West Area Combined Sewer Overflow Tunnel Storage and Treatment
What is a Combined Sewer System?
Combined sewers were built many years ago to carry wastewater and
stormwater from homes and businesses. Under dry conditions, wastewater
from homes and businesses flows through combined sewers to water
reclamation centers (WRCs) for treatment and eventual discharge
into a water body. When it rains, stormwater flows into the same
sewers, creating “combined flow.” The combined sewers
convey combined flow to the WRCs for treatment. To protect the WRCs
from flooding during heavy rain, regulators redirect the combined
flow to a combined sewer overflow (CSO) treatment facility. These
facilities treat the combined flows through screening and disinfection
prior to discharge into a river or stream. During larger storms,
flow can exceed the capacity of the CSO facility.
What is the Problem?
A 19-square mile area of the City of Atlanta (City), with downtown
as its center, is served by a combined sewer system. Combined sewers
were used in the late 1800s until the mid-1900s in many major metropolitan
areas and have since been discontinued.
In a combined system, a single sewer pipe carries both sewage and
stormwater to a wastewater treatment facility. When heavy rains
occur, exceeding the single pipe capacity, the flow is diverted
to one of seven CSO control facilities. When the rains exceed the
treatment capacity of the CSO control facilities, screened and disinfected
flows sometimes are discharged directly into a receiving stream.
In a separate sewer system, sewage and stormwater travel in separate
pipes, with sewage flowing to a wastewater treatment facility and
stormwater being discharged directly to receiving streams.
Atlanta is under a federal court order to bring the combined sewer
system into compliance with federal and state water quality laws
by mid-2007. The court order, or Consent Decree, requires the City
to develop a plan for achieving compliance by the deadline.
Combined Sewer System Improvement Plan
The Environmental Protection Agency (EPA) and the Georgia Environmental
Protection Division (EPD) have approved the City’s plan to
eliminate water quality violations from CSOs. This plan involves
a combination of tunnels, treatment, and separation of the combined
sewers in selected areas.
• A deep rock storage tunnel and treatment system will be
constructed to capture the combined stormwater and sewage flow for
conveyance to two new CSO treatment facility where the pollutants
will be removed before discharge to the Chattahoochee River and
Intrenchment Creek. The remaining overflows will be screened and
disinfected before discharge to area receiving streams. Approximately
100 CSOs occur annually at each overflow site. Under the approved
plan, all overflow volumes will be captured and treated, except
for approximately 4 large rain events per year, when the volume
of overflow exceeds the storage capacity. In these cases, the excess
volume will be disinfected and dechlorinated before discharge to
a receiving stream. This means that the discharge will meet water
• The Greensferry and McDaniel CSO Basins and a portion of
the Stockade sub-basin (Custer CSO Basin) willb e separated. This
will increase the City’s total separated area from 85% to
approximately 90% and eliminate CSOs in these basins, thereby eliminating
two CSO facilities and one regulator.
West Area CSO Storage Tunnel and Dedicated Treatment Plant
The new West Area CSO Storage Tunnel will be constructed deep below
the ground’s surface in bedrock (150 to 300 feet). The West
Area CSO Storage Tunnel will capture, store and convey CSO from
the Clear Creek, Tanyard and North Avenue CSO Basins. The Tunnel
will be approximately 8.5 miles long with a 24-foot finished diameter
sized to store up to 177 million gallons (MG) of overflow from a
rainstorm. When the rainfall is over, the overflow will be conveyed
to a dedicated CSO treatment plant for pollutant removal and
chlorination/dechlorination disinfection before discharge to
the Chattahoochee River.
A new 85-million gallons per day (MGD) submersible pump station
will be constructed at the end of the tunnel (at the R.M. Clayton
Water Reclamation Center [WRC]) to lift the stored flow from the
tunnel for treatment at the new dedicated CSO treatment plant. The
pump station will be sized to allow a full tunnel to be empties
within a 2-day period.
The new dedicated CSO treatment plant will be constructed on the
site of the decommissioned steam plant next to the existing R.M.
Clayton WRC. The plant will have a treatment capacity of 85 MGD.
The combined sewage stored in the tunnel system will undergo both
physical and chemical treatment prior to discharge into receiving
waters. The treatment processes will include clarification and filtration
to remove suspended solids and disinfection by UV light to destroy
viruses and bacteria to protect public health. The UV disinfection
is comparable in effectiveness to traditional chlorine disinfection
but is more protective of aquatic life. The treated overflow then
is discharged into the Chattahoochee River.
