Protection of an urban catchment area is essential for indirect wastewater reuse. On-line monitoring and real-time control techniques are gradually becoming accepted methods for optimising the operation of sewerage systems. The operation of combined sewer systems during rain events typically aims to decrease the number and volume of overflows and to transport a maximum of combined water to the wastewater treatment plant (WWTP). Integrated modelling approaches nowadays consider the output of both the combined sewer overflows and the effluent of WWTP, whereas the dynamic characteristics of the receiving water is rarely included. The theoretical investigation aims to analyse the dynamic interaction of all three subsystems by combining measurements and simulations. The application of a real-time control approach to the sewer system in Trebic is described. It enables the storage capacity of the sewer system to be better utilised, thus minimising combined sewer overflow.