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COMPREHENSIVE SEWER PLAN Infiltration and Inflow (1/1) September 15, 2020 o.0 INFILTRATION AND INFLOW (1/1) 5.1 GENERAL The Metropolitan Council instituted an Inflow and Infiltration (1/1) Surcharge Program (IISP) in 2006. The fundamental policy statement summarizing this program is that the Metropolitan Council "will not provide additional capacity within its interceptor system to serve excessive inflow and infiltration." The Council establishes inflow and infiltration thresholds for each of the communities that connect to its system. Communities that exceed the thresholds are required to eliminate the excess flow within a reasonable timeframe or pay a surcharge fee. 5.1.1 SOURCES AND EXTENT OF 1/1 Mounds View is a second -tier suburb, commencing installation of public sewer in the late 1960s. Approximately 90% of sanitary sewers in the City were constructed 30-50 years ago, with the remainder constructed more recently. Its system has been constructed to modern standards using iron, concrete, and plastic materials. The system is in good to very good condition overall with minimal backups. Currently, Mounds View generates average annual wastewater flow of approximately 90 gallons of wastewater per capita per day (gcd). The extent of 1/1 is not excessive, with no IISP exceedance events in the past ten years. The EPA Guide for Estimating Infiltration and Inflow (June 2014) was used to estimate the proportion of 1/1 contribution in the City's wastewater system. Monthly flow data were obtained from Metropolitan Council Environmental Services, for the period from 2009 to 2019. 2012 to 2016 monthly data were used to obtain monthly average flows for the five-year period from March to November (representative of a wet portion of the year) and December to February (representative of a dry portion of the year). It was determined that the wet monthly average flow (March -November) was 32.08 mg, and that the dry monthly average flow (December -February) was 31.59 mg. Thus, on average, 1/1 contributes roughly 0.49 mg monthly (roughly 2% of base flows). The peak monthly flow for the City of Mounds View is 38.60 mg in June 2014, which is roughly 22% above base flows. Potential sources of 1/1 could include • Non -compliant residential sump pump connections to the sanitary sewer • Leaky pipes and structures under groundwater conditions • Service line leaks • Inflow from floor drains of flooded structures • Private sanitary sewer systems No recent, detailed, citywide analysis of 1/1 has been completed. The City will perform specific evaluations in the future of areas with excessive 1/1 on an as -needed basis. B v:\1938\active\193804167\reports\2018 csp\submitted to city 9-8-2020\moundsviewcsp_formcesreview_09152020_updateddraft.docx 5.9