Harmonising Life Cycle Assessment of Biosolids Processing Systems: Reducing Variability and Identifying Key Sources of Variation.

A global transition in biosolids processing, driven by concerns over greenhouse gas emissions and emerging contaminants, demands robust sustainability assessments. While life cycle assessment (LCA) is widely adopted, methodological inconsistencies across studies have limited the comparability and reliability of results. This study addresses this gap by systematically reducing variability in published LCA estimates, providing a harmonized baseline for the environmental performance of biosolids processing technologies. The study adopted a harmonization approach to align methodological choices across 19 LCA studies covering four biosolid processing systems. Harmonisation significantly reduced variability and clarified comparative environmental outcomes: anaerobic digestion had the lowest mean harmonized global warming potential, while incineration and composting remained the highest-impact systems. Pyrolysis presented the widest postharmonization range, reflecting ongoing uncertainties and technological heterogeneity. A biosolids LCA benchmarking tool was also proposed to enable the development of customized harmonization results based on site-specific conditions. Moreover, a novel approach, partial harmonization, was introduced to serve as a sensitivity analysis that isolates the influence of individual LCA inputs to identify key sources of variation. Among 92 LCA inputs, this method identified inputs such as electricity mix, system boundaries for energy recovery, and fertilizer substitution assumptions as dominant drivers of variability.