Biogas circulation for improving the promotive effect of zero-valent iron on anaerobic digestion of swine manure

Abstract

Zero-valent iron (ZVI) supplementation for enhancing anaerobic digestion (AD) of swine manure (SM) has been widely studied. However, the low utilization efficiency of ZVI in AD process greatly limits the development of this technology. In this study, a promising approach, biogas circulation was proposed to promote ZVI dissolution during AD. Compared with the control with ZVI only, the CH4 yield was increased by 53.84 % in the digester with ZVI addition and biogas circulation. Biogas circulation apparently increased the system alkalinity and lowered the oxidation-reduction potential through promoting ZVI dissolution and enhancing gas-liquid-solid contact. Particularly, ethanol-type fermentation was favored, as evidenced by the 19.96–183.45 % higher ethanol concentrations in R-Circulation, which might provide more H2 and ethanol to consolidate the hydrogenotrophic pathways and direct interspecies electron transfer (DIET), respectively. Hydrogenotrophic Methanobacterium and Methanothrix that capable of mediating DIET were enriched by 38.81 % and 26.41 % with biogas circulation, respectively.