Development of a Decision Support Tool to Assess the Impact of Wastewater Reuse on Downstream Watercourses: STREAM—Support Tool for Reuse Evaluation and Management

Sustainable water management has become one of the main concerns in Europe, with water reuse practices as a key strategy for it. In this framework STREAM was developed, an open-access decision support tool (DST) designed to assess the hydrological, chemical, and agricultural implications of wastewater treatment plant (WWTP) effluent reuse in a real context. The DST simulates the interaction between WWTP effluent composition and the receiving water body in terms of hydrological balance, chemical composition and implications for agriculture. The outputs provide the effects on the ecological status of the river and the potential saving in fertilizers. To generalize system behaviour, a dimensionless ratio between river flow and WWTP discharge (R) is introduced, enabling the interpretation of reuse scenarios in terms of hydrological dominance. The DST was applied to three case studies in Luxembourg representing a range of WWTP capacities and river conditions. Results show that reuse feasibility is governed by the interaction between R and baseline water quality conditions, rather than by flow magnitude alone. Systems characterized by high R values exhibit greater dilution capacity and lower ecological risk, while systems with low R are more sensitive to effluent inputs. These findings position STREAM as a screening-level analytical framework that complements existing techno-economic DSTs, supporting early-stage decision-making in water reuse planning, particularly in contexts with limited data available.