ALCOSAN’s Water Quality Modeling Approach

Allegheny County Sanitary Authority, based in Pittsburgh, Pennsylvania, is implementing its Clean Water Plan to manage sewage overflows in compliance with a Federal Consent Decree. ALCOSAN and CDM Smith developed receiving water quality models to support alternative analysis and facility sizing. The technical approach involved statistical calibration for flow and bacteria utilizing an 11,000+ node PCSWMM model. The tributaries were simulated using SWMM and the main rivers using EPA's Environmental Fluid Dynamics Code. Covering 280 miles (451 kms), the model simulates hydrology, river hydraulics, bacteria loading, and temperature-dependent decay. A literature review identified prior studies describing model calibration assessment methods and calibration statistics for flow and nutrients with limited information available for bacteria. ALCOSAN and CDM Smith developed bacteria calibration methods and metrics to objectively assess model calibration suitability for intended purposes. Calibration results for bacteria show a strong fit to observed data, with the root mean square error (RMSE) of the log transformed counts of fecal coliform of between 0.51 and 0.95 and for E. coli of 0.57 and 0.83. Since the model simulated bacteria counts were within 1 log of the observed data, the model is suitable for the purpose of forecasting attainment with water quality standards over a typical year.