Improved life cycle assessment of wastewater and sludge management with resource recovery

University dissertation from Chalmers University of Technology

Abstract: Around the world every day, large amounts of wastewater are treated before release, to avoid impacts on humans and the environment. The treatment requires resources in the form of energy and chemicals, and it generates large amounts of sewage sludge, however, it can also serve as a source of energy, nutrients and carbon. These valuable resources can be recovered in many ways, including in the form of biogas, or through the use of sludge in agriculture or even, potentially, in form of biopolymer raw material. Life cycle assessment (LCA) can be used to quantify the life cycle impact of wastewater and sludge management with resource recovery, on humans and the environment, in order to evaluate their environmental performance and avoid sub-optimisation. LCAs of such systems face different types of methodological problems. This thesis focusses on two such problems. The first research topic concerns how to divide the environmental impact that results from a wastewater treatment process with the simultaneous production of a valuable by-product. Methodologies exist for handling such general situations, however, some properties inherent to wastewater and sludge management may result in complex allocation problems. This research identified the LCA of a system with wastewater treatment and simultaneous polyhydroxyalcanoate (PHA) production as particularly challenging, if PHA was considered as the main product. Three partly new allocation approaches were evaluated, and the choice of approach was found to influence the LCA results. A second research topic concerns the potential importance of assessing the risks of the pathogens that exist in wastewater and sludge management systems, which is not currently done within the LCA framework. This research has found that these risks are potentially important compared to other impacts on human health, both for wastewater and sludge management systems where sludge is incinerated or used for agricultural purposes.