FLOW MONITORING DATA ANALYSIS

Excessive wet weather sanitary sewer flows are a primary reason for recurring permit violations at wastewater treatment plants throughout the country. Accurate flow monitoring and rainfall measurement can help communities to quantify wet weather flows and pinpoint the location of infiltration and inflow problems.Effective strategies for organizing and utilizing the data gathered from these flow monitoring projects are essential. Equally important is the subject of what to do in areas where data is either incomplete or missing. Sufficient “up-front” knowledge of the sanitary sewer system, including sanitary pumping stations, is required for proper flow monitor placement and flow analysis.The first step in quantifying and determining the general location of infiltration and inflow is to develop an Infiltration and Inflow (I/I) Study. The I/I Study should include information on sanitary sewer flow monitoring, proper rain gauge placement and rainfall measurement, and accurate quantification and subdivision of extraneous flows into infiltration and inflow. The I/I Study should also exhibit comparisons of the relative magnitude of infiltration and inflow problems in different areas of the sanitary sewer system. Lastly, recommendations regarding where to look for physical infiltration and inflow sources should be included.Hydrographs are especially useful in analyzing dry weather and wet weather flows. Storm hydrographs should be subdivided between wet and dry weather flows and further subdivided between infiltration and inflow. Wet weather hydrographs must be converted to design storm hydrographs by extrapolating inflow values by the ratio of design storm rainfall to measured rainfall. Design storm hydrographs provide an “apples-to-apples” criteria for comparing the effects of various rainstorms on the sanitary sewer system.One common problem associated with flow monitoring data analysis is the fact that many wastewater treatment plants do not have sufficient influent flow metering capacity to accurately measure significant wet weather flow events. This was the case for a small community in eastern Ohio. Their plant's influent flow meter had a capacity of 6 mgd, but peak wet weather flows were in excess of 16 mgd. In order to accurately measure the wet weather flows, monitors were installed at the downstream ends of each of the six sewers that were tributary to the treatment plant. Hydrographs for each of these six monitoring sites were then added together to provide an accurate representation of the total flow that was received at the plant.Once infiltration and inflow have been quantified and assigned to various portions of the collection system, a Sanitary Sewer Evaluation Survey (SSES) can be performed to pinpoint the precise locations and causes of infiltration and inflow. Different methods of performing SSES field investigations include smoke testing, sanitary sewer televising, dyed-water downspout testing, manhole inspections and footer drain/sump pump investigations. The SSES provides calculations showing the total amount of achievable infiltration and inflow removal and the portion of the total infiltration and inflow that would be cost-effective to remove. Cost-effective infiltration and inflow removal is based upon anticipated flow removal volumes associated with the disconnection of catch basins, downspouts, footer drains, yard drains, etc. from the sanitary sewer system and the approximate cost of disconnecting these I/I sources. Through a detailed cost-effectiveness analysis, a breakpoint can be determined beyond which it is more economical to leave certain I/I sources connected to the sewer system, conveying their infiltration and inflow to the treatment plant for equalization storage and subsequent treatment. Recommendations are given in the SSES pertaining to those infiltration and inflow sources that are cost effective to disconnect from the sanitary sewer system. Cost curves developed for cost-effective I/I reduction and the associated design storm hydrograph showing anticipated system-wide reductions in infiltration and inflow are attached to this abstract.In summary, effective analysis of flow monitoring data will provide a cost-effective approach to removing excessive infiltration and inflow from the sanitary sewer system as well as identifying post-rehabilitation design flows at various points in the system and at the wastewater treatment plant.