Core Technologies and Engineering Application of Reverse Osmosis Membrane Treatment for Desulfurization Wastewater in Thermal Power Plants

Abstract

Wastewater generated from wet flue gas desulfurization (WFGD) in thermal power plants is characterized by its complex composition, high salinity, high hardness, high suspended solids, heavy metal content, and the strong corrosiveness of chloride ions. It represents a critical bottleneck for achieving "zero liquid discharge" (ZLD) or "near-zero liquid discharge" in power plants. Reverse osmosis membrane technology, as an efficient physical separation method, is a core process unit for deep desalination, volume reduction, and reuse of desulfurization wastewater. However, the high concentrations of scaling ions, organic matter, and solid suspended solids pose severe challenges to the stable operation of RO membranes. This article systematically elaborates on the core technologies involved in RO membrane treatment of desulfurization wastewater, including efficient pretreatment, selection of special membrane elements, anti-scaling processes, system integration optimization, and engineering application practices. The aim is to provide a systematic technical solution for the large-scale engineering application of this technology.

1. Water Quality Characteristics and Treatment Challenges of Desulfurization Wastewater

1.1 Water Quality Analysis

Desulfurization wastewater primarily originates from the slurry dewatering system of limestone-gypsum wet FGD processes. Its typical characteristics are as follows:

• High Salinity: Total Dissolved Solids (TDS) typically range from 20,000-50,000 mg/L or even higher. Main ions are Cl⁻, SO₄²⁻, Na⁺, Mg²⁺, Ca²⁺, etc. Among these, Cl⁻ concentration can reach 10,000-20,000 mg/L, a primary factor leading to corrosion and increased osmotic pressure.

• High Hardness and Scaling Tendency: High concentrations of Ca²⁺ and Mg²⁺ readily combine with SO₄²⁻ and CO₃²⁻ to form sparingly soluble scales like calcium sulfate, calcium carbonate, and magnesium silicate. This is the primary obstacle for membrane treatment.

• High Suspended Solids and Heavy Metals: Contains fine gypsum (CaSO₄·2H₂O) particles, unreacted limestone powder, fly ash, and trace amounts of heavy metals like mercury, arsenic, selenium, and lead.

• Chemical Oxygen Demand and Organic Matter: Contains partially unoxidized sulfites, additives (e.g., scale inhibitors, corrosion inhibitors), and organics introduced from the process.

1.2 Core Technical Challenges for RO Membrane Treatment

• Membrane Scaling Risk: Inorganic salts like CaSO₄, CaCO₃, SiO₂ easily become supersaturated and crystallize on the membrane surface, leading to irreversible hard scale, difficult cleaning, and permanent flux decline.

• Colloidal and Particulate Fouling: Fine gypsum particles, colloidal silica, metal hydroxides, etc., cause membrane surface fouling and feed channel clogging.

• High Osmotic Pressure and Energy Consumption: High salinity results in feed water osmotic pressure as high as 2.0-5.0 MPa, requiring high system operating pressure (often 8-12 MPa or higher) and leading to significant energy consumption.

• Chemical Tolerance Requirements: The wastewater is weakly acidic to neutral and may contain oxidizing substances, demanding membrane elements with good chemical stability and pressure resistance.

  

2. Core Technology System for RO Membrane Treatment

Addressing the above challenges requires the successful application of RO technology to construct a core technology system integrating the four pillars of: "pretreatment protection, membrane system optimization, fouling control, and concentrate disposal".

2.1 Enhanced Pretreatment Technology: The "Firewall" for Stable Membrane Operation

The pretreatment goal is to remove suspended solids and colloids, significantly reduce hardness and silica content, and condition the water quality to meet RO feed requirements (SDI<3, turbidity<0.2 NTU, controlled scaling ion saturation).

• Chemical Precipitation Softening: The core process. Employs a "Two-Stage Lime-Soda Ash Softening" process.

