Modelling of Combined Sewer Overflow for Future Climate Change and Urban Development in Trollhättan - A quantitative and water qualitative assessment for River Göta älv as a drinking water source

Abstract

Combined sewer overflows (CSOs) are an increasing challenge that the water manage ment in cities faces, as climate change and urbanisation increase pressure on the system. The consequences of CSO events are the release of untreated wastewater into water bod ies, leading to contamination and increased health risks. In this case study, hydraulic modelling is used to simulate CSO events in Trollhättan under future climate change and urban development scenarios with a focus on changes in the parameters volume, flow, and dilution. In addition, different rain events with varying duration and intensity are compared to find the most critical one for the sewer system. In the second part of this study, a quantitative microbial risk assessment (QMRA) was performed to eval uate the health risks for drinking water consumers downstream of Trollhättan for the pathogens Campylobacter, norovirus, Cryptosporidium. Two different drinking water treatment plants (DWTPs) were evaluated; one located in Gothenburg, and another ar bitrary DWTP lacking UV-disinfection. The results from the hydraulic modelling show that climate change will significantly increase the amount of CSO, and the proportion of impervious areas has a great impact where implementation of green infrastructure is shown to have a reducing impact. Another finding is that intense and short-duration rainfall events are more critical than longer and less intense rainfall events. From the QMRA, the health risk due to CSOs from the sewer network in Trollhättan does not pose a risk to the drinking water consumers in Gothenburg. However, without UV treatment, there is a risk for Cryptosporidium infection, which highlights the efficiency of the UV-treatment process and the importance of sufficient and efficient treatment barriers at DWTPs.