To waste, or to resource? A comparative Life Cycle Assessment of recovering bottom ash from waste incineration for use in road construction in Malmö.

Abstract: The low-carbon benefits and emissions savings from circular economy (CE) strategies are not yet well understood, yet are crucial for global development within Planetary Boundaries. Using the mineral fraction of waste incinerator bottom ash (MIBA) as construction material in road construction is one application of a CE strategy that could lead to environmental gains by substituting energy-intensive primary material and avoiding its alternative disposal in a landfill. The aim of this thesis is to improve the current understanding of, as well as quantify the resource efficiency and greenhouse gas (GHG) mitigation potential, from using MIBA as construction material, compared to virgin material. It contributes knowledge to local decision-making and to developing the CE concept more broadly. A Life Cycle Assessment (LCA) model to assess the environmental performance of a road built with MIBA, compared to a road built with virgin material, was developed and applied to a case study in Malmö, Sweden. Resource efficiency and GHG mitigation potential were assessed using a combination of two life cycle impact assessment methods: ILCD 2011 Midpoint+ and Cumulative Energy Demand. The results indicate less potential environmental impacts for the road scenario with MIBA in almost all impact categories. The analysis of the results shows that there can be some application contexts where using MIBA in lieu of primary material, creates larger benefits than in others. Important parameters were identified to be critical determinants for the environmental performance of the road with secondary material, including the transportation distance of materials, the types of substituted material, and specific properties of the secondary material. While the results are meant to support decision making, inherent limitations to the LCA methodology, must be considered when making decisions based on these results alone. Further research is needed to better account for resource-related impacts in a local context and to explore the effect of carbonation on the potential climate benefits of using MIBA.