In-Line Disinfection of a CSO First Flush Basin in Detroit

The Detroit Water and Sewerage Department (DWSD) developed a Long Term CSO Control Plan for the City of Detroit in 1996. That plan recommended several uncontrolled outfalls be connected to a CSO storage tunnel. However, the cost of the project escalated coincident with the global recession of 2009, a time that the City of Detroit and DWSD were experiencing unprecedented economic distress. In June 2009, DWSD submitted a request to the state regulating agency to pursue an analysis of alternative, less costly CSO control options for the Upper Rouge River outfalls. This request was subsequently granted to DWSD and the result of the analysis was a recommendation to construct seven “First Flush” Capture Basins for CSOs with In-Pipe Disinfection and Disposable Nets to treat the remaining overflow volumes associated with peak flow rates up to 10-year, 1-hour recurrence interval.First flush capture with in-pipe disinfection and netting of overflows is an unproven approach to controlling CSO's and achieving State of Michigan outfall standards. Disinfection of the CSO discharge will be achieved by dosing at an upstream in-pipe location(s) to allow the disinfectant contact time with the combined sewer flow to achieve an adequate kill of harmful organisms before overflow occurs to the river. The permitting agency approved this approach as a demonstration pilot facility to determine the effectiveness of these methods. This paper focuses on the challenges involved in assessing flow rates and correctly dosing and monitoring disinfection of highly variable sewer flows.Due to concerns with locating chemical storage facilities within the dense residential neighborhood surrounding the in-pipe application points, the chemical storage and pumping equipment will be centrally located at the first flush capture tank site. From this site, NaOCl will be pumped to the application points in the sewer system.CSO facilities operation is much different than wastewater treatment plant operations as the facility is operated intermittently and at higher flow rates. Due to the dilute nature of CSO flow, disinfection of CSO can be achieved with less contact time than wastewater treatment with various studies throughout the country suggesting that 5 minutes or less is adequate. A target contact time of five minutes was chosen for this facility.Due to the layout of the existing collection system in the area, a minimum of two dosing points were required to provide adequate detention time. The dosing points are located along two large sewers: one dosing point is 2100 LF upstream of the outfall on a 4′-0″ sewer and the other is 2700 LF upstream along a 6′-6″ cylindrical sewer. Each dosing location was determined by computing 5 minutes of travel time to the outfall through each leg of the sewer using the time of travel for the 10 year, 1-hour storm peak flow rate.