SSO LEVEL OF PROTECTION — A RISK BASED APPROACH

The definition of sanitary sewer overflow (SSO) level of protection is an inexact term that stemsfrom the many definitions in use and the fact that most of these definitions are indirectly related to the risk of SSOs. A definition for the level of protection (LOP) based on a direct measure of the risk of an overflow event is more useful for facility design and regulatory enforcement than indirect measures. Most of the approaches to level of protection are indirectly related to therisk of SSOs and are, therefore, poorly suited to the task of designing facilities that can achieve a desired level of protection with a higher degree of confidence. Direct measures of level of protection, based on the recurrence interval of the excess wastewater volume are directly relatedto the risk of SSOs. The excess wastewater volume is the volume of wastewater that exceeds the capacity of the conveyance/storage/treatment system. The excess wastewater volume becomes a SSO volume when the recurrence interval of the event exceeds the level of protection of the facilities. This paper discusses the use of hydraulic models of the conveyance system to simulate excess wastewater volume for a wide range of events using historic hydrologic conditions. The recurrence interval of the excess wastewater volume based on the simulated results is then compared to the recurrence interval of the 24-hour rainfall depth. This comparison shows the lack of correlation between the indirect measure of rainfall recurrence and the direct measure of SSO recurrence based onexcess wastewater volume. The excess wastewater volume is directly related to the task of sizing facilities that balance the level of protection and the cost to satisfy the regulatory requirements and the stakeholder goals. The conclusion is that a level of protection defined by the recurrence interval of excess wastewater volume is a more accurate and defensible method of managing therisk of an SSO than the recurrence interval of rainfall depth.