Regulation of microbial network and granular sludge stabilization: The role of iron-modified pumice in a UASB reactor treating incineration leachate under high organic loading

In anaerobic biotechnology, high organic loading rates (OLRs) often trigger microbial dysbiosis and system instability. This study investigated the methanogenic performance of an upflow anaerobic sludge blanket (UASB) reactor under progressively increasing OLRs and evaluated the role of iron-modified pumice (FMP) in enhancing anaerobic digestion. A stepwise OLR increase was applied to elucidate microbial adaptation mechanisms and system responses to high-load stress.Results showed that increasing OLRs induced marked shifts in microbial community structure and metabolic pathways. Hydrolytic-acidogenic bacteria transitioned toward enhanced macromolecular hydrolysis, with enrichment of Syntrophomonas , while methanogenesis evolved from hydrogenotrophic dominance to a multimodal pathway system centered on Methanofuran . FMP enhanced microbial resilience by promoting biomass retention, facilitating extracellular electron transfer, and stimulating extracellular polymeric substance and enzyme production. Consequently, the FMP-amended UASB achieved 90% COD removal and a methane yield of 14.56 L CH₄/L reactor/d ( 36.7% increase over the control)at an ultra-high OLR of 42.39 kgCOD/(m³·d). This study provides mechanistic insights into microbial regulation for stable high-load wastewater treatment. • Iron-modified pumice enhances syntrophic metabolism and mitigates acidification in UASB reactor. • Iron-modified pumice stimulates key enzyme activities across three major methanogenic pathways. • Over 90% COD removal achieved at 42.39 kgCOD/(m³·d) with high methane yield. • A synergistic core microbiome underpins high-load methanogenic performance.