The tunnels are part of a storage and treatment system that 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 ultra-violet disinfection before discharge to receiving waters. Two tunnels will be constructed.
One tunnel will store and convey overflows from the west area of
Atlanta to a new dedicated treatment facility located near the Chattahoochee
River. The other will store overflows from the east area of Atlanta
and convey the flows to the existing Intrenchment Creek CSO Treatment
Facility, which will be expanded and upgraded. At each of the CSO
treatment facilities, the combined sewage and stormwater will undergo
a higher level of treatment than that provided at existing facilities
before being discharged into receiving waters. The treatment components
include both physical and chemical processes to remove suspended
solids and metals, and disinfection by chlorination that
destroys virus and bacteria to protect public health.
As parts of the combined sewer system are separated, the existing
combined system can be used to treat storm water runoff from the
separated portions in addition to the combined sewage 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. Storm water is a significant source of pollution in
our streams and rivers.