Resolution No. 4793 - Laserfiche WebLink

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6 DEVELOPMENT AND EVALUATION OF ALTERNATIVES - Does not preclude conversion of the existing lagoon for additional FSL space in the future. 6.6 Development and Eva uation of System Alternatives The Oregon DEQ has mandated el/ruination of SSOs resulting from less than a 5-year, 24- hour rainfall event ha the wet season and a 10-year, 24-hour rainfall event in the dry season [OAR 340-41-0009 (6) and (7)].This nde will take effect by the year 2010. Influent wastewater flows up to these quantifies will be required to be treated to a level so that the effluent meets secondary treatment standards. The projected 2025 PWWF at the WPCF that is being used for planning purposes is 277 mgd. Collection system modeling efforts and studies have concluded that peak flows could be conveyed by the collection system to the WPCF. Once flows have been conveyed to the WPCF, significant modifications to the facility would be required to treat the PWWF flow. 6.6.1 Common Parameters Several improvements will be constructed/ndependent of the system alternative ultimately implemented, because they serve multiple benefits (e.g., ammonia removal, dry season mass limits, etc.) and/or they will still be required for all peak flow management alternatives. These improvements include enhancements to the existing primary and secondary darifiers, additional secondary clar~iers, new tertiary filters, expansion of the pretreatment facil/ty, primary sludge thickening, a new high-rate disinfection facility, a new effluent blending structure, new thermophilic digesters (or other digestion process to produce Class A biosolids), WAS thickemng facility expansion, odor control facilities, and reuse disinfection with UV. Only System Alternative I will not include implementation of these common improvements. All system alternal4ves assume the capacity of the existing four primary ctarifiers would be increased to exceed 160 mgd by adding energy dissipating inlets, flocculation feed wells, density current baffles, and by operation without a sludge blanket. All system alternatives also include an increase in secondary treatment capacity to 165 mgd by enhancing secondary clarifiers, modifying the aeration basins to a step feed configuration with anoxic selectors, and by constructing two additional secondary clarifiers. The evaluation was performed assuming that future permits would not include an increase ha allowable TSS and CBODs effluent mass limits. 6.6.2 System Alternatives Development Many alternatives were considered in develop/rig system alternatives. Previou~s sections of Chapter 6 and technical memorandums developed for the Fac/liPy Plan update deta/l alternatives analysis for trait processes in order to treat the projected peak week wet weather flow (PWWWF). The system alternatives proposed in this section focus on the treatment alternatives required to treat the entire PWWF flow to secondary treatment standards. The proposed system alternatives use a combination of pretreatment, prknary treatment, secondary treatment, and tertiary filtration in conjtmction with high-rate disinfection and MWMC_§ O_REVI ~.DOC 6-5'J