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70 Percent I/I Reduction - Sweet Home, OR
Author: Jeff Scarano; Mike Adams (City of Sweet Home, OR)
Date: 5/107
From the WEF Collection Systems Specialty Conference, May 2007

In Sweet Home, Oregon, flow monitoring and hydrologic modeling evaluations predict a 20:1 ratio of peak hour 5-year recurrence flows to average summer flows. These, and lower peaks overwhelm the capacity of the collection system and the wastewater treatment plant (WWTP), causing sanitary sewer overflows (SSOs) into the South Santiam River, a salmon-bearing water body that also serves as a drinking water source for several downstream cities. Faced with the choice of either upgrading the sewer system to reduce peak flows by 70 percent to match the existing WWTP capacity or constructing increased conveyance and WWTP capaci-ty, the City of Sweet Home (City) elected to go to the source of the problem. Over the past 5 years, the City has undertaken system-wide flow monitoring, infiltration/inflow (I/I) and hydraulic modeling, TV inspection, and condition assessment. Rehabilitation projects were then developed that incorporated the same rehabilitation strategy across each basin. To date, the City has spent approximately $4 million rehabilitating nearly 8 percent of its collection system and over 9 percent of the service laterals. A brief review of the modeling methodology employed is provided. It includes calibration of pre- and post-rehabilitation models considering various basin characteristics and hydrologic parameters, running a 50-year rainfall record through the models, and selecting a 5-year flow recurrence event through statistical analysis of the resulting long-term hydrographs. A compari-son of three separate flow monitoring technologies is also presented. In one basin, rehabilitation of publicly-owned sewer mains and privately-owned service laterals (from the main to the house) removed 88 percent of the peak I/I at a cost of $0.41 per gallon per day. In several other basins, rehabilitation of either mains or laterals removed, on average, 17 percent of the peak I/I at a cost of $27 per gallon per day.