Process Integration and Combined Applications of Industrial Reverse Osmosis Membrane in Wastewater Treatment

Abstract

With increasingly stringent environmental regulations and growing demand for water resource recycling, industrial reverse osmosis (RO) technology has become a core solution for advanced wastewater treatment and reuse. However, facing industrial wastewater with complex compositions and high pollution loads, a single RO unit often struggles to operate stably, efficiently, and economically. Therefore, scientifically integrating the RO membrane unit with other water treatment processes to build multi-barrier, synergistic treatment systems has become a mainstream technical direction in the field of advanced industrial wastewater treatment and resource recovery. This article aims to systematically explain the process integration models centered on RO, key combined technologies, and their application effectiveness, hoping to provide reference for engineering practice.

1. Necessity and Core Concept of Process Integration

Industrial wastewater typically features high salinity, high organic content, high hardness, and contains specific pollutants (e.g., heavy metals, refractory organic matter). Direct introduction into an RO system can easily lead to membrane fouling, scaling, performance decline, or even irreversible damage. The core concept of process integration is "graded treatment and synergistic enhancement." This means ensuring the "treatability" of RO feed water through pretreatment units, and achieving resource recovery and brine minimization through post-treatment or combined processes, ultimately constructing an efficient, stable, and economical full-process solution.

2. System Architecture of RO-Centered Process Integration

A complete integrated system typically consists of three main modules: pretreatment, the main RO unit, and post-treatment/brine management.

2.1 Front-End Pretreatment Module: The "Outpost" Ensuring Stable RO Operation

The goal of pretreatment is to provide the RO unit with stable feed water that meets its quality requirements (e.g., low turbidity, low SDI, low scaling potential, within threshold limits for specific pollutants).

• Conventional Physicochemical Pretreatment: Includes screens/sieves, coagulation sedimentation/flotation, multi-media filtration, etc., mainly used to remove suspended solids, colloids, large particles, and some COD.

• Advanced Oxidation Pretreatment: For refractory organic matter (e.g., characteristic pollutants), technologies like ozone catalytic oxidation, Fenton/photo-Fenton, and electrochemical oxidation are used. These employ strong oxidation to break molecular chains and rings, improving biodegradability or reducing membrane fouling potential.

• Biological Pretreatment: For organic wastewater with good biodegradability, processes like aerobic, anaerobic treatment, or Membrane Bioreactors (MBR) are used to effectively remove most BOD and part of the COD, reducing the organic load on the RO.

• Specialized Pretreatment:

  • For Hardness and Scaling Ions: Employ softening (ion exchange, lime-soda process) or antiscalant dosing to prevent scaling of CaCO₃, CaSO₄, SiO₂, etc., on the membrane surface.

  • For Specific Pollutants: Use activated carbon adsorption, ion exchange, specialized chemical precipitation, etc., to remove heavy metals, fluoride, boron, etc.

2.2 Core RO Unit: The "Main Force" for Deep Desalination and Purification

With adequate pretreatment, the RO unit undertakes the core task of deep desalination and removal of dissolved solids and small-molecule organics. Integration design requires optimization of:

• RO Configuration: Use multi-stage designs to improve system recovery rate; select specialized membrane elements (low-pressure, fouling-resistant, seawater, etc.) based on water quality.

• Energy Recovery: When treating high-salinity wastewater, integrate pressure exchangers (PX) or turbo-type energy recovery devices (ERD) to recover energy from the high-pressure concentrate, significantly reducing system energy consumption by 20%-40%.

2.3 Post-Treatment and Brine Management Module: The "Closed Loop" for Resource Recovery and Zero Liquid Discharge

• RO Permeate Post-Treatment: Depending on reuse standards (e.g., boiler feed water, process water), subsequent integration of ultraviolet disinfection, degasifiers, mixed-bed ion exchange, or Electrodeionization (EDI) for polishing may be required.

• RO Brine Treatment and Resource Recovery (Key Combined Processes):

  • Further Concentration Processes: Further concentrate RO brine using higher-pressure DTRO/STRO (Disc Tube/Spiral Reverse Osmosis) or Electrodialysis (ED) to reduce the volume sent to subsequent evaporation units.

  • Salt Separation and Resource Recovery Processes: Integrate a Nanofiltration (NF) unit before or after concentration. Utilizing NF's selective separation properties for monovalent/divalent ions, it achieves preliminary separation of salts like sodium chloride and sodium sulfate, laying the groundwork for subsequent evaporative crystallization and separate salt recovery.

