RDII Investigation and Quantification for Philadelphia's Wastewater Collection System

The Philadelphia Water Department (PWD) has developed a comprehensive Combined Sewer Overflow (CSO) Compliance Program in an effort to improve Philadelphia's water environment and consequently, meet NPDES permit conditions. A component of the CSO Program Long- Term Control Plan (LTCP) includes infiltration and inflow (I/I) investigation to assess CSO impacts. The PWD's I/I evaluation program includes a precipitation and flow monitoring data collection and quality assurance and quality control (QA/QC) process and the groundwater infiltration (GWI) and rainfall dependent inflow and infiltration (RDII) analysis. Camp Dresser & McKee (CDM) has combined a series of RDII-related programs designed to process a complete set of flow monitoring data into RDII Utilities. This compilation of processes includes flow and rainfall data preparation, dry-weather flow evaluations, wet-weather flow characterizations, and unit hydrograph curve fitting. Rainfall and sewershed characteristics data were used with existing hydrologic and hydraulic models to evaluate the wet-weather flows from the combined sewer system (CSS), while rainfall and flow monitoring data collection and RDII analysis were used to evaluate wet-weather flows to the interceptor from the separate sewer areas. Both the RDII and the GWI estimation methods used in the PWD CSO Program were well suited for the model-based estimations of the long-term mean overflow frequency, duration, and volume characteristics required to satisfy PWD's CSO NPDES reporting responsibilities. Identification, quantification, and reduction of RDII in separate sewer areas increase available capacity for combined wastewater in the collection system. This, in turn, reduces CSO discharges and enables greater pollutant capture in the CSS. Furthermore, reduced I/I decreases operating costs associated with pumping and treating these flows.