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Electrodialysis (ED)


Electrodialysis (ED) has been around nearly as long as RO, with its discovery in 1890 (Xu, 2005), and first installation for the desalination of seawater in the 1960s (Xu, 2005). While electrodialysis is a proven method for desalination, it is not as widespread as RO. A recent study estimated that electrodialysis accounted for 3.53% of the world's desalination capacity while RO accounted for 59.85% in 2012 (Ghaffour et al., 2013). Electrical potential, not pressure, is the driving force for ion transport through ED membranes.

Electrodialysis "stacks" are comprised of alternating cation- and anion-exchange membranes as shown inthe figure below. Commercial stacks will include hundreds of cell pairs. An aqueous solution, such as brackish water, is fed into alternating feed compartments. As an electrical potential is applied, anions move towards the positively charged anode and cations move towards the negatively charged cathode. Cations, shown moving to the right, will pass through the cation-exchange membrane but will be blocked by the anion-exchange membrane. Similarly, anions will pass thorough the anion-exchange membrane but will be blocked by the cation exchange membrane. This leads to changes in the salt content of alternating streams; one is a diluted stream (called diluate) and the other is a concentrated stream (called concentrate). As shown in the figure, all ions will concentrate in the ED system. The recovery will be limited by the solubility of species that form such as CaSO4.

chart


Oxidation and reduction reactions occur and gasses are generated at each of the electrodes. At the cathode, hydrogen gas is produced; at the anode, oxygen gas is produced. An electrode rinse stream flows across each electrode to provide a path for current to flow from the electrodes into the rest of the stack, as well as to cool both electrodes and sweep gases out of the stack. Gases generated in the electrode rinse compartments are vented to the atmosphere.

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