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This paper describes innovative opportunities for reducing total volumes and mitigating impacts of CSO occurrences within the Metropolitan Sewer District of Greater Cincinnati (MSDGC) collection system, by applying RTC techniques to interceptor system operational strategies. RTC facilities, strategies and performance criteria are described and RTC modeling results are compared with model results for the current system operations.

The three major drainage basins comprising the MSDGC service area are the Mill Creek, Little Miami, and Great Miami basins, comprising a total drainage area of approximately 400 mi2 and including approximately 230 Combined Sewer Overflows (CSOs). The primary objectives for implementing RTC in the MSDGC collection system are the reduction of total CSO volumes, and the maximizing of wet-weather flows to the Waste Water Treatment Plants (WWTPs). Greater total volumes being conveyed to the treatment plants indicate less total volume losses throughout the interceptor system, e.g. through manhole surcharging.

RTC opportunities in the Mill Creek basin were found in the potential for conveyance balancing between parallel interceptor sewers and in-system storage potential in large, relatively shallow-sloped combined trunk sewers that feed the interceptors. For example, the primary Mill Creek interceptor system is comprised of two parallel 72" sewers, the Mill Creek Interceptor and the Mill Creek Auxiliary Interceptor, which physically cross six times along their length, with existing gated connections between them at three locations.

In addition to conveyance balance opportunities, in-system storage opportunities were evaluated for all CSOs in the Mill Creek basin, in the form of potential storage volumes available upstream of CSO outfalls. The potential storage available was computed using ArcView GIS scripts to determine each CSO outfall crown elevation, and then trace the pipes upstream of the outfall to a point in the network where the pipe invert elevation was equal to the outfall crown elevation, as indicated in Figure A-1. The storage volume was then calculated using pipe geometry and the stored water depth, assuming that manhole surcharging or basement flooding does not occur for depths less than or equal to the outfall crown elevation. The four top Mill Creek storage locations selected for RTC analysis each have a storage potential volume greater than 1 MG.

RTC model simulations were carried out in the SewerCAT RTC modeling platform utilizing datasets developed and calibrated for MSDGC's System Wide Model (SWM) project, which utilized the EPA SWMM engine and MIKE SWMM model interface. Because the benefits realized by a given RTC strategy for controlling interceptor crossovers and other facilities within the RTC model network may vary with different spatial rainfall scenarios, coarsely-defined spatial rainfall variability was included in the initial simulations, to encompass the range of situations likely during a more realistically varying storm event. This allowed the model team to see the range of possible effects of RTC controls on the behavior of the interceptor system. A second stage of rainfall scenario testing was carried out utilizing lessons learned, to evaluate the RTC alternatives for a range of rainfall intensities, volumes, and spatial variability, in order to better quantify the benefits of RTC for the interceptor system over a range of more frequently-occurring conditions.

The greatest overflow reductions observed for Mill Creek were achieved by the combination of conveyance balancing and in-system storage controls. The storage controls at the four CSOs trapped or delayed the passage of peak flows, reducing interceptor surcharging and overflows during the storm peak and providing for the retained flows to be conveyed to the WWTP following peak periods. Simulated CSO volumes were reduced by up to 57% for the subset of CSOs directly influenced by interceptor system hydraulics, which is equivalent to a reduction in CSO volume of approximately 24% over all the major CSOs in the Mill Creek basin.

The studies indicated that RTC techniques can provide significant CSO reduction for the MSDGC interceptor system via both conveyance enhancement and use of trunk sewer storage. The benefits of using operational in-system storage as a CSO control alternative are the low costs of implementation (relative to more structure-intensive CSO controls, such as storage tanks); the accommodation of variability and high volumes associated with CSOs; the reduction in pollution entering receiving waters resulting from first flush capture and treatment; and the optimization of the volumes of combined sewer flows treated at the wastewater treatment facility. The project team has recommended to implement storage RTC on a limited (pilot) scale initially to field test the inflatable dam and gate technology and the effects of storage on the sewer infrastructure.

Document Type: Research Article


Publication date: 2003-01-01

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