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.

The evaluation of the long-term EDR operating condition, comprising mean residence time plus root of its variance and CaSO4 nucleation time, shows a large safety limit of the proposed equation. The demonstrated possibility of polarity cycle prolongation from 17 to 30 min, due to stagnant-regions reduction, enables a portion offspec product to diminish. At the same time the possibility of long-term stable operation at high CaSO4 supersaturation levels without any chemical additives was proved. Both features enable significant wastewater volume reduction that will allow zero-discharge technology to develop.