Synergistic enhancement of biomethanation integrating H2 injection with external CO2 in thermophilic anaerobic digestion of sewage sludge.

Integrating power-to-gas technology with anaerobic digestion offers a sustainable route for enhancing biogas production through H2 supplementation. This study aimed to develop an energy-efficient biomethanation strategy by identifying the operational limitations of thermophilic anaerobic digestion of sewage sludge, where maintaining stable performance remains a key challenge. Efficient gas-liquid mass transfer enabled minimal residual H2 (1.4 ± 0.8 %), resulting in a high CH4 content (86 ± 3 %) and stable biogas yield (0.65 ± 0.09 NL g-VS-added-1). Sudden H2 overloading reduced the CO2 levels and increased the pH, thereby triggering process inhibition. Recovery occurred instantly upon pH restoration and CO2 addition, likely due to the metabolic versatility and adaptive capacity of the microbial community. The dominance of bacteria within the orders Thermoanaerobacterales and Clostridiales, alongside hydrogenotrophic methanogens, efficiently directed H2 toward CH4 formation without compromising overall process stability. Importantly, CO2 addition stabilized the pH and enhanced methanation, underscoring its role in both process control and carbon capture. The H2S concentration in the H2-added reactor was minimal, ranging from undetectable to 25 ppm. These findings offer critical insights into H2-assisted thermophilic digestion of sewage sludge and demonstrate that external CO2 addition supports stable biomethanation while enhancing the sustainability of the system.