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MWMC FACILITIES PLAN Alternative 1 would not increase capacity through the primary clarifiers. Alternative 2 is expected to increase the allowable SOR of the existing clarifiers, and thus the primary clarifier capacity, by approximately 20 percent. Although it will not be adequate to fully address the peak flow issue with respect to blending while meeting the NPDES permit, this alternative does provide an excellent increase in capacity using existing infrastructure and can be done at a reasonable cost. It also provides increased reliability in performance; however, sludge blanket washout would still occur at peak flows, and eventual permit violations would be anticipated. Alternative 3 is anticipated to increase the allowable SOR of the existing clarifiers, and thus the clarifier capacity, by up to 100 percent over Alternative 1. The resulting capacity of the primary clarifiers would be comparable to the proposed secondary treatment capacity. Alternative 3 is recommended because it would increase the existing primary clarifier capacity and improve the reliability of effluent quality. Although it is the highest-cost alternative, most of the capacity gained is through existing infrastructure, which maximizes the plant's existing investment while occupying very little site space. The equipment associated with the elimination of the sludge blanket in the primary clarifier accounts for much of the cost. This alternative provides all the capacity and performance reliability benefits of Alternative 2, plus 60 percent increased capacity, excellent operational flexibility, high-quality PE at high SORs, and meets all the primary clarification needs of the proposed secondary treatment system. Baffling improvements, new primary sludge pumps, two new primary sludge thickeners, and a primary sludge pump station will be required for this alternative. 6.2.4 Preliminary Treatment The existing pretreatment facility at the WPCF has a peak capacity of 175 mgd. This is based on having one of the six screening channels and one of the four aerated grit chambers out of service during peak flow events. Equipment in the existing pretreatment facility is outlined in Table 3.2.2-1. The projected 2025 PWWF flow being used for facility planning analysis is 277 mgd. Pretreatment facilities, at a minimum, must be expanded to accommodate additional flows with the largest units out of service. Two alternatives have been evaluated to expand the existing pretreatment capacity, both of which are based on an expanded capacity of 160 mgd. An expanded capacity of 160 mgd is necessary to accommodate the projected peak dry weather flow. It was used as the design basis so that the expanded facility capacity would match the capacity of modified secondary treatment facilities. Using this approach, the new facilities would be used to treat normal plant flows and the existing pretreatment facility would be brought online only as necessary during wet weather events to treat peak flows. Alternative 1 - Expand Existing Pretreatment Facility Alternative I looks at expanding the existing pretreatment facility. New grit and screenings handling equipment would be located in a new building south of the existing pretreatment area. The influent channels of the expanded facilities would be hydraulically connected to the existing facilities so that flow could be transferred between them. The expanded facilities would use fine screening technology and the screenings would be handled separately from the existing facilities by new washer/compactors. This is necessary because the 6-14 MWMC_6.0_REV11.DOC