Stormwater retention basin Forckenbeckstraße

Behind the Kreuzkirche on Forckenbeckstraße is a water area, framed by a few trees and shrubs. It is used as a rainwater retention basin, in heavy rainfall prevents so overflow of the sewer system. The city center of Berlin has a mixed sewage system in which dirty water and rainwater are transported together. Cloudbursts overwhelm the capacity of the sewer network. It would be ideal to let the first "rinse" pass with dirty water and direct the incoming, only slightly dirty water directly into the receiving water, but this can hardly be realized in practice. Instead, the wastewater diluted by the rain is stored in retention basins and - if it allows the capacity of the pumping stations after the precipitation has subsided - is pumped to the sewage treatment plants.
Source: flanieren-in-berlin.de/bezirke/charlottenburg/militaerkommando-und-mediengarten.html

In the summer it sometimes smells terribly like a sewer and the water in the pool is almost always dirty. This unfortunately leaves a stain on the green area.