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4.2 Wastewater Treatment Facility Improvements — Liquid Process <br /> Based on the design flows and loadings presented in Section 2, liquid stream process <br /> improvements have been developed for both design options. A detailed discussion of the needed <br /> improvements for both design conditions is given below. <br /> 4.2.1. Design Option No. 1 <br /> Detailed descriptions of the improvements for each unit process for Design Option No. 1 <br /> are presented in the following sections. A site plan with the needed improvements is <br /> present in Figure 4.1 and an updated process flow diagram is presented in Figure 4.2. <br /> 4.2.1.1. Screening <br /> Ten States Standards states that the capacity of the screens should meet the design peak <br /> instantaneous flow with one unit out of service. By these criteria the screening process is <br /> 0.10 MGD short of capacity for Design Option 1. The existing screening process was <br /> installed in 2005 during the Headworks Improvements Project. Due to the age and <br /> capacity of the existing screening process, it is recommended that the City revaluate the <br /> screening process when the peak flows to the facility reaches 90% of the capacity of the <br /> existing screens. <br /> 4.2.1.2. Flow Measurement <br /> The existing facility has 12-inch throat parshall flume for flow measurement with <br /> maximum capacity of 10.4 MGD. <br /> The existing parshall flume would meet the flow measurement needs for Design Option <br /> No. 1. <br /> 4.2.1.3. Grit Removal <br /> The maximum capacity of the existing grit removal process is 7.0 MGD at PHWW flows. <br /> Therefore, the current grit removal system has adequate capacity for Design Option No. 1. <br /> A bypass is provided for peak instantaneous flows. <br /> Addition of a grit cyclone to the grit classifier will also be investigated to alleviate <br /> plugging issues that have occurred with the existing classifier. <br /> City of Elk River—T12.104758 Page 17 <br /> Wastewater Treatment Facility Plan Prepared by Bolton&Menk,Inc. <br />