Membrane Scaling in Electrodialysis Fed with High-Strength Wastewater

Membrane scaling problems can limit broad applications of electrodialysis (ED) for nutrients recovery from wastewater. In this study, we investigated the calcium- and magnesium-scale precipitation on ion-exchange membranes (IEMs) using a laboratory-scale ED reactor. Two high-strength wastewater streams, including municipal waste (MW) liquid digestate and food waste (FW) liquid digestate, were fed into the ED reactor. For the operation with MW liquid digestate, the cumulative Ca2+ loss increased with the increasing electric current, while the electric current conditions did not affect the cumulative Mg2+ loss. After 8-h operation, 60.1% of Ca2+ and 39.0% of Mg2+ in the MW liquid digestate were lost in the form of precipitates. Observed scalants on cation-exchange membranes were vaterite, amorphous calcium carbonate (ACC), and struvite, while ACC was not found on anion-exchange membranes. Observed scalants of calcium carbonate with MW liquid digestate (vaterite and ACC) were different from scalants (calcite) found with synthetic solutions. Among these scalants, struvite was formed as sharp (needle-shaped) crystals that can potentially damage the IEM. The gradual loss of Mg2+ was observed with FW liquid digestate because of high PO43− concentration, indicating the formation of struvite. The membrane with high selectivity for divalent ions resulted in the rapid decrease in electric current, implying serious membrane scaling on IEMs. These findings demonstrated that the membrane scaling problems by calcium and magnesium precipitation are ubiquitous in ED for nutrients recovery from wastewater.