A new process for resourse recovery from surface water RO brine from permeate remineralization

Abstract

Low-Pressure Reverse Osmosis (LPRO) membranes could be valuable alternatives to conventional surface water potabilization processes [1] thanks to a combined ability to: remove micropollutants, soften, disinfect and remove organic substances. However, these processes produce an aggressive permeate and a concentrated brine with discharge constraints [2]. Assisted Reverse Electrodialysis (A-RED, [3]) was tested for LPRO permeate remineralization by recovery of target minerals from the corresponding LPRO brine while maintaining the product water’s integrity with respect to the organic compounds present in the concentrate. Bench-scale experiments were carried out with SUEZ-WTS membranes (CR67T-AR204T), operating under equal permeate and brine flows (2 cm/s velocities). The process was first tested using synthetic salt solutions, with varying applied voltage, permeate and brine inlet conductivity and brine composition to study the selective transport of individual species. Tests were then performed with Seine River LPRO brine spiked with 17 micropollutants to investigate membrane retention capacity. Results showed that ion transport numbers and permselectivity were weakly influenced by inlet solution conductivity and applied current/voltage, while permselectivity was mainly influenced by brine composition. High passage of salts was observed, allowing for a significant increase of permeate mineral content from ≈20 mg/L CaCO3 up to ≈490 mg/L CaCO3 and from ≈120 µS/cm up to 1284 µS/cm for hardness and conductivity, respectively. Spiked brine runs highlighted low micropollutant passage with over 98% rejection for 15 out of 17 compounds while natural organic matter breakthrough averaged 2% (0.2 mg C/L). Following these positive results a pilot unit was set-up with two 70 cell-pair stacks in series, fed with LPRO permeate and brine at nominal flow rates of 3.6 m3/h. Preliminary results have confirmed process feasibility while long-term performance stability evaluation is ongoing. Overall, results of this study highlighted the viability of the proposed process for LPRO permeate remineralization to target potable levels.