The Allegheny County Sanitary Authority (ALCOSAN) divided its service area into seven regional planning basins and hired consultants to develop a wet weather plan for each basin. Wade Trim developed the plan for the Lower Ohio/Girty’s Run (LOGR) planning basin that included 18 municipalities’ combined and sanitary sewer overflow (CSO and SSO) discharges along the Allegheny and Ohio Rivers. A hydraulic model was the backbone of the technical analysis. We updated the model and worked with the local municipalities to ensure it accurately reflected local hydraulic conditions. The LOGR model was integrated into the overall ALCOSAN model that was used to develop the system-wide plan.
Technologies for CSO and SSO control and treatment were identified and screened in four categories: source control, collection system transport, storage, and treatment. The availability, location and size of potential sites and routes for the construction of wet weather control facilities were also identified, evaluated and screened. Based on the basin geometry and outfalls, routes for storage and conveyance tunnels and consolidation sewers are along the same general alignment (corridor). The physical practicality of constructing consolidation sewers and outfall conduits for remote treatment and storage facilities was considered on a site-by-site basis.
Alternatives were built using two different outlet conditions: basin based (controlled and treated entirely within basin) and regional based (convey wet weather flows to a regional system connection point). This approach provided the two extremes of wet weather treatment and control alternatives that were used in an evaluation iteration process to determine the right blend of regional conveyance and local treatment. Cross basin alternatives also revealed it was more often cost effective to combine treatment with an adjacent planning basin.
