Ferroferric oxide triggered possible direct interspecies electron transfer between Syntrophomonas and Methanosaeta to enhance waste activated sludge anaerobic digestion

ZVI was reported to enrich H 2 -utilizing methanogens that enhanced interspecies H 2 transfer, while Fe(III) oxide served as a conductive material to promote direct interspecies electron transfer (DIET). However, the interaction of these two modes in anaerobic digestion has not been clarified yet. In this study, when adding Fe 3 O 4 and ZVI simultaneously into an anaerobic digester, the abundance of hydrogenotrophic methanogens decreased drastically compared to ZVI-added digester and Fe-free digester. However, the methane production of ZVI + Fe 3 O 4 added digester were 68.9% higher than Fe-free digester and 20.0% higher than ZVI-added digester, respectively. Sludge reduction rate of these three digesters also showed similar results. These indicated that hydrogenotrophic methanogenesis was not the main reason for methanogenesis in Fe 3 O 4 -added digester. Instead, Syntrophomonas and Methanosaeta were specially enriched in Fe 3 O 4 -added digesters, which implied that the potential DIET between Syntrophomonas and Methanosaeta was likely a crucial reason for accelerating anaerobic digestion of waste sludge.