Enhancing the anaerobic digestion process through carbon dioxide enrichment: Initial insights into mechanisms of utilisation

Carbon dioxide (CO2) enrichment of anaerobic digesters (ADs) without hydrogen addition has been demonstrated to provide a potential solution to manage CO2 streams generated in the water and organic waste sectors, with concomitant increases in methane (CH4) production. This study investigates the CO2 utilisation mechanisms, by considering chemical and biological pathways in food waste and sewage sludge ADs. Methanosaetaceae was observed to be the dominant methanogen in sewage sludge ADs (Abundance of 83.8 – 98.8%) but scarce in food waste units (3.5 – 5.8%). Methanosarcinaceae was dominant in food waste (14.3 – 32.4%), likely due to a higher tolerance to the free ammonia nitrogen concentration recorded (885 mg·L-1). Ratios of RMethanosaetaceae (ratio of Methanosaetaceae fluorescence signal between test and control) of 1.45 and 1.79 were observed for sludge ADs enriched once and periodically with CO2, respectively (p-value <0.05), suggesting a higher Methanosaetaceae activity associated with CO2 enrichment. Reduction of CO2 by homoacetogenesis followed by acetoclastic methanogenesis was proposed as a CO2 utilisation mechanism, which requires validation by radiolabelling or carbon isotope analysis.