Robust biogas upgrading process via homoacetogens against ammonia and sulfide toxicities

Using hydrogen derived from surplus green energy (e.g., solar and wind) to convert carbon dioxide to acetate via homoacetogens represents a promising technology for simultaneous biogas upgrading and biochemical production. However, effects of hydrogen sulfide and ammonia on activities of homoacetogens remain unknown, hindering their applications in biogas upgrading. This study investigated the impacts of ammonia and sulfide on homoacetogen-dominated microbial community for biogas upgrading process by combining short-term batch tests and long-term membrane biofilm reactor (MBfR) operation. Results showed that sulfide concentrations ≤ 2 mM TDS (total dissolved sulfide) increased H₂ and CO₂ uptake rates and acetate production both in the short-term and long-term tests. The relative abundance of Acetobacterium (typical homoacetogens) in the MBfR also increased from 30 % without TDS addition to 40 % with the addition of 2 mM TDS. These results suggest that sulfide addition (≤ 2 mM TDS) likely promoted the growth of homoacetogens, thereby enhancing the biogas upgrading efficiency. In terms of ammonia, results suggested that 0.5 g NH₄⁺-N/L has negligible impacts on the homoacetogens' activities, while concentrations ≥ 1 g NH₄⁺-N/L significantly inhibited homoacetogens' activities, resulting in negligible acetate production during the short-term tests. However, the long-term biogas upgrading performance remained unaffected by 1 g NH₄⁺-N/L. Moreover, with the simultaneous additions of typical concentrations of sulfide (2 mM TDS, equivalent to the H₂S concentration of 0.8 % in biogas) and ammonia (1 g NH₄⁺-N/L, equivalent to the NH₃ concentration of 0.1 % in biogas) in raw biogas, our MBfR still achieved high H₂ and CO₂ utilization efficiencies (95 % and 97 %, respectively) and acetate production rate (550 mg/L/d). These highlight the robustness of MBfR against ammonia and sulfide toxicities. Additionally, the injection of extra H₂ could alleviate the ammonia and sulfide inhibitions on homoacetogens with acetate production increased by 13-80 times. This provides a new strategy to enhance the tolerance of homoacetogens against high concentrations of hydrogen sulfide and ammonia. Collectively, our findings advance the understanding of the response of homoacetogens to ammonia and sulfide stress and facilitate the development of a resilient and efficient homoacetogen-mediated bioprocess for upgrading biogas to biomethane and chemicals simultaneously.