Final Polishing Nanofiltration Membrane System Design Scheme for High-Purity Water

1.0 Design Basis and System Positioning

• System Positioning: Serves as the final polishing and customization unit within a high-purity water production system. It is positioned after primary desalination systems like Reverse Osmosis (RO) or Electrodeionization (EDI) to deliver high-quality water meeting specific requirements directly to the final points of use.

• Feed Water Source: Permeate from primary or secondary RO systems, or EDI product water.

  • Typical Feed Water Quality:

    • Conductivity: 1 - 10 µS/cm

    • Total Organic Carbon (TOC): < 50 ppb

    • Silica (SiO₂): < 0.5 mg/L

    • Total Bacteria Count: < 10 CFU/100 mL

• Core Functions and Objectives:

  1. Selective Separation: Efficiently removes residual divalent ions (e.g., Ca²⁺, Mg²⁺, SO₄²⁻), trace heavy metals, and low molecular weight organics (MW 200-1000 Da), while allowing partial passage of monovalent ions (Na⁺, Cl⁻), enabling "fine-tuning" of water quality.

  2. Pyrogen/Endotoxin Control: Acts as a reliable barrier for removing bacterial endotoxins (typically MW >10,000 Da), achieving a stable product water endotoxin level below 0.25 EU/mL.

  3. Water Quality Stabilization and Buffering: Provides a final safeguard against upstream water quality fluctuations, ensuring absolute stability of the final water.

• Typical Applications: Purification of Water for Injection (WFI), final polishing for electronic ultrapure water, high-end laboratory water, process make-up water requiring specific ionic composition.

2.0 Process Flow Design

This system adopts the core process route of "Constant Pressure Feed → Terminal Nanofiltration → Circulation & Disinfection," ensuring stable polishing under low-pressure, low-fouling conditions. The specific process is illustrated in the diagram below:

Step-by-Step Process Explanation:

1. Feed Stabilization and Protection Unit:

   • Feed water from the upstream high-purity system is first supplied by a VFD-controlled booster pump to ensure stable pressure and flow, overcoming the resistance of the NF system and subsequent distribution.

   • A plate heat exchanger precisely controls the feed temperature to 25±1°C, ensuring stable NF membrane separation performance (flux and rejection).

   • The final cartridge filter (1-5 µm absolute) acts as an absolute physical barrier, preventing any particulate matter from entering the NF elements.

2. Nanofiltration Core Polishing Unit:

   • Membrane Element Selection: Uses sanitary-grade, high-selectivity spiral-wound NF elements, optimized for low ionic strength feed, achieving >95% rejection of divalent ions and low MW organics while maintaining appropriate passage for monovalent ions.

   • Operation Mode: The system operates at a low recovery rate (typically 85-95%), primarily for "polishing" rather than concentration, minimizing membrane fouling and concentration polarization. The small volume of concentrate produced can be recycled to the feed of the upstream RO system for zero liquid discharge.

   • Core Separation Process: Under relatively low operating pressure, sieving and charge effects occur on the membrane surface. Target impurities are efficiently rejected, resulting in the final enhancement of product water quality.

3. Circulation Distribution and Microbiological Control Unit:

   • NF permeate enters a nitrogen-blanketed final product tank, isolating it from air to prevent CO₂ dissolution and conductivity increase.

   • A closed-loop circulation system continuously moves the water at a velocity >1.0 m/s, ensuring turbulent flow 24/7 and eliminating dead legs.

   • A 254nm ultraviolet sterilizer integrated into the loop provides continuous disinfection of the circulating water.

   • A 0.22 µm sterilizing-grade filter is installed before each point of use, providing a final barrier.

4. Online Maintenance and Monitoring Unit:

   • An integrated compact CIP system allows for periodic or on-demand gentle chemical cleaning of the NF membranes to restore performance.

   • An online integrity tester automatically performs regular tests (e.g., pressure decay) on the NF membrane assembly to ensure the absolute effectiveness of the separation barrier.

3.0 Key Design Parameters and Equipment Selection

4.0 Techno-Economic Analysis

• System Characteristics: As a final polishing unit, its core value lies in enhancing and guaranteeing final water quality, not processing high volumes. Therefore, evaluation focuses on water quality compliance rate, stability, and protective value for the final product.

• Investment Components: Primarily consists of high-precision NF membrane elements, sanitary-grade circulation/distribution systems, high-sensitivity online instruments, and automated control systems.

• Operating Costs: Mainly from electricity (circulation pumps, UV lamps), periodic membrane cleaning and replacement, and nitrogen consumption. The cost per unit volume of water is generally acceptable relative to the value it creates (e.g., ensuring yield in high-end manufacturing, meeting pharmacopeia compliance).

• Benefits Realized:

  • Quality Benefit: Provides absolutely reliable water quality assurance for high-end manufacturing or life sciences.

  • Risk Mitigation: Avoids product batch failure, research failure, or compliance risks due to final water quality fluctuations.

  • System Resilience: Serves as a final barrier, buffering potential minor fluctuations from upstream systems.

5.0 Scheme Summary

This final polishing NF system scheme is a "last-mile" solution for applications with extremely high water quality requirements. It performs the final refinement and fortification of already purified water at the molecular level through the selective separation mechanism of nanofiltration membranes, excelling particularly in controlling divalent ions, low MW organics, and endotoxins.

Keys to Successful Implementation:

1. Feed Water Stability: The upstream system (RO/EDI) must provide stable, high-quality feed water as the foundation for efficient NF operation.

2. Microbiological & Contamination Control: The entire system must adhere to sanitary design standards to eliminate risks of recontamination.

3. Precision Operation: Employing a gentle operating mode with low pressure, low flux, and low recovery, coupled with intelligent predictive maintenance, is central to ensuring long-term membrane performance and stable water quality.

This scheme is suitable for cutting-edge applications with stringent water quality requirements and zero tolerance for water quality risks. It is the ultimate component in building a reliable high-purity water supply system.