(301e) Electrodialysis Reversal Pilot Studies Performed At the Brackish Ground Water National Desalination Research Facility (BGNDRF) In Alamogordo, NM

Electrodialysis Reversal (EDR) technology uses voltage driving force and ion selective membranes to remove salt from water.  The “Reversal” in EDR refers to the process of periodically reversing the polarity of the electrodes in order to inhibit scale formation.  EDR has been an established desalination technology for almost 40 years, and yet the understanding of EDR is less developed than that of other technologies such as distillation and reverse osmosis.  For example, all the major suppliers of reverse osmosis membranes also provide advanced simulation tools that enable engineers to reliably design systems.  In contrast, there is no such design software publicly available for EDR and very few literature references that provide operating data for commercial sized systems.  EDR is cloaked in mystery.

Extensive studies were performed using full sized pilot EDR equipment, where the electrodes, spacers, and membranes were sized identical to commercial systems, while the number of cell pairs was typically 40, compared to 600 at most commercial installations.  The experiments were performed at BGNDRF utilizing the various sources of brackish water available on site.  Experimental variables included water chemistry, temperature, recovery, velocity, volts/amps, stack configuration, and membranes.  Thousands of individual experiments were performed and over 900 water samples were collected for analysis.

Results were analyzed in a number of ways.  First, the relationship between volts, amps, and cut rate of dissolved salts is defined as function of operating conditions.  Second, the selectivity for various ions was determined, indicating that selectivity was highest at high temperatures and low cut rates.  Third, limiting current was determined and found to have unexpected functional dependencies.  Temperature had very little effect over the range 7 – 40 C, the nature of the ionic species had no discernible effect, and limiting current was nearly linearly dependent on fluid velocity.  The quantitative coupling of operating conditions (temperature and flow velocity) with performance (volts, amps, conductivity) and water chemistry led to greatly increased understanding of EDR process and provided the foundation for an EDR process model.