Core Applications of Reverse Osmosis Membranes in the Resource Recovery and Reuse of Power Plant Wastewater

Abstract

With the increasingly severe issue of water resource scarcity and continuously rising environmental protection requirements, thermal power plants, as major industrial water consumers, face an imperative need for zero liquid discharge and resource recovery of their wastewater to achieve sustainable industry development. Reverse osmosis (RO) membrane technology, leveraging its high-efficiency desalination and stable reliability, has become the core process unit in advanced treatment and reuse systems for power plant wastewater. This article systematically elaborates on the key role, typical process flows, encountered technical challenges, and optimization strategies of RO membranes in various power plant wastewater reuse scenarios. It aims to provide technical reference and practical guidance for the intensive utilization of water resources in power plants.

1. Strategic Significance of Power Plant Wastewater Reuse and the Positioning of RO Technology

1.1 Strategic Significance

Thermal power plants have massive water consumption, primarily concentrated in circulating cooling water systems, boiler feedwater systems, desulfurization systems, and wet electrostatic precipitators. Promoting the advanced treatment and reuse of internal wastewater (including circulating cooling blowdown, desulfurization wastewater, and miscellaneous industrial drainage) can significantly reduce freshwater intake, minimize environmental risks associated with wastewater discharge, and represents a crucial measure for achieving "water conservation, emission reduction, and green development." This offers substantial economic, social, and environmental benefits.

1.2 Core Positioning of RO Technology

Within the technological framework for power plant wastewater reuse, reverse osmosis membrane technology assumes the core function of "advanced desalination and purification." It efficiently removes the vast majority of dissolved salts, ions, colloids, organic matter, and microorganisms from water. The produced water quality can meet or even exceed standards for high-quality industrial water (such as boiler feedwater or make-up water for circulating cooling systems). It is a critical step in transforming wastewater into high-value reuse water.

2. Typical Process Routes for RO Membrane Treatment of Different Power Plant Water Sources

For wastewater of different origins and qualities within a power plant, it is necessary to establish an integrated process following the principle of "separate collection, segregated treatment based on quality, and cascaded reuse." The RO membrane plays a pivotal role within this system.

2.1 Treatment and Reuse of Circulating Cooling Blowdown

• Source Characteristics: Large volume, relatively high salinity (TDS typically 1500-5000 mg/L), containing residual scale/corrosion inhibitors, biocides, and some suspended solids.

• Core Process Route:

  • Pretreatment: Employ a combined process of "coagulation-sedimentation + filtration + Ultrafiltration (UF)" to efficiently remove suspended solids, colloids, and some organic matter, ensuring effluent SDI <3 to meet RO feed requirements.

  • Desalination Core: Use a single-stage or two-stage RO system for deep desalination. Single-stage RO permeate can be directly reused as make-up water for the circulating cooling system; for higher quality requirements (e.g., reuse for boilers), a second-stage RO or electrodeionization (EDI) can be added for polishing.

  • Concentrate Management: RO concentrate can be further concentrated and reduced in volume via High-Pressure RO (HPRO) or electrodialysis (ED), with the final concentrated stream sent to an evaporation crystallization system for zero liquid discharge.

• Technical Advantage: Significantly reduces the salinity and scaling tendency of the make-up water for the circulating water system, allowing the cycles of concentration to increase to over 6 times, resulting in notable water savings.

2.2 Treatment and Reuse of Desulfurization Wastewater

• Source Characteristics: Complex water quality, high salinity (especially high Cl⁻ and SO₄²⁻), high suspended solids, high hardness, containing heavy metals and trace metals. It is one of the most difficult wastewaters to treat in power plants.

• Core Process Route:

  • Pretreatment and Softening: Use processes like "chemical precipitation softening (e.g., soda ash/sodium hydroxide softening) + Tubular Microfiltration" to remove calcium, magnesium, heavy metals, and most suspended solids, reducing scaling and fouling risks.

  • Membrane Concentration: The pretreated wastewater enters a specialty fouling-resistant RO membrane system or a Disc-Tube RO (DTRO) system for concentration and volume reduction. Due to the challenging water quality, specific antiscalants are often required, and recovery rates must be strictly controlled.

