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.