The Hubbell-Southfield Combined Sewer Overflow (CSO) Basin, built by the Detroit Water and Sewerage Department, received the 2000 Outstanding Civil Engineering Achievement Award from the Southeastern Michigan Branch of the American Society of Civil Engineers.
The Hubbell-Southfield basin is part of the Rouge River National Wet Weather Demonstration Program (RRNWWDP), a nationally funded program established to clean up CSO and other sources of pollution in the Rouge River. It is one of the largest and most complex CSO basins in the state and was constructed at a cost of $50 million. The 22 million gallon facility provides treatment for the largest CSO on the Rouge River through three different operational modes.
"Providing treatment to discharges from the Hubbell-Southfield location will go a long way to help restore the Rouge River. DWSD's commitment is to control and eliminate untreated CSOs," said Gary Fujita, DWSD Assistant Director of Wastewater Operations.
Despite its size, the basin represents the smallest design storm capture of the nine RRNWWDP detention basins. Surrounded by a championship golf course, wetlands and the Rouge River, it was designed as large as the site would allow. Special design considerations were included to satisfy the Michigan Department of Environmental Quality (MDEQ) criteria for sufficient treatment: 18-minute detention for the one-year, one-hour storm providing settling, skimming and disinfection.
A dual-purpose shunt channel was also designed to enhance the basin's effectiveness. Because the basin was constructed within the CSO outfall channel to the Rouge River, the shunt was originally built to maintain CSO flow to the river during construction. Once complete, the shunt channel became an integral part of basin operations.
The shunt protects the basin from blowing out when capacity is maximized and can be used to divert inflow around the two basin compartments to prevent negative treatment. Flow receives screening and disinfection in the shunt channel prior to discharge to the river. Negative treatment occurs when flow through the basin suspends previously settled solids and causes the effluent to be more polluted than the influent.
Another innovative basin feature is its decant Capabilities. Should rain events occur closely together and the Detroit Wastewater Treatment Plant be unable to accept all flow, decanting provides another operational option. It enables the top layer of captured flow to be released into the Rouge River after disinfection, providing significant settling time for treatment of flow from subsequent rain events.
"It was the partnering process that allowed DWSD to design and construct this facility with its shunt channel and decanting abilities," Fujita said. "MDEQ staff were involved from the project definition stage through the final stage."
During a two-year testing and evaluation period, the effectiveness of each operating mode will be measured. Test results will be used to determine if a smaller-sized basin with innovative design additions, such as the Hubbell-Southfield, can be as effective as larger basins. Regulatory agencies can then use this data to support and approve smaller, more cost-effective basin requirements for CSO treatment in other locations.
The Hubbell-Southfield Basin is part of the City of Detroit's Long Term CSO Control Plan to reduce CSO overflows from 78 outfalls along the Detroit and Rouge Rivers. Construction of end-of-pipe treatment facilities like the basin is just one of the strategies being used by DWSD. In-system storage devices are being built and a rain water control program is being evaluated.
