Electrodialysis reversal in high CaSO₄ supersaturation mode

In the case of the most troubling saline waters with CaSO₄ content close to saturation level, electrodialysis reversal (EDR) was found to be serviceable as a desalination method since its laboratory and commercial applications report stable long-term operation at high CaSO₄ concentrate supersaturation level. The available literature data exhibit stable long-term operation even at CaSO₄ supersaturation degree close to 175%. Previously the authors found that EDR of concentrated calcium sulfate solution might be carried out without scaling risk even at a higher CaSO₄ supersaturation degree, but a special effort must be addressed to lower the mean value and variance of concentrate channel residence time distribution. Also, special care must be taken with respect to CaSO₄ nucleation kinetics. Presently the authors have attempted to examine the previously observed relationship between EDR concentrate chamber residence time and calcium sulfate nucleation time. At first the maximum CaSO₄ supersaturation level was measured during long-term operation of a previously designed, low mean residence time EDR unit. These values were compared with those previously obtained from non-modified EDR units. It was found that the mean residence time reduction enables stable EDR long-term operation at high calcium sulfate saturation levels (326-364%) at a polarity cycle time prolonged from 17 to 30 min. The above data were then analyzed with respect to CaSO₄ nucleation kinetics.