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 this choice for a specific wastewater and sludge management system.