Is there a standard rainfall design frequency used for sewer design?

The answer depends on the type of sewer. For storm sewers, specified rainfall return frequencies are used to provide various levels of service and are often specified in local storm sewer design requirements. For combined sewers, specified rainfall return frequencies are often used as well and are derived from combined sewer overflow (CSO) discharge permit requirements. For sanitary sewers, there are no specified requirements. This topic has been discussed on-and-off for years within the industry and between municipalities and regulators, but no consensus has emerged. However, it makes sense to establish a level of service for sanitary sewer systems and use that as a basis to benchmark performance over time and plan system improvements with the desired level of service in mind.