Improving the reliability of sewer systems involves implementing measures and facilities to prevent the discharge of untreated sewage onto the ground or surface. These measures include the redundancy of both pressurized and non-pressurized pipelines within the network, as well as the use of their accumulating capacity and emergency wastewater tanks. The accumulating capacity of a gravityflow sewer system was examined in this work. An algorithm for calculating the accumulating capacity of existing sewer systems was proposed. Additionally, the study explored methods to enhance accumulating capacity by replacing smaller diameter manifolds with larger ones and implementing blocking devices (back-flow barriers) in control wells to prevent wastewater from reaching the surface and to transfer wastewater flow from gravity-flow to pressurized mode. In order to investigate the accumulating capacity of existing gravity-flow collectors in the sewerage handling area to maintain operations during a 6-hour pump station shutdown resulting from an accident, the sewerage pumping station-18 (SPS-18) in Irkutsk was taken as an example. On the basis of the performed calculations, it is suggested that five control wells be equipped with blocking devices in order to increase the accumulating capacity of the gravity-flow network to the required levels. The article offers a methodology for calculating and developing the required accumulating capacity of the designed, existing, and reconstructed sewer systems. The research findings can be useful for updating the development plan for the water supply and wastewater disposal systems of the city.