Creating a Roadmap for Cost-Effective Infiltration and Inflow Removal in King County, Washington

In 2007, the King County Wastewater Treatment Division began a study of methods for costeffectively reducing infiltration and inflow (I/I). Previous studies had identified needs for peakflow storage facilities to prevent sanitary sewer overflows in the County's regional conveyance system. The objective of I/I reduction was to remove enough I/I from local collection systems to allow the storage projects to be downsized, delayed or eliminated.Analyses were performed to quantify I/I in 11 sewer “mini-basins,” allocate the I/I among private facilities (laterals and side sewers) and public facilities (sewer mains and manholes), estimate peak-flow reduction from I/I rehabilitation, and compare costs to the cost of associated storage projects. A key objective was to develop a way to predict, investigate, and eliminate I/I from private property, which was believed to be the largest component of peak flows. The methodology employed in this project provides a time-saving approach for municipalities considering I/I removal.