Opportunity
In 1993, the city of Atlanta launched an aggressive ten-year capital improvement program, spending more than $1 billion on improvements to the city’s drinking-water and wastewater systems.
Among the city’s challenges was the elimination of combined sewer overflows (CSOs) to the environmentally sensitive Chattahoochee River and its tributaries. The Chattahoochee originates in northeast Georgia and runs for most of the length of the state, forming the southern half of Georgia’s border with Alabama. Because Atlanta is built on the crest of a ridge rather than in the Chattahoochee floodplain, the river retains much of its scenic character as it passes through the city. Therefore, maintenance of the river’s water quality is a paramount objective of the City and state.
Solution
In order to eliminate CSOs and protect the river, the city decided to build a water storage tunnel eight and a half miles long, and a pumping station that would convey the stored stormwater to a new CSO stormwater treatment plant. The plant would remove pollutants from the stormwater and disinfect it with UV light before discharging it into the Chattahoochee.
Working in a joint venture, Mott MacDonald was selected to design the storage tunnel and pumping station portion of the project. Mott MacDonald was assigned the process mechanical, electrical, and structural designs of the pumping station, and the storage tunnel design and construction management.
Storage tunnel
The West Area CSO Storage Tunnel is 8.5 miles long, with a 24-foot finished diameter. Constructed in bedrock about 170 feet deep, the tunnel can store up to 177 million gallons of overflow after a rainstorm. The storage tunnel captures and stores CSOs from the Clear Creek, Tanyard, and North Avenue CSO Basins. These facilities prevent any overflows from occuring during a typical rainfall year.
Pumping station
The R.M. Clayton Pumping Station is connected to the end of the storage tunnel at the site of the R.M. Clayton Wastewater Treatment Plant. It is designed to pump up to 100 million gallons per day (MGD) to the new CSO treatment plant. This allows the storage tunnel to be emptied within two days after a rainstorm, even if it has been completely filled.
The pumping station is a circular structure 66 feet in diameter and approximately 200 feet in depth. Two submersible dewatering pumps with a capacity of 4.3 MGD keep it empty of groundwater that may infiltrate the storage tunnel.
Under normal CSO conditions, the wet well level in the pumping station may rise as much as 66 feet above the empty level. If the storage tunnel becomes overfilled, the level may rise as much as 136 feet. During a storm, the CSO pumps start consecutively and ramp up in speed until a maximum of 10 pumps are operating, pumping approximately 100 MGD. The pumps are equipped with adjustable frequency drives, and their speed is controlled by a PLC-based system that responds to the wet-well level.
The inlet area of the pumping station includes a grit sump to collect grit and debris before it enters the pump chamber. The design allows grit to be removed after major rain events and discharged into roll-off containers for offsite disposal.
Outcome
In October 2010, Atlanta Mayor Kasim Reed presented a blueprint for making Atlanta one of the top ten US cities for sustainability. Atlanta, he said, should be a national leader in energy efficiency, water conservation, and the preservation of green space.
Atlanta’s CSO storage tunnel and pumping station have already made an important contribution toward that goal. The city’s Clean Water Atlanta program has said it plans to expand the original ten-year capital improvement program to “comply with federal consent decrees and a state consent order, improve water quality throughout metro Atlanta, secure drinking water facilities in accordance with national Homeland Security guidelines and provide holistic management of the City’s resource operation.”