1. Classification and Structural Characteristics of Nanofiltration Membrane Materials

1.1 Organic Polymer Nanofiltration Membranes

Organic polymer nanofiltration membranes are one of the most widely used types of nanofiltration membranes. The materials mainly include polyamide, poly-piperazine amide, sulfonated polysulfone, etc. These materials have good film-forming properties, chemical stability, and mechanical strength, meeting the needs of various water treatment applications.

Polyamide Composite Nanofiltration Membranes: Polyamide composite nanofiltration membranes are the mainstream products of commercialized nanofiltration membranes. The support layer is usually made of polysulfone materials, which have high mechanical strength and good chemical resistance, allowing stable operation within a pH range of 1 to 13. The polyamide composite layer is prepared by interfacial polymerization and has high selectivity and high flux. For example, the NF-50 and NF-70 membranes from FilmTec in the United States can achieve a rejection rate of over 95% for divalent ions, with a water flux between 0.5 and 2.0 L/(m²·h).

Poly-piperazine Amide Composite Nanofiltration Membranes: Poly-piperazine amide composite nanofiltration membranes have higher water flux and good anti-fouling properties. The composite layer materials are prepared by chemical synthesis and can effectively reject organic matter and divalent ions. For example, the SU-600 membrane from Toray in Japan has an organic matter rejection rate of over 90%, with a water flux between 1.0 and 2.5 L/(m²·h).

Sulfonated Polysulfone Nanofiltration Membranes: Sulfonated polysulfone nanofiltration membranes have good hydrophilicity and anti-fouling properties and are suitable for treating wastewater containing a large amount of organic matter. The membrane surface carries a negative charge, which can effectively reject negatively charged organic matter and divalent ions.

1.2 Inorganic Nanofiltration Membranes

Inorganic nanofiltration membranes mainly include ceramic nanofiltration membranes and metal oxide nanofiltration membranes. These materials have high mechanical strength, high-temperature resistance, and chemical corrosion resistance, making them suitable for special application scenarios.

Ceramic Nanofiltration Membranes: Ceramic nanofiltration membranes have good high-temperature resistance and chemical stability and can operate under extreme conditions. Their pore size distribution is uniform, allowing effective rejection of divalent ions and organic matter. For example, ceramic nanofiltration membranes can achieve a rejection rate of over 99% for heavy metal ions when treating heavy metal-containing wastewater.

Metal Oxide Nanofiltration Membranes: Metal oxide nanofiltration membranes have good hydrophilicity and anti-fouling properties and are suitable for treating wastewater containing a large amount of organic matter. For example, titanium dioxide nanofiltration membranes can achieve a rejection rate of over 95% for organic matter when treating dyeing wastewater.

1.3 Organic-Inorganic Hybrid Nanofiltration Membranes

Organic-inorganic hybrid nanofiltration membranes combine the advantages of organic polymer materials and inorganic materials, having comprehensive good properties. Their preparation methods usually include sol-gel method and in-situ polymerization.

Hybrid Nanofiltration Membranes Prepared by Sol-Gel Method: By introducing inorganic materials to the surface of organic polymer membranes through the sol-gel method, the hydrophilicity and anti-fouling properties of the membrane can be improved. For example, the poly-sulfone-silica hybrid nanofiltration membrane can achieve a rejection rate of over 98% for oil when treating oily wastewater.

Hybrid Nanofiltration Membranes Prepared by In-situ Polymerization: By growing organic polymer membranes on the surface of inorganic materials through in-situ polymerization, the mechanical strength and selectivity of the membrane can be enhanced. For example, the polyamide-titanium dioxide hybrid nanofiltration membrane can achieve a rejection rate of over 99% for heavy metal ions when treating heavy metal-containing wastewater.

2. Application Scenarios of Different Material Nanofiltration Membranes

2.1 Application Scenarios of Organic Polymer Nanofiltration Membranes

Organic polymer nanofiltration membranes, with their good film-forming properties, chemical stability, and mechanical strength, perform well in many water treatment scenarios. The specific application scenarios are as follows:

Drinking Water Purification: Polyamide composite nanofiltration membranes have a rejection rate of over 95% for divalent ions and a water flux between 0.5 and 2.0 L/(m²·h). This high rejection capability enables them to effectively remove hardness components (such as calcium and magnesium ions), sulfates, nitrates, and trace organic pollutants (such as pesticide residues and disinfection by-products) from water, thereby significantly improving the taste and quality of drinking water and meeting the demand for high-quality drinking water.

