Reevaluating the Role of Magnetite-Mediated Direct Interspecies Electron Transfer in Anaerobic Digestion of Swine Manure

Abstract

This study compared the effects of magnetite-mediated direct interspecies electron transfer (DIET) with conventional biogas mixing-driven interspecies hydrogen transfer (IHT) on anaerobic digestion (AD) of swine manure (SM). Compared with control, magnetite supplementation, biogas circulation, and their integration enhanced the CH4 yield by 19.3%, 25.9%, and 26.2%, respectively. Magnetite mainly enriched DIET-related syntrophic bacteria (Anaerolineae and Synergistia) and methanogens (Methanosarcina) to accelerate acidification and establish DIET, while biogas circulation mainly enriched hydrolytic bacteria (Clostridia) and hydrogenotrophic methanogens (Methanolinea and Methanobacterium) to promote hydrolysis and accelerate IHT. Coupling magnetite addition with biogas circulation led to the enrichment of the above six microorganisms to different extents. The effectiveness of the strategies for lowering the H2 pressure followed: magnetite + biogas circulation ≈ biogas circulation > magnetite. In short, the magnetite-mediated DIET is not always effective in improving AD of SM, and its promotion effect might be insignificant in well-mixed commercial digesters.

Keywords: anaerobic digestion, Biogas circulation, Direct interspecies electron transfer, Interspecies hydrogen transfer, Magnetite