(101b) Recovering/Reuse of Sodium Chloride from Zero Discharge Desalination Concentrate Streams

Zero Discharge Desalination (ZDD) technology is able to achieve up to 98% recovery by desalinating brackish groundwater. ZDD is typically comprised of a reverse osmosis (RO) or nanofiltration (NF) system and a unique variation of electrodialysis called electrodialysis metathesis (EDM). The ZDD technology increases the yield of fresh water from water supplies that contain enough CaSO₄ to cause precipitation problems when the water is treated with RO or NF alone.  EDM can remove CaSO₄ and other troublesome salts, using NaCl as the exchangeable ions.  NaCl recovery and return to the EDM system is important to advance the development of the EDM technology. Electrodialysis with monovalent selective anion and cation exchange membranes was explored to selectively remove sodium chloride from a feed stream saturated with calcium sulfate. Recovered sodium chloride was reuse in the EDM process. Experiments with simulated solutions and extended tests with the actual EDM concentrate streams were performed. Very good selectivity was observed for chloride over sulfate. The performance of the process with actual EDM concentrate streams shows great promise for rapid transfer to the pilot scale.