Water Softening: Poly-piperazine amide composite nanofiltration membranes have higher water flux and good anti-fouling properties, with an organic matter rejection rate of over 90% and a water flux between 1.0 and 2.5 L/(m²·h). When treating high-hardness water sources such as brackish water, they can effectively remove divalent ions from the water, reducing water hardness while maintaining low operating pressure and high water flux, reducing treatment costs and improving treatment efficiency.

Paper Mill Wastewater Treatment: Sulfonated polysulfone nanofiltration membranes have good hydrophilicity and anti-fouling properties and are suitable for treating wastewater containing a large amount of organic matter. Paper mill wastewater contains a large amount of organic pollutants such as lignin and chlorinated lignin, many of which are negatively charged. The sulfonated polysulfone nanofiltration membrane surface carries a negative charge, which can effectively reject these negatively charged organic matter while having better permeability to monovalent sodium ions, enabling resource recovery and reducing wastewater treatment costs.

2.2 Application Scenarios of Inorganic Nanofiltration Membranes

Inorganic nanofiltration membranes, with their high mechanical strength, high-temperature resistance, and chemical corrosion resistance, have unique advantages in some special application scenarios:

Heavy Metal-Containing Wastewater Treatment: Ceramic nanofiltration membranes have good high-temperature resistance and chemical stability and can operate under extreme conditions. Their uniform pore size distribution allows effective rejection of divalent ions and organic matter. When treating heavy metal-containing wastewater (such as electroplating wastewater and mining wastewater), they can achieve a rejection rate of over 99% for heavy metal ions, effectively removing heavy metal ions such as copper, zinc, and nickel from the wastewater, enabling the treated wastewater to meet discharge standards. Meanwhile, the recovered heavy metals can be reused, bringing significant environmental and economic benefits.

High-Temperature Wastewater Treatment: The high-temperature resistance of inorganic nanofiltration membranes enables them to meet the treatment needs of high-temperature wastewater. In some industrial production processes, such as petrochemical and steel industries, the wastewater generated has a high temperature. Traditional organic nanofiltration membranes may be damaged due to the inability to withstand high-temperature environments. In contrast, inorganic nanofiltration membranes can operate stably at high temperatures, eliminating the need for pre-cooling treatment of wastewater, saving energy and reducing treatment costs while improving treatment efficiency.

Dyeing Wastewater Treatment: Metal oxide nanofiltration membranes have good hydrophilicity and anti-fouling properties and are suitable for treating wastewater containing a large amount of organic matter. Dyeing wastewater contains a large amount of organic pollutants such as dyes and auxiliaries, which have complex chemical structures and high color intensity. Titanium dioxide nanofiltration membranes can achieve a rejection rate of over 95% for organic matter when treating dyeing wastewater, effectively removing organic pollutants from the wastewater, reducing the color intensity and chemical oxygen demand (COD) of the wastewater, and improving the biodegradability of the wastewater, creating favorable conditions for subsequent biological treatment.

2.3 Application Scenarios of Organic-Inorganic Hybrid Nanofiltration Membranes

Organic-inorganic hybrid nanofiltration membranes, combining the advantages of organic polymer materials and inorganic materials, have good comprehensive properties and are suitable for various complex water treatment scenarios:

Oily Wastewater Treatment: Hybrid nanofiltration membranes prepared by the sol-gel method, by introducing inorganic materials to the surface of organic polymer membranes, can improve the hydrophilicity and anti-fouling properties of the membrane. When treating oily wastewater, the poly-sulfone-silica hybrid nanofiltration membrane can achieve a rejection rate of over 98% for oil, effectively removing emulsified oil, dissolved oil, and other oil pollutants from the wastewater while reducing membrane surface fouling, extending the membrane's service life, and lowering the frequency of membrane cleaning and maintenance costs.

