Electrodialysis Reversal (EDR)
In Electrodialysis Reversal (EDR) system, the polarity of the direct current applied to the membrane electrodes is reversed 2 - 4 times per hour. When the electrode polarity is reversed, the desalted water and brine channels are also reversed by automatic valves that control the flows in the stack.
The fresh water channel becomes the brine channel and the brine channel becomes the fresh water channel. Both fresh water channel and brine channel have identical construction. For about 1 - 2 minutes, both streams which have high salinity, are automatically diverted to waste. This continues until the desired product conductivity is obtained, at which time the product stream is automatically diverted to storage.
By switching cells and reversing current direction, freshly precipitated scale is flushed from the membrane before it can solidify. The direction of movement of colloidal particulates drawn to the membrane by the flow is also reversed, so colloids do not form a film on the membrane.
The major advantage of the EDR process is that it eliminates the need to acidify or add inhibitors to the brine stream. The main disadvantage occurs during the polarity reversal where water is wasted and productivity is lost.