Infiltration/Inflow (I/I) Analysis


The infiltration/inflow analysis (I/I Analysis) may be included in other sewer system projects(such as SSES, Master Plan, Hydraulic Modeling, etc) but is usually the first step in evaluating the collection system. Installation of wastewater flow meters and rain gauges establishes the “heartbeat” of the collection system. Dry weather flows provide baseline data and capacity being utilized during dry weather in addition to evaluating groundwater infiltration that may enter the system. Rainfall creates the stress test and measures the leakiness of mainline pipes, manholes and private sector service lines. Using GIS rainfall distribution techniques, including NEXRAD radar images, each rainfall event can be analyzed to determine its impact on the collection system hydraulics. By establishing the relationships between dry and wet weather flows, the collection system can be effectively evaluated and areas prioritized based on capacity being exceeded, observed bottlenecks, volume of inflow, etc. The data from an I/I Analysis may be used to calibrate collection systemhydraulic models, project design storm flows, determine the current level of protection from SSO’s, establish customer city contributions, etc. Of prime importance will be establishing the priority ranking of each metered service area based on infiltration/inflow. The ranking should be used to initiate a phased evaluation to locate the sources of extraneous flows and establish least cost repair methods and costs.