City of Atlanta — East Area CSO Custer Avenue CSO Storage and Dechlorination Facility Background The City of Atlanta constructed its first CSO treatment facilities in the East Area Combined Sewer Basin in the mid-1980s
to address water quality problems in the South River, a tributary of theOcmulgee River and Lake Jackson. These facilities consisted of: The Custer Avenue CSO Diversion Facility The 26 foot diameter, 2 miles long
Intrenchment Creek Deep Rock Storage and Conveyance Tunnel The Intrenchment Creek CSO Treatment Facility The McDaniel Street CSO Storage Facility These facilities were a key component
that led to the achievement of water quality standards in the South River. Atlanta's CSO Control Program continued to develop in the early 1990s with construction of CSO Treatment facilities in the West Area Combined Sewer Area at the Clear Creek, Tanyard Creek, Greensferry and North Avenue
overflows. These CSO Control Facilities are located at the headwaters of small, urban streams that are tributary to the Chattahoochee River. Upon completion of construction of these facilities, Atlanta was in compliance with EPA's National CSO Control Policy. As State of Georgia and federal
water quality regulations subsequently changed, the City was required to continue its aggressive approach to mitigation of its CSOs. In 2002, theCity was mandatedunder a federal consent order to implement additional long-term planning and CSO controls throughout the entire combined sewer areas.
The Authorized Plan ( refer to Figure No. 1) included: The West Area CSO Storage and Transport Tunnel The new West Area CSO Treatment Facility The East Area CSO
Storage and Transport Tunnel Modifications to the Intrenchment Creek CSO Treatment Facility Partial sewer separation in each of the combined sewer basins. In the summer of 2002,
Mayor Shirley Franklin initiated a Clean Water Advisory Panel to review the plan before design and construction proceeded on these major facilities. This panel recommended sewer separation in three specific combined sewer basins instead of the proposed partial sewer separation in all of the
combined sewer basins also recommended was the modification of the storage capacity for the two tunnels. Two of the three areas to be separated were in the East Area — McDaniel and Stockade, resulting in a reduction of the length of the East Area tunnel to one mile. Before final design
of this facility began, the City's Department of Watershed Management conducted a Value Engineering (VE) Study of the CSO Control Plan. Recommendations from the VE Study included replacement of the East Area CSO Storage Tunnel with an above ground CSO storage tank and modifications to the
proposed CSO Treatment Facilities which resulted in estimated cost savings of approximately 150 million The East Area CSO The Clear Streams Objective JV (CSOJV) comprised of Parsons Brinckerhoff, Brown and Caldwell and Williams Russell and Johnson began the design of this storage
facility in the fall of 2003. During preliminary design, a review of operational hydraulics, available property, protection of the mature, stately trees in the area, impacts of construction and future operation to the area residents and cost, led to the consideration of options to provide
the required additional CSO storage in an underground configuration. These options included shallow wide shaft, deep narrow shaft andunderground cavern. The selected option was the underground cavern in the form of an underground linear storage facility (ULSF) with a capacity of ten (10) million
gallons to be excavated into the granite rock. This ULSF is connected to the existing tunnel and shaft and it can fill and drain by gravity. The existing Custer CSO screening facility is being modified to add new fine mesh 500 mgd rated drum screens to minimize debris and solids passing
into the tunnel. The existing tunnel has just been cleaned of large volumes of grit and the downstream pumping station redesigned to handle gritas well as stored CSO. A new dechlorination facility is included in the construction to treat the chlorinated flows coming to the Custer facility
from the Boulevard regulator upstream. There will also bework in the nearby Intrenchment Creek in the form of a new stormwater by-pass channel to carry the future separated storm water runoff from the upstream Stockade basin as well as bending weirs to control the flood levels in the creek
to ensure that the design flow is taken to the tunnel and Intrenchment Creek CSO Treatment Plant. The paper will outline all of these design features in detail, including discussion on flow requirements, screen design, rock excavation and support procedures, reduction of impact to the
neighborhood and cost comparisons as well as the operation of the CSO facilitiesin the East Area. The paper will also include construction photographs.
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