Combined Sewage Detention Tanks (CSDT) are components of combined sewage systems commonly used in Germany. A lack of knowledge of the processes occurring within these structures causes the engineer to apply conceptual approaches in the simulation of pollutant transport. The investigations presented were implemented in an integrated systematic way. Extensive analytical, experimental and numerical studies of steady and transient phenomena taking place in CSDTs were carried out simulating different levels of hydraulic load. The study of the flow field and the simulation of sedimentation and erosion under realistic conditions resulted in proposals for modification and optimisation of the function of CSDTs. A simple way of predicting the cleaning efficiency is presented.