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City of Arden Hills March 2008 o Estimate III rates and per connection contributions using hourly flow data and early morning minimum flow rates. . In general, the flow in a sanitary sewer during the early morning hours (3:00- 5:00 AM) is a reasonable estimate of infiltration because there is minimal sanitary use of the system at that time. This estimate can be affected by water softener recharge cycles or commercial! industrial uses that continue throughout the night . The early morning flow rate for a dry weather (non-rainfall) day is subtracted frorn the total flow for the day to calculate an average daily "wastewater" flow for a dry weather (non-rainfall) day. . The estimated infiltration rate is divided by the "inch-miles" of sewer pipe that are tributary to the location . The calculated wastewater flow rate is divided by the number of connections to determine a "per connection" rate. o Estimate Inflow rates. . By definition, inflow generates a peak flow condition as a result of a rainfall or runoff event. . During the temporary metering period, 2.05 inches of rain were received on August 27 and 28, 2007. A more significant 1.83 inch rainfall occurred on September 19, 2007 after the temporary meters were removed. The August event will be used for the temporary meter locations and the September event will be used for the MCES meter locations. . The inflow rate is estimated by noting the peak hourly flow rate during the rainfall period and the time of occurrence. . The flow rate for the same time period on a dry day is then subtracted from the peak hourly flow on the rainfall day to separate out the portion of the flow attributable to the rainfall event. This value is the peak hourlv inflow rate. o Calculate an adjusted peak flow rate. . Determine the normal peak hourly wastewater flow on a dry weather day. . Add the peak hourly inflow rate to the peak hour wastewater flow rate to determine a "worst case" peak rate that could occur if the two events were simultaneous. This value is the adiusted peak flow rate. . Calculate a peak flow to average flow ratio for comparison with MCES guidelines. o Calculate inflow rate in gpd per connection and per inch mile of sewer for comparison purposes. The remaining item in the data analysis is the review of the flow graph itself. The flow graph figures provide a graphic comparison of the dry weather flow versus the wet weather peak flow in each meter area. The shape of the graph orthe "flow response" can be an indicator ofthe type of III sources that may be responsible for the clear water flow. The rapid rise and decline as demonstrated in Figure 3 Meter 051 would suggest a direct type connection that contributes flow for a short time during the runoff event and then stops operating after the runoff ceases. A rapid increase and a slow decline demonstrated in Figure 11 Meter 054 suggests a more extended flow contribution from a very long service area (long transport time in the pipes), or a foundation drainl sump pump type connection that requires an extended time after the end of the runoff event for the flow to percolate through the soil to the collector pipe. Page 6 of 12