In recent years, with rapid economic growth, industrialization, and urbanization, the subsequent water pollution and its prevention have attracted widespread attention. On the one hand, natural water quality continues to deteriorate, and traditional treatment processes cannot meet the increasingly stringent water quality standards. In order to ensure that the water quality meets the standards, more effective water treatment technologies need to be adopted to remove water pollutants; On the other hand, in order to solve the problem of water shortage, expanding the treatment volume of sewage recycling and the use of reclaimed water has become one of the effective ways to reduce the pressure on the urban water environment. Compared with other treatment technologies, membrane technology has been widely used in water treatment and reuse in recent years due to its good and stable separation effect. A rolled nanofiltration membrane, as a membrane filtration technology between ultrafiltration and reverse osmosis, can efficiently intercept multivalent salts and organic pollutants in water. At the same time, combined with its relatively low rejection rate of monovalent salts, Filtration has good selective separation properties for monovalent and multivalent ion mixed systems. In recent years, it has been widely used in drinking water softening, advanced sewage treatment and reuse, and concentration and separation in industrial processes.
The roll nanofiltration membrane was first called "loose reverse osmosis membrane" or "dense ultrafiltration membrane". Later, due to general speculation that it may have pores of about 1 nm, it was changed to "roll nanofiltration membrane". But in fact, the origin of the name of the roll nanofiltration membrane is mainly because the particle size of the trapped particles is nanometer level (~1 nm, the corresponding molecular weight range is about 150~200 Da), rather than the membrane structure (such as the membrane pore size) itself is nanoscale. Rolled nanofiltration membranes have two typical characteristics: First, the molecular weight cut-off is between reverse osmosis membranes and ultrafiltration membranes. The molecular weight cut-off of nanofiltration membranes is usually in the range of 150~2000 Da, while the salt rejection of reverse osmosis membranes rate is usually above 90%, the molecular weight cut-off is below 50 Da, and the salt rejection rate of the ultrafiltration membrane is generally below 5%; The second is that the surface separation layer of the nanofiltration membrane is usually charged, and the charge interaction caused by the surface charge changes the mass transfer process of the nanofiltration membrane and the ability of the nanofiltration membrane to interceptions of different valence states. There is a negative charge. The positively charged ions in the aqueous solution will be attracted by the charge on the membrane surface, and the negatively charged ions will be repelled away from the membrane surface. This charge effect is called the Donnan effect. Nanofiltration affects water. The rejection rate of dissolved salt is often affected by the size and valence of salt ions. For example, the rejection rate order of standard substances for Na2SO4, CaCl2 and NaCl 3 common salt rejection tests is: Na2SO4>CaCl2>NaCl. So, people generally considered that a nanofiltration membrane is a charged separation membrane with a nano-scale microporous structure.
The application of roll nanofiltration membranes can be summarized into three aspects: ①The rejection rate of monovalent salt is not high (usually between 20% and 80%, which is closely related to specific water chemistry conditions and membrane types);②Separation and purification of different valence ions;③Separation and concentration of organic matter with relatively high and relatively low molecular weight. The current main application areas of nanofiltration are concentrated in drinking water treatment and advanced treatment and reuse of sewage. Nanofiltration can treat many water sources, including groundwater, surface water, sewage, and water sources as pretreatment processes for desalination processes. The use of nanofiltration as a pretreatment method for the desalination process is considered to be a breakthrough in the process. Nanofiltration can well remove turbidity, microorganisms, hardness, and a considerable part of dissolved salts in water. Compared with reverse osmosis, nanofiltration with these characteristics can significantly reduce the operating pressure of the desalination process, improve the energy utilization rate of the entire process, and reduce equipment investment and operating costs.
In recent years, the rolled nanofiltration membrane technology has been widely used in the upgrading and transformation of drinking water softening and purification treatment processes and has maintained a stable and reliable treatment effect in long-term operation. Rolled nanofiltration membranes can not only remove trace chemical substances (such as pesticides, pesticides, etc.) and disinfection by-products (trihalomethane, haloacetic acid, etc.) remaining in the water, but also retain algae, bacteria, and pathogenic microorganisms in the water to ensure biological safety. Besides, it can remove harmful multivalent ions such as heavy metals, retain some of the minerals in the water that are beneficial to the human body, and can also ensure the stability of the final water supply underwater quality fluctuations and emergency conditions, and meet the water demand under different water source conditions. The rolled nanofiltration membrane has a high flux under low pressure and has a high degree of selective separation of monovalent and multivalent ions. The actual energy consumption and cost during operation are lower than reverse osmosis membranes. The water source conditions are complicated and water requirements are required. In areas with higher economies and more developed areas, roll nanofiltration membrane technology may be a more suitable choice as an advanced drinking water treatment process.