Unravelling the inconsistency in organic matter removal and membrane fouling control in leather mixed wastewater treated by different O₃-AOPs

The secondary effluent from leather wastewater contains complex and recalcitrant organic matter, and conventional treatment often show limited removal effect, making it difficult to meet the standards required for water reuse. Therefore, advanced treatment is necessary to ensure effluent quality and promote water resource reuse. Ozone-based advanced oxidation processes (O₃-AOPs), owing to their strong oxidizing capacity, can effectively degrade refractory organics and mitigate water quality risks, laying a solid foundation for stable and reliable wastewater reclamation. Although O₃-AOPs have been widely applied, the molecular transformation of DOM under different processes and its relationship with membrane fouling remain insufficiently understood. In this study, spectroscopic and molecular-level characterization combined with surface chemical analysis was employed to systematically elucidate the transformation of DOM under various O₃-AOPs and their impacts on membrane fouling. The results showed unsaturated and aromatic compounds were preferentially oxidized across all systems, yet the composition of effluent DOM varied. Although the O₃/UV/H₂O₂ process achieved the highest mineralization efficiency (from 14 mg/L to 6.8 mg/L), its advantage in alleviating membrane fouling was not evident. In contrast, the O₃/UV efficiently degrade aromatic and peptide-like compounds with the mildest fouling observed, while the O₃/H₂O₂ effluents enriched with protein-like and hydrophobic compounds and exhibited the most severe membrane fouling. In summary, a lower DOC concentration is not necessarily the primary indicator of process efficiency, since the properties of residual organics can critically influence subsequent treatment, overall system performance, and energy consumption. This work clarifies the transformation and fouling control of O₃-AOPs and offers a theoretical basis for sustainable wastewater reclamation.