological improvement of SBR by inoculating salt-tolerant bacteria can achieve a better organic pollutant removal effect and strong resistance to salt shock.

Based on the inoculation of salt-tolerant bacteria, the effluent quality of the sewage treatment plant can be further improved by using SBR in combination with other technologies. Ferrer-Polonio et al. [58]. put the membrane filtration device into an SBR system, and studied the treatment effect of high-salinity wastewater from edible olive processing on a laboratory scale. It was found that the removal rate of COD and total phenol was about 80% and 71%, respectively. The final effluent COD concentration was lower than 125 mg/L, while turbidity, color, and phenolic compounds are completely removed. The purpose of adding a membrane treatment process is to completely remove microorganisms, color, turbidity, suspended solids, and other substances from biological treatment to create and achieve deep purification.

By inoculating and domesticating halotolerant bacteria to strengthen the SBR system, the efficiency and stability of the organic matter removal system in high-salinity environments can be improved and better applied to the treatment of high-salinity wastewater.

4.4. Biological Contact Oxidation Process

The biological contact oxidation process, also called a submerged biological filter, is a wastewater treatment process between the activated sludge process and the biofilm process. The pretreatment of influent by a hydrolysis acidification unit in a biological contact oxidation system can improve the salt tolerance of the system. In addition, the biological contact oxidation process has a high sludge concentration, a rich biological phase, high biological activity, high oxygen utilization efficiency, and strong shock load resistance, making it suitable for high-saline wastewater treatment [59].

Table 6. Performance and experimental conditions of the biological contact oxidation process for the treatment of saline wastewater.

Types of Wastewater	Technology	Wastewater Quality	Operating Conditions	Removal Efficiency	References
Synthetic wastewater	Biological contact oxidation	COD: 1500 mg/L NaCl: 14,000 mg/L	DO = 2–4 mg/L HRTA = 12 h HRTO = 24 h	COD: 83%	[60]
Ballast water	A/O biological contact oxidation	COD: 500 mg/L TN: 50 mg/L TDS: 3.5%	reflux ratio: 3:1 Filtration speed: 1.5 m/h	COD: 30.95 mg/L TN: 10.39 mg/L	[61]
Food manufacturing wastewater	Two-stage A/O biological contact oxidation	COD: 800–1500 mg/L NH3-N: 5–30 mg/L TDS: 2.8~4.7%	Vanaerobic:Vaerobic = 1:5	COD: 96% NH3-N: 87.5%	[62]
Oil field wastewater	Preprocessing + A/O biological contact oxidation	COD: 476–682 mg/L Cl −: 11,042–14,725 mg/L Volatile phenol: 0.98–1.37 mg/L Petroleum: 1.33–11.65 mg/L	hydrolytic acidification: HRT = 6–8 h contact oxidation: HRT = 12 h	Petroleum: 81.82% Volatile phenol: 95.01% COD: 85.19%	[63]

lists the research examples of the biological contact oxidation process in the field of high-salinity wastewater treatment.

The research basis of the biological contact oxidation process for treating high-salinity wastewater mainly lies in the inoculation of salt-tolerant microorganisms. Lin et al. [60] studied the effect of the biological contact oxidation process for treating high-salinity mixed chemical wastewater. After the domestication of biofilm inoculated with activated sludge from an ordinary domestic sewage treatment plant, it can achieve a good removal effect on high-salinity wastewater and form compact biofilm at the same time. In order to further optimize the performance of the biological contact oxidation process in treating high-salinity wastewater, the researchers divided the biochemical tank into two parts: an A-level tank (anaerobic section) and an O-level tank (aerobic section), which is called the A/O biological contact oxidation process. The A-level tank not only has certain organic matter removal functions, but also reduces the organic load of the subsequent O-level biochemical tank, so as to facilitate nitrification. Yu et al. [61] studied the performance of the A/O biological contact oxidation process in treating high-chlorine wastewater under different parameters, and the effluent quality of COD, NH₃-N, and TN of the reactor under the optimal conditions can meet the national Class A effluent standard.

