Electrodeionization

Continuous Electrodeionization (CEDI) is the process of removing ionized or ionizable substances from water using ion exchange membranes, electrically active media (typically ion exchange resin), and a DC electric potential. Continuous demineralization in the module consists of three coupled processes:

1) Feed – Water is feed through a series of alternating dilute and concentrate compartments with electrode compartments at either end. The feed water (RO permeate) to be deionized is fed through the dilute feed compartments with a fraction of the flow being diverted to the concentrate and electrode feed (internal) compartments. The ion exchange resin accepts the ions in the feed water just as in conventional ion exchange, subject to the usual mass transfer and thermodynamic equilibrium considerations.

2) Removal – In CEDI, the ion exchange resin serves as a bridge for the cations and anions to travel rather than the conventional method of ion exchange where they are ionically bound and require chemical to be removed. Electrodes at the end of each module produce a DC electrical field which cause the ions to migrate toward ion selective membrane (AEM and CEM) through to the adjacent concentrate compartments. Once in the concentrating compartment, the ions are carried away by the concentrate flow. This process is particular to electrodeionization. The reject stream is sent to drain or in some cases is recycled back into the process.

3) Regeneration – As the water become more and more pure further down into the dilute compartment, the electric potential also causes some of the water present to split into hydrogen and hydroxyl ions which regenerate the ion exchange resins continuously in-situ, without the addition of chemical reagents. This process is particular to electrodeionization and can proceed even in the absence of ions in the feed water. The result is a module that continually regenerates itself during operation.