Modeling an integrated urban wastewater system to assess (micro-)pollutant discharge under dry- and wet-weather: mitigation strategies and climate change scenarios

Integrated urban wastewater systems, including sewer networks and wastewater treatment plants (WWTPs), face critical challenges during wet weather, leading to combined sewer overflows and WWTP bypass that release untreated wastewater. Climate change exacerbates these issues through more extreme weather patterns. The recently approved recast of the European Urban Wastewater Treatment Directive (UWWTD) imposes a new limit requiring wet-weather discharges to be less than 2 % of the collected annual dry-weather loads, making modeling tools essential for assessing compliance and evaluating mitigation strategies. In this context, an integrated hydraulic and pollutant transport model was applied to an urban catchment near Milan, Italy. The model was calibrated using field measurements of water quantity and quality (including 4 conventional pollutants, 3 metals, 3 organic micropollutants) under dry- and wet-weather conditions. We assessed two mitigation strategies (end-of-pipe first-flush tanks; source control via sewer separation with blue-green infrastructure infiltration) and two climate change scenarios representing climate variability extremes. First-flush tanks reduced conventional pollutant loads by 5-42 % but showed limited effectiveness for organic micropollutants due to lower WWTP removal efficiency. Sewer separation with infiltration met UWWTD target only at 75 % catchment implementation, demanding extensive retrofitting efforts compared to first-flush tanks. This comparison underscores that no single mitigation strategy fits all situations. Environmental risk assessment across scenarios revealed dilution's crucial role, underscoring the need to consider both pollutant loads and hydraulic conditions. This study shows integrated modeling supports water managers in addressing pollution control and climate adaptation, enabling transitions towards sustainable urban areas.