DI and EDI
High-purity water production has traditionally used a combination of membrane separation and ion exchange processes. One well-known membrane separation concept is electrodialysis (ED), which uses an electrical potential to transport and segregate charged aqueous species. EDI is a further refinement of electrodialysis in that it combines the semi-permeable membrane technology with ion-exchange media to provide a high efficiency demineralization process. While the fundamental concept is somewhat simple with the basic desalting unit being an ED dilute cell filled with mixed-bed ion-exchange resin, some complex chemical reactions take place within the resin-filled cell. It is these reactions that help to produce the very high purity water required.