How to Pair Ultra-Pure Water Systems with Reverse Osmosis Membrane Systems
03 Jun 2025
1. Principles for Pairing Ultra-Pure Water Systems with Reverse Osmosis Membrane Systems
1.1 Raw Water Quality Compatibility
When pairing ultra-pure water systems with reverse osmosis (RO) membrane systems, the first consideration is the quality of the raw water. Parameters of raw water quality include hardness, turbidity, chemical oxygen demand (COD), and total dissolved solids (TDS). According to the "Hygienic Standard for Drinking Water" (GB5749-2023), the TDS value of raw water is generally between 50-1000mg/L, and hardness is usually between 50-300mg/L (calculated as CaCO₃). These parameters directly affect the performance and lifespan of RO membranes. For instance, raw water with high hardness can cause scaling on the surface of RO membranes, reducing their permeability and increasing operating costs. Therefore, when pairing systems, it is necessary to select appropriate pre-treatment equipment based on the raw water quality, such as softeners and activated carbon filters, to ensure that the water entering the RO membrane system meets the requirements. At the same time, the pore size of the RO membrane (generally between 0.1-10nm) also needs to match the particle size of pollutants in the raw water to achieve efficient filtration.
1.2 Complementary System Functions
Ultra-pure water systems and RO membrane systems have strong complementary functions. RO membrane systems are mainly used to remove dissolved solids, organic matter, and microorganisms from raw water, with a desalination rate that can typically reach 95%-99%. Ultra-pure water systems further remove ion impurities from water through ion exchange, mixed bed, and other technologies, elevating the water quality to ultra-pure water standards (resistivity ≥ 18.2MΩ·cm). When pairing systems, the RO membrane system, as a pre-treatment unit, can effectively reduce the burden on the ultra-pure water system and extend its service life. For example, in the electronics industry, ultra-pure water is used for chip manufacturing, where water quality requirements are extremely high. The RO membrane system can remove most impurities, extending the regeneration cycle of ion exchange resins in the ultra-pure water system by 30%-50%, thereby improving the overall system's operational efficiency and water quality stability. In addition, the ultra-pure water system can also monitor water quality parameters (such as conductivity, TOC, etc.) to provide feedback for the operation of the RO membrane system, achieving collaborative optimization of the systems.
1.3 Cost-Effectiveness Balance
In the pairing of ultra-pure water systems with RO membrane systems, cost-effectiveness balance is a key factor. The cost of an RO membrane system mainly includes equipment purchase costs, operating energy consumption, and maintenance costs. For example, for an RO membrane system with a processing capacity of 10m³/h, the equipment purchase cost is about 500,000 yuan, the annual operating energy consumption (electricity cost) is about 100,000 yuan, and the membrane replacement cost every 3 years is about 150,000 yuan. The cost of the ultra-pure water system is mainly focused on the replacement and regeneration costs of ion exchange resins. Reasonable system pairing can reduce overall operating costs while ensuring water quality. For example, by optimizing the recovery rate of the RO membrane system (generally between 70%-80%), wastewater discharge can be reduced, lowering water treatment costs. At the same time, selecting appropriate membrane materials (such as polyamide composite membranes) and design parameters for the ultra-pure water system (such as resin filling volume) can extend equipment lifespan and reduce maintenance costs while ensuring water quality. In addition, the degree of system automation also affects cost-effectiveness. Adopting an automated control system can improve operational efficiency, reduce human intervention, and lower operating costs.
2. Key Points for Reverse Osmosis Membrane System Configuration
2.1 Membrane Element Selection
The membrane element is the core component of the RO membrane system, and its selection directly affects the system's performance and operating costs.
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Membrane Material Selection: Common RO membrane materials include cellulose acetate (CA) and polyamide (PA). Polyamide membranes have a higher desalination rate (up to over 99.5%) and good chemical resistance, making them suitable for treating raw water with high salinity and hardness. For example, in seawater desalination, polyamide membranes are the preferred material. In contrast, cellulose acetate membranes have better chlorine resistance, making them suitable for treating raw water with higher chlorine content, such as municipal water supplies.
