A Simplified Approach to Pollutant Load Modeling

The Combined Sewer Overflow (CSO) Control Requirements imposed by the United States Environmental Protection Agency (USEPA) on several sewer district utilities within the US initially focused solely on 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 (LTCPs) for addressing the USEPA’s CSO Control Policies progress, 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 consideration of several alternatives, and the benefits provided by each are evaluated in addition to cost. While evaluating CSO Control Alternatives in Cincinnati, Ohio, a simplified approach for comparing 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 SWMM5, EMC assignments were applied to individual subareas based on land-use characteristics. The treatment effectiveness of both “grey” and “green” CSO and storm water treatment facilities were simulated using estimated pollutant removal efficiencies. A single design storm event as well as continuous 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 water quality benefits offered by CSO Control Alternatives within any CSS.