(from http://www.ionics.com/toolbox/edr.htm)

Over forty years ago, Ionics invented and introduced ion-exchange membranes and the electrodialysis (ED) process which incorporates such membranes. ED is a process in which solutions are desalted or concentrated electrically. Salts in water dissociate into positively and negatively charged ions. The key to the ED process in a semi-permeable barrier which allows passage of either positively charged ions (cations) or negatively charged ions (anions) while excluding passage of ions of the opposite charge. These semi-permeable barriers are commonly known as ion-exchange, ion-selective or electrodialysis membranes.

One of the problems in water desalination processes is that membranes and other active surfaces tend to become "fouled" or "scaled" over time by organic and inorganic substances present in the water. The electrodialysis reversal (EDR) process was developed and introduced by Ionics in the early 1970’s to deal with this problem. By reversing the electrical current and exchanging the fresh product water and the concentrate wastewater streams within the membrane stack several times per hour, fouling and scaling constituents that build up on the membrane surface in one cycle are removed in the next reversing cycle