Evaluating the Wet Weather Performance of the Sewer Collection System in Minneapolis after Combined Sewer Separation

The Metropolitan Council Environmental Services (MCES) and the City of Minneapolis are co-permittees of a National Pollution Discharge Elimination System (NPDES) Permit for a Combined Sewer Overflow (CSO) system located in the City of Minneapolis. For the past 20 years, the City has performed a series of sewer separation projects aimed a reducing the number of CSOs to area streams and the Mississippi River. Despite the expenditure of hundreds of millions of dollars, overflows persist at specific locations in the sewer system. This project was devised to gain a better understanding of where and how much flow enters the system, of the hydraulic conditions contributing to the overflows, and of modifications to the sewer system or additional inflow removal projects that could further reduce overflows.Flow monitoring data obtained from prior efforts to determine these locations is no longer current – City sewer separation projects and the growth of the suburban communities have rendered this data obsolete. New data would allow identification of areas with extraneous water sources, and permit quantification of the extent their disconnection from the system would reduce overflows to the Mississippi River. The new insights into the collection system hydraulics gained from this data will allow evaluation of structures within the system and to what extent these structures can be modified to improve the system hydraulics and reduce overflows.This paper presents the techniques used to gather appropriate data regarding the system operation and analyze the data to determine a prioritized set of steps for further reducing system overflows. Specific attention is paid to the difficulty of meter installation and the value of the data gathered as a result. The techniques used to analyze the data are presented, including field verification of some model-based conclusions.