The production of high purity water by continuous electrodeionization with bipolar membranes: Influence of the anion-exchange membrane permselectivity

Continuous electrodeionization is used widely today for the production of high purity water. The conventional electrodeionization process using mixed-bed ion-exchange resins has several shortcomings, such as incomplete removal of weak acids and diluate conductivity above that of completely deionized water. In this study, the performance of a modified continuous electrodeionization stack using separated beds of cation- and anion-exchange resins and a bipolar membrane as source of regenerating protons and hydroxide ions is considered. Two modifications of this design, one with and one without a so-called protection compartment are compared with each other. The role of the co-ion transport through an anion-exchange membrane of incomplete permselectivity on the process performance is discussed theoretically and shown experimentally as function of the current density, the hydrostatic pressure and the flow velocity in the concentrate compartment, as well as the nature of the protection compartment rinse water. Using the protection compartment, the co-ion transport into the diluate could be eliminated and a specific conductivity of the product water of less than 0.056 μS/cm was achieved.