Field-realistic accelerated aging protocol for spiral-wound reverse osmosis membranes: multi-scale diagnostics and end-of-life criteria

Reverse osmosis (RO) membranes dominate desalination and water reuse, yet their aging under realistic, chlorine-free conditions remains poorly investigated. Existing aging studies largely rely on flat-sheets, single foulants, and chemical oxidation, overlooking the complex dynamics dominating spiral-wound (SPW) elements. We introduce a chlorine-free SPW aging protocol that reproduces mixed fouling using humic-protein-silica feeds at low shear and cyclic fouling/harsh acid-base cleaning. Two commercial SPW elements (24-mil vs 14-mil feed spacers) were aged to a functional EoL defined by a post-clean FRR ≤70% and/or ≥ 5 percentage-point rejection decline. The protocol produced stepwise flux loss with partial recovery. EoL occurred after 5 cycles for SPW1 (24-mil) versus 4 cycles for SPW2 (14-mil) with modest rejection drift (∼4–5 pp) but large irreversibility (R_ir ∼ 31% vs ∼36%, respectively). Multi-modal characterizations (FTIR, XPS, SEM, AFM, contact angle) reveal compact, NOM–silica–mineral crusts that persist after acid/base cleaning, shield the polyamide (PA) layer, and delay PA oxidation until high oxidant challenge (200 k → 300 k ppm·h NaOCl). The thinner-channel element exhibited more severe localized fouling and deformation features and reached EoL earlier. A field EoL benchmark exhibits convergent chemical and morphological evolution, supporting protocol realism. Finally, a plant-configurable SEC–FRR map links declining FRR (cleanability loss) to rising energy/cleaning penalties and supports site-specific EoL thresholds. The protocol offers a reproducible SPW RO framework for assessing RO durability, optimizing cleaning, benchmarking aging on a days scale, and guiding circular reuse of aged membranes.