Abstract
The combined sewer overflow (CSO) control requirements imposed by the United States Environmental Protection Agency (USEPA) on approximately 1 100 sewer district utilities within the United States initially focused solely on the volumetric reduction of CSOs, with the assumption that a corresponding reduction in pollutant loads to a combined sewer system’s (CSS) receiving stream would result. As the development of CSO long term control plans for addressing the agency’s CSO control policies has progressed, the focus of CSO control has shifted, and assessing water quality benefits through quantitative analysis is becoming more common. Development of CSO improvements typically involves the consideration of several alternatives, and the benefits provided by each are evaluated in addition to its cost. While evaluating CSO control alternatives in Cincinnati, Ohio a simplified approach for com paring the relative water quality benefits achieved by each alternative was developed. Pollutant load event mean concentrations (EMCs) were developed for the pollutants of concern, based on available national average information. Within the existing conditions and alternatives models being evaluated utilizing USEPA’s Stormwater Management Model 5 (SWMM5), EMC assignments were applied to the rainfall derived infiltration and inflow, sanitary baseflow and individual subareas based on land use character istics. The treatment effectiveness of both grey and green CSO and stormwater treatment facilities were simulated using estimated pollutant removal efficiencies. A single design storm event as well as continuous annual simulation modeling over an entire year was performed using design storm rainfall and historical rainfall data. Pollutant loadings to the receiving stream were quantified and compared to assess the water quality benefit of each alternative. This study presents these results and provides an approach for making relative comparisons of the water quality benefits offered by CSO control alternatives within any CSS.
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