The sewer tank is a specific type of overflow tank. While sewer tanks with a surplus flow at the beginning of the sewer are regarded as uncritical, in sewer tanks with a surplus flow at the end of the sewer, sedimentation may be remobilised and transported into the receiving waters. For this reason, in Germany, additional capacity over and above the minimum requirements is demanded for these sewers.
In order to ensure the qualitative mode of action of sewer tanks with a surplus flow at the end of the sewer, pilot-scale investigations into a model sewer, with artificial combined water, as well as large-scale investigations under real life conditions, were carried out.
The findings of these investigations illustrated that a sedimentation of solids takes place in the majority of flow circumstances, including times of overflow. Thus, even at times of high stormwater flow, the outflow to the receiving waters contains only a low level of pollution. As well as storing the stormwater, the sewer tank with a surplus flow at the end of the sewer has the additional environmentally friendly effect of improving the quality of the outflow and reducing its pollution content.