Working Principle and Characteristics of Electrodialysis

Electrodialysis is a method that utilizes the selective permeability of a semi-permeable membrane to separate different solute particles (such as ions). During dialysis under the influence of an electric field, the phenomenon of charged solute particles (such as ions) in a solution migrating through the membrane is called electrodialysis. The technology of purifying and separating substances using electrodialysis is called electrodialysis method. It is a new technology developed in the 1950s, initially used for seawater desalination, and now widely used in chemical, light industry, metallurgy, papermaking, and pharmaceutical industries. It is particularly valued for producing pure water and treating waste in environmental protection, such as acid and alkali recovery, electroplating wastewater treatment, and the recovery of useful substances from industrial wastewater.

Compared to reverse osmosis, another membrane separation technology introduced in recent years, electrodialysis is cheaper, but has a lower desalination rate. Currently, domestically produced ion exchange membranes are of stable quality and easy to operate and manage. Automatic control of frequent electrode reversal electrodialysis (EDR) further facilitates operation and management. Raw water utilization can reach 80%, with a typical raw water recovery rate between 45% and 70%. Electrodialysis is primarily used for primary desalination of water, achieving desalination rates between 45% and 90%. It is widely used in seawater and brackish water desalination; primary desalination in the production of pure water; and desalination and softening of feedwater for boilers and power equipment.

Essentially, electrodialysis can be considered a desalination technology because various types of water (including natural water, tap water, and industrial wastewater) contain a certain amount of salt. The anions and cations that make up these salts migrate towards electrodes in opposite directions under the influence of a direct current electric field. If one anion exchange membrane and one cation exchange membrane are inserted into an electrodialysis unit, due to the selective permeability of the ion exchange membranes—that is, the cation exchange membrane only allows cations to pass freely, and the anion exchange membrane only allows anions to pass—the salt concentration in the middle compartment decreases due to the directional migration of ions. The two compartments closest to the electrodes act as concentration chambers for anions and cations, respectively, ultimately achieving desalination in the central desalination chamber.

In practical applications, an electrodialysis unit is not composed of a pair of anion and cation exchange membranes (because doing so would be very inefficient), but rather uses one hundred or even several hundred pairs of exchange membranes, thus greatly improving efficiency.