Electrodialysis membrane with concentration polarization – A review

Electrodialysis (ED) comprises of periodic cells featuring cation exchange membranes (CEM), anion exchange membranes (AEM), two electrodes (anode and cathode), and mesh/porous-filled spacers to maintain the intermembrane gap. Concentration polarization may lead in a decline of over than 50% in the usable membrane for forward osmosis (FO) and reverse osmosis (RO), as well as a 60% fall in system efficacy. The undesirable effect of concentration polarization in ED, on the other hand, are twofold. Membrane spacers are frequently employed in membrane modules to optimize flow dynamics and mass transfer patterns, which affect the impact of concentration polarization and permeate flux. The current systematic review seeks to address the issue, "Which choice is optimal for minimizing concentration polarization, increasing velocity, or improving spacer?". Innovative geometries and configurations of membrane spacers are required to reduce pressure drop and concentration polarization. Using carefully developed spacers to increase turbulence in the channel can serve to mitigate the polarization effects. Unfortunately, improved spacers continue to have detrimental consequences, such as the shadow effect, which decreases the ion exchange area and increases flow resistance. Increasing the flowrate may improve membrane performance, maximize recovery rate (or permeate rate), and lessen the built-in boundaries of spacer design (e.g., shadow effect, unsatisfactory cell hydrodynamics, and elevated concentration polarization). Increasing the flowrate has drawbacks that may be considerable in specific membrane applications. For example, with a higher Reynolds number, a spacer performs better with a higher Sherwood number, but at the tradeoff of a greater value of power number. Concentration polarization can significantly impact the effective membrane capability, lowering the gross power generated; however, the extent of the concentration polarization in the boundary layer and its significance to operation efficiency are dependent on the feed flowrate and spacer geometry. In fact, the extent of concentration polarization and cake layer development are heavily influenced by spacer design as well as the velocities of the dilute and brine fluids. Because convective motions effect mixing, the concentration domain within the cell is highly dependent on duct design and flow velocity.