Nanofiltration membrane is also called ultra-low pressure reverse osmosis membrane. The membrane with a molecular weight cutoff of 200-1000 and a NaCl retention rate of ≤90% can be considered as a nanofiltration membrane. Nanofiltration membrane separation technology has been separated from reverse osmosis technology and has become an independent separation technology between ultrafiltration and reverse osmosis technology. It has been widely used in many fields such as seawater desalination, ultrapure water production, food industry, environmental protection, etc., and has become an important branch of water treatment technology. In the field of drinking water, it is mainly used to remove trihalomethane intermediates, odor, chromaticity, pesticides, synthetic detergents, soluble organic matter, hardness components such as Ca and Mg, and evaporation residual substances.

The preparation methods of nanofiltration membrane mainly include phase inversion method, coating-crosslinking method, layer-by-layer self-assembly method and interfacial polymerization method. The phase inversion method is usually used to prepare asymmetric nanofiltration membranes, that is, the casting liquid is transformed from liquid phase to solid phase porous membrane material under certain conditions. The preparation method of composite nanofiltration membrane is divided into two steps: one is the preparation of the support layer, and the other is the preparation of the active functional layer. The support layer is prepared by phase inversion method, and the preparation of active layer mainly adopts coating-crosslinking method, layer-by-layer self-assembly method and interfacial polymerization method.

The coating-crosslinking method is to coat first and then crosslink, that is, to coat a layer of crosslinkable polymer or active monomer that can form an active layer under specific conditions on the polymer base film. The layer-by-layer self-assembly method is a method of assembling different molecules into an ordered structure through intermolecular forces (such as hydrogen bonds and electrostatic attraction of polyelectrolytes). At present, commercial nanofiltration membranes are mainly made by interfacial polymerization method, which is a method of generating an active selective layer by polymerization reaction at the interface of two immiscible solvents. Among them, the most commonly used monomers for polymerizing water phase are piperazine and its derivatives, polyethyleneimine, metaphenylenediamine and polyols, and the oil phase monomers are mostly benzoyl chloride, terephthaloyl chloride and isophthaloyl chloride.