CO₂-driven hybrid ion exchange: A resilient reverse osmosis pretreatment for high-recovery wastewater reuse

Municipal wastewater reuse is an essential strategy to address global water scarcity. However, its expansion is often constrained by reverse osmosis (RO) membrane scaling at high recovery. Achieving high RO recovery is critical for inland facilities to directly reduce the volume of concentrate requiring disposal. Conventional ion exchange (IX) pretreatment removes scaling ions and helps increase RO recovery, yet its reliance on chemicals produces additional salty spent regenerants that add to the RO concentrate burden, further complicating brine management challenges for inland wastewater reuse. In this study, we demonstrated a pressurized CO₂-driven hybrid IX process as a sustainable pretreatment to overcome these limitations. Using real and CaSO₄-enhanced secondary effluents from a municipal wastewater treatment plant, the IX system consistently removed multiple scaling ions, reduced aqueous pH, and enabled phosphorus recovery over 52 consecutive cycles with CO₂ as the regenerant. Regeneration efficiency of IX materials increased systematically with CO₂ partial pressure, exceeding 90 % above 10 bar, and was unaffected by the presence of nitrogen and oxygen. Comprehensive PHREEQC modeling and bench-scale RO validation confirmed that the pretreatment suppressed carbonate, sulfate, and phosphate scaling due to ion removal and pH reduction. For the CaSO₄-enhanced secondary effluent, RO permeate flux was stabilized for over 3 times longer duration than that without pretreatment, and the pretreatment enabled ∼90 % water recovery (<30 % if without pretreatment for the same water). Additionally, the process reduced reject-side osmotic pressure, thereby lowering operational energy demand. Overall, CO₂-driven IX provides a multifunctional pretreatment approach for enhancing RO recovery, promoting sustainable inland wastewater reuse.