Electrodialysis on RO concentrate to improve water recovery in wastewater reclamation

Over-consumption of groundwater in coastal areas causes seawater intrusion and soil salinization, which is a threat to residents, to agricultural activity and to the ecological system. In this study, a more sustainable approach is investigated based on groundwater recharge of the unconfined aquifer in the dune water catchment area in the western part of the Flemish coast. Ultrafiltration (UF) followed by reverse osmosis (RO) is currently applied to treat the secondary effluent from a wastewater treatment plant (WWTP) for infiltration (groundwater recharge). This paper investigates the feasibility of electrodialysis (ED) on the RO concentrate to reduce the volume of salty water discharge and to improve the overall water recovery to produce infiltration water for groundwater recharge. In the pilot system, the decarbonation process was used to reduce scaling potential of the feed or the concentrate stream of the ED. Based on various experiments in batch and in feed-and-bleed mode, ion transport mechanisms were studied to monitor the effluent water compositions. Meanwhile, a factor named critical scaling concentration (CSC) was established to predict the potential occurrence of scaling. Ozonation was used to improve the biodegradability of the ED effluent hence to reduce the potential of organic compounds accumulation in the recirculation system. Thus, ED was found to be a good option to treat RO concentrates.