Combined Sewer Overflow Facilities
GLWA operates nine of the 18 Combined Sewer Overflow (CSO) control facilities tributary to GLWA’s Regional sewer system in Wayne, Oakland and Macomb Counties. Of the nine CSO facilities, eight are leased by GLWA. The one that is not leased is the Belle Isle CSO Facility. GLWA operates this facility as prescribed in a shared services agreement. The facilities are an outgrowth of the Long Term CSO Control Plan started in 1993 to address CSO discharges from 78 outfalls along the Detroit and Rouge Rivers.
GLWA adopted a four-part strategy to address CSO:
CSO control is needed because the sewer system can become overloaded during heavy rain storms. In older, large metropolitan areas like Detroit, combined sewers are used to transport both wastewater and storm water in the same pipe. During rain storms, these sewers can receive many times the volume of flow that is normally transported on a dry day. CSO control facilities capture, store and treat these excess flows during wet weather to prevent the discharge of untreated CSO to a lake or river. Newer communities have two separate sewer systems: one to handle wastewater flow and the other for storm flow.
Two approaches are used to size CSO control facilities in Michigan: presumptive and demonstrative.
With the presumptive approach, the facility is designed to conservative engineering criteria established by the Michigan Department of Environmental Quality (MDEQ). With the demonstrative approach, different technologies are used to reduce capital and operational/maintenance costs or work with site restrictions.
Once construction is completed, the facility goes through an evaluation, typically two years long, where the performance is monitored to demonstrate that the treated effluent which is being discharged is adequate to protect the public health and meet the receiving water quality standards established for the presumptive approach. Continued adherence to these requirements is then monitored by MDEQ through GLWA/DWSD’s National Pollutant Discharge Elimination System (NPDES) permit. To date, all GLWA CSO control facilities have been designed using a demonstrative approach. Of the nine facilities, six are retention treatment basins (RTBs) and three are screening and disinfection facilities.
A CSO RTB is an underground tank that temporarily stores and treats combined sewage that previously was discharged through outfalls during storms. Flows diverted to the RTB are screened and treated with a disinfectant and discharged to the river if RTB storage capacity is exceeded. Materials removed by the screens are sent to the wastewater treatment plant (WWTP) for disposal. The stored flows are sent to the WWTP after the storm has subsided and capacity is available in the sewer system. Many times the flows are small enough to be completely captured and stored in the RTB.
Some RTBs have a first-flush compartment used to store flow with the highest level of pollutants from the first part of the storm. These pollutants include organic material, oil, sediment, salt and lawn chemicals that are picked up by the storm water as it runs off roads and lawns. Flows from this compartment are always stored and sent to the WWTP when the RTB is emptied.
A CSO screening and disinfection facility treats combined sewage without ever storing it. Called flow-through facilities, they use fine screens to remove solids and sanitary trash from the combined sewage. Flows are injected with a disinfectant to kill bacteria before it is discharged to the river. Materials removed by the screens are sent to the WWTP for disposal.
In addition to its CSO treatment facilities, GLWA maintains 13 in-system storage devices throughout central Detroit and seven in-system gates throughout the west side of Detroit to maximize the storage capacity of sewers during storms. The in-system storage devices are rubber, inflatable dams located inside large trunk sewers. The in-system gates are mechanical gates located inside outfall sewers. These devices are designed to temporarily retain flows in the sewer system during storm events up to a certain level before discharge to the river occurs. These devices operate automatically but are monitored by GLWA staff. These staff members coordinate and apply operational protocols prior to storm events to dewater the wastewater collection system and treatment facilities to maximize the available in-system storage capacity.