Opportunities of consequential and attributional modelling in life cycle assessment of wastewater and sludge management

Despite general agreement on the importance of adjusting each life cycle assessment (LCA) to its goal, the methodological choices in previously published LCAs on wastewater and sludge management systems are surprisingly similar, even when the information sought in the studies most likely differ. We argue that the potential of LCA may not currently be fully utilised, partly due to particular methodological challenges arising in both attributional and consequential LCAs for this type of systems. By developing the theory for handling of allocation problems in attributional LCAs, and by elaborating on the different possible foreseeable consequences in consequential LCA, we aim to facilitate both attributional and consequential LCAs, and to show the importance of such choices for a specific wastewater and sludge management system. We introduce and apply a distinction between physically and legally joint processes as basis for the allocation of resource use and emissions in attributional LCA, and suggest that, when the joint process is not driven by commercial interests, allocation factors could be identified and quantified through stakeholder priorities. In consequential LCAs, the substitution depends on the subjective view on what consequences are foreseeable, for example based on short- or long-term considerations. All of these modelling aspects can, as our case study illustrates, affect the LCA results.