On December 31st, 2008, the Ruhrverband in its catchment area operated 547 rainwater treatment plants with a total useful volume of 655,185 cubic metres. In these facilities, rainwater is gathered, stored during rainy periods, and later transferred to the sewage treatment plants for purification. Residual loads from diffuse sources also continue to be degraded in the river course and in the five Ruhr impounding lakes.
Out of these 547 plants, 203 were built exclusively as stormwater overflow tanks accounting for 377,709 cubic metres of useful volume. 316 rainwater treatment plants accounting for 231,658 cubic metres were built as storage sewers. Finally, 28 facilities were designed as a combination of stormwater overflow tanks and storage sewers with respective volumes of 32,779 cubic metres in stormwater overflow tanks and 13,039 cubic metres in storage sewers.
The following diagram illustrates the development of both the number and storage volume of rainwater treatment plants in the Ruhr catchment area.