  1. Primary Lime Softening: Dosing lime milk to remove Mg²⁺, part of SiO₂, and raise pH to promote heavy metal precipitation.

  2. Secondary Soda Ash Softening: Dosing sodium carbonate to further remove Ca²⁺, forming calcium carbonate precipitate. This process can reduce calcium hardness below 50 mg/L and significantly lower total hardness.

• Coagulation-Flocculation and Solid-Liquid Separation: Dosing iron/aluminum salt coagulants and polymer flocculants to form flocs for co-precipitation of heavy metals, colloidal silica, and organics. Employ "High-Rate Clarifiers" or "Tubular Microfiltration (TMF)" for efficient solid-liquid separation, yielding clarified effluent.

• Filtration and Guard Filtration: Subsequent installation of "Multi-Media Filters" and an "Ultrafiltration (UF)" unit. Among these, fouling-resistant, chemical-cleaning-tolerant tubular or externally pressurized Ultrafiltration is the key barrier, ensuring stable compliance of RO feed water SDI.

2.2 RO Membrane System Design and Optimization Technology

• Specialty Membrane Element Selection: Must select Seawater Desalination RO Membranes or High-Pressure Fouling-Resistant RO Membranes. These membranes offer higher salt rejection (>99.5%), stronger pressure resistance (up to 12 MPa or higher), and superior anti-fouling surface characteristics.

• Integrated Anti-Scaling Processes:

  • High-Efficiency Scale Inhibitor/Dispersant: For high SO₄²⁻ and residual Ca²⁺, use specialized scale inhibitors with extremely high inhibition efficiency against calcium sulfate and silica scaling. Precise dosing requires accurate calculation of ion product and saturation index via software simulation (e.g., GPSA, WT).

  • Softening-RO Coupling Optimization: By optimizing the residual hardness in the softened water effluent and synergizing with scale inhibitors, the RO system can operate at higher recovery rates while ensuring system safety.

• System Configuration and Energy Recovery:

  • High-Pressure Design: Use high-pressure pumps, piping, and valves capable of withstanding operating pressures above 10 MPa.

  • Energy Recovery: Integrate Pressure Exchangers (PX) or Turbo-type Energy Recovery Devices in the high-pressure RO stage. This can recover 30-40% of the concentrate pressure energy, a key technology for reducing system energy consumption (specific power consumption can be reduced to 8-12 kWh per ton of water).

  • Staging/Split-Stream Design: Employ configurations like "Primary RO + Concentrate RO" or "Multi-Stage RO in Series" to gradually increase the concentration factor and optimize overall recovery (up to 60-80%).

2.3 Intelligent Membrane Fouling Monitoring and Efficient Cleaning Technology

• Online Monitoring and Early Warning: Real-time monitoring of feed water pH, ORP, turbidity, hardness, and RO system normalized flux, salt rejection, and inter-stage differential pressure. Establish a data-model-based fouling early warning system.

• Specialized Chemical Cleaning:

  • Acidic Cleaning: For inorganic scale, use low-pH citric or hydrochloric acid solutions.

  • Alkaline Cleaning: For organic and microbial fouling, use high-pH NaOH solutions combined with surfactants.

  • Cleaning formulations and cycles must be personalized based on water quality and operational data.

2.4 Ultimate Disposal Technology for Concentrated Brine

RO permeate is reused, but it produces a concentrated brine (20-40% of the feed volume) with high TDS (up to 80,000-150,000 mg/L). Its disposal is the final hurdle for "zero liquid discharge".

• Further Volume Reduction: Use Disc-Tube RO (DTRO), Electrodialysis (ED), or Forward Osmosis (FO) to further concentrate the RO brine, reducing the volume for evaporation.

• Evaporation Crystallization and Solidification: Finally, feed the concentrate into a Multiple Effect Evaporation (MEE) or Mechanical Vapor Recompression (MVR) Evaporation system. Water evaporates, and salts crystallize out. This is currently the mainstream end-of-pipe process for achieving "zero discharge", capable of separating industrial-grade NaCl or mixed salts.