With the development of A/O biological contact oxidation technology, more and more research have been carried out on the treatment of high-salinity wastewater. Lv et al. [62] studied the saline effluent from a casing factory in Shanghai using a two-step A/O biological contact oxidation process. The treatment system has good tolerance to the impact load of organic matter, and the effluent standard meets the Class I standard of the integrated wastewater discharge standard. With the development of research, A/O biological contact oxidation technology has been widely used in actual wastewater treatment. At present, the China Xinjiang Oilfield Company has built and put into operation three high-salinity wastewater up-to-standard treatment stations at their heavy oil exploitation site, with a cumulative wastewater treatment volume of about 1.5 million m³, which has significant social benefits [63].

It is not difficult to find that the biological contact oxidation process often adopts the staged method to improve the purification capacity. In addition, through the combination of aerobic and anaerobic processes, the development of a single-stage and multi-stage biological contact oxidation combined process can significantly improve the treatment system to salinity and organic load impact resistance, which will also be one of the development trends of biological treatment technology of high-salinity wastewater.

4.5. Biological Aerated Filter

A biological aerated filter (BAF) is an aerobic biological reactor that uses granular media to immobilize biofilm. In terms of filler, compared with biological contact oxidation process, the filler of the biological aerated filter not only acts as a carrier, but it also has a physical interception effect on suspended solids, which combines the characteristics of the biological contact oxidation process and the rapid filter process. Because the filler of the biological aerated filter has a filtering function, the SS concentration of effluent is very low, and no sedimentation separation is required, so no secondary sedimentation tank is set. The BAF process relies on the high concentration of biofilm on the filter media to oxidize and break down the organic pollutants in the influent and leverages the small particle size of the filter media and the bio-flocculation properties of the biofilm to capture a large number of suspended solids in the effluent [64]. At present, the technology of biological aerated filters (BAF) has been successfully applied to the advanced treatment of high-salinity wastewater and has achieved good results.

Table 7. Performance and experimental conditions of BAF for the treatment of saline wastewater.

Types of Wastewater	Technology	Wastewater Quality	Operating Conditions	Removal Efficiency	References
Carboxymethyl cellulose wastewater	Up-flow BAF	COD: 200 mg/L Cl-: 9850 mg/L TDS: 1.8 g/L	HRT = 10 h Gas-water ratio: 4:1	COD: 60%	[65]
Aquaculture wastewater	BAF	TDS: 3–5% NH4+-N: 10 mg/L Permanganate index: 14	DO = 4–5 mg/L T = 26 ± 2 °C HRT = 4 h	NH4+–N: 95% Permanganate index: 80%	[66]
Petrochemical wastewater	BAF	COD: 58.1–114.1 mg/L NH3-N:1.2–19.0 mg/L TDS: 18,000–35,000 mg/L	q0 = 1.1 m3/(m2·d) DO = 2.0 mg/L HRT = 2.7 h	COD: 43.7% NH3-N:74.2%	[67]
Pharmaceutical wastewater	Catalytic ozonation -BAF	COD: 203–262 mg/L TOC: 79–101 mg/L NH4+-N: 14 mg/L	HRT = 4 h Gas-water ratio: 4/1	COD = 46 mg/L NH4+-N = 4.1 mg/L	[68]

lists the research examples of the BAF process in the field of high-salinity wastewater treatment.

Conventional biological ventilation filters are generally tolerant of highly saline effluents. Based on this, researchers have carried out a lot of research and optimized biological aerated filters for various effluents. Wang et al. [65] used biologically aerated up-flow filters to treat effluents from carboxymethylcellulose production inoculated with SEM salt-tolerant bacteria, where the effluent’s COD was less than 100 mg/L. Considering the small effect of denitrification of mariculture effluent, Duan et al. [66] placed marine tribes into the biologically aerated filter and found that for the biologically aerated filter under appropriate process conditions for mariculture effluent, the removal rates of ammoniacal nitrogen, total nitrogen, and the permanganate index were more than 95%, 93%, and 80%, respectively. In addition to inoculating salt-tolerant bacteria, BAF can also provide good treatment performance in combination with other methods. Wang et al. [67] used an anaerobic biological filter (AF) and aerated biological filter (BAF) process to treat high-salinity petrochemical wastewater such that the wastewater met the discharge requirements of COD < 60 mg/L and NH₃-N < 6 mg/L. According to the properties of complex components, strong toxicity, and difficult biodegradability of antibiotic effluent, He et al. [68] a combined process of ozonation-BAF, which uses ozonation to improve the biodegradability of wastewater and the stability is obviously stronger than that of the BAF process alone.

