Retention basin on Carl Blochs Gade

⛶i

Aarhus Municipality’s Wastewater Plan for 2006–2009 sets out a number of measures designed to reduce the discharge of nutrients into the surrounding aquatic environments.

In 2008–2009, a large retention basin will be constructed on, or rather beneath, the lawn along the stream, between the City Museum and Thorvaldsensgade. The basin will have a capacity of 15,000 cubic metres, or approximately 10 times that of the basin established beneath Mølleparken in connection with the opening of the stream and the restoration of the park.

The basin is being built because Aarhus city centre is a so-called combined sewerage area. In a combined sewerage area, rainwater and sewage are carried in the same pipes, unlike in a modern separate sewerage system, where rainwater is typically channelled directly into a receiving water body, such as a nearby river or stream.

The problem with combined sewerage is that all rainwater must pass through the treatment works, and although the Marselisborg Treatment Works has a high capacity, the sewerage system cannot cope when it is pouring with rain.

On several occasions, this has resulted in flooded basements and the discharge of mixed sewage into the Aarhus River and, consequently, the bay. This is not in line with the aim that Brabrand Lake, the Aarhus River and the inner city harbour should, in the long term, achieve bathing water quality standards.

To prevent the discharge of sewage, wastewater is collected in retention basins during cloudbursts, so that the water can later be pumped to the treatment plant.

The measures for this are set out in the wastewater management plan; in addition to retention basins to prevent rain-induced discharges, the plan is for the two treatment works discharging into the Aarhus River, Viby and Åby, which discharge into the Århus Å, will eventually (2017+) be decommissioned so that the wastewater from these sites is treated at the Marselisborg Treatment Plant. At that point, only the Egå Treatment Plant and the Marselisborg Treatment Plant will remain.