As an efficient separation technology, nanofiltration is widely used in drinking water treatment. Based on the new progress in the development and preparation of membrane materials in recent years, this review systematically summarizes the application of nanofiltration in the fields of surface water purification, groundwater treatment, water reuse, brackish water desalination, and drinking water terminals. In view of the treatment objectives of drinking water, on the basis of traditional water quality indicators such as salts, pathogens, natural organic matter, hardness, etc., it is proposed to consider the removal of new pollutants such as disinfection by-products and perfluorinated pollutants by nanofiltration membranes, and emphasize the importance of developing highly applicable nanofiltration membranes for different application scenarios. On the premise of ensuring stable, safe and reliable drinking water treatment, the development space of the new generation of nanofiltration membranes in membrane fouling control, chlorine resistance, integrity and selectivity is analyzed and prospected.
The chemical composition of nanofiltration membrane materials classifies the current nanofiltration membranes into polymeric and non-polymeric types. The main representative of polymer-based nanofiltration membrane is polyamide film composite membrane (TFC), which is prepared by interfacial polymerization, its surface morphology is relatively flat, and the structure is mainly composed of the top polyamide separation layer and the bottom porous support layer. The separation performance of traditional polyamide nanofiltration membranes is mainly limited by their permeability selectivity, and the material and structural properties of traditional polyamide nanofiltration membranes can effectively improve the separation performance. Non-polymeric nanofiltration membranes mainly include ceramic nanofiltration membranes and new two-dimensional material nanofiltration membranes.
In surface water purification, nanofiltration technology is mainly used to remove pollutants and natural organic matter in water to ensure the safety and stability of effluent quality. Pre-treatment steps such as coagulation-sedimentation, microfiltration, ultrafiltration and other steps can remove particulate matter and some natural organic matter in the raw water to reduce the risk of membrane fouling in the subsequent nanofiltration unit. The nanofiltration unit effectively removes most of the pollutants in the water and ensures that the effluent quality meets the relevant standards. In addition, nanofiltration membranes can efficiently remove natural organic matter from water, thereby effectively reducing the generation of highly toxic disinfection by-products in the subsequent disinfection process.
In groundwater treatment, nanofiltration membranes are mainly used to reduce hardness and soften water quality. Excessive hardness removal rates can lead to insufficient minerals such as calcium and magnesium in the effluent and a high risk of membrane fouling. Therefore, the selective removal of sulfate and the effective retention of minerals can be achieved through the regulation of the membrane's retention performance, which can increase the mineral content and reduce the possibility of membrane fouling. In addition, nanofiltration membranes can also achieve high removal of contaminants such as arsenic and perfluorinated substances from groundwater.
In terms of water reuse, the treatment targets of nanofiltration membranes are mainly toxic and harmful traces, new pollutants and pathogens. Due to the strong pore size sieving and charge repulsion effects, nanofiltration membranes tend to have a high removal rate for hydrophilic macromolecules and negatively charged new pollutants. However, the removal rate of hydrophobic and polar pollutants such as endocrine disruptors and disinfection by-products can be as low as 50%. The hydrophilicity and pore size of the membrane can be effectively improved by using the surface coating to control the removal of hydrophobic pollutants. In addition, the pore size of existing nanofiltration membranes allows for efficient removal of pathogens such as viruses. However, in recent years, it has been confirmed that nanodefects in nanofiltration membrane materials may increase the risk of virus transmembrane, so it is urgent to study effective membrane preparation and post-processing methods to improve the integrity of membrane materials and achieve complete virus retention.
In general, nanofiltration, as an efficient separation technology, plays an important role in surface water purification, groundwater treatment, water reuse and other fields. With the continuous progress of science and technology and the emergence of new materials, nanofiltration technology will further improve the separation performance and treatment efficiency under the premise of ensuring safety and reliability, and provide a more stable and efficient solution for drinking water treatment.