8.1. SR 03-18-2013 - Laserfiche WebLink

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4.2.2.1. Screening Ten States Standards states that the capacity of the screens should meet the design peak instantaneous flow with one unit out of service. By these criteria the screening process is 2.08 MGD short in capacity for Design Option 2. Addition of a new screen would require an addition to the pretreatment building with substantial capital costs. Therefore, a manual screen insert would be constructed that could be placed in the existing channel if one of the screens was down for maintenance. The addition of the manual screen will meet the design requirements. 4.2.2.2. Flow Measurement The existing facility has 12-inch throat parshall flume for flow measurement with maximum capacity of 10.4 MGD. The existing parshall flume would meet the flow measurement needs for the Design Option No. 2. 4.2.2.3. Grit Removal The maximum capacity of the existing grit removal process of 7.0 MGD at PHWW flows is slightly less that the design PHWW flow of 7.27 MGD for Design Option No. 2. Due to the age and the capacity of the existing grit removal process, it is recommended that the City reevaluate the grit removal process when peak flows to the facility reach 90 percent of the capacity of the grit system. Addition of a grit cyclone to the grit classifier will also be investigated to alleviate plugging issues that have occurred with the existing classifier. 4.2.2.4. Primary Clarifiers To meet the PHWW flow of 7.27 MGD at the design year the existing facility would need an additional capacity of 4.23 MGD of capacity. Due to land area constraints the maximum clarifier area that can be added is two 40 foot diameter clarifiers as was discussed in Design Option No. 1. Two 40 foot diameter clarifiers would provide an additional 3.02 MGD of capacity at PHWW. However, this capacity could increase if City of Elk River—T12.104758 Page 21 Wastewater Treatment Facility Plan Prepared by Bolton&Menk,Inc.