Carbon-based conductive material-assisted anaerobic digestion: meta-analysis, mechanisms and perspectives

The incorporation of conductive materials (CMs) has emerged as a sustainable and efficient means of optimizing the anaerobic digestion process. This review presents a meta and ecological analyses elucidating the relationship between the physicochemical properties of carbon-based CMs, microbial community dynamics, and thus process performance parameters. Meta-analysis indicates that medium-sized (1-2 mm) carbon-based CMs at 1 g/L are the most practical and effective for optimizing anaerobic digestion, likely owing to enhanced surface attachment that supports microbial interactions without impairing mass transfer or syntrophic exchanges. It was surmised that methanogenic influence of redox activity outweighs that of electrical conductivity (EC). Furthermore, buffer capacity, largely influenced by ash content and surface functional groups, stabilizes pH and mitigates VFA accumulation, enhancing process stability. However, variable sustained long-term effects subject to operational conditions have been reported. The biological effects primarily involve promoting microbial attachment, enrichment of syntrophic bacteria and methanogens, particularly Geobacter and Methanosarcina through surface active sites and gene upregulation linked to electron transfer and enzyme activity. Profiling of the microbial community revealed that Clostridium was the most recurrent syntroph often preferentially proliferated within CM-assisted systems. Furthermore, CM affinity is a key factor shaping ecological niche differentiation; biochar and activated carbon preferentially proliferating Methanothrix and Methanosarcina, respectively. Overall, while activated carbon offers higher conductivity and adsorption capacity, biochar's cost-effectiveness and modifiability make it the preferred choice for large-scale applications. These valuable insights should inform the strategic applications of carbon-based CMs in real-world settings.