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Membrane Element Form: Membrane elements come in various forms, including spiral-wound, plate and frame, tubular, and hollow fiber. Spiral-wound membrane elements are compact and cost-effective but require higher feed water quality. Hollow fiber membrane elements have a larger effective membrane area and are suitable for treating raw water with high pollution loads. For example, in RO systems treating oily wastewater, hollow fiber membrane elements can better withstand the impact of pollutants.
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Membrane Element Performance Parameters: Performance parameters of membrane elements include flux, desalination rate, and fouling resistance. Flux is an important indicator of the membrane element's water production capacity, generally ranging from 10-50L/m²·h. The desalination rate reflects the membrane element's ability to remove dissolved solids. Fouling resistance determines the membrane element's service life. In actual selection, these parameters need to be considered comprehensively based on the raw water quality and system design requirements. For example, in RO systems treating surface water, the fouling resistance of the membrane element is particularly important because surface water contains a large amount of organic matter and microorganisms.
2.2 High-Pressure Pump Matching
The high-pressure pump provides the necessary pressure for the RO membrane system to overcome the membrane's osmotic resistance and ensure normal system operation.
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Pump Flow and Head: The flow rate of the high-pressure pump should match the processing capacity of the RO membrane system. For example, for an RO system with a processing capacity of 5m³/h, the flow rate of the high-pressure pump should be slightly higher than 5m³/h to ensure sufficient feed water pressure for the system. At the same time, the pump's head needs to be determined based on the operating pressure requirements of the membrane elements. Generally, the operating pressure of RO membranes is between 1-5MPa, depending on the membrane type and the salinity of the raw water. For example, in RO systems treating high-salinity seawater, the operating pressure is usually around 5MPa.
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Pump Material and Corrosion Resistance: Since the raw water treated by the RO system may contain corrosive substances, the material of the high-pressure pump needs to have good corrosion resistance. Common pump materials include stainless steel and titanium alloy. Stainless steel pumps are suitable for treating general raw water, while titanium alloy pumps are more suitable for treating highly corrosive raw water, such as chlorinated seawater.
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Pump Efficiency and Energy Consumption: The efficiency of the high-pressure pump directly affects the system's operating energy consumption. High-efficiency and energy-saving high-pressure pumps can reduce the system's operating costs. For example, high-pressure pumps with variable frequency speed control technology can automatically adjust the flow rate and pressure according to the system's actual needs, thereby improving the pump's operating efficiency and achieving energy savings of 20%-30%.
2.3 System Layout Optimization
A rational system layout can improve the operational efficiency and operational convenience of the RO membrane system.
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Pre-treatment Unit Layout: Pre-treatment units (such as sand filters, activated carbon filters, softeners, etc.) should be arranged at the front end of the RO membrane system to ensure that the water entering the membrane system meets the requirements. The connecting pipes between pre-treatment units should be as short and straight as possible to reduce head loss. For example, in a typical RO system, sand filters and activated carbon filters are usually arranged in series to first remove large particulate impurities from the water and then remove organic matter and residual chlorine.
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Membrane Element Arrangement: There are two main ways to arrange membrane elements: in parallel and in series. Parallel arrangement can increase the system's water production capacity but requires higher feed water quality; series arrangement can improve the system's desalination rate but results in lower water production capacity. In practical applications, a combination of parallel and series arrangements is usually adopted. For example, in a large RO system, multiple membrane elements can be connected in parallel to form a group, and then several groups of membrane elements can be connected in series to achieve both high water production capacity and high desalination rate.
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System Piping Design: The system piping should minimize the number of elbows and valves to reduce head loss and system resistance. At the same time, the piping design should facilitate maintenance and inspection. For example, using detachable flange connections makes it convenient to replace damaged piping components and valves. In addition, the material of the system piping also needs to be selected based on the raw water quality and system operating pressure. Common piping materials include stainless steel and PVC.
3. Post-treatment Pairing for Ultra-Pure Water Systems
3.1 Application of Ion Exchange Technology
Ion exchange technology is a key part of the post-treatment process in ultra-pure water systems, mainly used to remove ion impurities from water and further improve water quality. Ion exchange resins remove calcium, magnesium, sodium, and other cations, as well as chloride and sulfate anions, through ion exchange reactions with ions in the water, bringing the water quality up to ultra-pure water standards (resistivity ≥ 18.2MΩ·cm).
