Sizing a New CSO Storage Tank using Continuous SWMM
The Region of Hamilton-Wentworth completed a Pollution Control Plan (PCP) in 1991 to address pollution from 23 combined sewer overflows (CSOs) which discharged to local receiving waters. The PCP recommended the construction of 10-12 underground storage tanks to intercept and detain CSOs at these outfalls during wet weather, for subsequent conveyance to and treatment by the Woodward Avenue Wastewater Treatment Plant during dry weather. The target CSO frequency from the tanks was 1-4 CSOs per year, and an 87-95% reduction in previously existing annual CSO volumes. The Region has made significant progress with the implementation of its PCP since 1991. Five CSO storage tanks are in operation. Together these facilities provide approximately 193,000 in 3 of additional CSO storage volume. Four tanks have been constructed since the completion of the PCP and were designed to reduce the number of CSOs at nine former outfalls from up to 29/y to 1-3/y (May to October).
Construction of the first tank, a 70,000 m3 facility located at the east end of Greenhill Avenue, was completed in 1988 prior to the completion of the PCP. This tank was designed to retain the runoff from a 15 mm storm. Storms of this size occur fairly frequently, and as a result, the Greenhill CSO tank typically overflows about 13 times/y (May to October). The Region has decided to construct a second storage tank to improve the level of CSO control at the Greenhill outfall and to meet new provincial CSO control objectives set out in the Ontario Ministry of the Environment's Procedure F-5-5. The sizing of the tank was done in-house by Regional staff using the USEPA Stormwater Management Model (SWMM) Version 4.0. This chapter will discuss in detail the methodology used to determine the size of the tank required to meet desired CSO control objectives. This methodology is based upon the premise that continuous modeling is the only reliable way to ensure that the tank performs to these control criteria under real operating conditions for long periods of time.
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