Controlled chlorination of polyamide reverse osmosis membranes at real scale for enhanced desalination performance

Abstract

State-of-the-art desalination and water purification processes use reverse osmosis and nanofiltration membranes. Their thin polyamide (PA) top-layers ensure concurrent high water permeances and salt rejections, but are also intrinsically sensitive to chlorine, originating from disinfectant added upstream. The chlorine resistance of PA-based membranes has been thoroughly studied at lab-scale, as opposed to industrial-scale membrane modules, where fundamental studies are lacking. Therefore, to better understand chlorine-induced changes in membrane performance and physicochemical properties at industrial scale, chlorination of commercial 8” elements was conducted at different pH (4-7-10) in pressurized modules with low chlorine concentrations (0, 1, 20, 50 ppm NaOCl) during 2.5 h. After 50 ppm acidic chlorination, water permeability decreased (−40%) but salt rejection increased significantly (+0.4%, i.e., salt passage decreased with −78.8%). Boron (+27%) and isopropanol (+8%) rejection also increased. Chlorination with 20 ppm NaOCl at pH 7 and with 50 ppm NaOCl at pH 10 caused boron rejection to drop with −17% and −33%, respectively, but had negligible influence on isopropanol rejection. However, neutral and alkaline chlorination drastically improved water permeability with +40% and salt rejection with +0.6% (i.e., salt passage decreased with −66.9%), approaching and in some cases even slightly exceeding the salt/water permselectivity limit. It can thus be concluded that, under controlled conditions, chlorination can boost the performance of membrane modules. Significant changes in the membrane physicochemical properties were observed at pH 4. At pH 7 and pH 10, a low chlorine-uptake in the PA network was observed, although no significant PA deterioration was observed with XPS and ATR-FTIR. This study is the first to fundamentally investigate chlorination of PA-based real-scale membrane modules as a function of feed pH. Furthermore, it provides a promising strategy to boost membrane performance at real scale and highlights the importance of chlorination conditions.