  • Final Treatment Processes: Send the highly concentrated brine to evaporative crystallizers (MVR/MED) or evaporation ponds to ultimately achieve complete water separation and crystallization/solidification of salts, achieving Zero Liquid Discharge (ZLD). The crystallized salts, after treatment, hold potential for resource recovery.

3. Typical Combined Process Application Models

Based on different wastewater qualities and treatment goals, several mature RO combination processes have been established.

3.1 "MBR/UF + RO" Dual-Membrane Combination

• Combination Form: Biological system (or MBR) / Ultrafiltration (UF) as precise pretreatment for RO.

• Application Advantages: MBR/UF can efficiently remove suspended solids, colloids, bacteria, and macromolecular organics, providing RO with high-quality feed water (SDI<3), greatly reducing RO fouling risk. Suitable for municipal wastewater reuse and advanced treatment of comprehensive wastewater in industrial parks.

• Efficacy: Permeate can meet high-quality reuse water standards with high system recovery rates.

3.2 "Advanced Oxidation + RO" Combination

• Combination Form: Coupling processes like Fenton oxidation, ozone catalytic oxidation with RO.

• Application Advantages: Advanced oxidation can effectively degrade refractory organics and characteristic pollutants in the RO feed, reducing their biotoxicity and membrane fouling tendency, ensuring long-term stable RO operation. Suitable for wastewater containing refractory organics from chemical, pharmaceutical, dyeing industries.

• Efficacy: Improves the RO system's resistance to organic fouling and extends cleaning intervals.

3.3 "NF + RO" Synergistic Salt Separation and Softening Combination

• Combination Form: NF placed before RO or used to treat RO brine.

• Application Advantages:

  • NF as RO Pretreatment: Can remove most hardness, sulfate, and divalent ions, significantly reducing RO scaling risk and allowing RO to operate at higher recovery rates. Simultaneously, NF's better permeability for some monovalent ions enables preliminary salt separation.

  • NF Treating RO Brine: A key step for salt resource recovery in ZLD systems, separating mixed brine into streams dominated by NaCl and by Na₂SO₄, facilitating subsequent separate crystallization.

• Efficacy: Increases overall system recovery rate, reduces operational risks, and creates conditions for resource recovery.

3.4 "RO + Evaporative Crystallization" Zero Liquid Discharge Combination

• Combination Form: RO significantly concentrates and reduces wastewater volume, and the brine is fed to an evaporative crystallization unit.

• Application Advantages: This is the ultimate combination for achieving industrial wastewater ZLD. RO undertakes the main task of water recovery and volume reduction (typically recovery rate >75%), greatly reducing the scale and operating costs of the subsequent energy-intensive evaporative crystallization unit.

• Efficacy: Ultimately achieves complete wastewater recovery and separation/disposal of solid salts (or mixed salts), meeting the strictest environmental requirements.

4. Optimal Design and Intelligent Control of Integrated Systems

• System Integration Design: Requires optimization of the scale, recovery rate allocation, and process interconnection points of each unit based on life-cycle cost analysis to achieve optimal energy consumption, chemical usage, investment, and operational maintenance costs.

• Intelligent Control System: Integrates online water quality monitoring (pH, ORP, turbidity, SDI, TOC, etc.), pressure/flow sensors, and automated chemical dosing/cleaning systems. Utilizes big data and AI algorithms to achieve real-time system diagnosis, early warning, adaptive optimization of process parameters, and predictive maintenance, enhancing system stability and energy efficiency.

5. Challenges and Outlook

The main challenges facing current process integration include increased costs from complex pretreatment, accurate prediction and control of membrane fouling, efficient and economical treatment of high-salinity brine, and the quality and market outlets for recovered crystalline salts. Future development trends will focus on:

• Developing more fouling-resistant, chemically durable specialized RO membrane materials.

• Innovating low-energy, high-efficiency concentration and salt separation combined processes (e.g., Forward Osmosis-RO, Membrane Distillation-RO).

• Deepening the application of intelligent operation and digital twin technologies in integrated systems for smart water plant management.

• Promoting high-value-added salt recovery technologies and industrial chain development to improve the economics of ZLD projects.

Conclusion

The integration and combined application of industrial reverse osmosis membrane processes in wastewater treatment is a systematic process of optimizing, matching, and linking various unit technologies for specific wastewater qualities and treatment goals. By scientifically protecting RO operation through pretreatment and achieving deep purification, resource recovery, and ZLD goals through innovative combined processes (especially coupling with NF, advanced oxidation, evaporative crystallization), it has become a technological pinnacle in the field of industrial wastewater treatment. With the continuous development of new materials, new processes, and intelligent technologies, RO-centered integrated systems will play an increasingly crucial role in the sustainable management of industrial water resources.