Making waves: Conductive materials in anaerobic digestion: A sustainable pathway or a hidden carbon burden?

Conductive material-mediated anaerobic digestion (AD) systems offer a promising solution to enhance methane production, yet its sustainability and economic viability require holistic evaluation. In this study, we systematically assess the typical conductive materials, namely, biochar, iron-based material, and biochar-iron composites, through integrated life cycle and cost-benefit analyses of 219 experimental cases. Biochar-iron composites achieved the highest methane yield improvement (36 %), while iron-based materials posed significant carbon burdens (contributing up to 44 % of system emissions). Crucially, material recycling (five cycles) reduced iron's carbon footprint by 72 %, and digestate valorization into biochar further lowered net emissions by 113.8-184.9 %. Economically, iron-based materials outperformed biochar in profitability (220 USD/ton volatile solids), and combining material recovery with digestate valorization boosted net profits by 191.8-264.8 %. The findings demonstrate that prioritizing biochar-iron composites for performance and iron-based materials with recovery for cost-effectiveness, alongside closed-loop design, can reconcile environmental and economic goals. This work provides actionable pathways to optimize conductive material-enhanced AD systems for scalable, sustainable waste-to-energy conversion.