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Comprehensive Sanitary Sewer Plan City of Arden Hills, MN WSB Project No. 3455-20 Page 17 I/I Analysis Arden Hills’ sanitary sewer system currently consists of approximately 49 miles of gravity main, 14 lift stations, and 3.6 miles of forcemain. Approximately 26 percent of the gravity mains have been lined in the last fifteen years, primarily in the southern half of the system. Approximately 38 percent of the residential housing in the City was constructed before 1970. The only I/I evaluation that has been done for pre-1970 era private services is the televising of their wye connection during the trunk main televising listed in Table 11 in the next section. I/I reduction activities are discussed in greater detail in the next section. The amount of clearwater flow generated within the City was estimated by calculating the average annual and peak month I/I rates, equal to the average wastewater flow minus the base wastewater flow, using data from 2012-2016. The average flow, both annual and monthly, was calculated from MCES meter data. The peak month flow was determined for each year from 2012-2016, and then those peak month flows were averaged to give the value listed in Table 10. The base flow was calculated as the difference between average dry weather flow and groundwater infiltration, based on hourly meter data from a period of nine days of dry weather (three days since a rain event) and high groundwater level (spring) in March 2017. The groundwater infiltration rate was calculated as the average flow from 1:00-6:00 AM over the same nine-day period. Table 10. Estimated I/I Rate Metershed M051 M052 M054 M059 Total Average Flow (MGD) 0.345 0.029 0.192 0.261 0.826 Peak Month Flow (MGD) 0.498 0.039 0.278 0.318 1.134 Base Flow (MGD) 0.142 0.015 0.078 0.108 0.344 Average Annual I/I Rate (MGD (%)) 0.202 (59%) 0.014 (48%) 0.114 (59%) 0.153 (58%) 0.482 (58%) Peak Month I/I Rate (MGD (%)) 0.356 (71%) 0.024 (62%) 0.201 (72%) 0.210 (66%) 0.790 (70%) There are approximately 11 miles of vitrified clay pipe (VCP) in the City that have not yet been lined, located primarily in the residential areas east of Lake Johanna and west of Valentine Lake. These pipe segments are more susceptible to infiltration and are likely the most significant public I/I source. All of these pipes are planned to be lined over the next five years. Private sources, such as leaking service laterals or unidentified, illicit sump pump or foundation drain connections, likely contribute a minimal portion of the remaining I/I. The City televises the wye connection of service laterals in conjunction with its sewer lining and rehabilitation projects in an effort to identify private I/I sources, however there may be faults in private systems that are not captured at the time of televising. The City may consider conducting flow monitoring immediately before and after its sewer lining projects to quantify the impact of the lining and to identify private I/I sources.