The Challenges of Abating a Very Large Combined Sewer Overflow - Conner Creek, Detroit, Michigan
- Gary Fujita, Detroit Water and Sewerage Dept.
- Mirza Rabbaig, Detroit Water and Sewerage Dept.
- Peter E. Moffa, Brown and Caldwell
- Daniel P. Davis, Brown and Caldwell
- Curtis D. Courter, Hazen & Sawyer
- Charles Hocking, Hazen & Sawyer
This presents the challenges faced in abating a large combined sewer overflow. Prior to construction of the Detroit Water and Sewerage Department’s (DWSD) Conner Creek CSO Control Facility, three combined sewer outfalls regularly discharged large volumes of untreated combined sewage into Conner Creek, a ½-mile long tributary to the Detroit River. On average, these combined sewer overflows resulted in discharges of over 2 billion gallons per year to the receiving waters, with a peak rate of flow in excess of 13,000 cfs for the 10-year, 1-hour storm event.
In 1994 the Michigan Department of Environmental Quality (MDEQ) adopted a presumptive treatment design criteria, which required skimming, settling and disinfection, with thirty minutes of detention time, to treat the 10-year, 1-hour storm. Concurrently, the MDEQ allowed permittees to propose alternative levels of control provided that the facility “demonstrates” that it is capable of achieving water quality standards at times of discharge.
In 1996 the DWSD completed its Long Term Combined Sewer Overflow (CSO) Control Plan (LTCP), which recommended additional CSO control facilities along the Rouge and Detroit Rivers, the major one being at the Conner Creek. However, due to site constraints, and acknowledging the large incremental cost to benefit relationship of providing additional storage at this location, the DWSD-LTCP concept for the Conner Facility was to provide disinfection of the 10-year, 1-hour storm substituting rapid mixing followed by only ten minutes detention time in lieu of the MDEQ’s thirty minute presumptive criteria. In addition to bacteria, the other major pollutant concerns noted in the DWSD-LTCP are floatables control and suspended solids.
The DWSD subsequently negotiated an agreement with the MDEQ to study, design, and construct a pilot CSO treatment facility to achieve daily and monthly fecal coliform limits of 400 and 200 (average geometric mean) colony forming units per 100 milliliters (cfu/100 ml), respectively, with only five minutes of contact time at the 10-year, 1-hour peak flow of 13,262 cubic feet per second (cfs). In 1997, the DWSD’s National Pollution Discharge Elimination System (NPDES) Permit was revised to reflect this agreement, and require construction of an approximately 27-million gallon retention basin, to provide settling, skimming, and disinfection of all transported combined sewage discharged from the three existing Outfalls to the Conner Creek by December 31, 2004.
The design and construction of the Conner Creek CSO Control Facility posed many significant challenges in addition to providing high rate disinfection of the very large peak flow, some of which are discussed below.
The project plan required that the entire range of flows, from less than 500 cfs to over 14,000 cfs (the 10-year, 24-hour storm hydraulic design criteria) be conveyed by gravity through the facility, utilizing only the one-foot of available head. Site constraints limited the final layout of the facility and required a 1000-ft long by 80-ft wide by up to 45-ft deep cofferdam. Also, several tank configurations were evaluated to determine the optimum configuration to hydraulically handle the range of flows.
Because of existing hydraulic constraints, screening facilities had to be designed for not more than one foot of head loss at the design flow and a minimum design velocity of about 2 fps in connecting chambers so that sedimentation does not occur at this point in the new CSO facility. Catenary, Climber, Romag, and Copa screens were all reviewed for their applicability.
Settleability tests were conducted and the results used to analyze the alternative facility layouts considered including USEPA swirl concentrators, settling tanks, and combinations thereof to determine the degree of settling that could occur upstream of screening and in the contact tank. The results were also utilized to determine the benefit of providing first flush capture versus complete flow through in the preferred retention basin alternative. The final basin physical configuration and operational plan provides approximately 45% solids removal on an average annual basis.
Field pilot testing of the high-rate disinfection was completed on actual combined sewer overflows from the three outfalls to determine the dosing requirements to meet the effluent fecal coliform limit with a five-minute contact time. Based on the test results, the effluent fecal coliform limits could always be achieved with a maximum Cl2 dose of 25 mg/l. However, due to the extreme range of flows to the facility, and the infrequency of the design storm, a much lower dose can be utilized for most events. Therefore, the disinfection system was designed to feed based on a concentration times contact time (C x T) setpoint of 125, where the required dose is directly linked to the influent flow rate. The use of the C x T setpoint reduces the volume of sodium hypochlorite (NaOCl) required to treat most events and the total residual chlorine in the facility’s effluent.
Also due to the large range of possible flows, the NaOCl feed system needed to be designed to handle turndown of approximately 26:1. The system was designed to handle this turndown utilizing constant speed magnetic drive centrifugal pumps. Feed rates are controlled by feed and recirculation control valves, which modulate at separate times to deliver only the required dose to a system of 10 low-level diffusers and 60-submersible induction mixers. Additional field pilot tests were designed and conducted to confirm the results of the computerized modeling of the full-scale feed control system and to determine the extent and effects of NaOCl degradation.
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