From flocs to interfaces: mechanism analysis of Fe(II)/Peracetic acid as ultrafiltration pretreatment for achieving low fouling and high virus removal

Ultrafiltration (UF) faces the dual challenges of membrane fouling and low small-sized virus removal. This study found that Fe(II)/peracetic acid (F/P) pretreatment significantly reduced irreversible resistance while achieving high virus removal (∼5.71 log, with the pretreatment inactivation approximately 3.67 log), thereby breaking the traditional trade-off between high virus removal and high irreversible fouling. From the perspective of floc regulation to interface optimization, and comparing with the conventional Fe(II)/H₂O₂ (F/H) system, we elucidated the synergistic mechanism of F/P pretreatment through integrated water quality analysis, floc characterization, interfacial thermodynamics, fouling modeling, and Spearman correlation analysis: (i) efficient degradation of hydrophobic organic matter, reducing irreversible fouling; (ii) enhanced membrane hydrophilicity, weakening hydrophobic adhesion and delaying dense fouling layer formation; (iii) formation of a loose, particle-stacked cake layer by iron flocs, trapping viruses within its deep pores, with irreversible retention being the primary contributor to virus removal during UF. Spearman correlation analysis further confirmed that virus removal rate was significantly positively correlated with DOC removal (r = 0.838), polysaccharide removal (r = 0.672), and standard blocking R² (r = 0.790), while negatively correlated with irreversible resistance (r = −0.779), reflecting the high oxidation-coagulation intensity of F/P pretreatment simultaneously promoted water purification, fouling mitigation, and virus removal. The extremely low concentration of free viruses in backwash water reduced the biosafety risks of backwash water reuse. This study provides a theoretical basis for developing low-energy, high-safety UF pretreatment technologies.