Appendix-A: Combined Sewer System

Combined sewer systems are systems that convey both sanitary sewerage and storm water through the same pipes. Interceptors are pipes designed to capture 100% of the sanitary flows during dry weather periods and convey them to a WWTP. During periods of moderate or heavy rainfall, however, the wastewater volume in the combined sewer system can exceed the capacity of the interceptor or the WWTP. For this reason, combined sewer systems are designed to discharge the excess wastewater directly to a nearby stream or water body through diversion regulators. Figure 3.1 shows a schematic representation of a combined sewer system and CSO occurring in the system. The figure shows how for wet-weather flows the interceptor at the bottom is able to convey only part of the flow into the WWTP and CSOs occur. However, in this tutorial, the excess water is diverted to the nearby storm water conveyance pipes.

SWMM Representation

To create a combined sewer system in SWMM one adds dry weather wastewater flows into the appropriate nodes of a previously created stormwater conveyance system. These nodes typically represent locations where collector sewers discharge into trunk sewers. Their number and location will depend on the level of aggregation used to combine individual wastewater sources (homes, businesses, etc.) together. The sidebar below explains how to use a node’s Inflow Editor to specify the time series of dry weather flow entering the node.