Enhanced methane production from propionate in anaerobic digestion using high-porosity PVAG gel beads embedded with conductive materials

Preventing propionate accumulation is essential for stabilizing methane fermentation. In this study, conductive materials (magnetite, activated carbon, and green tuff) were added to polyvinyl alcohol (PVA) gel beads and increased-porosity gelatin-modified PVA (PVAG) gel beads characterized by larger pore sizes serving as carriers. Batch methane fermentation experiments under mesophilic conditions (37 °C) using propionate as a substrate (1670 mg-COD L⁻¹) showed that adding conductive materials, particularly magnetite to PVAG beads shortened lag time by 71%, improved methane production rate by 1.9 times, and enhanced microbial attachment by up to 5.7 times compared to controls. Microbial analysis identified Pelotomaculum and Candidatus Cloacamonas as key bacterial genera, along with three methanogenic genera: Methanothrix, Methanoculleus, and Methanolinea. While adding conductive materials to PVA gel beads did not increase the relative abundance of syntrophic propionate-degrading microorganisms, adding them to PVAG beads achieved a 2.7-fold increase. Predictive analysis of microbial function suggested that conductive materials increased the abundance of genes encoding enzymes involved in methanogenesis, in particular those responsible for direct interspecies electron transfer (DIET). These findings indicate that adding conductive materials to those porous PVAG carriers enhances microbial retention and potentially enriches syntrophic consortia via DIET, accelerating propionate degradation and stabilizing methane fermentation.