Physical morphology of conductive carriers shapes anaerobic biofilm development and methanogenesis

The role of conductive materials (CMs) in enhancing anaerobic digestion (AD) is widely recognized. However, there is still limited understanding of how CMs' physical morphology, particularly pore characteristics, influences biofilm development and methanogenic activity. In this study, five CMs with distinct pore structures were used as biofilm carriers: macroporous bamboo biochar (BB) and straw biochar (SB), microporous peach shell biochar (PSB) and coconut shell biochar (CSB), and non-porous granular graphite (GG). To isolate the effects of porosity and morphology, the materials were also ground into powders and fabricated into planar carriers for comparative experiments. The physicochemical properties of both granular and planar carriers were comprehensively characterized, and their biofilm performance was evaluated. Results showed that granular BB and SB exhibited similarly high methanogenic activity, followed by PSB and CSB, which also performed comparably, while GG showed the lowest activity. In contrast, the four biochar-based planar carriers, which had comparable morphology, exhibited similar methane production, whereas the rough-surfaced graphite planar carrier performed best. Further analysis revealed that carrier morphology significantly influences biofilm biomass, extracellular polymeric substance (EPS) composition, bacterial community diversity, and methanogen abundance, thereby regulating the methanogenic function of the biofilm. Correlation analysis identified macropore-related features in granular carriers and surface roughness in planar carriers as the variables most strongly associated with biofilm development, suggesting a potential enhancing role of macroporous structure and surface roughness in biofilm performance. This study highlights the importance of physical-property analysis in related research and provides guidance for optimizing conductive carriers.