Modeling additional waterflows in sewage systems in Sweden – An outlook on the impact of climate change.

Sammanfattning: This study assesses the phenomenon of additional water intruding into sewage sys-tems in different areas of Sweden. Additional water means in this case the non-foul water that can originate from storm water runoff, which is either supposed to be con-veyed into the pipe system or intrudes it by wrongly connected drains. It can also re-sult from in-seepage of groundwater due to imperfections of the pipe system itself. It is intended to analyze how different features of the areas have an impact on the extent of this phenomenon. Further, an estimation of the conditions in future scenarios will be obtained. This includes an indication about the potential risk of a sewage system overflow, the expected volume of inflow at treatment plants and thus about the sys-tem´s suitability for the future. Numerical, hydrological compartment models for 19 cities in Sweden were set-up. The used parameters were obtained from previous studies by the Swedish Environ-mental Protection Agency. In these models present and future climate data were ap-plied. For future scenarios three different climate change projections were used which contain bias corrected climate data timeseries for each study area. The climate change scenarios were supposed to represent an "optimistic", "average" and "pessimistic" outlook. By assessing the outputs of the climate models, it was concluded that signifi-cant differences can occur, depending on exact geographical location and chosen cli-mate models. A sensitivity analysis was conducted of how geology, climate and status of the pipe system have an impact on the extent of additional water flows. It revealed that the status of the sewage system has by far the biggest impact. Finally a discharge analysis showed a potential outlook of future development of additional water flows for the chosen study sites, yielding highest increase for the sewage systems of Kiruna, Karlskoga and Sundsvall.