Increasing water reuse is at the heart of Europe's climate change adaptation strategy to counter water scarcity. Sustainable and safe reuse is hampered by a complex mixture of contaminants that reach recipient waters due to their incomplete elimination in wastewater treatment plants (WWTPs). Natural biodegradation by microorganisms is a major determinant of the environmental impact of these contaminants and a precondition for safe water reuse. Microbial diversity and availability of natural organic carbon appear to promote degradation (Posselt et al. 2020, Schaper et al. 2019). However, due to the lack of knowledge onthe interplay of contaminantproperties, microbial activity, background carbon and nitrogen loads, and changing environmental conditions, it is still not possible to reliably estimate current, let alone future, in situ degradation rates. In this 2-year project, we will consolidate expertise from three research institutes at Stockholm University to investigate the contribution of natural contaminant biodegradation to municipal wastewater purification for water reuse. Our overall aim is to, combine chemical analytical, biogeochemical and molecular tools togenerate andlink knowledge on the interplay of riverine microbial communities, carbon cycling and contaminant biodegradation -which is a crucial prerequisite for safe and sustainable reuse.