Process Optimization of Reverse Osmosis Membranes in Reclaimed Water Reuse Treatment for Power Plants

Abstract

With tightening water resource constraints and rising environmental requirements, achieving internal water resource recycling within thermal power plants has become key to sustainable development, cost reduction, and efficiency improvement. Reverse osmosis (RO) membrane technology, as the core process for advanced desalination and purification, is a critical guarantee for ensuring that reclaimed water (primarily consisting of circulating cooling blowdown, desulfurization wastewater, and miscellaneous industrial drainage) can be reused for high-quality purposes such as boiler feedwater and circulating cooling water. However, power plant reclaimed water is characterized by complex water composition, high salinity, and significant fluctuations in organic matter and scaling ion concentrations, posing challenges to the stable, efficient, and economical operation of RO systems. Addressing the characteristics of power plant reclaimed water, this article systematically elaborates on process optimization strategies for RO systems in terms of pretreatment enhancement, membrane system design, operating parameter control, concentrate management, and intelligent management. The aim is to construct an optimized operating mode that balances efficiency, cost, and reliability, providing a technical pathway for the intensive utilization of water resources in power plants.

1. Characteristics of Power Plant Reclaimed Water and Core Challenges for RO Treatment

1.1 Main Water Sources and Water Quality Characteristics

• Circulating Cooling Blowdown: Large volume, relatively high salinity (TDS typically 1500-5000 mg/L), containing residual corrosion inhibitors, scale inhibitors, and biocides, moderate suspended solids and colloid content, relatively high water temperature.

• Desulfurization Wastewater: Complex composition, high salinity (especially Cl⁻, SO₄²⁻), high suspended solids, high hardness, containing heavy metals (e.g., mercury, arsenic) and trace unreacted absorbents; one of the more difficult wastewaters to treat.

• Miscellaneous Industrial Drainage (including boiler blowdown, floor wash water, etc.): Relatively better water quality, but may contain oils and residual chemicals.

• Comprehensive Characteristics: Water quality and quantity fluctuate significantly with unit load, season, and coal quality; presence of special pollutants like silica, boron, and organic matter.

1.2 Core Challenges for RO Systems

• High Salinity and High Osmotic Pressure: Constrains system recovery rate and increases energy consumption.

• High Scaling Risk: High content of scaling ions like calcium, magnesium, sulfate, and silicate, easily leading to inorganic salt scaling.

• Organic and Microbial Fouling: Residual chemicals, organics, and suitable water temperatures facilitate microbial growth, leading to biofouling.

• Water Quality Fluctuation Impacts: Places high demands on the stability of chemical dosing and operating parameters.

  

2. Enhancement and Optimization of Pretreatment Processes

Optimized pretreatment is the cornerstone for the long-term stable operation of the RO system, aiming to produce a stable water source meeting RO feed requirements (SDI<3, controlled scaling tendency and fouling risk).

2.1 Separate Treatment and Water Source Optimization

• Strategy: Classify, collect, and perform separate pretreatment on reclaimed water of different qualities. For example, perform independent advanced pretreatment (e.g., chemical softening + tubular microfiltration) on high-salinity, high-pollution desulfurization wastewater; combine and treat relatively cleaner circulating blowdown and miscellaneous drainage. This reduces the treatment difficulty and chemical consumption of the mixed water.

2.2 Efficient Solid-Liquid Separation and Colloid Removal

• Technology Optimization:

  • Enhanced Coagulation: Optimize the type and dosage of coagulants (e.g., PAC, PFS) and flocculants (PAM) through jar testing; use efficient sedimentation equipment (e.g., high-rate clarifiers, magnetic coagulation).

  • UF as Core Protection: Employ fouling-resistant, high-flux externally pressurized or submerged Ultrafiltration (UF) as the core pretreatment unit for RO. Optimize UF operating flux, backwash and air-scour frequency, and chemical cleaning protocols to ensure its effluent SDI is stably <3, effectively retaining colloids, bacteria, and macromolecular organics.

2.3 Control of Scaling Ions and Specific Pollutants

• Softening/Hardness Removal: For high-hardness water, use processes like "lime-soda ash softening" or "ion exchange softening" to effectively reduce calcium and magnesium ion concentrations.

• Silica Control: Enhance silica co-precipitation removal by adjusting pH, dosing magnesium agents, or specialized coagulants.

• Organic Matter Reduction: When necessary (e.g., high raw water COD), add units like "ozone catalytic oxidation" or "biological activated carbon" to degrade refractory organics and reduce membrane fouling risk.

3. Design and Operation Optimization of the RO Membrane System

3.1 Membrane System Configuration and Selection Optimization

• Membrane Element Selection: For high-salinity, high-fouling-risk water, select wide-feed-spacer, fouling-resistant brackish water or seawater desalination RO membranes. For strict requirements on specific pollutants (e.g., boron), high-boron-rejection membranes can be selected.

• System Design Optimization:

  • Recovery Rate Staged Design: Design recovery rates in stages or for separate systems based on feed water salinity. Combinations like "Primary RO + Concentrate RO" or "Primary Two-Stage RO" can be used to maximize overall system recovery (up to 75%-85%) while ensuring product water quality.