Complex Industrial Wastewater Treatment: Hybrid nanofiltration membranes prepared by in-situ polymerization, by growing organic polymer membranes on the surface of inorganic materials, can enhance the mechanical strength and selectivity of the membrane. When treating complex industrial wastewater (such as electronic and pharmaceutical wastewater), the polyamide-titanium dioxide hybrid nanofiltration membrane can achieve a rejection rate of over 99% for heavy metal ions while effectively removing organic pollutants, antibiotics, hormones, and other trace pollutants from the wastewater, ensuring that the treated wastewater meets strict discharge standards and protecting the environment and ecological safety.

Seawater Desalination Pretreatment: Organic-inorganic hybrid nanofiltration membranes have good anti-fouling properties and high water flux, making them suitable for seawater desalination pretreatment. Seawater contains a large amount of salts, microorganisms, organic matter, and other impurities. Traditional pretreatment methods are difficult to effectively remove these pollutants and are prone to membrane fouling. Hybrid nanofiltration membranes can effectively remove most of the salts, organic matter, and microorganisms from seawater, reducing the hardness and turbidity of seawater, improving the quality of seawater, and providing high-quality feed water for the subsequent reverse osmosis seawater desalination process, improving the efficiency and stability of seawater desalination, and reducing the risk of pollution and damage to reverse osmosis membranes.

3. Application Scenarios of Different Structural Nanofiltration Membranes

3.1 Application Scenarios of Spiral-Wound Nanofiltration Membranes

Spiral-wound nanofiltration membranes are one of the most widely used nanofiltration membrane structures, with advantages such as low cost, high flux, and low operating pressure, making them suitable for various water treatment scenarios:

Large-Scale Drinking Water Purification: The high flux characteristic of spiral-wound nanofiltration membranes enables them to efficiently treat large amounts of drinking water. For example, in some large water plants, spiral-wound nanofiltration membrane systems can effectively remove hardness components, sulfates, nitrates, and trace organic pollutants from water, with a treatment capacity reaching thousands of cubic meters per day. Their operating pressure is generally between 1 and 3 MPa, with low energy consumption, significantly reducing water treatment costs.

Industrial Water Softening: In industrial production, many processes require softened water. Spiral-wound nanofiltration membranes can effectively remove divalent ions from water, reducing water hardness to meet the requirements of industrial water. For example, in the chemical and textile industries, spiral-wound nanofiltration membranes can be used to soften boiler water, preventing boiler scaling and extending equipment service life.

Wastewater Advanced Treatment: Spiral-wound nanofiltration membranes can be used for advanced wastewater treatment to remove residual organic matter and heavy metal ions from wastewater. For example, when treating dyeing wastewater, spiral-wound nanofiltration membranes can further remove organic pollutants from wastewater, reducing the COD and color intensity of wastewater to meet higher discharge standards.

3.2 Application Scenarios of Plate and Frame Nanofiltration Membranes

Plate and frame nanofiltration membranes have advantages such as simple structure, strong anti-fouling properties, and easy cleaning, making them suitable for water treatment scenarios with higher membrane performance requirements:

Seawater Desalination Pretreatment: Plate and frame nanofiltration membranes can effectively remove organic matter, microorganisms, and part of the salts from seawater, reducing the turbidity and hardness of seawater and providing high-quality feed water for the subsequent reverse osmosis seawater desalination process. Their strong anti-fouling properties enable them to maintain stable operation in high-salinity, high-organic-content seawater, reducing membrane fouling and cleaning frequency.

Electronic Industry Wastewater Treatment: Electronic industry wastewater contains a large amount of heavy metal ions, organic pollutants, and tiny particles. The high selectivity and anti-fouling properties of plate and frame nanofiltration membranes make them effective in removing these pollutants from wastewater, ensuring that the treated wastewater meets strict discharge standards. Moreover, plate and frame nanofiltration membranes are easy to clean and maintain, reducing the operating costs of wastewater treatment systems.

Biopharmaceutical Wastewater Treatment: Biopharmaceutical wastewater contains a large amount of proteins, antibiotics, hormones, and other bioactive substances. Plate and frame nanofiltration membranes can effectively reject these substances while removing organic pollutants and microorganisms from wastewater, ensuring the safe discharge of wastewater. Their good anti-fouling properties and easy cleaning features give them a significant advantage in treating complex-component biopharmaceutical wastewater.