3. Typical Engineering Application Case Analysis

Case: ZLD Project for Desulfurization Wastewater at a 2×1000 MW Ultra-Supercritical Coal-Fired Power Plant

• Design Flow Rate: Desulfurization wastewater 20 m³/h.

• Core Process Flow:

  "Desulfurization Wastewater → Flow/Quality Equalization → Two-Stage Chemical Softening (Lime+Soda Ash) → Coagulation-Sedimentation → Tubular Microfiltration (TMF) → Primary Seawater RO (60% Recovery) → Concentrate DTRO (50% Recovery) → MVR Evaporation Crystallization"

• Key Technical Points:

  1. Tubular Microfiltration replaces traditional clarifiers+filters, providing superior and more stable effluent quality with a smaller footprint.

  2. Primary RO uses seawater membranes to tolerate high salinity and pressure.

  3. DTRO provides ultimate concentration of the primary RO concentrate, tolerating high turbidity and high fouling potential feed.

  4. MVR Evaporation Crystallization ultimately yields mixed salts, with condensate reused.

• Operational Performance:

  • Overall system water recovery > 90%. Product water is reused in the circulating cooling water system.

  • Zero liquid discharge of wastewater achieved.

  • Membrane system operates stably, with cleaning cycles approximately every 2-3 months.

4. Techno-Economic Analysis and Development Trends

4.1 Economic Analysis

• Capital Costs: Includes pretreatment, membrane system, evaporation crystallization, and auxiliary systems, resulting in relatively high unit investment. The evaporation crystallization unit accounts for the largest share.

• Operating Costs: Primarily consist of electricity (high-pressure pumps, evaporator), chemicals (scale inhibitors, acids/bases), membrane replacement, and maintenance. Operating cost per ton of water is approximately 30-60 RMB.

• Benefits: Achieves zero liquid discharge for the entire plant, meeting environmental requirements and eliminating discharge fees; recovers water resources, offering significant environmental and social benefits. It is necessary and has promotion value in regions with strict environmental policies and water scarcity.

4.2 Technology Development Trends

• Membrane Material Innovation: Research and develop specialty RO membranes with higher salt rejection, stronger fouling resistance, and lower operating pressure.

• Intelligent Pretreatment: Develop new, efficient, and low-cost chemicals and processes for softening and silica removal.

• Low-Energy Concentration Technology: Develop new concentration technologies like Forward Osmosis-RO coupling and Membrane Distillation to reduce energy consumption in the pre-evaporation stage.

• Resource Recovery: Explore separation and purification technologies for valuable components in the concentrated brine, such as sodium chloride, sodium sulfate, and gypsum, to achieve "waste salt" resource recovery.

• Smart Operation and Maintenance: Utilize digital twins and AI algorithms to achieve intelligent full-process monitoring, optimized operation, and predictive maintenance.

Conclusion

Reverse osmosis membrane technology is the core technical link for treating desulfurization wastewater in thermal power plants, achieving deep volume reduction and reuse. Its successful application relies on a systematic engineering solution tailored to the characteristics of high salinity, high hardness, and high scaling tendency. The key lies in efficient and reliable pretreatment for softening and hardness removal, optimized design of specialty high-pressure fouling-resistant RO membrane systems, precise anti-scaling strategies, and the ultimate evaporation crystallization disposal of concentrated brine. Through technology integration and optimization, the RO system can operate stably at high recovery rates, laying the foundation for subsequent ZLD processes. In the future, with advancements in materials science, process coupling, and intelligent technologies, the application of RO in desulfurization wastewater treatment will continue to develop towards lower energy consumption, higher reliability, and stronger resource recovery. It will provide solid technical assurance for the green transformation of the thermal power industry and water environmental security.