Fouling dynamics of forward osmosis membrane during multi-cycle concentration of hydrolysed and stabilized real human urine

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 by real human urine, demonstrating how particulate load and stabilisation chemistry influence biofouling progression, and fouling reversibility.