Differential Effects of GAC and Graphite on Interspecies Electron Transfer during Anaerobic Digestion of Waste Activated Sludge for Methane Production

Abstract

This study investigated the effect of the particle size of granular activated carbon (GAC) and graphite on methane production efficiency. Additionally, differences in the mechanisms by which GAC and graphite-enhanced anaerobic digestion (AD) of waste activated sludge (WAS) were evaluated. The results showed that 150 μm GAC and 38 μm graphite had the best promoting effect, increasing cumulative biogas production by 14.6% and 12.3%, respectively, compared with blank control. GAC advanced the peak daily biogas production to day 8. Scanning electron microscopy revealed the formation of a dense bacterial biofilm on the GAC surface. Microorganisms on the graphite surface had more conductive pili than those on the GAC and blank surfaces. GAC enriched Methanobacterium (hydrogenotrophic methanotrophic archaea) while maintaining the relative abundance of existing acetate methanogenic bacteria. Conversely, graphite reduced the abundance of hydrogenotrophic methanogenic bacteria and enriched Methanosaeta, which participate in direct interspecies electron transfer. GAC increased the abundance of key enzymes in the hydrogenotrophic methanogenesis pathway and alleviated the inhibition of nitrate nitrogen and H2 in the methanogenesis reaction system. In contrast, graphite increased the abundance of pili genes and promoted electron exchange between symbiotic bacteria and methanogens, thereby accelerating substrate consumption and methane production.