Abstract: Capital intensive and path-dependent sectors are highly vulnerable to changing framework conditions such as demographic change, technological advancements, a stricter legislation or climate change. Decision-makers are often not aware of the long-term dynamics of systems and mostly rely on forecasts and the assumption that the future can be predicted based on extrapolations of past trends. We present a scenario study on the future of waste water treatment in the year 2050 in which we investigated the possibilities of scenario planning. We conducted a case study in East Germany due to the challenges this sector faces: Important parts of the sanitation system were modernized due to legislative requirements, but a decreasing population, reinforced by structural and economic changes, was not expected. Following a Delphi-based scenario approach we first conducted a PESTEL-analysis and subsequently applied Fuzzy Cognitive Mapping for developing four scenarios: Business as usual, Watershed-first, Recycling-first and Mega-City.