Abstract

Many combined sewer communities in the United States require reporting combined sewer overflow (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 develop the CSO discharge monitoring reports (DMR).

Reliable and consistent approaches are critical in preparing ongoing CSO discharge reports to satisfy NPDES permit requirements and to track long-term trends in discharges. Outfall flow monitoring presents many challenges and relying upon the outfall monitoring equipment alone to estimate overflow volumes may lead to inconsistent and unreliable information. The DMR data reported based entirely on monitoring data may add more complexity (in addition to the rainfall variations, system changes, etc.) when assessing the efficacy of implemented CSO controls. Continuous simulations with the interceptor hydraulic model supported by CSO activation monitoring data is the most cost-effective, reliable, and consistent method for generating continuous CSO discharge data to meet permit requirements.

This chapter will present a case study based on extensive experience in preparing CSO DMRs for the City of Indianapolis, Indiana by applying the calibrated sewer system hydraulic model on a continuous basis. Continuous modeling techniques and protocols developed for a complex Indianapolis sewer system are presented along with important lessons learned.

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