12-years of CSO Discharge Reporting/Tracking/Trending using an Integrated Monitoring and Modeling Approach

Many combined sewer communities in the United States require reporting CSO discharges to a state regulatory agency on a regular basis to satisfy National Pollution Discharge Elimination System (NPDES) permit requirements. Typically, these requirements include gathering discharge data using automatic flow meters in each outfall and developing the CSO discharge monitoring reports (DMR). The city of Indianapolis, Indiana is required to report CSO discharges to the Indiana Department of Environmental Management (IDEM) on a bi-annual basis.During the early stages of implementing CSO discharge reporting, the city envisioned that reliable and consistent approaches are critical in preparing DMRs to satisfy NPDES permit requirements and to track long-term trends in CSO discharges in a meaningful way. In 1999, the city has adapted an integrated monitoring and hydraulic modeling approach for developing DMRs to meet these needs and establish a sound basis to track CSO reductions as they implement future CSO control projects. This approach involved continuous monitoring to characterize the system flows and overflows, and application of this data for maintenance/recalibration of the city's hydraulic model that reflects ongoing operational changes as well as system improvements. The DMRs were prepared on 6-month intervals based on hydraulic model simulations on continuous basis. This practice of generating DMRs continued, and is currently ongoing to meet the NPDES permit requirements.Experiences gained over a dozen years proved that reliable and consistent approaches are paramount for accurate CSO discharge reporting to satisfy NPDES permit requirements, and to track long-term trends in discharges. The existing system performance and the efficacy of CSO long-term control projects (LTCP) must be evaluated on an average annual basis. Wide variations are observed in year-to-year precipitation patterns and associated CSO discharges from the combined sewer systems. Long-term DMR data based on an integrated monitoring and modeling approach provides a sound and consistent method for evaluating overflow characteristics in relation to precipitation conditions, and assessing the performance and effectiveness of the system improvements.