Effect of Free Chlorine Exposure on Fouled Engineering-Scale Reverse Osmosis Membranes and Their Surface Characterization

Reverse osmosis (RO) membrane technology has proven to be an essential approach to ensuring high quality water for potable reuse. The disposal of end-of-life (EoL) membranes continues to be a critical challenge. RO membranes are periodically disposed of, typically in landfills, creating a global environmental and economic concern. Chlorination has emerged as a potential solution for extending the lifetime of RO membranes. Engineering-scale results showed that chlorination does not compromise selectivity while restoring permeability. To further explore how chlorination changes EoL RO membrane characteristics and mass transfer properties, two EoL RO spiral-wound membranes were removed from the first (PV1) and last (PV6) pressure vessels of an engineering-scale system, to represent organic fouling (PV1) and inorganic scaling (PV6) propensity. Then, the membranes were unrolled, cut into coupons, oxidized with chlorine at different doses, and tested in a crossflow configuration bench-scale system with reclaimed water effluent. The fouled coupons were oxidized by progressive exposure to free-chlorine doses (up to 48,000 ppm-h, namely: 1k, 2k, 4k, 6k, 12k, 24k, and 48k ppm-h). The water samples were analyzed for organic matter content in relation to chlorine dose using UV254 absorbance, EEM fluorescence, and total organic carbon (TOC). Water was tested for inorganic material by measuring conductivity, total dissolved solids, and ion concentration. The membrane surface modification was assessed by measuring Zeta potential and contact angle. Organic results show a significant increase of UV254 absorbance and TOC with chlorine dose increase, suggesting that the membrane surface was progressively modified. At the 4k ppm-h dose, both permeate samples had similar organic and inorganic content to the 0k dose, indicating minimal degradation of the polyamide layer. EEM fluorescence also significantly increased with chlorine dosage, consistent with TOC and UV254 absorbance results. Inorganic results showed an increase of ion presence with chlorine dose, more pronounced after 6k. TOC results were correlated to sulfates and indicate a linear relationship between inorganic and organic matter. Finally, membrane analysis revealed an overall decrease in zeta potential and water contact angle with the 0k (no chlorine exposure) PV1 and PV6 membranes measuring charges of -4.88 and -24.59 mV, respectively, while the 4k membranes reached -30.42 and -33.57 mV. Contact angles for PV1 and PV6 were reduced by 23.8% and 29.8% from 0k to 6k. The outcomes of this study provide insights into mass transfer mechanisms of chlorinated membranes and the chlorine dosage at which a compromise between permeability and selectivity is achieved.