In the municipality of Vänersborg there is a number of pumping stations from which overflows may occur. Based on the proximity of these pumping stations to the river Göta älv and the data on the total volumes of overflows from each pumping station during year 2011, several pumping stations could be potentially relevant for the water quality in the river Göta älv. For the purpose of this AG 825 study, one pumping station, located approximately 3 km upstream the Överby DWTP was selected.
Using the modelling results and the quantified concentrations in the sewage, the peak norovirus concentrations in the source water at the intake were calculated for each loading event.
2.4. Quantitative microbial risk assessment
The hydrodynamic model provided a time series of concentration at the water intake in response to each simulated contamination condition. For each peak contamination level, Eq. (3) was used to find the required Log10 reduction to achieve the health target. The health target suggested by the World Health Organization for drinking water was used: 1 × 10− 6 DALY (disability adjusted life years) per person per annum (WHO, 2011), which can be translated to 2.74 × 10− 9 DALY per person per day. By complying with the daily target, an even stricter requirement than the annual target is fulfilled.equation(3)Healthtarget=DALY×1−e−r×C×V×10−LR