Due to the large backwash water volume and pressure drop, as well as the stringent SS inflow requirements, BAF technology must be combined with the appropriate pre-treatment process, which increases operational costs [69]. At the same time, the sludge production of the biological aerated filter is slightly larger than that of the activated sludge method, and the sludge stability is not good, making the sludge treatment difficult. We should further optimize the structure and filter media of BAF to increase the operational stability of the BAF process and make it more suitable for the treatment of high-salinity wastewater.

4.6. Up-flow Anaerobic Sludge Bed/Blanket

In various anaerobic treatment processes, the up-flow sludge bed (UASB) reactor has been widely used because of its simple design, convenient maintenance, low operating cost, and flexibility to environmental conditions [70]. During the operation of the UASB reactor, raw water flows from the bottom to the top, and solid–liquid–gas three-phase separation can be realized in the reactor. In the process, granular sludge is formed to improve biomass and treatment efficiency.

Table 8. Performance and experimental conditions of UASB for the treatment of saline wastewater.

Types of Wastewater	Technology	Wastewater Quality	Operating Conditions	Removal Efficiency	References
Heavy oil production wastewater	UASB	COD: 350–640 mg/L Oil content: 112.5–205.4 mg/L TDS: 11.5–14.6 g/L	T = 30.1 °C HRT ≥ 24 h	COD: 65.08% Oil content: 74.33%	[71]
Synthetic wastewater	UASB	COD: 2000 mg/L NaCl: 0, 10, 25, 50 g/L	T = 37 °C HRT = 1 d	COD: 72–92%	[72]
Dye wastewater	UASB	Dye concentration: 1000 mg/L TDS: 15–17. 5%	T = 30–50 °C pH = 7	Rapid decolorization	[73]
Pharmaceutical wastewater	UASB + SBR/MBR	TDS: 4.96–24.90 g/L COD: 16,547 ± 1827 mg/L	OLR = 8.11 ± 0.31 g COD/L/d HRT = 48 h	COD: UASB + MBR: 94.7% UASB + SBR: 91.8%	[74]

lists the research examples of the UASB process in the field of high-salinity wastewater treatment.

Liu et al. [71] used an up-flow sludge bed anaerobic reactor (UASB) to treat wastewater from heavy oil production. The highest removal rates of crude oil and COD by the UASB reactor were 65.08% and 74.33%, respectively. However, it is difficult to achieve sludge granulation in conventional UASB under the condition of high-salinity, because the salinity inhibits the growth of microorganisms and leads to the disintegration of flakes and particles. Aslan et al. [72] studied the effect of high-salinity (0–50 g/L NaCl) on the UASB reactor and observed that the removal rate of COD gradually decreased with increasing salinity. The decrease in the COD removal rate was mainly due to the desiccation and death of anaerobic bacteria. To improve the salt tolerance of UASB, inoculation of salt-tolerant bacteria is the choice to effectively improve the salinity shock resistance of microorganisms. Masoomeh et al. [73] treated wastewater with 1000 mg/L azo dye by inoculating geometric Berlin salt bacteria through static experiments, and the wastewater could be decolorized quickly under optimal conditions. There are also researchers who are improving the salt tolerance of UASB systems by coupling other processes. Shi et al. [74] used UASB to treat high-saline pharmaceutical wastewater, and the UASB treatment unit is followed by SBR and MBR reactors, respectively, and the COD removal rate increased to 91.8% and 94.7%, respectively.

Although high-salinity can inhibit the growth of microorganisms, domesticated microorganisms can gradually adapt to a high-salinity environment and be used for the treatment of high-salinity wastewater. However, more research is needed to investigate different types of target pollutants and the interaction between salt and specific pollutants in activated sludge. There is a need for a thorough understanding of the effects of salinity on microbial activity and the process of adaptation of microorganisms to a saline environment.

5. Hybrid Treatment Processes

Combined process treatment is a method of combining two or more treatment technologies to treat wastewater. Whether it is a physicochemical process or a biological process, it is difficult to achieve the best performance alone in the treatment of saline wastewater due to the influence of many factors. Therefore, after understanding the water quality characteristics of high-saline wastewater, a physicochemical process and a biological process are often combined in a certain order to overcome the shortcomings of the two processes and achieve efficient treatment of high-saline wastewater. Physical–chemical–biological combined treatment technology is often used in the treatment of high-salinity wastewater due to its low salt content and good cleaning effect.

