Abstract

The Retention Treatment Facility (RTF) for the Twelve Towns Drainage District of the Southeastern Oakland County Sewage Disposal System (SOCSDS) in Michigan is authorized to discharge treated combined sewer overflow (CSO) to the Red Run Drain through a National Pollutant Discharge Elimination System (NPDES) permit. In an effort to comply with the requirements of the Federal Clean Water Act (PL 92-500 of 1972), Oakland County needed to evaluate the performance of the RTF and determine any necessary improvements to achieve that compliance.

A computer model was developed to assess the performance of the RTF and evaluate alternatives to comply with the requirements of the NPDES permit that governs the RTF. The computer model provided a mechanism to perform the assessment of the most cost-effective and feasible means for implementing system improvement to reduce the number and volume of CSO. The improvements to the system were accomplished by additional storage of CSO, removal of storm water inputs to the RTF, and improvements to maximize the use of the existing interceptor system.

The analysis of the RTF and proposed improvements was conducted with respect to the state regulatory agency, the Michigan Department of Environmental Quality (MDEQ) presumption approach for "adequate treatment." MDEQ defines its presumptive "adequate treatment" criteria as total capture of the 1-y, 1-h storm event and 30-min detention (for settling, skimming, and disinfection) of the 10-y, 1-h storm event.

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