Selectivity Studies On Pilot-Scale Electrodialysis Reversal

Although Electrodialysis Reversal (EDR) technology has been commercially used since the early 1960s, the fundamental understanding of this technology is less developed than that of other technologies such as distillation and reverse osmosis.  Selectively removal of some ions of interest compared to other ions which present in water has not been studies in details specially in pilot-scale.  Groundwater resources, which are very important sources of drinking water in many parts of the world as well as southwest region of the United States, have various water chemistries. Depending on several parameters, various amounts of anions such as chloride, sulfate, and alkalinity species that contribute to high total dissolved solids (TDS) and/or salinity can be found in groundwater samples.  Understanding of selective removal of ions and implication of this understanding would help to reduce the cost of desalination by reducing the number of moles of ions that is removed to get appropriate product water. The objective of this study was to evaluate: a) effect of salt removal par electric stage, b) ionic strength and composition of feed water, c) operating conditions including temperature, and d) anionic exchange membrane fouling on selectively removal of some ions of interest compared to other ions. 

Full pilot-scale studies were performed on a GE electrodialyser with the capacity of 10 gpm influent flow rate at the Brackish Groundwater National Desalination Research Facility (BGNDRF) in Alamogordo, New Mexico.  Brackish feed-water was provided from aquifers in the Tularosa Basin with conductivity of 1700-6600 µS/cm, which are within the spectrum for water quality in the southwestern region of the United States.  The experiment operated with single/double electrical/hydraulic stage(s) using CR67HMR or CR64LMR cation-exchange membranes, AR204 or AR908 anion-exchange membranes, and Spacer-Mark IV with an effective membrane surface area of 3,000 cm².  Selectivity of EDR stack for different monovalent and divalent ions was evaluated by water analysis of feed, concentrate, and product water samples under various salt removals par stage, various ionic strengths and compositions of feed water, various operating conditions and new/aged anionic exchange membranes.  

Preliminary data analysis indicated that all the above-mentioned parameters have direct effect on selectively removal of some species compared to other species.  The results showed that higher temperature increases the selectivity for divalent ions compared to monovalent ions.  Interestingly, the results indicated that generally aged anionic membranes have less selectivity compared to new anionic membranes.   The results of this study could be used to help EDR designers/operators to selectively target the ions of interest and it would result in reducing the cost of desalination by reducing the number of moles of ions that is removed to reach the targeted product water.