Hydrodynamic and water quality modelling of a free water surface constructed wetland for urban runoff mitigation

• nTIS approach outperforms simple CSTR in reproducing wetland hydrodynamics. • 3P-TIS ( n = 4) model best represents and reproduces wetland hydraulic behaviour. • Water quality model simulated pollutant removal processes in CW under CSO events. • Simplified modelling provides a basis for future wetland design and management. Free water surface constructed wetlands (FWSCWs) replicate the ecological functions of natural wetlands, providing multiple ecosystem services, including water quality improvement. The Tancat de la Pipa constructed wetland (TPCW), located in Albufera Natural Park, Valencia, Spain, was designed to mitigate lake eutrophication. Its role in treating urban runoff, particularly from combined sewer overflows (CSOs), was recently assessed. Hydrodynamic modelling revealed that three-parallel lines with tanks-in-series configuration best represents the wetland’s flow dynamics. By applying the water quality model, the main mechanisms enabling the TPCW, originally not designed for runoff control, to mitigate pollution peaks from CSO discharges were identified. Ammonium removal was primarily driven by washout and nitrification, while sedimentation dominated TSS reduction. These findings emphasize the critical role of FWSCWs in managing diffuse pollution from CSOs and highlight the potential of predictive water quality models to optimize wetland performance under variable inflow conditions.