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Resin Type Selection: Common ion exchange resins include cation exchange resins and anion exchange resins. Cation exchange resins are mainly used to remove cations such as calcium and magnesium from water; anion exchange resins are used to remove anions such as chloride and sulfate. In practical applications, a combination of cation and anion exchange resins is usually used to achieve more efficient ion removal. For example, in the electronics industry, to ensure the high purity of water used in chip manufacturing, a dual-bed ion exchange system is often used. First, cation exchange resins remove cations, followed by anion exchange resins to remove anions, ultimately bringing the water quality up to ultra-pure water standards.
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Resin Regeneration and Replacement Cycle: The regeneration and replacement cycle of ion exchange resins directly affect the system's operating costs and water quality stability. Resin regeneration typically uses acid and alkali solutions, with a regeneration cycle generally between 1-3 months, depending on the feed water quality and system design. For example, when the feed water quality is poor, the resin regeneration cycle may be shortened to 1 month; when the feed water quality is good, the cycle can be extended to 3 months. The service life of resins is usually 3-5 years, after which they need to be replaced. By optimizing the resin filling volume and regeneration process, the service life of the resins can be extended, reducing operating costs.
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System Design and Optimization: The optimized design of the ion exchange system can improve water quality stability and operational efficiency. For example, using a mixed bed ion exchange system can further improve water quality, bringing the resistivity above 18.2MΩ·cm. The mixed bed system combines cation and anion exchange resins, which can more quickly remove ion impurities from the water. In addition, by reasonably designing the resin bed height and water flow velocity, ion exchange efficiency can be improved, reducing resin usage and lowering system costs. For example, a resin bed height of 1.5-2.0m and a water flow velocity controlled at 10-20m/h can achieve good ion exchange results.
3.2 Integration of Electrodeionization Technology
Electrodeionization (EDI) technology is a new type of water treatment technology that combines ion exchange resins with electrodialysis and is widely used in the post-treatment process of ultra-pure water systems. EDI technology removes ion impurities from water through the action of an electric field while utilizing the adsorption effect of ion exchange resins to further improve water quality.
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Advantages of EDI Technology: Compared with traditional ion exchange technology, EDI technology has many advantages. First, EDI technology does not require acid and alkali regeneration, reducing the use and discharge of chemicals, lowering operating costs and environmental risks. Second, EDI technology has high operational efficiency and stable water quality, capable of continuously producing ultra-pure water to meet the continuity requirements of industrial production. For example, in the semiconductor manufacturing field, EDI technology is widely used in the post-treatment process of ultra-pure water systems, capable of stably providing ultra-pure water with a resistivity of ≥ 18.2MΩ·cm, meeting the strict water quality requirements for chip manufacturing.
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System Configuration and Parameter Optimization: The configuration and parameter optimization of EDI systems have an important impact on system performance and operating costs. The main parameters of EDI systems include current density, voltage, and flow rate. Current density is generally controlled between 0.5-2.0A/cm², voltage between 1-3V, and flow rate is determined based on system design and water demand. For example, for an EDI system with a processing capacity of 10m³/h, the current density can be set at 1.5A/cm², voltage at 2.0V, and flow rate at 10m³/h, achieving good water quality treatment results. In addition, the number and arrangement of EDI membrane stacks also affect system performance. A parallel connection of multiple membrane stacks is usually adopted to increase system water production capacity and operational stability.
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Operation, Maintenance, and Cost Analysis: The operation and maintenance of EDI systems are relatively simple but require regular inspection and maintenance of components such as electrodes, membrane stacks, and ion exchange resins. The cleaning cycle for electrodes is generally 3-6 months, the replacement cycle for membrane stacks is 3-5 years, and the replacement cycle for ion exchange resins is 1-2 years. The operating costs of EDI systems mainly include electricity costs and consumable replacement costs. For example, for an EDI system with a processing capacity of 10m³/h, the annual electricity cost is about 50,000 yuan, and the annual consumable replacement cost is about 100,000 yuan. By optimizing system design and operating parameters, operating costs can be reduced and system economy improved.
3.3 Ultraviolet Sterilization and Terminal Filtration
Ultraviolet sterilization and terminal filtration are the final barriers in ultra-pure water systems, mainly used to remove microorganisms and particulate impurities from water, ensuring the sterility and stability of water quality.
