INFILTRATION AND INFLOW (I/I) REDUCTION IN 10 PILOT PROJECTS KING COUNTY, WASHINGTON

Infiltration and inflow (I/I) of clean water into the sanitary sewer system affects the required capacity of wastewater conveyance and treatment facilities. Faced with the high cost of adding system capacity, the Wastewater Treatment Division in King County, Washington initiated a multi-year Regional I/I Control Program. Ten “pilot projects” were conducted in selected drainage basins to demonstrate the cost effectiveness of rehabilitating the existing sewer system compared with the cost of new facilities. Pre-rehabilitation flow monitoring data were collected to target locations for the pilot rehabilitation projects, and the County collaborated with local agencies to select candidate pilot projects.Based on detailed Sanitary Sewer Evaluation Surveys (SSES), designers selected sewer mains, manholes, laterals, and side sewer components for rehabilitation, and used primarily trenchless rehabilitation techniques and technologies (cured-in-place pipe (CIPP), pipe bursting, chemical grouting of manholes, service connection liners (SCLs), and service connection and lateral liners (SCLLs) to gain first-hand experience and information about sewer system repair. Pre- and post-rehabilitation rainfall and meter flow data were utilized in a model to determine rehabilitation effectiveness. Rainfall was quantified using data from a County-wide rain gauge network and estimated using CALAMAR (calculating rainfall with the aid of radar). MOUSE modeling software was used for continuous simulation of rainfall-dependent I/I and for quantifying the I/I entering the sewer system in each basin.Rehabilitation technologies produced measurable I/I reduction in 8 of the 10 pilot projects. The results suggest that a high percentage of I/I originates on private property. Where only sewer mains or manholes were rehabilitated, IyI reduction was less. The final construction cost for the pilot projects was 7.8 million. Experience some of the pilot projects showed that rehabilitating sewer mains at the same time that side sewers and laterals are rehabilitated can be done for a relatively small increase in cost.