Selecting the Appropriate Storm Criteria for Establishing Peak Flows in Sanitary Sewer Systems to Meet CMOM Requirements

A sanitary sewer system should transport only sanitary wastewater, not infiltration and inflow (I/I). However, I/I significantly increases flow in some sanitary sewer systems during storms. The I/I rates can be several times the dry-weather flow rate causing sanitary sewer overflows (SSOs). The Environmental Protection Agency's (EPA's) proposed Capacity, Management, Operation, and Maintenance (CMOM) regulations contain strict guidelines on SSOs. The EPA requires municipalities to demonstrate that their systems have adequate capacities to convey design flows. Municipalities therefore share a liability under the CMOM regulations when setting peak flow standards in a sanitary sewer collection system. Almost universally, and inevitably, wet-weather conditions will cause sanitary sewer system flows to exceed the system's design capacity. Therefore, municipalities must balance:Appropriate sizing of sewer facilities to mitigate SSOsCapital funding and prioritization of relief facilities to mitigate SSOsRisks of public health and environmental degradationThreats of third-party lawsuitHydraulic design criteria for sizing sewer facilities (e.g., average/peak dry-weather flows, I/I allowance, peaking factors, wet-weather flows, velocities) are well established and widely accepted. The most problematic variable is the amount of I/I that enters sanitary sewer systems during wet weather. Excessive I/I in sanitary sewer systems creates challenges when designing relief facilities to mitigate SSOs (relief sewers, equalization facilities, and rehabilitation). In these situations, peak flow projections must be based on the existing system flow data, primarily from the results of hydraulic modeling. An appropriate design storm must be chosen before evaluating the existing system to meet CMOM requirements. Because avoiding all SSOs is impossible, municipalities will likely choose a design storm that can economically mitigate most of the SSOs.This paper evaluates available approaches to selecting the appropriate design storm criteria and assesses techniques for establishing peak flows in sanitary sewer systems.