Abstract
Highlights • FHU prefiltration cut FO flux decline to 37% vs 60% UHU • Proteinaceous biofouling dominated, bacteria vs yeast by urine pH • Sequential cleaning restored 91-98% flux despite residual organics • RSF from 5 M NaCl increased Na⁺ 4.8x VCF in concentrates • Particulates + chemistry control FO fouling reversibility Abstract Forward osmosis (FO) enables efficient human urine concentration for nutrient recovery, but fouling hinders implementation. This study investigated multi-cycle FO fouling with real urine: filtered hydrolysed (FHU), unfiltered hydrolysed (UHU), and citric-acid-stabilised urine (SU, pH ≈ 5). FHU showed lower flux decline (37%) due to particulate removal, while UHU suffered severe fouling (60% loss, 43% recovery) and SU exhibited intermediate behaviour. Membranes rejected ≥90–95% of PO₄³⁻, Ca²⁺, Mg²⁺, and COD, with moderate K⁺/NH₄⁺ passage. Membrane autopsy revealed biofouling dominance: rod/cocci bacteria in alkaline FHU/UHU, yeast-like structures in SU, with minimal inorganic scaling. Sequential physical–chemical cleaning restored 91–98% of flux, although residual foulants indicated partially irreversible fouling. Reverse salt flux from 5 M NaCl increased concentrate salinity. Overall, this research provides valuable insights into the fouling mechanisms within FO caused 2