ALCOSAN Overflow Control Facility Full-Scale Demonstration Project

The Allegheny County Sanitary Authority (ALCOSAN) provides wastewater collection and treatment for the Greater Pittsburgh area. ALCOSAN owns and operates about 90 miles of interceptors that convey sewage from combined and separate sewer systems that are owned and operated by surrounding municipalities. Collected flows are treated at ALCOSAN's wastewater treatment plant (WWTP) located on Pittsburgh's north side. Over 300 regulator structures provide relief of the collection system during periods of wet weather, when flows tributary to the system exceed the conveyance and treatment capacity. Excess flow is discharged via CSO and SSO structures to three major rivers and their tributaries.In April 1999, ALCOSAN completed preparation of a wet weather concept plan as a step towards compliance with the National CSO Control Policy. In June 2007, ALCOSAN completed negotiation of a federal Consent Decree which sets forth a series of planning, design, and construction requirements which must be completed in accordance with an extensive schedule of interim milestones. The Consent Decree was officially entered into court on January 23, 2008. In anticipation of these consent decree requirements, ALCOSAN began the implementation of a full-scale demonstration project involving two end-of-pipe CSO treatment facilities in November, 2005. The information gained from implementation and demonstration testing of these two facilities, which are consistent with the CSO control approach established in the April 1999 wet weather concept plan, is intended to help to shape the planning and implementation of region-wide CSO controls under the Consent Decree.The process of selecting an appropriate overflow control technology for both demonstration projects followed a detailed and objective planning process. A total of six technology options were sized and developed as part of the process of evaluating and comparing alternatives. Each technology was sized for a range of peak flow rate and/or overflow volume corresponding to zero, one, two, four, or seven untreated overflows in an average rainfall year.This paper presents the process followed to size, develop, evaluate, and compare each site, technology, and sizing option considered for both the Allegheny and Monongahela River overflow control facility demonstration projects. The process starts with model development, then proceeds to the field sampling program, development of alternatives and evaluation of alternatives. The paper concludes with current project status and lessons learned.