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4.2.1.4. Primary Clarifiers <br /> To meet the PHWW flow requirements for Design Option No. 1 the existing facility <br /> would need an additional 2.66 MGD of capacity. This could be accomplished by the <br /> addition of two 40-foot diameter clarifiers that could be brought in to service in two <br /> phases. One additional 40-foot clarifier would provide enough total capacity for a PHWW <br /> flow of 4.52 MGD. Once PHWW flows reached 90% of the capacity of the three <br /> clarifiers the fourth clarifier could be brought on line which would increase the total <br /> PHWW capacity of the primary clarifiers to 6.03 MGD. With all four primary clarifiers <br /> online the system would meet the design requirements for Design Option No. 1. Staging <br /> the addition of the third and fourth clarifiers would provide easier operation at current <br /> flows. Sizing of the additional clarifiers was based on Ten States Standards which states <br /> that the primary clarifiers receiving waste activated sludge would be designed for a surface <br /> overflow rate of 1200 gpd/ft2. <br /> The new 40-foot diameter clarifiers will be sited just south of the existing primary <br /> clarifiers as shown in Figure 4.1. <br /> 4.2.1.5. Trickling Filters <br /> Improvements to the trickling filters will include modifications to the trickling filter <br /> operation and recirculation piping. The modifications would include operation of the two <br /> trickling filters in parallel with pumping and piping modifications to achieve a recirculation <br /> rate of 100 percent. Other possible modifications include improvements to the distribution <br /> arm, controls, ventilation, and access ways. When the trickling filters are run in the above <br /> mentioned configuration, each trickling filter would be able to treat 1.43 MGD. The <br /> remaining flow will be by-passed to a new activated sludge process. <br /> 4.2.1.6. Activated Sludge Process <br /> Since the existing trickling filters do not have enough capacity to treat all the flows and <br /> loadings, an additional biological wastewater treatment process is required. In order to <br /> provide a reliable, easy to operate system, an extended aeration activated sludge process <br /> would be constructed. <br /> City of Elk River—T12.104758 Page 18 <br /> Wastewater Treatment Facility Plan Prepared by Bolton&Menk,Inc. <br />