Research Progress in the Treatment of High-Salinity Wastewater

The generation of high-salinity wastewater is closely associated with various industries, containing a plethora of dissolved salts such as chlorides, sulfates, and carbonates, which pose a significant threat to the environment and human health. Consequently, the treatment of high-salinity wastewater has emerged as a pivotal environmental challenge in contemporary society. This review aims to elucidate the sources and characteristics of high-salinity wastewater, as well as the current status and trends in the field of high-salinity wastewater treatment. First and foremost, we explore the means by which high-salinity wastewater is generated in different industries and the principal salt components it contains. We revisit conventional methods for high-salinity wastewater treatment, including chemical precipitation, ion exchange, evaporation-crystallization, and reverse osmosis, while emphasizing the application of biotechnological approaches. This includes the utilization of salt-tolerant microorganisms, biological adsorption, biodegradation processes, and various types of bioreactors. Advanced oxidation technologies also play a crucial role in high-salinity wastewater treatment. We introduce advanced oxidation techniques such as ozone oxidation, UV-catalyzed oxidation, and high-pressure water oxidation, emphasizing their potential in degrading organic compounds and reducing salt concentration. Furthermore, we discuss hybrid approaches, such as case studies combining biological treatment with advanced oxidation technologies, and the advantages and challenges associated with these integrated techniques. Finally, we provide an outlook on future trends, including research on novel and efficient biodegrading agents, advancements and innovations in advanced oxidation technologies, and the development of intelligent high-salinity wastewater treatment systems. Policy and industry trends will also influence the direction of high-salinity wastewater treatment field development. In summary, high-salinity wastewater treatment represents a complex and pressing environmental challenge. However, through the integration of diverse treatment technologies and ongoing research efforts, it is anticipated that more effective, cost-efficient, and environmentally friendly solutions can be developed to mitigate the impact of high-salinity wastewater on ecosystems and society.


Introduction
The treatment of high-salinity wastewater is a matter of growing environmental concern, given its implications for various critical domains, such as environmental conservation, resource recovery, and sustainable development [1].The gravity of this issue continues to escalate as industrial processes expand, and climate change leads to increased instances of seawater intrusion and rainwater infiltration.High-salinity wastewater is characterized by its complex composition, including salts, organic compounds, heavy metals, and various contaminants, which pose potential threats to water resources and ecosystems [4].
The sources of high-salinity wastewater are diverse, encompassing discharges from various industrial sectors, including mining, chemical manufacturing, pharmaceuticals, and food processing, among others.Furthermore, urban areas face challenges associated with high-salinity wastewater due to rainwater infiltration [7].Addressing these wastewater streams presents environmental and socio-economic challenges, as effective treatment demands not only advanced technologies but also appropriate regulatory and policy support [10].
The purpose of this review is to delve into the current state of high-salinity wastewater treatment, the challenges it poses, innovative methods, and the impact of environmental regulations and policies in different countries.By gaining a comprehensive understanding of various aspects of high-salinity wastewater treatment, we can better address this environmental challenge, fostering sustainable water resource management and ecological preservation.

Source and Characteristics
High-salinity wastewater derives from a wide range of sources, encompassing multiple sectors and scenarios.Industrial wastewater stands as a prominent contributor, spanning industries such as mining, chemical processing, textiles, food production, and many others [11].Wastewaters generated by these industrial processes typically contain substantial amounts of salts and various chemical compounds.Rainwater infiltration is another common source, as rainwater permeates storage or treatment facilities, dissolving soil salts and carrying them into wastewater systems [13].Furthermore, natural groundwater sources themselves may possess high salinity, and water extracted from wells or underground sources may exhibit elevated salinity levels [14].Desalination wastewater arises from the desalination of seawater and typically includes highly saline effluents.Agricultural drainage in certain regions can also lead to salt concentration in soil, ultimately entering wastewater systems [15].The diversity of high-salinity wastewater sources and their widespread origins make its treatment a complex endeavor, necessitating the adoption of appropriate techniques and methods tailored to specific circumstances to ensure effective treatment and environmental preservation.The management of high-salinity wastewater holds significant importance in the realms of environmental conservation, resource reclamation, and sustainable development.
High-salinity wastewater comprises primary constituents including salt substances, organic compounds, heavy metals, and other contaminants.Salt substances such as chlorides, sulfates, nitrates, exist in the form of cations and anions, elevating water's electrical conductivity and salinity concentration [16].Organic compounds may originate from various industrial processes, bearing potential toxicity and pollution risks [17].Additionally, heavy metals like lead, mercury, cadmium may be present in high-salinity wastewater, stemming from industries such as mining and electronics manufacturing, posing threats to aquatic ecosystems [18].Furthermore, high-salinity wastewater may also contain other pollutants, including fats, dissolved gases, and microorganisms, with the types and concentrations of these components varying according to the wastewater's source and characteristics [19].Consequently, the treatment of high-salinity wastewater necessitates the application of suitable treatment methods and technologies tailored to specific situations to maintain the health and sustainability of aquatic environments.

