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MWMC FACILITIES PLAN effluent blending to treat the peak flows. Current effluent blending policy is still evolving; however, system alternatives 3, 4, and 5 assume that some level of effluent blending will continue to be an acceptable approach for treating PWWFs. For all alternatives it has been assumed that the existing primary clarifiers and secondary treatment have been modified to treat a peak flow of 165 mgd. An additional 112-140 mgd [277 mgd (or 300 mgd to account for uncertainty) less 165 mgd] of treated flow must be combined with the secondary effluent flow of 165 mgd from the existing facilities, and the combined flow must be blended to meet the existing NPDES permit requirements and secondary treatment standards. Prescreening of system alternatives led to the development of five system alternatives: 1) no action, 2) addition of primary and secondary treatment, 3) addition of primary clarifiers, 4) use of high-rate clarification, and 5) parallel primary and secondary treatment. System Alternative I - No Action Alternative No improvements would be made for system alternative 1. The existing WPCF facilities would continue to operate with no increase in system capacity. Upgrades and replacement of facilities and equipment would occur under normal maintenance with no major expansion or improvements. Although modifications could result in greater efficiencies and higher treatment capacities, the projected PWWF flow of 277 mgd would exceed the existing capacity. System Alternative 2 - Additional Primary and Secondary Treatment and Disinfection Four new primary clarifiers, one new aeration basin, and six new secondary clarifiers would be constructed to provide another primary and secondary treatment train used in conjunction with existing treatment facilities to treat the entire 277-mgd PWWF. Pretreated effluent in excess of 134 - 165 mgd would be split to the new treatment train. Primary influent flow would be split with a new primary influent splitter box and would flow by gravity to both the new and existing primary clarifiers. After disinfection, SE from new and existing treatment trains would be combined at the blending box prior to discharge into the Willamette River. This alternative would be required if a "no blending" policy was adopted by DEQ or imposed by EPA. System Alternative 3 - Additional Primary Clarifiers Four new primary clarifiers would be constructed and used in conjunction with the existing primary clarifiers to treat the entire 277-mgd PWWF. Pretreated effluent in excess of 134 - 165 mgd would be split to the new primary clarifiers and PE would be combined. PE in excess of 134- 165 mgd would be diverted around the secondary treatment system. The primary and secondary effluents would be disinfected separately and blended prior to discharge to the river. This system alternative essentially continues MWMC's current approach to accommodating peak flows. Figure 6.6.2-1 shows the process flow schematic for system alternative 3. 6-52 MWMC_6.0_REV11.DOC