Sewage sludge co-pyrolysis with agricultural/forest residues: A comparative life-cycle assessment

This study aims to determine the sustainability and energy efficiency of co-pyrolysis scenarios as treatment processes for municipal sewage sludge through a life cycle assessment (LCA). In addition, sensitivity and energy recovery analyses are conducted to determine the possible methods for optimizing the co-pyrolysis process from a circular bioeconomy perspective. Corncob and wood residue have been selected as potential co-feed materials for co-pyrolysis with sewage sludge at three mixing ratios (25, 50, and 75 wt%). The functional unit (FU) for this study is 1000 kg of dried single or mixed feedstock. LCA results indicate that sewage sludge, in a singular pyrolysis scenario, demonstrated the most unfavorable outcome by causing a rise in all negative environmental indicators. In contrast, the overall environmental impacts are reduced by up to 48 %, when the sewage sludge is mixed with co-feed biomass (wood or corncobs), with corncob co-pyrolysis performing better than wood residue in most impact indicators. Energy recovery from a gas turbine provides significant benefits, generating about nine times of the required energy for gas turbine operation and supplying sufficient energy to sustain the whole process. This is notably evident for corncob co-pyrolysis, where the energy produced from gas recovery is equivalent to 59-181 % of energy requirement of the whole process and achieved the highest net positive energy balance (+1368 kWh/FU). Sensitivity analysis indicates that co-pyrolysis is more sensitive to bio-oil yield fluctuations and feedstock transportation. In conclusion, this study establishes that sewage sludge co-pyrolysis is a more environmentally friendly treatment approach when compared to single pyrolysis.