A WATERSHED APPROACH TO SSO MANAGEMENT

A watershed approach to Sanitary Sewer Overflow (SSO) management was applied as part of an SSO elimination program for the Vallejo Sanitation and Flood Control District in Northern California. This approach was applied to estimate the total pollutant loads from SSO's, urban stormwater, and flows from the Napa River watershed. Flows generated from this approach were coupled with water quality data from a sampling program to project concentrations and loads for the three wet weather components (SSO, stormwater and upstream watershed discharges). This study is currently being used to set permit limits for a design wet weather event.The watershed approach used a combination of monitoring and modeling techniques to estimate the improvements to water quality for a variety of SSO elimination programs. Monitoring data included the use of radar and ground-level rain gages, flow monitoring data, and water quality samples. Modeling consisted of a detailed sanitary sewer collection model, an urban stormwater model, and a watershed model. These models were calibrated using available monitoring data to project flows for a variety of design events (i.e. 1-year, 5-year, and 20-year).This paper outlines the process used to project flows and water quality impacts for assessment of SSO's within a watershed. The modeling included 51 years of continuous flow simulation. A modified version of EPA SWMM coupled with the MOUSE model (Danish Hydraulic Institute) was applied as the collection system model. This method can be used to accurately predict wet weather flows within +⊘−5 percent of measured flows.By applying a holistic approach to SSO management programs can be developed that provide cost-effective solutions for protecting water quality and public health. While wet weather SSO's can only be managed up to a given design event (i.e. 5-year event), they can not usually be eliminated for every wet weather event. However, by taking a watershed approach to SSO management, the impacts to both the public and the receiving waters can be minimized to a cost-effective level. This technique can be applied to any collection system (and upstream watershed) to help set pollutant discharge limits that comply with local permitting regulations. By providing the “whole picture” to a municipal district and a regulatory agency, permits can be set for specific facilities and can be used in the future to potentially allow pollutant trading between pollutant sources.