Coupling Anaerobic–Oxic and Integrated Fixed–Activated Sludge Processes for Removal and Transformation of Refractory Organic Matter in Livestock and Poultry Digestate

Effective treatment of livestock and poultry digestate remains challenging due to its high content of refractory organic matter. In this study, a coupled anaerobic–oxic (A/O) and integrated fixed–film activated sludge (IFAS) system was investigated to evaluate its capacity for organic removal and molecular transformation. The IFAS system outperformed the A/O system, achieving an organic removal efficiency of 43%, compared to 17% in the A/O process. Molecular analyses revealed distinct transformation pathways: the A/O process tended to increase the molecular weight and structural complexity of dissolved organic matter, whereas the IFAS process promoted oxidative fragmentation, resulting in the formation of smaller, oxygen–rich compounds. These changes were reflected in key molecular indices (DBE, NOSC, AI _mod ) and the accumulation of carboxyl–rich intermediates. Microbial community analysis associated Bacteroidota with the degradation of oxygenated compounds via hydrolytic processes, while Actinobacteria and Deinococcota were linked to the transformation of lignin– derived aromatics. The integration of advanced molecular characterization with microbial profiling highlights the IFAS system’s enhanced capability for both structural simplification and biodegradation of complex organics, providing a promising strategy for the treatment of high–strength organic wastewater.