5.8. SR 12-20-2004 - Laserfiche WebLink

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detention time of 3 to 5 minutes at design peak hourly flows. Assuming a 3-minute detention time, the peak flow rate through the existing tank is 0.56 MGD, which is well below the CUlTent peak day flow. Additionally, head10ss through the grit tank at higher flow rates becomes problematic upstream. The grit tank is structurally sound and is in working order, but at higher flow rates, it is expected to become less effective and to cause hydraulic problems. Grit is pumped ITom the grit tank sump using one of two 200 gallons per minute (GPM), 16-foot total dynamic head (TDH), 5 horsepower (hp), 1,725 revolutions per minute (rpm), horizontal vortex type pumps. One grit pump is duty and one is standby. The grit is dewatered through a grit classifier and hydro cyclone installed in 2002 and discharged to disposal containers. The grit cyclone, in combination with the grit washing and classification equipment, is capable of removing at least 95 percent of the 150-mesh grit having a specific gravity of 2.65 or greater ITom the underflow of the grit tank. The classifier and cyclone are sized to handle a grit flow of 200 GPM. Grit is currently discharged into 20-gallon plastic garbage pails and collected in the grit building for disposal. The grit equipment collects approximately 20 gallons of grit each day. Plant staff gather the garbage pails weekly and dispose the grit at the hmdfil1. This is a high maintenance process for plant staff and a safety hazard due to ITequent changing of garbage pails. FUTURE WASTEWATER FLOW Operating data, population projections, and Ten State Standards provide a basis to project the design flow for the year 2025 for the Elk River Headworks design concept. Operating data show the 2003 A WW flow was 1.1 MGD with a PHWW flow rate of 1.8 MGD, thus giving a peaking factor of 1.6. Howard R. Green Company evaluated the ultimate plant size based on land available for expansion of various process units. The existing plant site has the capacity to accommodate an average flow of 4.8 MGD limited by final clarifier capacity. Figure I shows a site layout of the Elk River Wastewater Treatment Plant with tentative future expansion and the current and expanded flow rates of various processes. The City of Elk River population for the year 2025 is projected to be 35,000. Based on Ten State Standards, the average per capita flow is 100 gallons per day, which gives a total average flow of3.5 MGD for Elk River. Using a conservative peaking factor of2, peak flow rate projects to 7.0 MGD for the year 2025. General Improvements As indicated by Ten State Standards, unit processes should be designed to handle hydraulic peak flows. The proposed bar screens and grit units have been designed to accommodate a peak flow of 7.0 MGD. Bar screen channels have been evaluated at a flow of7.0 MGD to verify that the flow velocity and ITeeboard meet Ten State Standards. The existing Parshall Flume is sized for a flow of 10.4 MGD and replacement has not 4