Evaluating the Application of “Grey”and “Green” CSO Control Alternative Solutions in a Heavily Urbanized Environment

Addressing its combined and separate sanitary sewage overflow problem is a priority for the Allegheny County Sanitary Authority (ALCOSAN) and the 83 municipalities comprising its service area, which includes a population of 896,500 and 250 combined sewer overflows (CSOs). In 2008, a Consent Decree (CD) was issued with a mandate by the United States Environmental Protection Agency (USEPA) for ALCOSAN to prepare and implement a comprehensive wet weather plan to greatly reduce the annual discharge of untreated sewage into area waterways by 2026. When implemented, the Regional Long Term Wet Weather Control Plan (“Control Plan”) will be the largest public works project ever completed in Allegheny County.To efficiently manage the control alternatives evaluation process, ALCOSAN divided the service area into seven separate planning basins with detailed basin facilities plans being developed for each. This paper focuses on the Main Rivers Basin (MRB), which contains a primarily combined sewer system bracketed by three large rivers in the City of Pittsburgh and encompasses 101 CSO outfalls (Figure 1). While the project area is a large metropolitan area, the approach to developing and evaluating grey and green infrastructure alternative solutions for CSO control has direct application to small and medium sized urban areas, as well.