Electrodeionization

Electrodeionization (EDI) is an electrochemical process that combines the two methods of electrodialysis and ion exchange. With this method, the most extensive removal of ions and ionizable substances from water is possible.

The basic element of an EDI module is the EDI cell. The cell is surrounded by semipermeable membranes, with an anion-selective membrane on one side and a cation-selective membrane on the other side. The cell is filled with a mixed bed resin, i.e. a resin bed of anion and cation exchange resin. Several of these cells form an EDI module, which in turn is enclosed on one side by an anode and on the other side by a cathode.

The water, which has been desalinated, flows through the cells. The electrical voltage applied to the electrodes drives the ions through the semipermeable membranes. Consequently, there is always a concentrate chamber between two cells with diluate or ultrapure water, from which the ions are flushed out of the module.

At the same time, the application of an electrical voltage in the resin bed causes water to split into H + and OH ions. The H+ ions regenerate the cation exchange resin, the OH ions regenerate the anion exchange resin. This regeneration process runs continuously and parallel to the operation of the module and does not require the use of chemicals.

The electrodeionization is generally used after a reverse osmosis plant (single-stage or two-stage permeate stage) for further desalination. As a result, conductivities in ultrapure water of < 0.06 µS / cm can be achieved.