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MWMC FACILITIES PLAN subsequent anoxic selector of the following cell where denitrification occurs. Therefore, the capital and operating costs associated with the nitrified mixed liquor pumping equipment are eliminated. RAS pumping is required and RAS performs a dual function of biomass return as well as nitrate recirculation from the secondary clarifiers. The step-feed anoxic selector process provides for control of sludge settleability through selective organism growth, effectively reducing the sludge volume index (SVI). The reduction in SVIs enables the secondary clarifiers to be rated for a higher design solids loading rate. It is the effect of this increase in biological capacity in combination with improved sludge settleability that would allow the WPCF to increase its capacity using the existing aeration basin volume and secondary clarifiers. It was assumed that as part of the secondary treatment modifications, the secondary clarifiers would be modified to include baffling to increase their capacity and provide a more reliable effluent quality, as outlined in section 6.2.2. To implement this technology at the WPCF, the existing complete-mix/plug-flow aeration basins would have to be significantly modified to provide complete mixed anoxic selector zones physically separated from the aerobic zones. This may be accomplished by constructing a new dividing wall along the length of each cell in each basin to provide an anoxic selector having approximately 20 percent of the total aeration volume. This would provide four identical cells in each basin containing an anoxic selector followed by an aerobic zone. Primary effluent could then be distributed to each of these cells in various quantities to accommodate seasonal effluent requirements. Dry weather operations requiring nitrification could distribute primary effluent in the proportion of 25 percent to each cell, whereas wet weather operations where no nitrificaiton is required could be distributed in the proportion of 33 percent to each of the last three cells in the aeration basin. Table 6.2.1-1 summarizes the dry and wet season design criteria used for analysis of the step-feed anoxic selector process. TABLE 6.2.1-1 Design Criteria: Step-Feed Anoxic Selector Process MWMC Facilities Plan, Eugene-Springfield Parameter Dry Season Wet Season Temperature, degrees C 15.6 12.5 Nitrification Safety Factor 2.0 N/A Aerobic Volume 80% 80% Aerobic SRT, days 7.8 4 Anoxic Volume 20% 20% Anoxic MCRT, days 1.7 1.0 PE Flow Split 25% to 4 cells, or 33% to 3 cells 33% to 3 cells SVI, ml/g 110 120 Table 6.2.1-2 summarizes the facility requirements for the modified system to meet the 2025 projected flow and loads. 6-8 MWMC_6.0_REVf 1.DOC