3.3 Application Scenarios of Hollow Fiber Nanofiltration Membranes

Hollow fiber nanofiltration membranes have advantages such as large specific surface area, high flux, and backwashability, making them suitable for water treatment scenarios with high water quality requirements and frequent cleaning needs:

Drinking Water Advanced Purification: Hollow fiber nanofiltration membranes can effectively remove trace organic pollutants, disinfection by-products, and hardness components from drinking water while retaining some beneficial minerals for the human body. Their backwashable characteristic enables them to maintain high flux and water quality stability during operation, extending the membrane's service life and reducing operating costs.

Food and Beverage Industry: In the food and beverage industry, hollow fiber nanofiltration membranes can be used for the clarification and concentration of products such as juice and milk. For example, in juice production, hollow fiber nanofiltration membranes can remove suspended solids, microorganisms, and pigments from juice, improving the clarity and quality of juice. Their backwashable characteristic can reduce membrane fouling and ensure the stability of product quality.

Biopharmaceutical Industry: Hollow fiber nanofiltration membranes can be used for protein separation, concentration, and purification in biopharmaceuticals. Their high selectivity and backwashable characteristic enable them to maintain high recovery rate and purity when processing bioactive substances while reducing membrane fouling and cleaning frequency. For example, in antibody production, hollow fiber nanofiltration membranes can effectively separate and concentrate antibodies, improving production efficiency and product quality.

4. Conclusion

Nanofiltration membrane technology, with its unique separation performance, shows a broad application prospect in the field of water treatment. By conducting a detailed analysis of the application scenarios of different materials and structures of nanofiltration membranes, the following conclusions can be drawn:

4.1 Summary of Application Scenarios of Different Material Nanofiltration Membranes

Organic Polymer Nanofiltration Membranes: Suitable for drinking water purification, water softening, and paper mill wastewater treatment. Their advantages lie in good film-forming properties, chemical stability, and mechanical strength, which can effectively remove hardness components, sulfates, nitrates, and trace organic pollutants from water while maintaining high water flux and low operating pressure.

Inorganic Nanofiltration Membranes: Perform well in special application scenarios such as heavy metal-containing wastewater treatment, high-temperature wastewater treatment, and dyeing wastewater treatment. Inorganic nanofiltration membranes have high mechanical strength, high-temperature resistance, and chemical corrosion resistance, enabling them to operate under extreme conditions with high rejection rates for heavy metal ions and organic pollutants.

Organic-Inorganic Hybrid Nanofiltration Membranes: Combining the advantages of organic polymer materials and inorganic materials, they are suitable for complex water treatment scenarios such as oily wastewater treatment, complex industrial wastewater treatment, and seawater desalination pretreatment. They have comprehensive good properties, including good anti-fouling properties, high water flux, and selectivity.

4.2 Summary of Application Scenarios of Different Structural Nanofiltration Membranes

Spiral-Wound Nanofiltration Membranes: With advantages such as low cost, high flux, and low operating pressure, they are widely used in large-scale drinking water purification, industrial water softening, and wastewater advanced treatment. Their high flux characteristic enables them to efficiently treat large amounts of water, reducing water treatment costs.

Plate and Frame Nanofiltration Membranes: Simple in structure, strong in anti-fouling properties, and easy to clean, they are suitable for water treatment scenarios with higher membrane performance requirements, such as seawater desalination pretreatment, electronic industry wastewater treatment, and biopharmaceutical wastewater treatment. Their anti-fouling properties and easy cleaning features can reduce operating costs.

Hollow Fiber Nanofiltration Membranes: With a large specific surface area, high flux, and backwashability, they are suitable for water treatment scenarios with high water quality requirements and frequent cleaning needs, such as drinking water advanced purification, food and beverage industry, and biopharmaceutical industry. Their backwashable characteristic can extend the membrane's service life and reduce operating costs.

In conclusion, different materials and structures of nanofiltration membranes each have their unique properties and application scenarios. In practical applications, the appropriate nanofiltration membrane material and structure should be selected based on specific water quality conditions and treatment requirements to achieve the best treatment effect and economic benefits.