Theory of Operation

In an EDI device, the space within the ion depleting compartments (and in some cases in the ion concentrating compartments) is filled with electrically active media such as ion exchange resin. The ion-exchange resin enhances the transport of ions and can also participate as a substrate for electrochemical reactions, such as splitting of water into hydrogen (H+) and hydroxyl (OH-) ions. Different media configurations are possible, such as intimately mixed anion and cation exchange resins (mixed bed or MB) or separate sections of ion-exchange resin, each section substantially comprised of resins of the same polarity: e.g., either anion or cation resin (layered bed or LB and single bed or SB).

The electrodeionization process uses a combination of ion-selective membranes and ion-exchange resins sandwiched between two electrodes (anode (+) and cathode (-)) under a DC voltage potential to remove ions from RO-pretreated water.

Ion-selective membranes operate using the same principle and materials as ion- exchange resins, and they are used to transport specific ions away from their counter ions. Anion-selective membranes are permeable to anions but not to cations; cation-selective membranes are permeable to cations but not to anions. The membranes are not water-permeable.

By spacing alternating layers of anion-selective and cation-selective membranes within a plate-and-frame module, a "stack" of parallel purifying and concentrating compartments are created. The ion-selective membranes are fixed to an inert polymer frame, which is filled with mixed ion-exchange resins to form the purifying chambers. The screens between the purifying chambers form the concentrating chambers.

This basic repeating element of the EDI, called a "cell-pair", is illustrated in Figure 1. The "stack" of cell-pairs is positioned between the two electrodes, which supply the DC potential to the module. Under the influence of the applied DC voltage potential, ions are transported across the membranes from the purifying chambers into the concentrating chambers. Thus, as water moves through the purifying chambers, it becomes free of ions. This stream is the pure water product stream.

Most commercial EDI devices comprise alternating cation- and anion-permeable membranes with spaces in between configured to create liquid flow compartments with inlets and outlets. The compartments bound by a positively charged anion exchange membrane (AEM) facing the positively charged anode (+) and a negatively charged cation exchange membrane (CEM) facing the negatively charged cathode (-) are diluting or purifying compartments. The compartments bound by an anion membrane facing the cathode and a cation membrane facing the anode are concentrating compartments. To facilitate ion transfer in low ionic strength solutions, the dilute compartments, and sometimes the concentrate compartments, are filled with ion exchange resins. A transverse DC electrical field is applied by an external power source using electrodes at the bounds of the compartments such that ions in the liquid are attracted to their respective counter electrodes. The result is that the diluting compartments are depleted of ions and the concentrating compartments are concentrated with ions. Figure 1 is a representation of the process showing two diluting compartments and one concentrating compartment.


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FIGURE 1.

 

NOTES:

CEM = Cation exchange membrane.
AEM = Anion exchange membrane.
The product stream may also be referred to as the dilute stream.
The reject stream may also be referred to as the concentrate stream.
To facilitate ion transfer in low ionic strength solutions, the dilute compartments, and sometimes the concentrate compartments, are filled with ion exchange resins. A transverse DC electrical field is applied by an external power source using electrodes at the bounds of the compartments such that ions in the liquid are attracted to their respective counter electrodes. The result is that the diluting compartments are depleted of ions and the concentrating compartments are concentrated with ions. The figure above is a representation of the process showing two diluting compartments and one concentrating compartment.