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. Faced with estimated “unaccounted for” flows comprising approximately 30 percent of incoming flows to the City of Detroit WWTP
during dry weather conditions, leading to disputes with wholesale customers over sewer rates, the Detroit Water and Sewerage Department (DWSD) 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 portion of the Greater Detroit Regional Sewer System (GDRSS). This major undertaking was broken down into over ten total tasks. Included in these tasks were the following key subtasks: Extensive
records review complemented by targeted field investigations to delineate appropriately sized dry weather sewersheds (meter districts), select suitable flow monitoring sites, and prepare sites for temporary flow monitor installation. Cleaning of selected
sewers and manholes to remove sediment and debris from key meter sites in advance of flow monitoring. Installation, calibration, confirmation, maintenance and data download of ten (10)-minute interval depth and velocity reads for 193 meters installed
and concurrently operational for a 45-day flow monitoring period. Compiling population, employment, Significant Industrial User (SIU), and water consumption data for the entire study area, and employing Geographical Information System (GIS) tools to allocate
the various parameters to the project-specific meter districts. Compiling population, employment, Significant Industrial User (SIU), and water consumption data for the entire study area, and employing Geographical Information System (GIS) tools to allocate
the various parameters to the project-specific meter districts. Processing of over 80,000 discreet flow monitoring data points and generating final calculated flow parameters in spreadsheet and color graph format for each meter district. Screening of all flow data to remove data affected by rainfall or meter problems, leaving only DWF data. Evaluation of various calculation methodologies for determining DWI/I. Selection of a calculation methodology best suited
to determining comparatively high DWI/I, and generating DWI/I results for all meter districts. Determination of inch-diameter-miles (IDM) of sewer for all meter districts, and presenting comparative levels of DWI/I in gallons per day per IDM
for all meter districts in tabular, colored map and scatter plot formats, to facilitate comparison of DWI/I levels. Designing a field investigation program of manhole inspections, sewer evaluations, and other tests to assess system conditions in the
high I/I areas, as a basis of determining causes of I/I, potential methods of correcting I/I, and generating a cost effectiveness analysis for potential improvements. 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.
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