Magnetite Enhances upflow anaerobic sludge blanket resilience to progressive organic overloading by Preserving granule matrix integrity and redox function

Progressive organic overloading can destabilize anaerobic granules and constrain syntrophic conversion in upflow anaerobic sludge blanket (UASB) reactors, yet whether and how magnetite retention within granules contributes to reactor resilience remains unclear. This study examined the structural, redox, and microbial responses of control and magnetite-amended UASB reactors subjected to stepwise increases in organic loading rate (OLR). The control reactor showed pronounced deterioration at 4.0-5.0 g COD/L/d, and its operation was terminated at 6.0 g COD/L/d. In contrast, the magnetite-amended reactor maintained methane yields near 300 mL CH4/g CODadded through 6.0 g COD/L/d and remained operational at 8.0 g COD/L/d. At 6.0 g COD/L/d, total residual organic acids reached 2,675 ± 338 mg COD/L in the control reactor but only 709 ± 35 mg COD/L in the magnetite-amended reactor. Propionate in the magnetite-amended reactor remained below 89 mg COD/L during the higher loading stages. The persistent distribution of Fe-bearing material within the granule matrix, consistent with magnetite retention, coincided with sustained granule size, more coherent EPS-associated spectral responses, a smaller increase in humic- and fulvic-like fluorescence, stronger electrochemical responsiveness, and higher electron transport system activity. Metagenomic profiles further showed that methanogenic taxa and genes related to EPS biosynthesis, redox metabolism, and methanogenesis persisted to higher OLRs in the magnetite-amended reactor. Acetate accumulation at the highest OLR indicated that magnetite delayed rather than prevented the eventual limitation in acetate conversion. These results link granule-associated magnetite retention with maintenance of structural stability, redox activity, and microbial functional potential during progressive organic overload.