Traditional high-salt wastewater treatment methods
Traditional methods for treating high-salinity wastewater encompass chemical precipitation, ion exchange [20], evaporation crystallization [22], and reverse osmosis [23] .These methods are capable of reducing the salt concentration in high-salinity wastewater to some extent.Chemical precipitation involves the addition of chemical reagents to precipitate salts into solid forms, yet it entails issues related to high reagent costs and residual waste management.Ion exchange technology utilizes resins or membrane filtration to remove salts, but necessitates frequent material replacement and regeneration.The evaporation crystallization process entails the evaporation of wastewater and the collection of salt residues, albeit it is associated with high energy consumption.Reverse osmosis, which employs semi-permeable membranes to separate salts from water, requires high pressure and energy, and is susceptible to membrane fouling issues.These conventional approaches play a role in high-salinity wastewater treatment, but they present a range of challenges such as high costs, energy consumption, and waste generation, thereby driving ongoing research and exploration for more environmentally friendly, cost-effective, and efficient treatment technologies.

Biotreatment Technology
The biotechnological treatment of high-salinity wastewater has emerged as a pivotal approach in addressing the challenges associated with such wastewaters.These techniques heavily rely on halophilic microorganisms, such as halophilic algae and halophilic bacteria, which exhibit the capacity to thrive and reproduce in high-salinity environments [24].This biotechnological treatment encompasses two major mechanisms: biodegradation and biosorption.Through these mechanisms, the effective removal of organic compounds and salinity from high-salinity wastewater can be achieved.Bioreactors, including submerged bioreactors and suspended-growth bioreactors, have found wide applications in high-salinity wastewater treatment.These bioreactors provide a conducive environment for microbial growth and activity, thereby facilitating the treatment process [26].Despite the remarkable performance of biotechnological approaches in dealing with high-salinity wastewater under specific conditions, they also face several challenges.These challenges encompass issues related to the salt tolerance and stability of microorganisms, as well as reactor design and operational conditions.Nonetheless, these biotechnological methods still hold significant promise due to their ability to simultaneously reduce organic compounds and salinity levels [27].They contribute to cost reduction, decreased reliance on chemical agents, and energy conservation, making them pivotal players in the treatment of high-salinity wastewater.

Electrochemical Treatment Technology
The electrochemical treatment technology for high-salinity wastewater is a method that employs electric current to facilitate chemical reactions in wastewater for the removal of salts and organic compounds [28].This method primarily encompasses two major applications: electrochemical desalination and electrochemical oxidation.Electrochemical desalination involves the application of electric current to guide ions to move from wastewater, thereby effecting the removal of salt content [30].This process generally entails an electrolytic cell where one electrode serves as the cathode, attracting cations, while the other electrode functions as the anode, attracting anions.This results in the precipitation of salt ions on the electrodes or their removal from the water through other means.It is an effective method, particularly suitable for desalting purposes.Electrochemical oxidation, on the other hand, entails inducing redox reactions at the electrodes to degrade organic substances present in wastewater [31].This can be achieved by applying an electric current to generate active oxidants such as hydrogen peroxide or hypochlorous acid.These oxidants swiftly oxidize organic compounds, converting them into water and carbon dioxide.Electrochemical oxidation is proficient at removing organic pollutants, although its application in high-salinity wastewater may be constrained by salinity levels.

Advanced Oxidation Technology
Advanced oxidation technologies for high-salinity wastewater have emerged as a significant treatment method to address the intricacies and recalcitrance associated with such wastewater [32].These advanced oxidation technologies encompass ozone oxidation [33], ultraviolet photocatalytic oxidation [34], and high-pressure water oxidation [35], among other methods.Ozone oxidation harnesses ozone gas to generate highly oxidative oxidants, capable of swiftly oxidizing and decomposing organic pollutants within high-salinity wastewater.Ultraviolet photocatalytic oxidation utilizes ultraviolet light to activate catalysts, such as titanium dioxide, in order to generate reactive oxidants for the degradation of organic compounds within wastewater.High-pressure water oxidation, as an oxidation technique under high-pressure conditions, relies on the generation of hydroxyl radicals to oxidize organic constituents while simultaneously reducing salinity concentrations.These advanced oxidation technologies typically require minimal addition of chemical agents, thereby mitigating the chemical reagent costs associated with wastewater treatment processes.Although these technologies may entail relatively higher costs and demand intricate operational and maintenance procedures, they exhibit potential advantages in addressing recalcitrant organic constituents and high-salinity wastewater.Consequently, advanced oxidation technologies have evolved as a pivotal research focus in the realm of high-salinity wastewater treatment, contributing significantly to enhanced treatment efficiency and reduced reliance on natural resources, thus making valuable contributions to environmental preservation and sustainable development.

