Effects of Conductive Materials on Anaerobic Digestion of Polysaccharides and Proteins

Anaerobic digestion is a promising technology for the resource recovery from sewage and organic solid waste. Recent studies have shown that conductive materials can enhance anaerobic digestion, but their specific effects on different types of substrates remain unclear. In this study, starch and soy protein isolate were selected as representative substrates for polysaccharides and proteins, and biochar and iron powder were used as conductive materials in biochemical methane potential (BMP) tests, either individually or in combination. The results revealed a distinct double-plateau pattern in methane production from polysaccharides, whereas methane production from proteins was relatively smoother. The kinetics were well described (R² > 0.99) by a combination of two modified Gompertz models, which were then used to analyze methane potential and production rate. The findings indicated that under the mediation of conductive materials, biochar significantly increased the methane potential, whereas the addition of iron powder enhanced the methane production rate. Specifically, the addition of 10 g/L biochar increased the methane potential of polysaccharides from 275.8 mL/g to 292.6 mL/g, and in the protein group, the methane potential increased from 286.7 mL/g to 302.4 mL/g. The addition of iron powder slightly reduced the maximum methane potential of polysaccharides (260.3 mL/g) but had no significant effect on the methane potential of proteins (284.7 mL/g). During the early stages of polysaccharide degradation, pH decreased and butyrate accumulated, while the conductive materials accelerated pH recovery and butyrate degradation. During the protein hydrolysis process, the release of ammonia nitrogen maintained a stable pH, while the addition of iron powder slightly increased the pH. No significant accumulation of volatile fatty acid (VFA) was observed in the protein experiments. Microbial community analysis revealed considerable differences in the inoculated sludge cultured with polysaccharides and proteins. In the protein experimental group, a higher abundance of the hydrogenotrophic methanogen Methanobacterium was observed, suggesting that hydrogen, serving as an electron donor for CO2 reduction, may be the primary pathway for methane production during protein degradation. Iron powder promoted the enrichment of Methanobacterium regardless of the substrate. Additionally, in the polysaccharide experimental group, Clostridium_sensu_stricto_1, a butyrate-producing bacterium, showed significant enrichment, and the addition of biochar, either alone or in combination with iron powder, led to in a notable increase in its abundance. This suggests that biochar facilitated the acidification process during polysaccharide degradation, correlating with the accumulation of VFA. This study elucidates the anaerobic degradation processes of two representative substrates, polysaccharides and proteins, and explores the influence and mechanisms of conductive materials in these processes. The findings provide new insights into enhancing the efficiency of anaerobic conversion.