Comparing the effects of magnetite-mediated direct interspecies electron transfer with biogas mixing-driven interspecies hydrogen transfer on anaerobic digestion.

Although the promotive effect of direct interspecies electron transfer (DIET) on methane production has been well-documented, the practical applicability of DIET in different scenarios have not yet been systematically studied. This study compared the effects of magnetite-mediated 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. Under stress-free environment, the enhancement effect of magnetite-induced DIET was not even as pronounced as biogas circulation-a simple and common mixing strategy in commercial AD plants, and the promotion effect of magnetite was insignificant in the well-mixed digesters. In short, the magnetite-mediated DIET is not always effective in improving AD of SM.

Biological nitrogen removal mechanisms during anaerobic digestion of swine manure: Effects of biogas circulation and activated carbon addition.

This study investigated the biological nitrogen removal mechanisms during the anaerobic digestion of swine manure and the effects of biogas circulation and activated carbon (AC) addition. Biogas circulation, AC addition, and their combination increased the methane yield by 25.9%, 22.3%, and 44.1%, respectively, when compared to the control. Nitrogen species analysis and metagenomic results indicated that nitrification-denitrification was the dominant ammonia removal pathway in all digesters with little oxygen, while anammox did not occur. Biogas circulation could promote mass transfer and induce air infiltration to enrich nitrification- and denitrification-related bacteria and functional genes. And AC might act as an electron shuttle to facilitate ammonia removal. The combined strategies showed a synergetic effect on the enrichment of nitrification and denitrification bacteria and functional genes, significantly lowering the total ammonia nitrogen by 23.6%. A single digester with biogas circulation and AC addition could enhance methanogenesis and ammonia removal via nitrification and denitrification.