Electrically Driven Separation
Electrodialysis separates ions from water using an electric field and ion-selective membranes. A stack of alternating cation-exchange membranes (CEM) and anion-exchange membranes (AEM) creates a series of channels. When voltage is applied, cations migrate toward the cathode through CEMs and anions migrate toward the anode through AEMs. Due to membrane selectivity, ions accumulate in concentrate channels and are depleted from dilute channels, producing desalinated water.
Current Efficiency & Limiting Current
Not all electrical current contributes to salt removal. Back-diffusion of ions, osmotic water transport, and co-ion leakage reduce current efficiency below 100%. Most critically, when current density exceeds the limiting value, ion depletion at the membrane surface causes water splitting into H+ and OH- ions. This wastes energy, changes pH, and can damage membranes. Operating below 70-80% of the limiting current density ensures stable, efficient performance.
Energy Consumption
ED energy scales linearly with the amount of salt removed, making it energy-efficient for low-salinity feeds. For brackish water at 2-5 g/L, ED consumes 0.5-2 kWh/m³ — competitive with or better than RO. However, for seawater at 35 g/L, the high salt load makes ED impractical, and RO dominates. This complementarity means the optimal technology depends on feed salinity.
Advanced ED Processes
Electrodialysis reversal (EDR) periodically switches electrode polarity to flush scale-forming deposits, enabling operation on challenging feed waters without chemical cleaning. Bipolar membrane electrodialysis (BMED) produces acids and bases from salt solutions, finding applications in food processing and chemical production. Capacitive deionization, a related technology, stores removed ions in porous carbon electrodes, offering energy-efficient treatment of very low salinity streams.