Free chlorine exposure dose (ppm·h) and its impact on RO membranes ageing and recycling potential

Spiral wound thin film composite (TFC) polyamide (PA) membranes dominate the desalination market. Despite the proven performance of reverse osmosis (RO) membranes, they are periodically replaced and usually disposed of in landfills. The present study investigates the effect of using a diverse combination of free chlorine solutions (from 1 to 12,240 ppm) and exposure times (from 0.5 to 6500 h) with two purposes, namely: i) to simulate accelerated membrane ageing (low free chlorine concentration solutions) and ii) to optimize the existing recycling process of end-of-life RO membranes. Membrane coupons were taken from 8″ diameter modules (pristine and old). Membrane permeability and rejection coefficients were obtained by filtering synthetic brackish water (BW). Membrane surfaces were characterized by SEM and ATR-FTIR techniques. This work shows distinct PA vulnerability depending on membrane design (brackish water (BWRO) and seawater (SWRO) reverse osmosis membranes). Results reveal that the use of the exposure level parameter (ppm·h) as an independent basis for comparing free chlorine exposure must be carefully employed during ageing assessments and monitoring transition between nanofiltration (NF)-like performance to ultrafiltration (UF)-like performance. However, it can be used consistently to convert end-of-life RO membrane into NF-like recycled membranes using concentrations higher than 10 ppm.