LESSONS LEARNED ON A PILOT I/I REMOVAL PROJECT

The Skyway Water and Sewer District (WSD) Basin 6 Sewer Rehabilitation Project was selected as one of the ten pilot projects funded by King County to determine the effectiveness of different infiltration and inflow (I/I) removal options as part of the King County Regional I/I Control Program. As identified in previous I/I field studies conducted by King County and Brown and Caldwell and verified by the current King County program, the Skyway WSD sanitary sewer collection system contributes significant quantity of I/I to the King County interceptor and wastewater treatment system. Of the ten basins that make up the Skyway WSD system, Basin 6 contributed the highest peak flow per acre of all basins draining to the King County system. The previous study indicated that most of the I/I from Basin 6 resulted from defects in the side sewer laterals located on private property and connected to the Skyway WSD's sewer mains.Basin 6 includes 170 side sewer connections spread over 48.4 acres and was sewered with approximately 9,600 ft of 6-inch and 8-inch diameter concrete sewer mains and 39 manholes. The sewer mains and most of the side sewers had been constructed in the 1940s and early 1950s. The pilot project for Basin 6 included the replacement of all manholes in the basin and the replacement of sewer mains and side sewers using the pipe bursting technology. The design of the replacement project was completed in February 2003 by the King County I/I program team. The contract was awarded to the pipe bursting contractor, Buno Construction, in April 2003, and Buno began construction on May 8, 2003. Installation of the replacement pipelines was completed by September 30, 2003.There is minimal consistent data for both the cost and effectiveness of flow reduction associated with I/I removal and the institutional issues related to working on private property. The King County program was developed to fill the information gaps, provide good data on flow reduction, and provide solid information on different construction technologies to determine what works. The project was also intended to develop good baseline cost data for I/I removal. To accomplish this, King County provided public funding for ten pilot projects.The Skyway WSD Basin 6 was chosen to be one of these projects and was an excellent location for study of I/I removal because the previous studies of the system provided historical flow data, smoke testing and dye testing data, and condition assessment information that gave a detailed look at the preconstruction condition of the system. Also, the basin was large enough and the Basin's preconstruction I/I contribution was large enough to provide a good look at the effectiveness and the costs of flow reduction programs.Basin 6 also provided a good pilot project because sewer replacement was driven strictly by I/I removal and not to accommodate population growth. As a result, the replacement sewers were not upsized and therefore allow for a more accurate assessment of flow reduction efficiency. The project was also intended to provide hard data on the use and performance of pipe bursting in minimizing impacts to private property. Other reasons for selecting the Basin 6 project included the willingness of the Skyway WSD to conduct work on private properties and to deal with all the issues that working with homeowners requires, as well as the District's willingness to provide extensive community outreach to eliminate the “surprise” of construction on private propertyThe project had many challenges, including:Designing a replacement project without reliable data on the locations of the facilities being replaced and other utility information.Coordination design and sharing responsibility among King County, Skyway WSD, and multiple consulting firms.Maintenance of good customer relations during the disruptive construction period.Using the pipe bursting construction method, a relatively new rehabilitation technology.Following construction of the Skyway pilot project and after a season of flow monitoring to measure the post-construction flows, preliminary data indicates that the Skyway project was successful in reducing I/I flows into the Skyway and King County conveyance systems. The project reduced peak storm-related flows from the basin by 83 to 85 percent.