7C, Final Draft of I&I Report - Laserfiche WebLink

Home Browse Search

City of Arden Hills March 2008 I. Introduction and Purpose Infiltration I Inflow (III) is defined as clear water that enters a sanitary sewer system. Infiltration is clear water that seeps into the pipe network from groundwater, and Inflow is clear water that is diredly related to rainfall or runoff events. III is a term used to describe all clear water without attempting to differentiate its source. This clear water does not require treatment until it is mixed with the wastewater already in the sanitary sewer pipe. Once it is mixed, the combined flow must be conveyed to a treatment facility for treatment before it can be discharged back into the surface water network. The III in the system can increase the normal flow in the system enough to stress the conveyance capacity of the pipe network, pumping stations, and the wastewater treatment facilities. In the seven county metropolitan area, the Metropolitan Council Environmental Services (MCES) provides an interceptor sewer system to coiled the wastewater from over 100 communities and convey it to one of eight MCES wastewater treatment facilities located around the metropolitan area. Communities own and operate their own local sanitary sewer systems to coiled the wastewater from individual buildings within their service area. Any clear water (III) entering a local colledion system is conveyed to and through the MCES system and diredly impacts the available capacity of the MCES interceptor and treatment facilities. The Metropolitan Council appointed a Task Force to identify solutions to the regional III problem. The Task Force worked from April 2003 to May 2004 and developed recommendations which were adopted by the Metropolitan Council as Policy in 2004. The Policy established the III Surcharge Program that is now in effed with the intent to avoid "hundreds of millions" of dollars in regional infrastrudure costs. The MCES currently monitors wastewater flows from all the tributary communities in the service area. With the III Surcharge Program an additional step was initiated to monitor peak flow conditions. Briefly, the program establishes a three year average flow for each community or "metershed" within the system. A peaking factor (multiplier) is applied to the average flow to determine an allowable peak flow. The peaking fador was previously adopted by the MCES for design purposes and it ranges from 4.0 for average flows less than 110,000 gallons per day (gpd) to 1.7 for flows over 30.0 million gallons per day (mgd). The MCES reviews hourly flow data from each meter during runoff events and compares the actual "peak hourly" flow to the allowable peak flow. If the actual peak is higher than the allowable peak, the extra flow is considered "excessive" and subjed to the III surcharge rate of $350,000 per mgd (this rate is annually increased by an inflation fador). The 1/1 surcharge is calculated for the highest peak recorded by the MCES and the surcharge amount is subdivided into five (5) years of annual payments. If a greater peak is recorded in the five year period, the surcharge amount can be recalculated and increased for the remaining years in the 5 year program term. Surcharges began in 2007 and will run through 2011. The City of Arden Hills is subdivided into three "metersheds" for measuring and recording wastewater flow. The combined total" excess" peak flow of these three metersheds was 1.43 mgd on October 4, 2005. Using this peak flow rate, the City's III surcharge was calculated to be $500,500 which is to be colleded at an annual rate of $100,100. The City has the option to pay the surcharge amount to the MCES, or make III redudion improvements to their system that will diredly off set the surcharge amount. If the City expends less than the annual surcharge amount, the balance will be colleded by the MCES and held in an account for future III improvement work. At the end of the five year program if there are any funds remaining in the community's MCES account, they will be turned over to the MCES as a "demand" charge and the MCES will use the funds for regional system improvements. At the end of the live year III Surcharge Program, the MCES will continue annual charge communities for peak flows in excess of the allowable flow, but the funds will go diredly to the MCES budget and will not be available to the Community for III redudion efforts. The goal is to make enough III improvements between 2007 and 2011 to eliminate the excess peak flow and therefore eliminate the III surcharge entirely. Page 3 of 12