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Aarhus - Rainwater management against   combined sewerage overflow events

The River   Aarhus serves as a natural structure connecting the centre of Aarhus,   Denmark’s second largest city, with the port. To respond to the severe   pollution of the river and to promote infrastructure development, the River   Aarhus was converted into a covered concrete channel in the 1930s. In 2010,   still about half of the water in the river consisted of treated wastewater   and around 55 combined sewage overflow (CSO) systems discharged into the   river (Basso, 2010).

The city   authorities have implemented a series of measures to uncover the river, with   the purpose to enhance the aesthetics of the city, to promote recreation, to   enhance climate adaptation and flood protection and to reduce the frequency   of sewage overflows during extreme rainfall events. These measures included   the establishment of two upstream lakes to reduce nitrogen and   phosphorus flows into the Bay of Aarhus, the construction of new rainwater   retention basins and the implementation of an integrated real time control   system to allow for coordinated operation of the sewer systems and wastewater   treatment plants.   Additionally, a water quality early warning system was installed in Lake   Brabrand, River Aarhus and the harbor (Stahl Olafsson et al. 2015). So far,   these measures have resulted in a significant change in the way citizens and   visitors experience the river, which now forms a blue corridor lined with new   waterfront amenities and the harbour where bathing has now become safe   (Hvilsoj & Klee, 2013; Aarhus Municipality, 2008).