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Since 1994 the Miami-Dade Water and Sewer Department (WASD) has been engaged in an extensive sanitary sewer system evaluation and rehabilitation program in an effort to reduce system infiltration, exfiltration and inflow (I/E/I). Under the I/E/I Program, the entire sanitary sewer collection system, which represents approximately 3,930 km (12.9 million feet) of gravity sewer lines and 58,000 manholes was evaluated by July 1997. The Program sewer evaluation consisted of cleaning and televising 100 percent of the gravity lines, the visual inspection of each manhole and the smoke testing of the entire system to identify defects. A total of 32,194 defects were identified and repaired.

The I/E/I Program has been highly successful with system flows to the regional treatment facilities reduced by approximately 100 mgd. Although the system-wide infiltration was greatly reduced, Rainfall Dependent Infiltration / Inflow (RDII) and the various pump station and force main improvements have continued to increase the peak flows to the treatment facilities during heavy rainfall events. Since the mainline sewer components were fully investigated and laterals from the main sewer to the house (house lateral) have only been evaluated in close proximity to the mainline sewer, the sewer house laterals have been identified as the only system component not fully investigated.

The house laterals above the normal water table were, therefore, considered the source of the RDII. In 1999, WASD developed an Initial Lateral Pilot Program to determine if house laterals could be the cause of the RDII and to quantify the effectiveness and cost of a lateral evaluation and repair program.

Three collection basins were selected for the initial pilot program. Although all main sewers were repaired under the I/E/I Program, all basin main sewers were re-televised and smoke tested and all manholes were again visually inspected for defects. The identified repairs were completed and the manholes were sealed to reduce system inflow. After a significant rain event, rainfall dependent infiltration / inflow hydrograph signatures were obtained for the “before” baseline flow. Each basin lateral, both public and private sides, were then pressure tested for leaks and the defects were identified and repaired. Figures 1, 2 and 3 illustrate the effectiveness of the program where the RDII signature is reduced after repairs are made.

Document Type: Research Article


Publication date: January 1, 2005

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