Selection of a Sanitary Sewer Overflow (SSO) Design Condition With Credibility

The City of Toledo, Ohio is implementing sanitary sewer overflow (SSO) control storage facilities in areas of the system impacted by high quantities of rainfall dependent inflow and infiltration (RDII). These facilities are designed to control SSOs up to a 10-year recurrence interval event. The hydrologic sizing methodology can significantly impact the facility design volume. This paper describes the approach used in Toledo, which considered the probabilistic nature of rainfall, the variable hydrologic response of the system during different times of the year and the difference between control of a 10-year flow condition and a 10-year precipitation condition. The resultant design approach looked independently at volumetric sizing and hydraulic sizing. It resulted in a storage facility that both the City and the regulatory agencies felt was the “right size”, while providing system protection against basement backup events for more intense conditions. The approach gave confidence both to the regulatory agencies, who must approve the design, and to the project owner, who must pay for the proposed facilities.