Approximately 100 CSOs occur annually at each overflow site. Under
the approved plan, all overflow volumes will be captured and treated;
except for approximately 4 large rain events per year, when the
volume of overflow exceeds the storage capacity. In these cases,
the excess volume will be disinfected and dechlorinated before discharge
to a receiving stream. This means that the discharge will meet water
What Property Owners Might Experience During Tunnel Construction
The West Area CSO Storage Tunnel project is in the final design
phase. Currently, surveyors and engineers are surveying areas around
the existing CSO facilities in preparation for the construction
of various shafts associated with the storage tunnel. These field
crews are marking the CSO tunnel route, as well as conducting rock
borings to determine subsurface conditions.
Most of the tunnel is planned for routing within public right-of-way
and should not directly affect private property. However, occasionally
inspection personnel will cross property lines briefly in order
to perform a thorough inspection or to drive stakes into the ground
or cut undergrowth to clear a path. Personnel request permission
to enter private property; and all personnel will carry and present,
upon requeset, documentation showing that they have been authorized
by the City to conduct this work.
The soil boring work involves the use of a drilling rig (about
the eize of an average-sized truck) and supporting equipment. The
drilling rig will remain set up over the borehole for several days
until drilling is completed. The hole will be capped with a steel
plug and then plugged with cement a few days later. Soil and rock
drill cuttings and excess cement will be placed in steel drums for
disposal. These drums, which will be prominently labeled as equipment
used for the Clean Water Atlanta CSO project, may sit at the curb
for several days before they are picked up by a separate crew.
Final design is expected to be completed in January 2004, and then
actual construction of the tunnel is expected to begin by July 2004.
About Tunnel Construction:
Experience has shown that deep rock tunnel construction has little
impact on above-ground structures and minimal environmental and
community impact. In contrast, construction of conventional open-cut
sewer options typically requires clear cutting large parcels of
land and crossing streets and arterials, resulting in torn up pavement
and disruption to traffic for months at a time. Open-cut construction
also has a greater environmental impact on sensitive streams and
results in tree loss along the alignment. By contrast, the tunnel
option restricts open-cut construction to the pump station, construction
shaft sites, and intake sites. This limits the amount of land required
for construction and also limits construction activity.
The West Area CSO tunnel will be excavated using a Tunnel Boring
Machine (TBM). The TBM will be lowered in sections and assembled
at the bottom of a construction shaft and will chip off sections
of bedrock through the continuous rotation of a series of steel
cutting tools (cutters) mounted on a large-diameter, full-circular,
welded steel cutter head. The machine body of a TBM, which can be
as long as 50 feet, is mounted behind the cutter head. It contains
the drive motors and other electrical, mechanical and hydraulic
equipment that provide the necessary thrust and torque that is transmitted
to the cutters through the cutter head. The TBM replaces conventional
drilling and blasting technology and allows tunnel workers to excavate
at more than twice the rate as in water tunnel construction through
drilling and blasting methods. Another important advantage of using
the TBM is that , as it bores into the rock, there is less damage
at the point of excavation and no noise at the surface to disturb
Tunnel Shaft Construction:
Some blasting will be necessary to penetrate the bedrock and allow
the shafts to be construction. While detailed information will be
provided to residents prior to the beginning of any blasting activity,
the following should be noted:
• Blasting is done because it is the most cost-effective
way to fracture rock. More advanced blasting compounds have been
developed that are much more reliable and easily controlled than
other forms of blasting materials used historically (dynamite, etc.)
• Prior to any blasting activity, pre-blast surveys will
be taken of homes and other structures within the “zone of
• Blasting experts have determined the level at which blasting
can be done to prevent damage to homes. During blasting, seismographs
measure ground vibrations to ensure this level is not exceeded.
Vibrations may be felt during blasting, but it should be noted that
everyday occurrences in our homes – such as doors slamming,
children running in the house, running up and down stairs, pounding
nails, outside temperature, wind, humidity and soil moisture changes
– produce strains greater than what would be experienced within
legal blasting limits.
If You Have Additional Questions:
Please contact the Clean Water Atlanta information line at 404-529-9211,
if you have further questions about the West Area CSO Storage Tunnel
construction or other projects under the Clean Water Atlanta Initiative.