EDI theory and practice have been advanced by a large number of researchers throughout the world. It is believed that EDI was first described in a publication by scientists at Argonne Labs in January 1955 as a method for removal of trace radioactive materials from water (Walters, et. al.). One of the earliest known patents describing a EDI device and process was awarded in 1957 (Kollsman). It is thought that the first pilot device incorporating mixed resins was developed by Permutit Company in the United Kingdom in the late 1950's for the Harwell Atomic Energy Authority, as described in a paper (Gittens and Watts) and in more than one patent (Kressman; Tye). One of the first detailed theoretical discussions of EDI was written in December 1959 (Glueckauf). In April 1971, a Czechoslovakian researcher reported results of his experimental and theoretical work that advanced the theory of ionic transport within a EDI device (Mateka). Layered bed devices were described in the patent literature in the early 1980's (Kunz).
EDI devices and systems were first fully commercialized in early 1987 by a division of Millipore that is now part of United States Filter Corporation (Ganzi et. al., 1987). Since then, the theory and practice of EDI has advanced worldwide, and commercial EDI devices are now manufactured by a number of companies (Towe et. al.; Parsi et. al.; Rychen et. al., Stewart and Darbouret). There are now several thousand EDI systems in commercial operation for the production of high purity water at capacities ranging from less than 0.1 to more than 250 m3/h. This includes EDI systems that have been in continuous operation for nearly ten years, producing makeup water for high pressure boilers.
Continuous Electrodeionization or Ion Exchange?
Companies are constantly working to reduce operating costs, improve efficiency and eliminate the use of hazardous chemicals in the workplace. Such goals have caused an increase in the use of continuous electrodeionization technology to produce high-purity water.
Continuous electrodeionization (EDI) uses a combination of ion exchange resins and membranes, and direct current to continuously deionize the water without regeneration chemicals.
The principle behind ion exchange is that polymer resin beads are chemically structured to provide either positively or negatively charged fixed functional groups that attract and remove certain contaminant ions from the water. Conventional ion exchange technology can remove dissolved inorganics such as minerals and salts and some dissolved organics. It does not remove particles, colloids, bacteria or pyrogens.
Cationic resins remove positively charged ions such as calcium, magnesium and sodium, replacing them with hydrogen (H+) ions. Anionic resins remove such negatively charged ions as chloride, nitrate and silica and replace them with hydroxide (OH-) ions. The hydrogen and hydroxide ions then unite to form water molecules.
When the water passes through a tank containing a mixture of both cation and anion exchange resins, the process is called mixed-bed ion exchange. Mixed-bed systems can produce very high-quality water with resistivities up to 18.2 megohms-cm.
Over time, however, the resin beads become saturated with contaminant ions and become less effective at treating the water. Also, the high-purity water flowing past these saturated resins may actually extract trace amounts of contaminant ions by "chromatographic effects," causing a decline in water quality. The exhausted resins must be chemically regenerated off-line before reuse. During regeneration, the cation resin beads are restored to their hydrogen form by treating them with hydrochloric acid (HCl) or sulfuric acid (H2SO4). The anion resin beads are restored to their hydroxide form by treating them with caustic (NaOH).