Effect of Operating Conditions On Selectivity In Pilot Scale Electrodialysis Reversal Process

Electrodialysis Reversal (EDR) is a membrane based separation processes in which cations and anions pass through cation and anion exchange membranes, respectively, under the influence of an electric field. EDR processes have several practical applications that water desalination, the food and chemical process industry can be mentioned. EDR process has been utilized for over 50 years for the drinking water production from brackish water sources. Brackish ground water sources can have various amounts of ions depending on several parameters. When the amounts of some of the ions are higher than established levels of national primary/secondary drinking water standards, they are should be decreased to their standard levels. Based on this purpose, the ions should be separated selectively via ion exchange membranes in EDR process. Although a limited number of studies have been done for selectivity studies in EDR process, the lack of comprehensive research for selectivity of ion exchange membranes in EDR process, especially in pilot scale cannot be neglected. In this research, three sets of experiments were done in order to determine the effects of temperature, ion cut rate, dilute flow rate and the feed water chemistry on selectivity of divalent and monovalent ions in a pilot scale EDR process. The EDR pilot scale set up was installed in Brackish Ground Water Desalination Research Facility (BGNDRF), Alamogordo, New Mexico. The EDR stack was composed of 40 cell pairs in which CR67 and AR204 were used as cation and anion exchange membranes, respectively.  The feed water was provided from two wells with different water compositions located at BGNDRF. In order to find the effects of operating conditions on selective removal of ions, three levels of temperature of 10, 24 and 31°C, different levels of cut rate were selected according to feed water composition. Also, three levels of product flow rate of 5.3, 7.5 and 12 GPM were chosen to be studied. Based on the results of the experiments, at high values of cut rates all the monovalent and divalent ions tend to pass though membranes with almost the same rate; however, at lower cut rates such as 30% divalent ions such as Calcium and Strontium are separated more than monovalent ions. It seems that the effects of temperature and cut rate on selectivity are more significant than flow rate and feed water chemistry. At lower level of cut rate by increasing the temperature the selectivity of divalent ions to monovalent ions increases while at higher level of cut rate this effect cannot be detected clearly and all ions pass through the membranes with a nearly same rate. In addition, the selectivity values for divalent ions to monovalent ions at lower flow rates are greater than higher flow rates. In fact, based on the excess amount of one or some species in different brackish water sources, it is valuable to study the selective removal of some target ions in order to minimize operating costs. Selective removal of ions by choosing the optimum operating conditions can be considered as one of the most potential approaches for decreasing operating costs in EDR processes.