CAN DRY WEATHER INFILTRATION AND INFLOW BE COST EFFECTIVELY REMOVED FROM A COMBINED SEWER SYSTEM?

Flow Monitoring and Infiltration and Inflow (I/I) evaluations in urban combined sewer systems have historically focused on determining the quantity of rainfall induced infiltration and inflow (RDI/I). Less common are large-scale efforts to quantify the constituent components of dry weather flows. Dry weather evaluations have unique characteristics and present different challenges than those present in wet weather studies.“Unaccounted for” flows that enter the sewer system through leaking joints, cracks and brakes, or porous walls has been estimated to be 30 percent of incoming flows to the City of Detroit WWTP during dry weather conditions. This estimate has led to considerable discussions between the Detroit Water and Sewerage Department (DWSD) and the wholesale customers over sewer rates. Therefore, DWSD has embarked on a significant evaluation to locate, quantify and develop cost effective measures for elimination of excessive dry weather Infiltration and Inflow (DWI/I) from the City of Detroit's portion of the Greater Detroit Regional Sewer System (GDRSS).The result was the largest Inflow/Infiltration Study in Michigan, with almost 200 wastewater flow meters monitoring the flow at one time. A Sewer System Evaluation Survey (SSES) program followed in the areas with the highest DWI/I, which resulted in a one million dollar pilot program to demonstrate DWI/I removal techniques and measure the effects of the rehabilitation. This paper discusses some of the considerations involved in determining dry weather I/I rates for a large urban combined wastewater collection systems. The paper also reviews the pre and post rehabilitation engineering and cost-effectiveness analyses performed on field investigation and testing data - to determine if a specified amount of Dry Weather I/I (DWI/I) can be cost-effectively removed from a combined sewer system.This paper describes the key challenges of performing the DWI/I evaluation in DWSD's large urban combined sewer system, and presents the key project task methodologies utilized to meet these challenges. This paper also discusses the results to-date for this ongoing evaluation to find and correct DWI/I sources.