Zhan et al. [75] developed a combined process for oil field wastewater as shown in Figure 4. The removal rate of COD in the whole treatment system is about 55-70%. Finally, the adsorption of macroporous adsorption resin further improves the wastewater quality and makes the salt tolerance of the whole system excellent. At present, this technology has been applied in the Huanshilian Heavy Oil Wastewater Treatment Plant in Liaoning Province, China, and good economic benefits have been obtained. By combining different technologies, not only can the salt tolerance be improved, but the operation of the plant can also be further optimized. In the study by Song et al. [76], for example, to improve the denitrification capacity in high-salinity environments, a moving bed membrane bioreactor (AF-MBMBR) equipped with an anoxic biological filter and its performance in the treatment of high-salinity effluent from mariculture was studied, as shown in Figure 5. The preset ultrafiltration device cannot only improve the denitrification effect of MMBBR but it can also reduce membrane fouling and extend the life of the treatment device. However, due to the high sensitivity of phosphorus-accumulating bacteria to high-salinity, the phosphate removal rate of the system is low. In another study, Xu et al. [77] built up wetlands and microbial fuel cells to treat high-salinity wastewater. The results showed that salinity had no significant inhibitory effect on the removal rate of TP and COD. The salinity not only increased power generation but also promoted the removal of pollutants. To further elucidate the mechanism of the coupling system of electrochemistry and biotechnology to improve nitrogen and phosphorus removal ability, Liu et al. [78] used a new bio-electrochemical system for treating high-salinity petrochemical wastewater and the removal rates of COD and phenol were about 90.3% and 89.1%, respectively. The removal rate is significantly higher than using SBR or an electrochemical reactor alone. The increase in removal efficiency is mainly due to the different valence states generated by iron electrolysis, which as a carrier of electron transfer accelerates the efficiency of biological and chemical reactions, thus improving the nitrogen and phosphorus removal efficiency of the system. At the same time, electrolysis improves the salt tolerance of microorganisms and enriches the microorganism species.

However, since the combined system contains multi-seed processes, its maintenance and operation costs are significantly higher than other treatment methods, and the overall processing effect of the combined system depends on the performance of each sub-process. As shown in

Table 9. Performance and experimental conditions of hybrid treatment processes for the treatment of saline wastewater.

Types of Wastewater	Technology	Wastewater Quality	Operating Conditions	Removal Efficiency	References
Oil field wastewater	Electrochemistry + Coagulation + MBBR + MBR + macroporous adsorption resin	EC: 4.5 mS/cm COD: 212 mg/L	PFS: 500 mg/L HRT = 12 h Adsorption resin: SD300	COD: 20 mg/L	[75]
Aquaculture wastewater	AF + MBMBR	TOC: 100–125 mg/L TN: 40–50 mg/L TDS: 34.5‰	HRT: 134d Vaerobic:Vanaerobic = 10/4 Q = 10 (L/m2h) T = 25 °C	TOC: 92.8–96.2% TN: 93.2%	[76]
Synthetic wastewater	CW-MFC	NaCl: 5 g/L	T = 30±3 °C HRT = 3 d	TP: 86.64 ± 0.29% COD: 68.20 ± 1.15%	[77]
Petrochemical wastewater	Bioelectrochemical systems	COD: 1130 mg/L Phenol: 200 mg/L TDS: 12,000 mg/L	E = 0.1 V/cm T = 25 ± 2 °C, DO = 3.0 ± 0.5 mg/L HRT = 24 h	COD: 90.3% Phenol: 89.1%	[78]

, most of the combined saline wastewater treatment methods do not take into account the large differences in the quality of different types of saline wastewater, and the relevance of different types of saline wastewater treatment is not sufficient. According to the water quality characteristics of different types of high-saline sewage, combined with the advantages of different technologies, a suitable combination method should be selected to achieve the best treatment effect.

6. Scaling and Corrosion

Scaling and corrosion are key issues for the treatment/concentration of saline wastewater. High-salinity wastewater contains many kinds of inorganic salt ions, and the differences in the composition and concentration of anions and cations between water bodies cause chemical reactions to generate crystal precipitation, resulting in incompatibility or poor compatibility between water bodies and scaling phenomenon [79]. In the process of desalination or treatment of high-salt wastewater, which are affected by thermal conditions such as temperature, pressure, and pH, the ion balance state in water is broken and the scaling components are supersaturated and separated out in the solution to form a crystal nucleus, which grows and accumulates continuously and precipitates, finally forming scale, causing pipeline blockage and equipment scaling, affecting the normal operation of the system [80]. Currently, effective control methods for high-salinity wastewater scaling include chemical scale inhibition and physical scale inhibition. Chemical scale inhibition can cause corrosion damage to pipeline equipment to a certain extent due to the acid cleaning process. At present, more physical methods are used for scale prevention at home and abroad, mainly including the high-intensity sound shock wave treatment method, the permanent magnet descale method, and descaling by use of crystallization kinetics in water.

