Prospective life cycle assessment of waste management technologies and systems

Our society is currently facing a severe climate crisis, and to mitigate its adverse impacts, significant societal changes are expected in the coming years. Most of our actions carry a climate impact, and to mitigate the climate crisis, we must decrease those impacts, for instance by introducing efficiency measures, changing to renewable energy sources and increasing resource circularity. Here, waste management plays an important role, as it allows the recovery of resources and energy from objects which have already been discarded. This raises the question: how do we best treat our waste to protect the environment? To answer this question, quantification of the environmental impacts of waste systems is necessary. One option to quantify environmental impacts is life cycle assessment (LCA), which systematically quantifies potential environmental impacts and offers a safe space to rehearse changes before they are applied in real life. Prospective LCA (pLCA) has the additional goal of explicitly assessing a future version of the system. Thus, if pLCA is used in waste management planning, it may allow for choosing sustainable options that adhere to current environmental standards as well as future expectations.

This PhD thesis aims to quantify future environmental impacts of waste systems using pLCA; evaluate whether pLCA is a suitable methodology for waste management, and construct a framework identifying aspects requiring special reflections in waste pLCA. To that end, several prospective methods were tested throughout the four academic articles, which form this thesis's foundation. More specifically, three methods relevant for data scarcity in the prospective foreground modelling were tested: expert opinion, modelled inputs and learning curves. Moreover, two methods relevant to prospective background modelling were tested: stylised background scenarios and inclusion of integrated assessment modelling. The articles focus
on the use of biogas including carbon capture and storage or utilisation, treatment of the liquid fraction of digestate from biogas production, paper production and recycling, and biomass-based heat generation with carbon capture and storage.

During this PhD thesis, the selected pLCA methods were evaluated, and it was concluded that they were all suitable for assessing future environmental impacts of waste management. Further, it was concluded that the climate change impacts of waste management are today highly influenced by energy system impacts and choices for substitution, which are both expected to change substantially in the future. Overall, the climate impacts of waste management are expected to decrease as society moves towards a more renewable energy system. Nevertheless, burden shifting to other environmental impact categories may be an issue, also in the future.

On a more methodological level, it is recommended to assess several developmental future pathways, as the future is, of course, unknown. Hence, assessing a wide range of future pathways may give a more complete picture of possible futures. It is also recommended that results be reported as a range instead of a single value and that robustness assessment be used to quantify the sample space around the result. Uncertainties are inherent to prospective assessments, but meaningful conclusions can be drawn from pLCA by identifying robust tendencies over time. It is easy to overstate the certainty of pLCA results, but we must show knowledge humility and transparently report on limitations and shortcomings of the modelling. If waste pLCA is used within its limitations, it is a valuable tool to help quantify potential, future environmental impacts to guide decision makers towards a more sustainable future.