Inorganic Scaling Mechanisms During Forward Osmosis Concentration of Fresh vs. Hydrolysed Urine: Theoretical Modelling and Experimental Validation

Forward osmosis (FO) offers a promising route for urine concentration and nutrient recovery, yet inorganic scaling under high water recovery remains a significant challenge. This study systematically investigated scaling during FO treatment of synthetic fresh urine (SFU) and synthetic hydrolysed urine (SHU) over three consecutive cycles to 80% water recovery. SFU exhibited moderate flux decline (~14.4 → 4–5 LMH), with minimal hydraulic resistance from sparse calcium-deficient Ca–P deposits (Ca:P ≈ 1.2; ACP/OCP-like). In contrast, SHU caused severe cumulative scaling, progressively reducing flux from 19 → 14.46 → 1.3 LMH, dominated by struvite (Mg:P ≈ 1.02) and mixed Mg–carbonate phases. Visual MINTEQ thermodynamic modelling correctly identified the dominant mineral families in both feeds, while kinetic effects governed the formation of metastable phases, demonstrating that equilibrium modelling and experimental characterisation are complementary tools for scaling prediction under transient FO conditions. Physical cleaning restored ~98–99% of water flux for both feeds, confirming that even severe SHU-induced scaling is largely hydraulically reversible. High rejection of multivalent ions (PO43−, Mg2+, and Ca2+) was maintained throughout, confirming membrane integrity was preserved despite severe scaling. These findings demonstrate that urine hydrolysis fundamentally governs scaling pathways, severity, and reversibility in FO systems, and that simple hydraulic flushing is an effective fouling-control strategy, providing practical guidance for operating condition selection and cleaning strategy design in FO-based urine treatment applications.