Desalination using pressure or electric field? A fundamental comparison of RO and electrodialysis

Reverse osmosis (RO) is the dominant desalination technology today since it is the most economical. Electrodialysis (ED) is a competing technology that removes ionic solutes using an applied electric field. An analogy is derived between the flux of water in RO (pressure-driven) and salt in ED (in the form of an electric current), which helps define appropriate conversion factors for direct comparison between equivalent terms, e.g., electrical conductivity of ED and permeability of RO. Optimized batch and single-stage configurations of both technologies are considered for cross-comparison. The ratio of specific costs (per unit system area) of ED and RO to achieve similar overall cost of pure water is inversely proportional to the feed salinity squared, resulting in ED's relative advantage for lower-salinity/brackish-water desalination. This study provides a guideline on the competitiveness of ED for various desalination applications as a function of technology (membrane, channel resistance) and commercial (cost, plant life, interest rate) factors. For example, to outperform RO for seawater desalination, the amortized cost of ED or the overall cell pair resistance has to be reduced by a factor of around 20 relative to values reported in the literature.