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Ultraviolet Sterilization Technology: Ultraviolet sterilization technology destroys the DNA structure of microorganisms through ultraviolet radiation, rendering them unable to reproduce and thereby achieving sterilization. The wavelength of ultraviolet germicidal lamps is generally 254nm, with a sterilization efficiency of over 99%. In ultra-pure water systems, ultraviolet sterilization is usually installed before terminal filtration to ensure the sterility of water quality. For example, in the pharmaceutical industry, ultra-pure water is used for the preparation of injection water, where the requirement for water sterility is extremely high. Ultraviolet sterilization technology can effectively remove bacteria, viruses, and other microorganisms from water, ensuring that water quality meets pharmacopoeia standards.
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Terminal Filtration Technology: Terminal filtration is mainly used to remove particulate impurities from water to ensure water quality stability. Common terminal filtration devices include micro-porous filter membranes and ultrafiltration membranes. The pore size of micro-porous filter membranes is generally between 0.1-5.0µm, which can effectively remove suspended particles and microorganisms from water; ultrafiltration membranes have smaller pores, generally between 0.01-0.1µm, which can remove colloids and large-molecule organic matter from water. In practical applications, terminal filtration devices are usually selected based on water quality requirements. For example, in the electronics industry, to ensure the high purity of water used in chip manufacturing, 0.1µm micro-porous filter membranes are often used for terminal filtration to ensure water quality stability.
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System Integration and Optimization: The integrated design of ultraviolet sterilization and terminal filtration can improve the overall performance and operational efficiency of the system. For example, by installing ultraviolet germicidal lamps and micro-porous filter membranes in series at the terminal of the ultra-pure water system, microorganisms and particulate impurities in water can be effectively removed, ensuring the sterility and stability of water quality. At the same time, by reasonably designing system piping and equipment layout, head loss and system resistance can be reduced, improving system operational efficiency. In addition, regular maintenance and replacement of ultraviolet germicidal lamps and filter membranes are also important measures to ensure the normal operation of the system. The replacement cycle for ultraviolet germicidal lamps is generally 6-12 months, and the replacement cycle for micro-porous filter membranes is 3-6 months. By optimizing the maintenance cycle, operating costs can be reduced and system reliability improved.
4. System Operation and Maintenance Strategies
4.1 Pre-treatment Maintenance
The pre-treatment unit is a key part of the stable operation of ultra-pure water systems and RO membrane systems, and its maintenance work directly affects the overall performance and service life of the system.
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Sand Filter Maintenance: Sand filters are mainly used to remove large particulate impurities from water, such as silt and suspended solids. The maintenance cycle is generally once a month, including backwashing and filter media inspection. During backwashing, the water flow velocity should reach 10-15m/h to ensure that impurities on the surface of the filter media are thoroughly washed away. The replacement cycle for filter media is generally 3-5 years, depending on the feed water quality and backwashing frequency. For example, when treating raw water with a high sand content, the replacement cycle for filter media may be shortened to 2 years.
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Activated Carbon Filter Maintenance: The main function of the activated carbon filter is to remove organic matter, residual chlorine, and odors from water. The maintenance cycle is once every quarter, including backwashing and activated carbon replacement. During backwashing, the water flow velocity should be controlled at 15-20m/h to ensure that impurities on the surface of the activated carbon are removed. The replacement cycle for activated carbon is generally 1-2 years, depending on the feed water quality and backwashing effect. For example, when treating municipal water supplies with high chlorine content, the replacement cycle for activated carbon may be shortened to 1 year.
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Softener Maintenance: Softeners remove calcium, magnesium, and other hardness ions from water through ion exchange resins to prevent scaling on RO membranes. The maintenance cycle is once a month, including resin regeneration and inspection. Resin regeneration usually uses brine solution, with a regeneration cycle generally between 1-3 months, depending on the feed water hardness. The service life of resins is usually 3-5 years, after which they need to be replaced. By optimizing the regeneration process and resin filling volume, the service life of the resins can be extended, reducing operating costs.
4.2 Reverse Osmosis Membrane Cleaning and Replacement
The cleaning and replacement of RO membranes are important for ensuring the efficient operation of the system, and the maintenance strategy directly affects the performance and service life of the membranes.
