High-salt, high-concentration wastewater treatment technology

High-salinity, high-concentration wastewater, due to its extremely high salt and pollutant concentrations, is difficult to treat and has become a thorny problem in the field of industrial environmental protection. Its treatment technology requires the synergy of multiple processes to form a systematic solution. With industrial development and increasingly stringent environmental requirements, the treatment of high-salinity wastewater is not only related to enterprises' compliant discharge but also closely linked to resource recycling and sustainable development. This article will delve into its treatment technology system, current applications, and future directions.

1.Pretreatment: Laying the Foundation, Solving Front-End Problems

Pretreatment is a crucial step in ensuring the stable operation of subsequent processes. First, physical methods such as screens and sedimentation tanks remove large suspended solids and floating matter to prevent equipment blockage. Subsequently, coagulation and sedimentation technology, through the addition of flocculants (such as polyaluminum chloride and polyacrylamide), causes colloidal particles and tiny suspended solids to coagulate and settle, significantly reducing wastewater turbidity. For high hardness issues, chemical precipitation methods (such as the lime-soda process) effectively remove calcium and magnesium ions, preventing membrane scaling or evaporator scaling. In terms of organic matter removal, advanced oxidation technologies such as Fenton oxidation (utilizing Fe²⁺ and H₂O₂ to generate highly oxidizing hydroxyl radicals) can efficiently degrade recalcitrant organic matter and improve wastewater biodegradability. For wastewater with a salt concentration ≤5% and some biodegradability, salt-tolerant microbial pretreatment (such as halophilic bacteria bioreactors) is an economically feasible option, as it decomposes organic matter through microbial metabolism, but strict control of salt concentration fluctuations and influent load is required. Furthermore, dissolved air flotation (DAF) technologies can further remove grease and fine suspended solids, creating favorable conditions for subsequent processes.

2.Core Desalination: Diverse Technologies Adaptable to Different Scenarios

Core desalination technologies need to be flexibly selected based on wastewater characteristics, treatment scale, and cost requirements. Evaporation crystallization technology, as the mainstream process, evaporates water through heating, causing salt to crystallize and precipitate. Multi-effect evaporation (MEE) utilizes the steam from the previous effect as the heat source for the next effect, utilizing heat energy step by step, significantly reducing energy consumption, and is suitable for wastewater with large treatment volumes and simple salt compositions. Mechanical vapor recompression (MVR) uses a compressor to pressurize and heat the secondary steam generated by evaporation for recycling, consuming only half the energy of MEE. It is particularly suitable for large-scale high-salt wastewater treatment, but requires higher equipment investment and is subject to stringent requirements regarding material corrosivity. In membrane separation technologies, reverse osmosis (RO) uses high pressure to drive water molecules through a semi-permeable membrane, achieving a desalination rate of 95%-99%. It is suitable for pretreatment or deep desalination of wastewater with medium to low salt concentrations, but in high-salt environments, the membrane is prone to scaling and flux reduction, requiring frequent cleaning, and the concentrate needs further treatment. Electrodialysis (ED) uses an electric field to drive ion migration, exhibiting good tolerance to high salt and relatively low energy consumption, but suffers from electrode corrosion and concentration polarization problems. Membrane distillation (MD) uses the temperature difference across a hydrophobic membrane to drive water vapor transfer, capable of treating saturated solutions and even crystallizing wastewater, but its low membrane flux limits its large-scale application. Furthermore, forward osmosis (FO) technology extracts water molecules from wastewater using a high-osmotic-pressure draw solution, consuming little energy, but draw solution regeneration is complex, and it is currently mostly in the pilot-scale stage.

3.Deep Treatment and Resource Utilization: Closed-Loop Cycle, Value Extraction

Deep treatment and resource utilization are the core objectives of high-salinity wastewater treatment. Even after desalination, the treated water still requires further purification: activated carbon adsorption removes residual trace organic matter and color, ion exchange resins deeply desalinate and regulate water quality, and ultraviolet disinfection or advanced oxidation technologies ensure microbial safety. Concentrate treatment is key to "zero discharge." Salt separation and crystallization technology, through a combination of evaporation, cooling crystallization, and nanofiltration/electrodialysis, achieves the separation and resource utilization of salts such as sodium chloride and sodium sulfate. For example, coal chemical wastewater can be treated using a "pretreatment + RO + MVR evaporation crystallization + salt separation" process to recover industrial-grade salt products, reducing disposal costs. Furthermore, concentrated wastewater from some industries (such as pharmaceuticals and chemicals) may contain high-value components (such as organic solvents and precious metals), which can be recovered through extraction, adsorption, or bioconversion technologies to improve economic benefits. Treatment processes often require combined optimization. For example, a combination of "coagulation sedimentation + Fenton oxidation + RO + MVR" can simultaneously achieve organic matter removal, desalination, and energy saving, while an "ED + MD" combination can reduce energy consumption and treat highly concentrated wastewater.

4.Challenges and Future Trends: Technological Breakthroughs and Green Transformation

High-salinity wastewater treatment still faces many challenges: high energy consumption leads to high costs; membrane fouling and equipment corrosion affect stability; and insufficient purity of salt separation crystallization limits resource recovery value. Future development directions focus on the following areas:

4.1.Material Innovation: Developing fouling-resistant, high-flux membrane materials (such as graphene composite membranes and nanofiber membranes) to reduce membrane separation energy consumption and maintenance costs; researching and developing corrosion-resistant, high-heat-transfer-efficiency evaporation crystallization equipment materials.

4.2.Process Optimization: Through mathematical modeling and artificial intelligence, precisely controlling treatment parameters (such as temperature, pH, and flow rate) to achieve dynamic optimization operation; promoting integrated technologies for the entire process of "pretreatment-desalination-resource recovery" to reduce losses in intermediate stages.

4.3.Salt Separation and Deep Resource Utilization: Employing novel crystallization technologies (such as fluidized bed crystallization) to improve the purity of salt products and exploring high-value utilization of salts (such as the preparation of battery materials and snow-melting agents).

4.4New Energy Coupling: Utilizing solar energy, geothermal energy, and industrial waste heat to drive evaporation systems to reduce dependence on fossil fuels; combining water electrolysis for hydrogen production to synergize wastewater treatment with renewable energy production.

4.5.Bioaugmentation Technology: Selecting and cultivating microbial strains that are tolerant to high salt levels and efficiently degrade specific pollutants; developing biofilm reactors and electrochemical coupling technologies to reduce pretreatment costs.

5.Case Study Implications

Integration of Technology and Practice Taking a petrochemical company's "zero-emission" project as an example: using a process of "coagulation sedimentation + ozone catalytic oxidation + RO + MVR evaporation crystallization + mixed salt pyrolysis," 100,000 tons of wastewater are treated annually, recovering sodium chloride with a purity of 99.5%. Mixed salts are harmlessly disposed of through pyrolysis, saving over ten million yuan in disposal costs, achieving a win-win situation for both the environment and the economy. This case demonstrates that customized technology combinations and full life-cycle cost management are key to the successful treatment of high-salinity wastewater.

The treatment of high-salt, high-concentration wastewater is shifting from "meeting discharge standards" to "resource recycling." Continuous technological innovation and multidisciplinary integration will drive the industry to overcome bottlenecks and contribute to the green transformation of industry and the achievement of "dual-carbon" goals. In the future, policy support, industry-academia-research cooperation, and the improvement of market mechanisms will further accelerate the application of advanced technologies and build a new sustainable water resource management system.