  • Inter-stage Flux Balancing: Optimize the number and arrangement of membrane elements to avoid excessively high flux in front-end elements (exacerbating fouling) and underutilization due to low flux in rear-end elements.

  • Energy Recovery Integration: Integrate Pressure Exchangers (PX) or other energy recovery devices in high-pressure RO stages (e.g., concentrate RO) to recover pressure energy from the concentrate, reducing system power consumption by 20-35%.

3.2 Fine Control of Operating Parameters

• Dynamic Optimization of Flux and Recovery Rate: Based on online water quality monitoring data (conductivity, pH, temperature), dynamically adjust system operating flux and recovery rate via model calculations or expert systems to keep operation within the "optimal efficiency zone" for current water conditions, avoiding overload or inefficient operation.

• Precision Chemical Dosing:

  • Antiscalant Optimization: Based on real-time water quality (ion concentration, pH, temperature) and system recovery rate, use software simulation to calculate scaling tendency and dynamically adjust the type and dose of antiscalants, achieving "dosing on demand."

  • Reducing Agents and Non-Oxidizing Biocides: Precisely control dosage to balance membrane protection and microbial inhibition.

4. Membrane Fouling Control and Cleaning Strategy Optimization

4.1 Intelligent Fouling Early Warning and Diagnosis

• Data-Driven Early Warning Models: Real-time monitoring of key parameters like normalized flux, salt rejection, and inter-stage differential pressure. Use trend analysis or machine learning algorithms to establish early warning models for membrane fouling, enabling a shift from "scheduled cleaning" to "predictive cleaning."

4.2 Optimization of Efficient Cleaning Protocols

• Accurate Pollutant Analysis: Periodically sample and analyze cleaning solution composition to determine the main fouling type (inorganic scale, organics, biofilm).

• Cleaning Formulation and Procedure Optimization:

  • For inorganic scaling, use acidic cleaning (e.g., citric acid, hydrochloric acid).

  • For organic and biological fouling, use alkaline cleaning (NaOH with surfactants, EDTA) and non-oxidizing biocide cleaning.

  • Optimize cleaning temperature, flow rate, and soak time to improve cleaning efficiency. Explore enhancement techniques like ultrasound assistance or online coagulation cleaning.

5. Optimization of Concentrate Management and Resource Recovery Pathways

5.1 Concentrate Volume Reduction

• Use technologies like High-Pressure RO (HPRO), Electrodialysis (ED), or Forward Osmosis (FO) to further concentrate the RO brine, drastically reducing the treatment load and scale of subsequent evaporation/crystallization units. This is key to reducing the total investment and operating cost of zero liquid discharge systems.

5.2 Salt Separation and Resource Recovery

• Introduce a Nanofiltration (NF) unit before or after concentration to achieve preliminary separation of sodium chloride and sodium sulfate. Subsequently, use processes like MVR evaporative crystallization coupled with freeze crystallization to separately produce industrial-grade sodium chloride and sodium sulfate, enhancing resource value and reducing solid waste disposal costs.

6. Intelligent Operation Management and Benefit Assessment

6.1 Intelligent Management and Control Platform

• Build an integrated smart water treatment system encompassing data acquisition, intelligent analysis, optimized control, and mobile operation and maintenance. Achieve visual monitoring of the entire process, adaptive optimization of key parameters, predictive diagnosis of equipment faults, and digital management of inspections and work orders.

6.2 Comprehensive Benefit Assessment

• Economic Benefits:

  • Direct Water Savings: Reclaimed water reuse rates can reach 70%-90%, significantly reducing freshwater intake and water fees.

  • Energy Saving and Consumption Reduction: Through process optimization and energy recovery, reduce specific power consumption per unit of water by 10%-25%.

  • Chemical Savings: Precise dosing reduces chemical consumption by 10%-20%.

  • Life Extension and Cost Reduction: Optimized operation and cleaning extend membrane life by 1-2 years.

• Environmental and Social Benefits: Achieve wastewater reduction or near-zero discharge,减轻 (alleviating) environmental pressure; enhance the plant's water self-sufficiency and risk resilience; align with green power plant construction requirements.

7. Conclusion and Outlook

Process optimization of RO membrane technology in reclaimed water reuse treatment for power plants is a systematic engineering project covering the entire chain of "water source - pretreatment - membrane system - concentrate - intelligent operation." The core lies in ensuring feed water quality through refined pretreatment, improving separation efficiency and economics through system design and operation optimization, and achieving stable and reliable operation through intelligent management and control. In the future, with the continuous development of higher-performance fouling-resistant membrane materials, lower-energy membrane concentration processes, and more intelligent AI algorithms and digital twin technologies, the efficiency, economics, and automation level of RO-based reclaimed water reuse systems in power plants will be continuously enhanced. Through ongoing technological optimization and management innovation, RO technology is destined to play an irreplaceable core role in helping power plants achieve the goals of "water saving, emission reduction, cost reduction, and efficiency improvement, and green development," providing a solid foundation for water resource security for the sustainable future of the power industry.