Click here to enlarge imageFigure 1 illustrates how feed water enters the dilute channels, then feed water ions pass out of the dilute channels and into the concentrate channels through the appropriate membranes due to their attraction to the oppositely charged electrode.The result is a product water from the dilute channels and a waste stream from the concentrate channels. It’s common to recirculate a portion of the concentrate back to the concentrate channels and only send a relatively small amount of concentrate to drain.Electrodialysis ReversalIt was found that operating ED units on most brackish waters caused too much scaling in the concentrate channels. The requirement for chemical cleaning was too high. The solution was to reverse the polarity of the electrodes every few minutes and change the valving so that concentrate channels became dilute channels, and dilute channels became concentrate channels. This keeps electrodialysis reversal (EDR) units relatively clean.The difference between ED and EDR, therefore, is that in ED units the electrodes have fixed polarity, dilute channels are always dilute channels and concentrate channels are always concentrate channels. Figure 2 illustrates the process with the polarity of the electrodes reversed compared to Figure 1.