Abstract
Forward osmosis (FO) is widely explored for nutrient recovery from human urine, yet its operation is constrained by limited understanding of how fouling interacts with solute transport under realistic conditions. In particular, the role of critical flux (CF) in governing both fouling behaviour and nitrogen-species transport in hydrolysed urine remains unclear. Here, we establish a direct mechanistic link between CF operation, fouling evolution, and nitrogen transport during FO treatment of real hydrolysed urine. A CF of ∼9 LMH was identified using a draw-solution stepping method, followed by 120 h experiments below, at, and above this threshold. Below CF, flux remained stable with minimal fouling; above CF, a dense bio-organic layer formed, causing ∼70% flux decline. Nitrogen transport was governed by pH-dependent speciation, with NH3 as the dominant permeating species and NH4+ largely retained. Despite draw solution alkalinisation, nitrogen accumulation was limited by thermodynamic constraints. Operation above CF coincided with a nitrogen-rich fouling layer and significant unaccounted nitrogen losses (31–44%), suggesting fouling-associated interfacial nitrogen retention as a contributing mechanism. These results show that CF governs both hydraulic performance and nutrient transport, highlighting CF-based operation as essential for maximising stability and nitrogen recovery in FO systems.
•First determination of critical flux (∼9 LMH) for real hydrolysed urine in FO•Fouling transitions from sparse deposits below CF to dense bio-organic cake above CF•NH3 governs membrane transport; NH4+ largely retained across all flux conditions•DS TAN accumulation limited by thermodynamic equilibrium and DS alkalinisation•Above CF, nitrogen-rich fouling layer drives unaccounted TAN loss to 31–44%