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Combined Sewer Overflows (CSOs)

Background

In July 2001 - after a 3-year process of study and citizen input - the federal EPA and state EPD authorized the City to implement its plan to eliminate water quality violations from combined sewer overflows (CSOs). The City's plan involved a combination of tunnels and separation of selected sewer areas. The City submitted a refined plan to EPA and EPD that would increase the water quality benefits of proposed portions of the plan and reduce the lengths of the proposed CSO tunnels. On January 22, 2003 the EPA and EPD authorized the City to implement the revised plan.

Under the Refined CSO Remediation Plan, the City will:

• Separate the Greensferry and McDaniel CSO Basins and the Stockade Sub-basin (Custer CSO Basin). This plan would increase the city's total separated area from 85% to about 90%, and eliminate two CSO facilities.
  
• Construct a deep-rock tunnel storage and treatment system to capture and store combined stormwater and sewage flow for conveyance to two new CSO treatment facilities for near-secondary treatment before discharge to the Chattahoochee or South Rivers.
  
• The number of overflows will be reduced from 60+ per year at 6 existing facilities to an average of only 4 per year at the 4 remaining facilities. The remaining overflows will be screened, disinfected and dechlorinated before discharge to a receiving stream, and will meet water quality standards.

Storage and Treatment System

The storage and treatment system involves capturing and storing combined sewer overflows. The overflows are stored in large underground excavated tunnels in bedrock similar to the rock of Stone Mountain. When the rainfall is over, the captured CSO volume is conveyed to a separate treatment system for removal of pollutants and chlorine disinfection before discharge to receiving waters.

The West tunnel will store and convey overflows from the west area of Atlanta to a new dedicated treatment facility located on the site of the decommissioned steam plant next to the existing R.M. Clayton WRC. At this facility, the combined sewage and stormwater will undergo far more advanced treatment than that provided at existing facilities before being discharged into receiving waters. The advanced treatment includes both physical and chemical processes that remove more suspended solids and metals, filtration and disinfection by chlorination that destroys virus and bacteria to protect public health.

The East CSO tunnel, which is an extension of an existing tunnel, will convey flows from the east area of Atlanta to the Intrenchment Creek CSO facility for treatment before discharge to Intrenchment Creek in Dekalb County. The Intrenchment Creek CSO facility will be upgraded to provide enhanced treatment.

As parts of the combined sewer system are separated, this same system can be used to treat stormwater runoff from the urban core portion of the CSO area, which includes the central part of Atlanta, encompassing downtown, Midtown (near Piedmont Park), the Georgia Tech and Georgia Dome areas, and parts of east Atlanta near Grant Park. Stormwater is a significant source of pollution in our streams and rivers.