Combination Technology
In the realm of high-salinity wastewater treatment, innovation and progress are manifested through the integration of diverse treatment technologies to enhance treatment efficiency and reduce costs [36].One exemplary illustration of such integrated techniques involves the amalgamation of biological treatment and advanced oxidation technologies [38].This approach typically entails initial biological pretreatment within a bioreactor to effect partial removal of organic constituents and reduction of salinity concentrations, followed by the deployment of advanced oxidation technologies for further organic degradation, concomitant with salinity reduction.This synergistic treatment strategy overcomes the inherent limitations of individual technologies when dealing with high-salinity wastewater, ultimately enhancing the overall effectiveness of wastewater treatment.Furthermore, other hybrid techniques are continually emerging, such as the integration of electrochemical technologies with biological treatment to enhance desalination and decontamination capabilities, or the combination of chemical oxidation and membrane separation technologies to achieve more efficient wastewater treatment and reclamation [39].These integrated approaches not only contribute to improved treatment efficiency but also hold promise for reducing the overall costs associated with wastewater treatment, fostering resource recovery and sustainable development.In the future, as research and practical implementation of additional integrated techniques advance, the field of high-salinity wastewater treatment is poised to continue its trajectory of innovation and progress, addressing this significant environmental challenge.

Challenges and Difficulties
The treatment of high-salinity wastewater presents numerous challenges and complexities.Foremost among these challenges is the technical intricacy due to the diversity and variability in the composition of high-salinity wastewater, necessitating tailored treatment methods [40].High energy consumption stands as another inhibiting factor, as some treatment technologies require substantial energy inputs, increasing costs and posing adverse implications for sustainability.Wastewater concentration may trigger further processing or disposal requirements to prevent environmental contamination [42].Organic compound degradation is also challenging, as the high salinity environment inhibits microbial degradation processes.Regulatory and policy requirements add complexity to management, involving emissions standards, permits, and environmental regulations.Economic feasibility constrains the adoption of highly efficient treatment technologies, necessitating a comprehensive consideration of cost-effectiveness [43].Finally, water resource recovery may offer a solution, but demands addressing water quality and salinity requirements.Therefore, the treatment of high-salinity wastewater necessitates a multifaceted approach that encompasses technological, economic, environmental, and regulatory considerations to achieve effective wastewater management and resource reclamation, fostering sustainability.

Future Development Trend
The future development trends in the field of high-salinity wastewater treatment encompass multiple facets with the objective of enhancing treatment efficiency, reducing environmental impact, and fostering sustainability.Firstly, research into novel and efficient biodegrading agents will continue to propel the evolution of biotechnological treatment methodologies.This entails the discovery and utilization of more halotolerant microorganisms to adapt to diverse types of high-salinity wastewater [44].
Secondly, advanced oxidation technologies will undergo further refinement and innovation to enhance their performance and reduce operational costs [45].Examples of such enhancements include the adoption of solar energy or photocatalytic techniques for the degradation of organic compounds and pollutants within wastewater.The design and development of intelligent high-salinity wastewater treatment systems is another key trend for the future [46].These systems leverage automation and data analytics technologies for real-time monitoring and adjustment of treatment processes, thereby enhancing efficiency and controllability.Support from governmental policies and industry standards will play a pivotal role in the management of high-salinity wastewater, stimulating technological innovation and sustainable practices.Furthermore, wastewater reclamation and resource utilization will assume prominence in the future, transforming high-salinity wastewater into reusable water resources and valuable salt components to mitigate water scarcity and environmental burdens.In summary, the future of high-salinity wastewater treatment will be characterized by the development of innovative technologies, the application of intelligent systems, the promotion of policies and standards, all aimed at achieving efficient and sustainable high-salinity wastewater management and resource recovery.

Conclusions
High-salinity wastewater treatment presents a significant environmental challenge that spans multiple industries and contexts, characterized by its complexity and diversity, rendering its management a complex endeavor.In this comprehensive review, we delve into the sources, constituents, treatment technologies, as well as the challenges and intricacies associated with high-salinity wastewater.We underscore the significance of high-salinity wastewater treatment, as it not only pertains to environmental preservation but also involves resource recovery and sustainable development.High-salinity wastewater emanates from a plethora of sources, including industrial effluents and rainwater infiltration, carrying a diverse array of pollutants such as salts, organic compounds, and heavy metals.Treatment technologies for high-salinity wastewater are multifarious, encompassing conventional methods like chemical precipitation and ion exchange, as well as advanced techniques such as reverse osmosis and evaporation crystallization.Nevertheless, the technical complexities, cost implications, environmental considerations, and regulatory and policy requirements present formidable challenges to the treatment process.In grappling with these challenges, it is imperative to comprehensively assess the applicability of different technologies, striving to reduce treatment costs, minimize energy consumption, and curtail environmental impacts to the greatest extent possible.Concurrently, water resource reclamation and sustainable treatment approaches will play pivotal roles in the future.Ultimately, the treatment of high-salinity wastewater necessitates integrated efforts across technological, policy, and societal domains to ensure sustainable water resource management and environmental health.We hope that this review will draw increased attention to the issues surrounding high-salinity wastewater, motivating further research and innovation to discover more effective solutions.

Acknowledgments
The study was supported by the Scientific Research and Development Foundation of Hefei University (No. 20ZR07ZDB), Scientific Research Foundation of Education Department of Anhui Province of China (No. KJ2021A1012), Talent research Foundation of Hefei University (No. 20RC46)