South Bend CSO Long Term Control Plan Optimization: Comparison of Grey-Only vs. Grey-Green Infrastructure Least-Cost Solutions

This paper presents the methodology and results of the South Bend Combined Sewer Overflow (CSO) Long Term Control Plan Optimization project completed during 2011 and 2012. The project showcases innovative planning techniques and smart technology which enabled the City of South Bend to determine least-cost solutions to achieve complex planning objectives. In 2011 the City of South Bend, Indiana was completing negotiations on its Consent Decree with Federal and State regulators aimed at alleviating the frequency and volume of CSOs into the St. Joseph River. The City recognized that the estimated $510 million cost of the planned Long Term Control Plan (LTCP) would burden its rate payers with rising monthly utility bills. The City took a decisive step to undertake an optimization study to review and modify its existing LTCP hoping to significantly reduce the cost of the plan and also to efficiently stage the optimized LTCP improvements.This paper describes the comprehensive analyses completed by the project team utilizing a powerful Optimized Decision Support (ODS) software tool called Optimizer WCS. The specific steps in gathering data, formulating the optimization model, developing alternative solutions, performing sensitivity analyses and prioritizing capital improvements are reviewed. Particular focus is given to the inclusion of green infrastructure/LID (low impact development) options, balancing the grey and green infrastructure improvements, and incorporating real-time controls into the plan.The cost of South Bend's existing EPA-approved CSO LTCP was updated by the project team amounting to an estimated life-cycle cost of $412 million, excluding the wastewater treatment plant upgrade cost. A range of alternative optimized solutions were developed in two project phases including a grey infrastructure-only solution estimated at $350 million and a grey-green infrastructure solution estimated at $300 million. These optimized solutions represented cost savings of $62 million (15%) and $112 million (27%) compared to the existing plan. Each of the optimized solutions was carefully evaluated to verify that the service levels and performance achieved were the same or better than the City's existing CSO LTCP.