Electro Dialysis

Electrodialysis is a membrane-based process involving transport of ions through semipermeable membranes using an applied electric field. The applications employing electrodialysis include desalination, table salt production, wine stabilization, whey demineralization, and pickling bath recovery. The process involves an electrodialysis cell in which the feed stream and the permeate stream are separated by polyelectrolyte membranes. The membranes employed are anionic polyelectrolytes (such as poly(styrene sulfonic acid) and cationic membranes such as poly(vinyl benzyl trimethyl ammonium hydroxide). The anionic polyelectrolyte is the cation exchange membrane, and the cationic polyelectrolyte is the anion exchange membrane. As these polymers are water soluble or dispersible, they are often cross-linked for stability in the electrodialysis process. The anion exchange membranes permit the passage of anions but reject cation permeability, and the cation exchange membranes permit the passage of cations but reject anion permeability.

Electrodialysis (ED) was the first commercialized membrane-based desalination technology. It currently serves a relatively small percentage of the drinking water industry having been displaced by the preferential adoption of RO. ED involves the use of ion-selective membranes in conjunction with an electric potential to separate ions from water. ED uses a stack of alternating cation- and anion-selective membranes placed under an electric field gradient. The end compartments contain the anode and cathode along with their respective electrolyte solutions. Feedwater flows in the space between the membranes and is subjected to a potential gradient. Under the influence of the electric field, the cations permeate through the cation-selective membrane toward the cathode, whereas the anions migrate toward the anode. As the ion-selective membranes are in an alternating arrangement, cations migrating into an adjacent compartment are restricted from further movement toward the cathode by the intervening anion-selective membrane and vice versa. The migrating cations and anions are trapped in the intervening channels forming the concentrate compartments. The feed solution is depleted of both cations and anions forming the product stream or diluate.

The degree of desalination achieved is a function of the feed concentration, the current density, and residence time in the stack. The current density cannot be increased above a threshold called the limiting current density due to ion depletion at the solution–membrane interface. The limiting current density is proportional to the target concentration in the diluate stream. Therefore, a very low TDS specification in the diluate will usually mean a low operating current density.

The area of the ED stack is inversely proportional to the current density. A low current density will increase membrane area requirements and capital costs. The energy costs increase with current density. As current density influences both the capital and operating costs but in opposite directions, ED stack design usually involves determining the optimal current density.

The amount of energy used in electrodialysis is dependent on the quantity of salt to be removed, the current density, and the resistance of the cell pair. Highly saline waters, such as seawater, require more salt to be removed than brackish waters and, therefore, require more energy.