Assessing the Capacity of a Small Community’s Sanitary Sewer System
In 2010, the U.S. Environmental Protection Agency (EPA) notified the City of Oak Ridge, Tennessee (City), that it must upgrade its sanitary sewer system to eliminate sewer overflows. The notice was a “tall order” indeed. The major components of the Administrative Order were to:
- Document previous sanitary sewer overflows;
- Prepare a sewer overflow response plan, or SORP;
- Implement a management, operations and maintenance program, or MOM;
- Complete a system evaluation and rehabilitation plan, or SERP; and (5) implement the rehabilitation plan.
A key element of the overall program, included in the SERP, was the completion of a system capacity assessment, or CA. The objective of the CA was to evaluate the hydraulic limitations of the existing system that cause surcharging and overflows, and to identify what remediation measures are required to eliminate overflows under future design conditions. The CA required an evaluation of 1.3 million feet of sanitary sewer and almost seven thousand manholes.
The sewers and manholes were surveyed, mapped in GIS, and inspected. Rain gauges were installed at multiple locations within the sewer basin, and flow meters were installed at more than 50 locations (on a rotating basis). The rainfall and flow data were critical to determining normal “dry-weather” flows, and the extent to which rainfall runoff and groundwater make their way into the sewers through cracks, leaky joints, and other entry points.
Using the data gathered in the surveys, inspections, and monitoring, computer models of the major sewer system were developed. The models were used to identify existing “bottlenecks” in the system, to evaluate alternatives and help select the best remediation plan. The resulting plan consists of upsizing approximately 6,000 feet of sewer, constructing 2,000 feet of new sewer, rehabilitating selected manholes and sewers, modifying two force mains and constructing three flow equalization basins. The resulting plan was approved by EPA in 2012, and design and construction are presently underway. Completion of the program is scheduled for 2015.
The assessment team consisted of the staffs of the City of Oak Ridge and Lamar Dunn and Associates, Inc. (LDA Engineering). The success of the assessment can be largely attributed to the close collaboration between all members of the team throughout the process, including City leadership and administration, program management, system operators, engineers, field inspectors, GIS/CAD personnel, surveyors, modelers, and data managers.
In addition to close coordination of the team, some of the other items critical to success were accurate surveys and mapping, careful selection of flow meter locations and a good flow meter deployment tracking system, and a robust data quality review and management system.