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DWSD.ORG, the official website of the Detroit Water and Sewerage Department.
DWSD Facilities

DWSD
CSO
Facilities

Baby Creek Screening and Disinfection Facility
Baby Creek Screening and Disinfection Facility


Belle Isle CSO RTB Belle Isle
CSO RTB


Conner Creek CSO RTB
Conner Creek
CSO RTB


Hubbell-Southfield CSO RTB
Hubbell-Southfield CSO RTB


Leib Screening and Disinfection Facility
Leib Screening and Disinfection Facility


Oakwood CSO RTB
Oakwood CSO RTB


Puritan-Fenkell CSO RTB
Puritan-Fenkell
CSO RTB


Seven Mile CSO
Seven Mile
CSO RTB


St. Aubin CSO
St. Aubin Screening and Disinfection Facility


Combined Sewer Overflow Facilities

DWSD operates nine of the 18 Combined Sewer Overflow (CSO) control facilities tributary to the DWSD sewer system in Wayne, Oakland, and Macomb Counties. The facilities are an outgrowth of the Detroit Long Term CSO Control Plan started in 1993 to address CSO discharges from 78 outfalls along the Detroit and Rouge Rivers.

DWSD adopted a four-part strategy to address CSO:

  Source reduction – reduce the amount of storm flow that enters the wastewater system.

  In-system storage – maximize the use of existing storage space in the sewer system during storms.

  Wastewater treatment plant expansion – expand capacity of primary treatment from 1.5 to 1.7 billion gallons per day to treat more flows during
   storms.

  End-of-pipe treatment – construct facilities to store and treat the combined sewage, preventing it from entering area waterways unless treated
   and disinfected.

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 up to three 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 to prevent basement flooding. 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 DWSD’s National Pollutant Discharge Elimination System (NPDES) permit. To date, all Detroit CSO control facilities have been designed using a demonstrative approach. Of the nine facilities owned, operated and under construction by DWSD, 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, DWSD maintains 13 in-system storage devices throughout central Detroit and 7 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 DWSD staff. DWSD 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.