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West Area Combined Sewer Overflow Tunnel Storage and Treatment Projects


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 quality standards.

• 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 quality standards.

What Property Owners Might Experience During Tunnel Construction

Pre-construction Survey:

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 surrounding communities.

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 influence.”

• 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.

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