Economic optimization analysis of combined sewer overflow controls in the Narragansett Bay, Rhode Island

A combined sewer overflow (CSO) facilities plan is being developed for the Narragansett Bay Commission's (NBC's) Bucklin Point Service Area (BPSA) in Pawtucket and Central Falls, Rhode Island. An economic optimization analysis was performed to evaluate the impacts of BPSA system on CSO controls on the Narragansett Bay. The NBC prioritizes CSO projects for its two service areas, which include six sewer-sheds in the Providence area and three sewersheds in the former BPSA service area. The economic optimization methodology described in this paper and applied to the NBC's Bucklin Point Service Area CSOs is a useful tool to set priorities and evaluate the benefits of successive control levels. Control levels that are of interest are those required by the Rhode Island Department of Environmental Management's CSO Policy, which states that equivalent primary treatment must be provided for CSO discharges resulting from all storms of frequency greater than 1-year, 6-hour design storm, and those at the 'knee-of-the-curve' on the marginal cost curve. This paper describes how production theory was used to develop optimal cost curves relating the least cost control level to the CSO control performance. This approach was used to estimate the least cost mix of pairs of storage and treatment that provide a desired level of annual CSO volume capture. From the results of this optimization, three alternatives were selected for site specific development. The continuous simulation of CSO discharges was used to estimate the number and volume of CSO discharges at each site. In addition, the impact of CSO controls on receiving waters was determined and is described for a series of overflow events in terms of the water quality criteria violations resulting in shellfish bed closures. These violations are expressed as acre-days of fecal coliform criteria exceedance.