“GRIT HAPPENS” – HOW CLEANING A TUNNEL REDESIGNED A PUMP STATION
Authors: Matthews, Sharon; Barnes, George; Cook, Aaron; Days, Paula; McKern, Russ
Source: Proceedings of the Water Environment Federation, WEFTEC 2005: Session 81 through Session 90 , pp. 6677-6687(11)
Publisher: Water Environment Federation
Abstract:The City of Atlanta was ahead of its time in treating combined sewer overflow (CSO). A deep tunnel system was constructed in the early 1980's to capture, store, and treat CSO from the approximate 3,900-acre Custer combined sewershed located in southeast Atlanta and DeKalb County. In its proactive efforts to protect local receiving waters and improve water quality, they were met with an unanticipated challenge of managing the excessive quantity of grit that invades the tunnel system during each overflow event.
Approximately 1.5 billion gallons of CSO volume is generated within this sewershed annually, of which approximately 67% is captured within the tunnel system. The Intrenchment Creek CSO tunnel system includes a 35-ft diameter, 100-ft deep pump station that lifts captured and stored CSO from the 26-ft diameter, two-mile Intrenchment Creek CSO Storage Tunnel at a peak capacity of 20 million gallons per day (MGD). The flow is routed to the dedicated Intrenchment Creek CSO Treatment Plant.
The original design of the tunnel, pumping station and CSO treatment facilities was based on the concept of initiating pumping from the tunnel as soon as flow entered the pumping station. Under this concept, grit and other solids entering the storage tunnel would be flushed from the upstream tunnel segments under high flow conditions and would flow to the pump station to be pumped to the static micro screens, which comprised the first stage of the treatment facilities, for grit and trash removal.
During the first five years of operation the system functioned as designed and little accumulation of grit was observed in the storage system. Subsequently, the static screen system was replaced with a vortex type grit removal system and problems were encountered in maintaining the design flow rate from the storage system through the treatment facilities. The operating mode was changed to reflect the reduced treatment capacity and sufficient velocities were not obtained in the storage system to prevent the accumulation of grit in the tunnel. Accordingly, large quantities of grit had accumulated in the tunnel and immediately upstream of the pumping station.
The grit load has impacted facility operations with grit accumulation in the downstream treatment processes. It also led to accelerated wear on pumps and other mechanical components within the treatment plant.
In 2002, the City of Atlanta was mandated under a federal consent order to implement additional long-term planning and CSO controls throughout the entire combined sewer system. In response to this initiative, additional control measures including sewer separation, additional deep tunnel storage systems, and dedicated CSO treatment facilities were the key components identified for implementation in the Authorized CSO Plan as illustrated on Figure 1.
Document Type: Research Article
Publication date: 2005-01-01
- Proceedings of the Water Environment Federation is an archive of papers published in the proceedings of the annual Water Environment Federation® Technical Exhibition and Conference (WEFTEC® ) and specialty conferences held since the year 2000. These proceedings are not peer reviewed. WEF Members: Sign in (right panel) with your IngentaConnect user name and password to receive complimentary access.
- Subscribe to this Title
- Membership Information
- About WEF Proceedings
- WEFTEC Conference Information
- ingentaconnect is not responsible for the content or availability of external websites