Electrodeionization (EDI) is an electrically-driven water treatment technology that uses electricity, ion exchange membranes and resin to remove ionized species from water.

The EDI module consists of a set of chambers filled with ion exchange resins and separated by ion-exchange membranes. Water enters the module, where an applied electrical field at right-angles to the flow forces ions to move through the resins and across the membranes. These impurity ions are not permanently bound to the media but instead are collected into concentrate streams which can be directed to drain or recycled. The deionized product water can be used directly or undergo further treatment for enhanced water purity.

The EDI module acts, in effect, as an ion exchange bed which is continuously regenerated electrically. When the ions are moved through the resins and between the cation or anion selective membranes, they are exchanged for H+ and OH- ions. Ions that become bound to the ion exchange resins eventually migrate to a separate chamber under the influence of the externally applied electric field; this also produces the H+ and OH- ions necessary to maintain the resins in their regenerated state. Ions in the separate chamber are flushed to waste.

The limitations of EDI differ from conventional ion exchange. In the latter scenario product water ionic purity is essentially limited by the total number of ions taken up by the resins. EDI is limited by the maximum rate of arrival of ions. Too high an ionic load will tend to overload the module. EDI is, therefore, often used after reverse osmosis and, if the water is very hard, with degassing to remove carbon dioxide.