Influence of ethanol supplementation on sulfate reduction and methanogenesis in UASB reactors

This study evaluated the performance of an anaerobic reactor at low carbon-to-sulfate ratios with various sulfate concentrations and carbon sources. The results indicated that ethanol addition enhanced both sulfate removal and methane production, achieving 80 % sulfate removal and over 40 % methane content in the biogas. When mixed ethanol with starch hydrolysate in a weight ratio of 1, the methane content increased to 55 %. However, using 100 % ethanol reduced COD removal to <20 % and inhibited biogas production. Further analysis showed that ethanol promoted sulfate removal as sulfide in the liquid phase and altered electron flow between sulfate reducing bacteria and methanogens, with methanogens predominantly capturing the electrons. The addition of ethanol also reduced extracellular polymeric substance levels, suggesting decreased microbial stress. The addition of ethanol selectively enriched Desulfovibrio-based incompletely oxidizing sulfate-reducing bacteria. These microbes efficiently metabolized ethanol into acetic acid, which served as both an electron donor for sulfate reducers and a substrate for methanogens. As a result, over 50 % of the electron flow was directed toward methane generation at a carbon-to‑sulfur (C/S) ratio of 2:1, significantly increasing the methane content in the biogas to 55 %.