Methanogenic degradation of volatile fatty acids can be enhanced by using highly conductive materials or by forming electroactive biofilms on non-conductive materials.

To gain more insights into the impact of various additives on the stimulation of direct interspecies electron transfer (DIET), this study examined the anaerobic digestion (AD) performance of volatile fatty acids (VFAs) using different conductive (carbon felt (CF), stainless steel mesh (SM)) and non-conductive (polyester felt (PF), fiberglass mesh (FM)) carrier materials. The CH₄ yield in the PF and SM groups did not show a significant difference and was highest at 243.5 ± 6.0 and 224.7 ± 2.1 mL CH₄/g COD, respectively, representing an increase of 47 % and 35 %, respectively, compared to the control group (p < 0.001). The enhanced methanogenic activity of the SM group could be attributed to the highest conductivity of 11.1 ± 1.1 S/cm in SM and the associated DIET process. In contrast, the physicochemical properties of non-conductive PF, combined with the presence of specific functional groups, led to increased biofilm formation and a two-fold increase in extracellular polymeric substances. A significant improvement in the PF biofilm's conductivity and capacitance was observed, reaching 230 % and 20 % respectively, compared to the control group. Additionally, the presence of c-type cytochromes, a high abundance of pili-like structures, a diverse population of hydrogenotrophic methanogens, and the enrichment of syntrophic and potentially electroactive groups suggests the formation of an electroactive community on the PF. These findings imply that non-conductive materials with appropriate physicochemical properties may also foster the development of specific electroactive biofilms and enhance AD through the DIET pathway, similar to highly conductive materials.