High efficiency electrodialysis reversal of concentrated calcium sulfate and calcium carbonate solutions

Electrodialysis reversal (EDR) of high calcium sulfate/carbonate containent low-salinity waters was examined in the laboratory. The previously developed ED stack, equipped with a 0.19-mm-thin spacer, was modified to achieve low mean residence time and low residence time distribution. As a consequence of the above modification, higher than reported for stable EDR unit operation gypsum supersaturation levels (up to 368%) and a longer polarity cycle time (up to 22 min) were observed. At the same time the increase in stable operation LSI from 1.92 to 2.12 was observed. It was also shown that controlled gypsum crystallization from the concentrate stream resulted in an 80–83% reduction in calcium content and simultaneously a 57–60% reduction in sulfate content. This should enable concentrate re-circulation and high overall desalted water recovery using a modified EDR unit.