The electrodeionization process uses a combination of ion-selective membranes and ion-exchange resins sandwiched between two electrodes(anode (+) and cathode (-) ) under a DC voltage potential to remove ions from RO-pretreated water. lon-selective membranes operate using the same principle and materials
as ion exchange resins, and they are used to transport specific ions away from their counterions.

Anion-selective membranes are permeable to anions but not to cations; cation-selective membranes are permeable to cations but not to anions. The membranes are not water- permeable.

This basis repeating element of the EDI, called a “cell-pair”, is illustrated in Figure1.
The “stack”of cell-pairs is positioned between the two electrodes, which supply the DC potential to the module. Under the influence of the applied DC voltage potential, ions are transported across the membranes from the purifying chambers into the concentrating chambers. Thus, as water moves through the purifying chambers, it becomes free of ions. This stream is the pure water product stream.