In addition to the scaling problem, most of the desalination and treatment systems of high-salinity wastewater have corrosion problems in different degrees. Solution pH, carbon dioxide, bacteria, temperature, and flow rate are all important factors affecting the corrosion of pipelines and equipment [81]. Firstly, the high-salinity wastewater itself has strong erosion and penetrability. Secondly, CO₂ mainly appears in the form of bicarbonate in the produced water, which reduces pH and easily causes acid corrosion. After scaling in high-salinity wastewater, it is easy for a micro-anaerobic zone to form under the scale shell, which accelerates the corrosion of anaerobic bacteria. According to the influencing factors of corrosion, the selected anti-corrosion measures include improving the corrosion protection technology of the inner wall of the water injection pipeline and adding a corrosion inhibitor. For water with excessive bacteria and hydrogen sulfide content, a certain amount of bactericide and sulfur remover shall be added [82]. Among them, a corrosion inhibitor is a kind of chemical substance to slow down corrosion, which is the most cost-effective method to prevent or control steel pipe corrosion [83]. Different corrosion inhibitors should be selected according to different corrosion mechanisms.

In conclusion, pH, temperature, flow rate, and other factors have an impact on corrosion and scaling during the desalination and treatment of high-salinity wastewater. In future research, we can find the most appropriate control method for these common factors, so that the corrosion degree can be reduced to the lightest, and the amount of scaling can be reduced to the minimum.

7. Conclusions

It is very important to use reasonable methods to treat high-salinity wastewater, especially to realize zero discharge and water recycling. The comprehensive treatment of high-salinity wastewater includes desalination and advanced treatment. The traditional desalination technologies for high-salinity wastewater include membrane technology and thermal concentration technology. As for membrane technology, the core of membrane separation technology is membrane material, and the preparation of a membrane with high flux, high rejection, low membrane pollution, high mechanical strength, and low cost is the key to industrial application. Thermal concentration technology can completely desalinate, but the scaling problem makes the system less stable, and the construction and maintenance cost of the lengthy treatment process and energy consumption also cause its poor economy. How to effectively recycle the concentrated water/mixed salt generated after wastewater desalination is also a difficult problem. The advanced treatment of high-salinity wastewater includes physicochemical methods and biological methods. Advanced oxidation processes (AOPs) have great potential in the treatment of high-salinity wastewater and are important means for the removal of organic compounds in high-salinity organic wastewater. Among them, catalytic ozonation has been widely studied because of its strong mineralization, low cost, reduced energy consumption, simple operation, wide application, and its lack of secondary pollution. Developing new materials as catalysts and improving the mechanical strength and stability of catalysts are one of the research directions of catalytic ozonation technology in the future. Biological treatment is an economic alternative to high-salinity wastewater treatment. The research of biological treatment technology mainly focuses on the cultivation and domestication of halophilic microorganisms and the research of different biological treatment processes. The use of halophilic bacteria has been shown to be effective in improving the performance of conventional aerobic and anaerobic biological systems. Coupling technology is a hot research topic in the field of advanced treatment of high-salinity wastewater, which has good salt tolerance and treatment stability and can still have a high removal rate of organic pollutants even under high-salinity. According to the characteristics of wastewater produced by different industries, a high-efficiency and energy-saving physical–chemical–biochemical combined process is explored, so that the treatment efficiency of high-salt organic wastewater is improved, and the treatment cost is reduced. However, the complexity of the system increases, and the simplicity of operation and maintenance is the key point to be solved. The scaling and corrosion problems of equipment and pipelines in the treatment process cannot be ignored. In order to realize desalination and advanced treatment of high-salinity wastewater more efficiently, future research should focus on seeking clean energy to replace traditional energy, developing new membrane materials and catalyst materials, strengthening the research on salt-tolerant microorganisms, and developing coupling of multiple processes to make up for the defects and deficiencies of a single unit.