Electro deionisation (EDI) or continuous electro deionisation (CEDI) are used interchangeably and are techniques for making very pure water (demineralised water). The technique consists of a combination of membrane filtration and ion exchange. Unlike a traditional ion exchanger, this regeneration system does not require any chemicals. The ion exchange resin is located alternately between a cation selective and an anion selective membrane. These are the product and reject compartments.

By applying a DC voltage to the CEDI module, the ions in the feed water are transported in the direction of the ion-selective membranes. This conveys them to the surface of the ion exchange resin. The ions pass through the membrane and end up in the reject compartments. They are then disposed of. What remains is very clean water. The direct voltage causes the water to split into H+ and OH- ions. These in turn regenerate the ion exchange resin. Because the selective membranes only allow ions to pass through, a small part (5-10%) of the feed water is used to remove the ions from the reject compartments as a concentrated stream.