  • Salt Separation and Crystallization: The RO concentrate can be further processed via Nanofiltration (NF) for salt separation, producing separate concentrated streams primarily containing sodium chloride and sodium sulfate, which are then sent to evaporative crystallizers respectively. This enables salt resource recovery and achieves the final goal of zero liquid discharge.

• Technical Advantage: Solves the challenging treatment of desulfurization wastewater, achieving zero liquid discharge and partial salt resource recovery.

2.3 Treatment and Reuse of Miscellaneous Industrial and Sanitary Wastewater

• Source Characteristics: Includes boiler blowdown, floor wash water, initial rainwater, etc. The water quality is relatively better, though organic content might be higher.

• Core Process Route:

  • Biological + Advanced Treatment: Typically employs a combined process of "biological treatment (e.g., Membrane Bioreactor - MBR) + UF + RO". MBR ensures good biodegradation and solid-liquid separation, while UF acts as a precise barrier for RO.

  • Reuse Destination: The high-quality RO permeate can be used as make-up water for circulating cooling systems, feedwater for demineralized water systems, or even, after mixed-bed or EDI polishing, as boiler feedwater, achieving the most efficient internal water reuse.

3. Key Technical Considerations for RO System Application in Power Plants

3.1 Adaptability Selection of Pretreatment Processes

For different source water qualities, it is essential to design efficient and stable pretreatment processes, which form the cornerstone for ensuring the long-term stable operation of the RO system. Key control parameters include Silt Density Index (SDI), turbidity, hardness, organic matter, and oxidizing agents.

3.2 Targeted Membrane Material and System Design

• Membrane Selection: For water sources with high fouling risk, opt for low-fouling, wide-feed-spacer RO membrane elements.

• System Design: Rationally calculate inter-stage flux ratios and the number of stages based on feedwater salinity and recovery rate requirements. For high-salinity wastewater, configurations like inter-stage boosting or multi-stage series can be used. Integrating efficient energy recovery devices can significantly reduce system energy consumption.

3.3 Chemical Dosing and Scale Inhibition Control

Based on water quality analysis results, precisely dose antiscalants, reducing agents, and non-oxidizing biocides. Special attention must be paid to preventing silica scaling, calcium sulfate scaling, and organic fouling. Establish intelligent dosing strategies based on real-time monitoring.

3.4 Intelligent Operation and Maintenance

Establish an intelligent monitoring platform based on parameters such as online water quality instruments, pressure, flow, and conductivity. Use parameters like normalized flux, salt rejection, and differential pressure for performance evaluation and fouling prediction. Implement predictive maintenance and optimized chemical cleaning protocols to extend membrane lifespan.

4. Economic Analysis and Technological Development Trends

4.1 Economic Analysis

Although RO systems involve initial investment and operating costs, their comprehensive benefits are significant:

• Water Savings: Reduces freshwater intake and associated water fees.

• Emission Reduction Benefits: Lowers wastewater discharge fees and environmental risks.

• Operational Benefits: Increases cycles of concentration in the cooling water system, reducing chemical consumption.

  Typically, the payback period for such projects ranges from 3 to 6 years, demonstrating good economic feasibility.

4.2 Technological Development Trends

• Membrane Material Innovation: Development of new RO membranes with higher salt rejection, lower energy consumption, and stronger anti-fouling and oxidation resistance.

• Process Integration and Optimization: Coupling RO with cutting-edge technologies like forward osmosis (FO) and membrane distillation (MD) to treat high-salinity concentrates, pursuing lower energy consumption and higher recovery rates.

• System Intelligence: Deep integration of Internet of Things (IoT), big data, and artificial intelligence (AI) technologies to achieve intelligent monitoring, optimized operation, and fault self-diagnosis across the entire process from source water to product water, building "smart water treatment workshops."

Conclusion

Reverse osmosis membrane technology is the core enabling technology for power plants to achieve their strategic goals of wastewater resource recovery, reuse, and zero liquid discharge. By constructing quality-specific treatment and cascaded reuse process chains centered on RO, tailored to the characteristics of different water sources, power plants can convert the vast majority of their internal wastewater into high-quality reuse water. This effectively addresses water resource constraints and environmental pressures. In the future, with continuous advancements in membrane technology and increasing levels of intelligence, RO will undoubtedly play an increasingly critical role in water resource recycling within power plants and across broader industrial sectors. It will serve as a core driver in promoting green industrial transformation and sustainable development.