Application Status of Nanofiltration Membrane Technology in Advanced Drinking Water Treatment

With socio-economic development and the improvement of living standards, public demand for drinking water quality has shifted from "safe water" to "healthy water." Traditional water treatment processes (coagulation-sedimentation-filtration-disinfection) struggle to effectively remove trace pollutants such as dissolved organic matter, pesticides, endocrine-disrupting chemicals, and viruses from water. Against this backdrop, nanofiltration (NF) membrane technology, as an efficient advanced treatment method, demonstrates significant application potential and value in the drinking water treatment sector due to its unique separation capabilities.

I. Overview of Nanofiltration Membrane Technology and Its Core Advantages

Nanofiltration is a membrane separation technology situated between reverse osmosis (RO) and ultrafiltration (UF), with a pore size of approximately 1 nanometer. It exhibits high rejection rates for divalent ions and organic compounds with a molecular weight of 200-1000 Daltons, while allowing partial passage of monovalent ions (e.g., sodium, potassium, chloride ions).

In advanced drinking water treatment, the core advantages of NF technology are:

1. Exceptional Purification Capacity:

   • Inorganics: Effectively removes excessive hardness (calcium, magnesium ions - scale) and elements like fluoride, iron, and manganese from water, significantly reducing the total dissolved solids (TDS).

   • Organics: Can almost completely remove natural organic matter (e.g., humic acids, fulvic acids), as well as trace harmful organic compounds like pesticides, antibiotics, and endocrine-disrupting chemicals, effectively reducing the formation potential of disinfection by-products.

   • Microorganisms: Completely removes pathogenic microorganisms such as bacteria, viruses, Cryptosporidium, and Giardia, ensuring high biosafety.

2. Relatively Low Operating Pressure: Compared to RO, which requires high operating pressure (typically >10 bar), NF operates at lower pressures (typically 5-10 bar), resulting in lower energy consumption and operational costs.

3. Retention of Beneficial Minerals: NF membranes do not highly reject monovalent ions (e.g., potassium, sodium), allowing the product water to retain a portion of minerals beneficial to human health. This avoids the health debates associated with the near-pure water produced by RO and offers better taste.

II. Current Application Status

The application of NF technology in drinking water treatment has progressed from research and demonstration phases to large-scale, commercial implementation, primarily evident in the following areas:

1. High-Quality Drinking Water Plants Using Surface Water/Reservoir Water as Source:

   • For reservoir or lake water affected by non-point source agricultural pollution or eutrophication, NF is one of the optimal technological choices for producing high-quality drinking water. For example, the Wuzhen Tongfu Water Plant in Zhejiang Province is China's first large-scale (100,000 tons/day) drinking water plant utilizing a "UF-NF" dual-membrane advanced treatment process. It effectively removes organic matter and odors from the source water, significantly improving the quality and taste of the water supply.

2. Treatment of Hardness/Fluoride in Groundwater Sources:

   • For groundwater with high hardness or high fluoride content, NF technology can economically and effectively achieve softening (descaling) and defluoridation, producing stable, high-quality water superior to that treated by traditional chemical precipitation methods.

3. Advanced Purification and Emergency Treatment of Micro-Polluted Source Water:

   • In the event of sudden source water contamination incidents (e.g., chemical spills), NF systems can serve as a robust barrier, ensuring water supply safety. They offer broad-spectrum removal capability for organic pollutants with molecular weights above several hundred Daltons.

4. Direct Pipeborne Drinking Water and Community Dual-Water Supply Systems:

   • In settings such as high-end residential communities, schools, and office buildings, NF technology is the core process for pipeborne drinking water systems. It produces high-quality drinking water containing appropriate mineral levels and a sweet taste, directly meeting residents' demand for healthy drinking water.

III. Main Challenges and Countermeasures

Despite its clear advantages, the widespread application of NF technology still faces challenges:

1. Membrane Fouling and Cleaning:

   • Challenge: Colloids, organic matter, microorganisms, and inorganic salts in the feed water can deposit on the membrane surface and within pores, leading to decreased flux and increased energy consumption. Fouling is a key factor affecting system stability and cost.

   • Countermeasures: Efficient pretreatment (e.g., UF, activated carbon filtration) is essential for stable NF operation. Additionally, establishing rational chemical cleaning cycles and protocols based on water quality is crucial.

2. Brine/Concentrate Management:

   • Challenge: The NF process generates a concentrate stream (typically 15%-25% of the feed flow) containing concentrated impurities from the feed water. Direct discharge can adversely impact the environment.

   • Countermeasures: Proper disposal of NF concentrate is a bottleneck for its broader adoption. Current solutions include discharge to wastewater treatment plants, use for flushing or irrigation, and further volume reduction via technologies like evaporation crystallization, but these methods add cost.

3. Operational Costs and Energy Consumption:

   • Challenge: Although energy consumption is lower than RO, the capital and operational costs of NF systems (including electricity, membrane replacement, and chemical cleaning) are still higher than those of conventional processes.

   • Countermeasures: Continuous cost reduction efforts include employing energy recovery devices (for high-pressure NF applications), optimizing operational parameters, and developing new NF membranes with anti-fouling properties and lower energy requirements.

4. Precise Control of Mineral Content:

   • Challenge: Precisely controlling the mineral content in the product water to ensure it is both safe and healthy is a current research focus. Significant variations in raw water quality across regions demand membranes with higher ion selectivity.

   • Countermeasures: Developing "selective nanofiltration" membranes with specific ion selectivity or employing integrated processes combining NF with other technologies (e.g., mineral addition/post-treatment).

IV. Future Development Trends

1. Membrane Material Innovation: Research and development of new NF membrane materials (e.g., thin-film composite membranes based on carbon nanotubes, graphene) with high flux, high selectivity, strong anti-fouling properties, and oxidation resistance.

2. Process Integration: Optimized combination of NF with other processes like UF, advanced oxidation, and UV disinfection to form multi-barrier systems for synergistic effects.

3. Intelligent Operation: Utilizing big data and artificial intelligence for precise chemical dosing, predictive cleaning, and energy optimization of NF systems to enhance operational efficiency.

4. Brine Resource Recovery: Shifting the focus from "concentrate treatment" to "resource recovery," exploring the feasibility of recovering valuable substances (e.g., lithium, bromine) from the concentrate.

Summary

Nanofiltration membrane technology has matured into a highly competitive solution in the field of advanced drinking water treatment. It effectively ensures the biological and chemical safety of drinking water while retaining beneficial minerals, perfectly aligning with the current public demand for healthy water. Although challenges remain in membrane fouling control, concentrate management, and cost, continuous technological advancements and innovations will undoubtedly see NF technology play an increasingly vital role in the future upgrading of urban water supply systems and ensuring the provision of high-quality drinking water.