Real Time CSO Control in Philadelphia — A Real Balancing Act

In accordance with its NPDES permits, the City of Philadelphia Water Department (PWD) is conducting a comprehensive Combined Sewer Overflow (CSO) Compliance Program. As part of its Long Term Control Plan (LTCP), the Philadelphia Water Department (PWD) is implementing a number of technology-based capital improvement projects to further increase its ability to store and treat combined sewer flow, decrease CSO volumes, improve receiving water quality, and eliminate flooding due to surcharging. As part of the implementation of the LTCP, PWD has completed a facility plan for the implementation of a Real Time CSO Control system for its Southwest Drainage District. The implemented RTC system provides the capability to prioritize capture of combined flow in the collection system, to minimize overflows to sensitive receiving streams, and to optimize the use of available interceptor sewer-system storage during wet weather. The project had the following phases:Development of a public domain RTC modeling framework using the Extended Transport Block (EXTRAN) of the US EPA Stormwater Management Model (SWMM).Facility plan development including evaluation of CSO reduction alternatives, conceptual design plans for the proposed RTC facilities, and specifications for the RTC Center.Development of a Decision Support System (DSS) to integrate RTC with enhancements to the overall collection system management capabilities.This paper will focus on phase 2. After development and testing of a dynamic link library (.dll) module of the EXTRAN executable the model was used to analyze various alternatives for addressing the following alternatives:SWMM model simulations were performed to quantify the benefits of RTC implementation in PWD's SWDD. Model analyses were performed for single-event and continuous simulations using a multiple rain gage network to document the differences in these two characterizations of expected average annual reductions in CSO volumes due to RTC.The selected RTC strategy relies on systems and facilities throughout the SWDD. These facilities can be separated into three groups: 1) Collector system facilities, 2) The Fox Street Central Control Facility, and 3) The Southwest Water Pollution Control Plant (SWWPCP) RTC station. The primary objective of the SWDD RTC Project is to minimize CSO discharges to the Cobbs Creek. This objective is achieved by prioritizing capture of combined flows in the Cobbs Creek High Level (CCHL) intercepting system. A second objective of the RTC Project is to reduce or maintain existing CSO discharge volumes in the SWDD.