Maximizing Flow to the ALCOSAN Woods Run Plant

The Allegheny County Sanitary Authority provides wastewater conveyance and treatment for 83 communities in southwestern Pennsylvania, including Pittsburgh. Treatment is provided at its Woods Run facility, which is a conventional activated sludge facility with a rated capacity of 250 mgd. Influent tunnel capacity to the Woods Run plant is estimated to be approximately 1,000 mgd. As part of a 1997 plan to maximize flows to the treatment plant during wet weather to reduce the discharge of sewage to the rivers and streams, ALCOSAN proposed that preliminary treatment capacity be expanded to 875 mgd, primary treatment capacity be expanded to 600 mgd, and secondary treatment capacity be expanded to 275 mgd. ALCOSAN further proposed that full secondary treatment be provided for flows up to 275 mgd, while conventional bypass be used for flows between 275 and 600 mgd and a “split-flow” bypass be used for flows between 600 and 875 mgd. The “split-flow” bypass would involve secondary treatment of 275 mgd of headworks flow and primary treatment of the remaining headworks flow (325 to 600 mgd), with the primary effluent and secondary effluent being combined after disinfection and then discharged. These bypass scenarios were proposed in response to space limitations on the plant site that make it unfeasible to expand the conventional secondary treatment process. The Woods Run plant currently does not bypass during wet weather and wastewater flow entering the plant is limited via the influent pumping station to the secondary treatment capacity. It is anticipated that the proposed wet weather bypass meets the guidelines of the EPA's Combined Sewer Overflow Policy The expansion of the Woods Run plant wet weather capacity to 875 mgd is expected to provide 65 percent capture of combined sewer overflows.Because there were no known large treatment plants using a split-flow approach to wet weather treatment, pilot testing was performed from February through May 2005 and again from January through April 2006 to determine whether the existing secondary treatment system could adequately treat primary influent. In addition, full-scale testing of primary settling was conducted between April and October 5, 2005 and again between November 2005 and August 2006 to determine the capacity of the primary sedimentation tanks at projected wet weather wet flow conditions. The pilot study results suggest that the plant's existing activated sludge process, operating under steady state conditions and at normal mixed liquor concentrations of 1,500 to 2,000 mg/L range and solid retention times of two to four days, would be adequate for achieving effluent requirements during wet weather flows. Further, the pilot testing demonstrated adequate reduction of biodegradable material during non-steady-state wet weather conditions, over the limited number of low-intensity wet weather events that occurred during the pilot testing. The results of the full-scale primary testing support the concept of using the primary sedimentation tanks for wet weather treatment at flows up to the hydraulic capacity of each process unit (60 mgd/tank or 480 mgd firm capacity overall).