Substitution modelling in life cycle assessment of municipal solid waste management

Life cycle assessment (LCA) is gaining importance worldwide in guiding waste management policies. The capacity of co-products such as recycled materials and recovered energy to avoid primary production of equivalent products largely determines the environmental performance of waste treatment technologies. Estimating the reductions in resource use, emissions, and impacts enabled by this substitution of primary production is often the most influential and controversial factor in quantifying the overall environmental performance of a waste management strategy. This study aims to critically evaluate the modelling of substitution in LCAs of recovered material from municipal solid waste management systems (MSWMS) by answering two questions. First, to what extent is substitution modelling transparently documented in the literature? Second, are the substitution ratios justified to represent physically realistic replacement of one product by another? To address these questions, we performed a systematic analysis of 51 LCA studies on MSWMS published in the peer-reviewed literature. We found that 22% of the substitution ratios are only implicitly expressed. A significant proportion of substitution ratios is not justified (65%), while for the remaining 35%, justifications do not represent physically realistic substitutions. We call for more rigor and transparency, and we propose guidance for the documentation of substitution ratios, with the aim of reaching more credible and robust analyses. For the justification of a substitution ratio to be considered physically realistic, information should notably be provided concerning loss of quality, the function performed by substitutable materials, and the sector of use.