Electro dialysis is a technique that employs an electrical current and special membranes, which are semi permeable to ions, based on their charge. Membranes that permeate cations and membranes that permeate anions are placed alternately, with flow channels between them, and electrodes are placed on each side of the membranes. The electrodes draw their counter ions through the membranes, so that these are removed from the water.
The basic principles of the electrodialysis (ED) treatment process are similar to ion exchange in that dissolved ions present in water have either a positive or negative charge and are attracted to electrodes with an opposite electric charge. ED differs from a normal ion exchange process by utilizing both cation and anion selective membranes to segregate charged ions extracted from a water solution. In ED, membranes that allow either cations or anions (but not both) to pass are placed between a pair of electrodes. An improvement to ED, referred to as Electrodialysis Reversal (EDR), utilizes the same concept except that in EDR, there is periodic automatic reversal of polarity and cell function to reverse the flow of ions across the membrane. This returns anions across the anionic membranes and helps break up scale formed on the concentrating face of the membranes and minimizes membrane fouling. As with RO, a small pump is required to move the water through the membranes to overcome the resistance of the water as it passes through the narrow passages.
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