5.8. SR 12-20-2004 - Laserfiche WebLink

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I Bar Screens and Grit Removal . Design peak hourly flow must be considered in evaluating unit processes, pumping, piping, etc. Some of the standards may not be amenable to the building layout and existing plant hydraulics. If standards are not feasible, a variance may be requested by the City. EXISTING PLANT Wastewater Flow Wastewater flows into the facility have steadily increased over the past ten years. The flow rate over the last decade has risen to an average wet weather (A WW) flow of 1.1 miIlion gallons per day (MGD) and an average dry weather (ADW) flow of 1.0 MGD. The 2002 population for the City of Elk River is reported as 18,000. According to the City's land use plan, nearly 70% of the population is connected to the sanitary sewer. Based on the average wet weather flow and the connected population, the average flow per person is almost 90 gpdlcapita. A peak hourly wet weather (PHWW) flow of 1.8 MGD was recorded during a considerable rainstorm Based on the data, the PHWW to A WW flow ratio is roughly 1.6. Raw wastewater enters the plant via force mains. The lift stations leading to the plant provide some reservoir capacity, which helps to equalize the flow and appears to reduce the peaking factor slightly. Pretreatment Building The existing Pretreatment Building is 34' -8" long by 29' -4" wide. The south half of the building is 34' -8" long by 16' wide by 9'-4" high and houses one mechanical screen, one manual screen, and flow measurement equipment. The north half of the building is 34'- 8" long by 11'-6" wide by 26' -0" high, has two levels, and houses the grit equipment. Mechanical and electrical upgrades were performed on the existing Pretreatment Building in 2000. The City replaced the building's roof and HVAC during the upgrades. Existing Screening The primary bar screen is a mechanical climbing bar screen with 3/4-inch openings. A manual screen provides redundancy to the mechanical screen. Bypass piping, isolated by slide gates, directs flow to a Sh011 channel which contains a manual bar screen with I-inch openings. The hydraulic capacity of the mechanical screen is based on the 2'-0" wide x 3'-0" deep channel. If one channel is used solely for redundancy and the screen is unchanged, the channel capacity is approximately 4.0 MGD. If both channels are used, the channel capacity is approximately 8.0 MGD. The existing mechanical screen is 30 years old and needs to be replaced. The photo on the top right shows the screening room looking west and shows 2