Advancing Circular Bioeconomy through a Systems-Level Assessment of Food Waste and Industrial Sludge Codigestion

Disposal of food waste (FW) in landfills remains an unsustainable practice for organic waste management. Simultaneously, pulp and paper mills produce significant amounts of recalcitrant organic waste that is difficult to decompose due to its high lignocellulosic content. In this study, we developed an innovative approach to improve the digestion of pulp and paper mill sludge (PPMS) by amending FW to produce a low chemical oxygen demand (COD) sludge while recovering methane in the process. This codigestion process was evaluated through lab-scale biogas production experiments coupled with a comprehensive economic and environmental sustainability assessment. Biomethane production results revealed that the FW-PPMS codigestion methane yield was 36% higher on average than the PPMS monodigestion. Additionally, metagenomic analysis revealed that microbial communities for both systems transitioned from highly heterogeneous to more adapted uniform communities after digestion. Improved microbial communities contributed to higher COD removal (92%) in the FW-PPMS system compared to monodigestion (80% removal). The sustainability analysis revealed that the codigestion of FW-PPMS had median costs of 236.64 USD·tonne^-1·day^-1 and emissions of 228.30 kg CO₂ eq·tonne^-1·day^-1, a significant reduction compared to directly disposing the FW in landfills (median costs of 405.13 USD·tonne^-1·day^-1 and emissions of 556.27 kg CO₂ eq·tonne^-1·day^-1). A nationwide contextual analysis revealed that out of six regions, the US Northeast had the lowest median costs and emissions, while the Mountain Plains region had the highest, highlighting the importance of geographical and infrastructural factors in implementation. Overall, codigesting FW with PPMS is revealed to be a sustainable waste management option to decrease landfill disposal of valuable organic waste.