The densely populated Emscher basin (~ 800 km2) is embedded between the Ruhr and Lippe rivers in the North-West Germany. The sewerage from 2.5 million inhabitants, from a diversity of industrial plants and storm water is conveyed by a combined open sewer system to a central sewage treatment plant located at the mouth of the Emscher. The use of the natural drainage system as an open sewer was due to severe land subsidence caused by underground hard coal mining, which had a most deteriorating influence on flow conditions. After closing down most mines, land subsidence has been greatly reduced: consequently the open sewer concept can be abandoned and a general rehabilitation of the Emscher system is now in progress. The rehabilitation concept is based on the separation of natural river flow and sewage, which will be conveyed via modified combined systems to decentralized treatment plants. A step-wise programme is necessary to complete the basic system rehabilitation through “learning by doing”. The realization period is expected to be 25-30 years. For the construction of sewer systems and plants, including stormwater treatment facilities, and for the reconstruction of the surface drainage system in a natural form, including flood retention measures, the expected investment cost amounts to about 8 billion DM (~ 4.7 billion US $).