Assessing combined sewer overflow performance under climate projections and urban development

Graphical abstract of the paper: Assessing combined sewer overflow performance under climate projections and urban development • CSO emissions assessed under future climate and urban development scenarios. • Increased precipitation and impervious area may cause failure to meet regulations. • Decreased impervious area partly counters climate change effects and area increase. • 16–27 % impervious area reduction to sustain past CSO volumes in future climates. • 6–11 % impervious area reduction to sustain past pollutant loads in future climates. Apart from collecting and discharging wastewater from urban areas, drainage systems can reduce flooding and ensure acceptable water quality to minimize pollution of receiving waters. Combined sewer systems discharge diluted wastewater via combined sewer overflows (CSOs) into receiving waters during heavy rainfall, regulated by local directives. However, progressive urbanization is leading to an increase in the degree of surface sealing and associated changes in surface runoff. Although various surface decoupling measures, such as decentralized stormwater systems, are already being used to address these challenges, climate change-induced increase in frequency and intensity of precipitation events will make it more difficult to meet future demands. This study uses a conceptual model (KAREN) to examine CSO challenges under climate and urban growth projections, analyzing modeled CSO performance in Innsbruck (Austria) in light of the European Urban Wastewater Treatment Directive (UWWTD) and the Austrian directive. Results show that increase in impervious area and climate-driven precipitation change CSO frequency and volume. The UWWTD supports climate change adaptation by responding to a decrease in impervious area. To maintain historical CSO volumes, a reduction in the impervious area of 16 % (near future: 2031–2060) and 27 % (far future: 2071–2100) would be required. To maintain historical pollutant loads, reductions in connected area of 5.11 % (for Ammonium (NH 4 ), near future: 2031–2060) to 11.40 % (for Total Suspended Solids (TSS), far future: 2071–2100) would be required. Urban development and climate change pose challenges to combined sewer systems globally, with European cities facing particular difficulties in meeting the revised UWWTD requirements.