• • Both functional additives (NaHCO3 and FS) enabled UASB reactors to achieve rapid startup and granulation under stepwise increased organic loading rates up to 30 g COD·L−1·d−1, with HRT reduced to as low as 0.10 d, demonstrating operational resilience and high-rate treatment capability.
• • At HRT 0.2 d, Acetobacterium relative abundance reached 30.2% (UASB-A) and 36.9% (UASB-B), indicating that NaHCO3 supplementation strongly enriched acetogens, which is critical for establishing syntrophic acetate oxidation coupled with Methanothrix.
• • FS addition significantly increased Sporomusa abundance, establishing a new syntrophic partnership with Methanothrix, thereby diversifying metabolic pathways and enhancing sludge granulation—a key factor for reactor stability and biomass retention.
• • The combined use of NaHCO3 and FS resulted in higher TSS and VSS concentrations and larger granule sizes compared to NaHCO3 alone, underscoring the synergistic effect of inorganic carbon and amino acid supplements on biomass accumulation and granulation.