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Physical Cleaning: Physical cleaning removes pollutants from the membrane surface through hydraulic flushing, generally performed every 1-2 weeks. During cleaning, the water flow velocity should reach 2-3m/s to ensure that pollutants are thoroughly washed away. Physical cleaning can effectively remove suspended matter and some biofilms from the membrane surface, but its removal effect on organic matter and inorganic scale is limited.
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Chemical Cleaning: Chemical cleaning removes organic matter, inorganic scale, and biofilms from the membrane surface using chemical agents. Common chemical cleaning agents include acidic cleaners (such as citric acid, hydrochloric acid) and alkaline cleaners (such as sodium hydroxide, potassium hydroxide). Acidic cleaners are mainly used to remove inorganic scales, such as calcium carbonate and silicates; alkaline cleaners are used to remove organic matter and biofilms. The cleaning cycle for chemical cleaning is generally 3-6 months, depending on the degree of membrane fouling. For example, when treating raw water with high hardness and high organic matter content, the chemical cleaning cycle may be shortened to 2 months.
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Membrane Replacement: The service life of RO membranes is generally 3-5 years, depending on the membrane type, raw water quality, and cleaning and maintenance effectiveness. When the desalination rate of the membrane drops below 90%, or the pressure difference across the membrane increases to more than 1.5 times the initial value, it is necessary to consider replacing the membrane elements. For example, polyamide composite membranes used in high-salinity seawater treatment can have a service life of up to 5 years due to their high desalination rate and good chemical resistance; however, in oily wastewater treatment, the service life of membranes may be shortened to 3 years due to the complexity of pollutants.
4.3 Ultra-Pure Water System Monitoring and Calibration
Monitoring and calibration of ultra-pure water systems are important means to ensure water quality stability and system reliability, and the maintenance strategy directly affects the operational effectiveness of the system.
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Water Quality Monitoring: Water quality monitoring in ultra-pure water systems mainly includes real-time monitoring of parameters such as conductivity, total organic carbon (TOC), and particulate matter. Conductivity is an important indicator of water purity, which should reach above 18.2MΩ·cm; TOC reflects the content of organic matter in water, which should be below 5ppb. Monitoring equipment should be regularly calibrated, generally once every quarter, to ensure the accuracy of measurement results. For example, in the electronics industry, the conductivity and TOC monitoring data of water used in chip manufacturing are key parameters in the production process. Real-time monitoring and calibration can effectively prevent production accidents caused by water quality issues.
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Equipment Calibration: Key equipment in ultra-pure water systems, such as ion exchange resin beds, EDI systems, and ultraviolet germicidal lamps, needs to be regularly calibrated and maintained. Calibration of ion exchange resin beds mainly includes optimization of resin filling volume and regeneration process; calibration of EDI systems includes adjustment of current density, voltage, and flow rate; calibration of ultraviolet germicidal lamps mainly involves checking the power and radiation intensity of the lamp tubes. The calibration cycle for equipment is generally once every six months, depending on the usage frequency and operating environment of the equipment. For example, in the pharmaceutical industry, ultra-pure water is used for the preparation of injection water, and calibration and maintenance of equipment are important guarantees to ensure that water quality meets pharmacopoeia standards.
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System Optimization: Based on water quality monitoring data and equipment operation conditions, regularly optimize and adjust the ultra-pure water system. For example, by adjusting the regeneration cycle and process of ion exchange resins, water quality stability and resin service life can be improved; by optimizing the operating parameters of EDI systems, operating costs can be reduced and water quality improved; by reasonably arranging system piping and equipment layout, head loss and system resistance can be reduced, improving system operational efficiency.
5. Summary
Pairing ultra-pure water systems with RO membrane systems is a complex and meticulous engineering task that involves collaborative optimization of multiple links. By thoroughly analyzing the compatibility of raw water quality, the complementary functions of systems, and the balance of cost-effectiveness, efficient and economical water treatment solutions can be provided for different application scenarios. In the configuration of RO membrane systems, key points such as membrane element selection, high-pressure pump matching, and system layout optimization ensure the efficient operation of the system. The post-treatment process of ultra-pure water systems, including the integrated application of ion exchange technology, electrodeionization technology, and ultraviolet sterilization and terminal filtration, further improves water quality to meet high-purity water demands. The operation and maintenance strategies of the system, including pre-treatment maintenance, RO membrane cleaning and replacement, and ultra-pure water system monitoring and calibration, are important guarantees for the long-term stable operation of the system.
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