Our article focuses on the current issue concerning the adaptation of existing sewage systems to increasingly more restrictive regulations related to discharge of combined sewage to surface waters. The example of Przemyśl urban catchment was used to present the impact of a hydraulically overloaded sewage system on the San River and the operation of the sewage pumping station. The authors also discuss issues related to mathematical modeling and development of software used for the simulation and design of retention reservoir cooperating with the existing sewage pumping station in the part of Przemyśl located on the river’s left-hand side bank.