Distinct hydrolyzed Al-Ti species govern the coagulation behavior and subsequent anaerobic fermentation of CEPS...

Hydrolyzed species critically govern the coagulation behavior of Ti-based coagulants in chemically enhanced primary sedimentation (CEPS), thereby influencing the subsequent anaerobic fermentation of CEPS sludge. However, whether and how preparation-induced Al-Ti speciation modulates solid-liquid interfacial interactions and substrate accessibility during acidogenic fermentation remains unclear, limiting the simultaneous optimization of pollutant removal and resource recovery. Here, polyaluminum-titanium chloride (PATC) composite coagulants with distinct hydrolyzed species distributions were synthesized by slow alkaline titration (S-PATC) and electrodialysis (E-PATC). Compared with S-PATC, E-PATC achieved 99.52%, 68.60%, and 71.38% higher removal efficiencies for PO₄^3--P, UV₂₅₄, and COD, respectively. The superior CEPS performance was attributed to the synergistic effects of highly charged small species and highly polymerized macromolecular species. Crucially, the hydrolyzed Al-Ti species introduced during CEPS acted as a front-end regulator of sludge fermentability by reconstructing solid-liquid interfacial interactions and modulating substrate accessibility. Compared with S-PATC-loaded sludge, E-PATC-loaded sludge exhibited weaker hydrophilic repulsion and higher volatile fatty acids (VFAs) production, which was associated with enhanced interfacial mass transfer. Under alkaline conditions, enhanced hydrophilic repulsion coexisted with electrostatic repulsion in the E-PATC system, with the latter predominating and promoting greater sludge disintegration. Consequently, the maximum concentration of soluble organic matter increased from 764 mg/L to 2222 mg/L on Day 4, providing abundant substrates for acidogenic microorganisms. As a result, the VFAs yield reached 1250.83 mg-COD/L, which was 6.67% higher than that obtained with S-PATC-loaded sludge. This work provides evidence that hydrolyzed Al-Ti species play a pivotal role in coagulation behavior and subsequent sludge fermentation.