Electrodeionization is a water treatment technology that utilizes an electrode to ionize water molecules and separate dissolved ions (impurities) from water. It differs from other water purification technologies in that it is done without the use of chemical treatments and is usually a tertiary treatment to reverse osmosis (RO). There are also EDI units that use a small bed with ion-exchange resin to enhance the deionization further, this is often referred to as Continuous electrodeionization (CEDI) since the electric current regenerates the resin mass continuously. CEDI technique can achieve very high purity, with conductivity being below 0.1uS/cm.

The red membranes are cation-selective membranes and the blue membranes are anion-selective membranes. The negatively-charged anions (e.g., Cl^-) are attracted to the anode (+) and repelled by the cathode (-). The anions pass through the anion-selective membrane and into the adjacent concentrate stream where they are blocked by the cation-selective membrane on the far side of the chamber, and are thus trapped and carried away by the carrier water in the concentrate stream. The positively-charged cations (e.g., Na⁺) in the purifying stream are attracted to the cathode (-) and repelled by the anode (+). The cations pass through the cation-selective membrane and into the adjacent concentrate stream where they are blocked by the anion-selective membrane and are carried away.

Benefits of CEDI