Nanofiltration technology is a membrane separation technology between ultrafiltration and reverse osmosis. Its molecular weight cut-off is in the range of 80-1000, and the pore diameter is several nanometers. Therefore, it is called nanofiltration. The ability of intercepting soluble salts is 2-98%, the desalting of the monovalent anion salt solution is lower than that of the polyvalent anion salt solution.
Nanofiltration technology is a membrane separation technology separated from reverse osmosis technology and is a continuation and development branch of ultra low pressure reverse osmosis technology. Now, nanofiltration technology has been separated from reverse osmosis technology and has become an independent separation technology between ultrafiltration and reverse osmosis technologies. It has been widely used in fields of seawater desalination, ultrapure water production, food industry, environmental protection, etc., which has become an important branch of membrane separation technology.
The key to the nanofiltration process is the nanofiltration membrane. The requirements for membrane materials are: good membrane shaping, thermal stability, chemical stability, high mechanical strength, acid and alkali resistance and microbial attack, chlorine and other oxidizing substances resistance, high water flux and high salt rejection rate, good colloid and suspended matter pollution-resistance. the current nanofiltration membranes are mostly aromatic and polyacid hydrogen composite nanofiltration membranes. The composite membrane is an asymmetric membrane composed of two parts: one part is the porous membrane for supporting, the mechanism is sieving; the other part is a thin and dense membrane for separation, whose separation mechanism can be explained by the theory of solubility and diffusion. For the composite membrane, the material and structure of the skin layer and the support layer acting as separation can be optimized respectively, and a composite membrane with excellent performance can be obtained. The membrane types can be divided into hollow fibers, spiral wound, plate-and-frame and tube etc.. Among them, the hollow fiber and the spiral wound membrane modules have high filling density, with low cost and good hydrodynamic conditions in the membranes; however, the two types of membranes have high manufacturing technical requirements, difficult sealing, poor anti-pollution ability in use, and high requirements for the solution pretreatment. While, the plate-and-frame and tube membrane modules are easy to be cleaned and good resistance to contamination, but have low filling density and high costs. Therefore, hollow fiber or spiral wound membranes are often used in nanofiltration systems.