(378e) Purity of NaCl in the Electrodialysis Metathesis (EDM) Process for Water Desalination

Electrodialysis metathesis (EDM) is a component in the zero discharge desalination (ZDD) process that maximizes water production and salt recovery from desalination.  In the EDM process the concentrated effluent from reverse osmosis (RO) or nanofiltration (NF) is treated to recover minerals, such as gypsum and magnesium chloride, as well as additional pure water.  The minerals are separated and collected in two of the four compartments of the EDM stack that are separated by ion exchange membranes. One compartment will contain a concentrated mixture of sodium salts and another compartment will contain a concentrated mixture of chloride salts. The third compartment will contained the minerals-rich feed water to be treated. The fourth compartment is filled with sodium chloride to provide the ions for the metathesis reaction. The concentrated mixtures are generated when a voltage is applied to the repeated four-compartment units of the EDM stack.  Once the concentrated streams exit the system they can combine to precipitate gypsum.

The early precipitation of calcium sulfate in one of the concentrate compartments can adversely affect the potential recovery of other useful salts. Furthermore, the EDM stack can be clogged and damaged. Commercially available sodium chloride may contain sodium sulfate and other minerals that will promote the formation of gypsum in the stack. In this study we are adding impurities to the sodium chloride compartment to determine the lowest purity allowed by the system.  Preliminary experiments have shown that sodium chloride with concentrations of 1,000 mg/L of sodium sulfate doesn’t affect the performance of the salt separation process.  This results suggest that the cost of EDM can be further reduced by reducing the need of adding high quality chemicals to the process.