Electrodialysis Reversal System (EDR)

Electrodialysis Process

Pretreatment Typical operation requires: the addition of a scale inhibitor to prevent scaling and reduce the concentrate LSI below 2.1 in the concentrate stream, residual chlorine concentration of 0.5 mg/L to prevent biological growth, and a cartridge filter (10-20 μm) prior to the ED/EDR system. Air stripping can also be used prior to ED/EDR in order to remove H2S. Also, the feed water must be within the limitations of an ED/EDR system.

Technology Description Electrodialysis is a process that depends on the principal that most dissolved salts are positively or negatively charged and they will migrate to electrodes with an opposite charge. Selective membranes that are able to allow passage of either anions or cations make separation possible. ED uses these membranes in an alternating fashion to create concentrate and product streams.

The anions are able to pass through the anion-selective membrane, but are not able to pass by the cation-selective membrane, which blocks their path and traps the anions in the brine stream. Similarly, cations move in the opposite direction through the cation-selective membrane under a negative charge and are trapped by the anion-selective membrane. An ED unit is able to remove from 50% to 94% of dissolved solids from a feed water, up to 12,000 mg/L TDS [3,7]. Voltage input, and process configuration (number of stacks or stages dictates the viable percent removal. TDS removal is generally limited by economics. The cost of ED increases as the feed water TDS increases. The typical operating conditions are 1,200 mg/L TDS, high hardness and high silica.

Electrodialysis Circuit

A typical ED system includes a membrane stack with a number of cell pairs, each consisting of a cation transfer membrane, a demineralized flow spacer, an anion transfer membrane, and a concentrate flow spacer. Compartments for the electrodes are at opposite ends of the stack. The electrodes are continually flushed to reduce fouling or scaling.

Recycling the concentrate stream and discharging concentrate to waste, or blowdown, is common and called feed-and-bleed mode. This is necessary because of the fact that there are sharp differences in flow rates between the product and brine streams. Diluate flow is about 10 times the flow of the brine stream; this difference in flows creates pressure imbalances, requiring concentrate recycle.

Membranes are usually made out of cation- or anion-exchange resins made into sheet form. ED spacers are made out of HDPE, and the electrodes are composed of an inert metal. Membrane selection is based on careful review of raw water characteristics.

Electrodialysis Reversal (EDR) is similar to ED but the polarity of the electrodes is regularly reversed, thereby freeing accumulated ions on the membrane surface. This process minimizes the effect of inorganic scaling and fouling by converting product streams into waste streams. This process requires additional plumbing and electrical controls, but increases membrane life. EDR does not require added chemicals, and eases cleaning as well.