Nanofiltration is a membrane separation technology which is driven by pressure difference and the particle size of the trapped water between ultrafiltration and reverse osmosis is nanometer particles. The pore size and surface characteristics of nanofiltration membrane determine its unique properties, and it has different removal effects for ions with different charges and different valence Numbers. The separation mechanism of nanofiltration membrane is the coexistence of screening and dissolution and diffusion, and it also has charge exclusion effect. It can effectively remove divalent and polyvalent ions and all kinds of substances with molecular weight greater than 200Da, and can partially remove monovalent ions and substances with molecular weight less than 200Da.

Reverse osmosis (RO) is a membrane separation operation driven by pressure difference to separate solvent from solution. It is called reverse osmosis because it is opposite to the direction of natural osmosis. According to the different osmotic pressure of various materials, the reverse osmosis pressure greater than osmotic pressure can be used, that is, reverse osmosis method, to achieve the purpose of separation, extraction, purification and concentration.

ED membrane reactor uses anion and cation exchange membrane arranged alternately between positive and negative electrodes and separated by a special separator to form two systems of desalination and concentration. When brine is supplied to the compartment, the cation migrates to the cathode under the action of the dc electric field and can only pass through the cation exchange membrane. The anions migrate to the anode and can only pass through the anion exchange membrane, thus desalting the brine in the light chamber and concentrating the brine in the thick chamber.

Bipolar membrane is a new type of ion exchange composite membrane, which is usually composed of cation exchange layer, interfacial hydrophilic layer and anion exchange layer. Under the action of direct current electric field, the bipolar membrane can be hydrolyzed away, and H+ and OH- can be obtained on both sides of the membrane. Based on this characteristic, bipolar membrane and anion exchange membrane were combined into A two-compartment BPED system (2CBPED-A). The bipolar membrane and cation exchange membrane were combined into a two-compartment BPED system (2CBPED-C).