Potential microbial mechanisms driving granular activated carbon-enhanced syntrophic propionate degradation under elevated organic loading rates

Anaerobic digestion is a sustainable technology for organic wastes treatment and energy recovery. However, propionate accumulation under environmental stress disrupts microbial balance, resulting in reduced methane yield and system instability. In this study, the effect of granular activated carbon (GAC) on propionate degradation under high organic loading rates (OLRs) was investigated. In the long-term operation, the GAC-amended reactor enhanced methane yield and propionate removal efficiency by 7.4% and 11.6%, respectively, at an OLR of 7 g chemical oxygen demand/L/d. Short-term GAC addition showed a stronger effect in sludge from the GAC-amended reactor, with 16.2–34.1% increases in propionate degradation rates and 27.0–33.9% enhancements in maximum methane production rates compared with the sludge from the reactor without GAC addition under N 2 or high hydrogen partial pressure. Additionally, GAC maintained a high microbial diversity under high-OLR conditions and enriched Smithella and Mesotoga . GAC could facilitate propionate conversion via key metabolic pathways, including propionate oxidation and syntrophic acetate oxidation. Furthermore, GAC may alleviate oxidative stress and enhance bacterial membrane resilience, increasing microbial resistance to high OLR stress. Genes associated with extracellular electron transfer were increased by 14.3–65.8% in the presence of GAC. Collectively, GAC enhanced methanogenic propionate degradation and overall system stability. This study highlights GAC-driven microbial adaptations under environmental stress and provides guidance for applying conductive materials to optimize high-loading anaerobic reactors.