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wasting was no longer done to the primary clarifiers. Ten States Standards states that the <br /> primary clarifiers not receiving waste activated sludge would be designed for a surface <br /> overflow rate of 1,500—2,000 gpd/ft2 instead of 1200 gpd/ft2 for primary clarifiers that <br /> receive waste activated sludge. Using this design standard, the total capacity of four, 40 <br /> foot diameter clarifiers would be 7.54 MGD which would meet the design conditions for <br /> Design Option No. 2. Consequently,modifications to the sludge handling process would <br /> be required as waste activated sludge would no longer be sent to the primary clarifiers. <br /> The new 40 foot diameter clarifiers will be sited just south of the existing primary <br /> clarifiers as shown in Figure 4.3. <br /> 4.2.2.5. 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 <br /> recirculation rate of 100 percent. Other possible modifications include improvements to <br /> the distribution arm, controls, ventilation, and access ways. When the trickling filters are <br /> run in the above mentioned configuration, each trickling filter would be able to treat 1.43 <br /> MGD of flow. The remaining flow will be by-passed to a new activated sludge process. <br /> 4.2.2.6. 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 /> The extended aeration basin will be designed with a solids retention time of 20 days, <br /> organic loading of 15 lb/day/1000 cubic feet of basin volume, a foot to mass (F/M)ratio <br /> between 0.04-0.10 lbs CBOD5/lbs MLVSS/D,MLSS between 2,000-5,000 mg/l and 1.1 <br /> lbs 02/lbs of peak hourly CBOD5. A series of concrete aeration basins with a total <br /> volume of 2.6 MG would be provided to meet the design loading conditions. The basins <br /> City of Elk River—T12.104758 Page 22 <br /> Wastewater Treatment Facility Plan Prepared by Bolton&Menk,Inc. <br />