EDI, also referred to as continuous deionization (CDI), is a chemical-free process that removes ionized and ionizable species from solution using ion exchange resins that are continuously regenerated by an electric current. Purified water produced by EDI is commonly referred to as type 2 water. To understand the process of EDI, it is necessary to first understand the configuration of a typical EDI module. EDI modules, also referred to as stacks, are comprised of cell pairs. Each cell pair contains an anode on one end and a cathode on the other end. (Figure 1) In between the anode and cathode are 3 compartments, all filled with mixed bed ion exchange resins. Each compartment is separated by alternating selectively permeable anion or cation membranes that are manufactured from ion exchange resins. The center compartment is termed the dilute (product water) compartment and the two compartments on either end are termed the concentrating (waste) compartments.Water enters both the dilute and concentrating compartments. When a direct current (DC) is passed through the cell the cations migrate from the diluting compartment through the cation permeable membrane towards the cathode, the anions migrate towards the anode through the anion permeable membrane and both are removed in the concentrating streams. The cations from the left hand concentrate compartment are impeded from passing into the dilute stream towards the cathode by the selectively permeable anion membrane. Likewise, in the right hand side concentrate compartment anions are impeded from passing into the dilute stream towards the anode by the selectively permeable cation membrane. The end result is deionized water passing through the dilute stream in the range of 10-15 MΩ× cm.