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6. DEVELOPMENT AND EVALUATION OF ALTERNATIVES the potential of not having to construct a fifth FSL. Consequently, to provide for future flexibility of solids at the WPCF, both pasteurization emd TPAD should be considered and evaluated further. 6.5.3 FacultativeSludge Lagoons Futtrre capacity m*~d reduction of the current overloaded solids inventory are the primary issues that alternatives should address with regards to the FSLs. Capacity - VSS Loading Projected upper limit flows and loads presented in Chapter 4.0 were used to determine future capacity requirements for the FSLs. It is conservatively assumed that a fourth mesophilic digester will be online between 2010 and 2012 and that the digested sludge contains 63 percent VSS. This value is based on the annual average VSS content of digested sludge over the past 3 years. Figure 6.5.3-1 illustrates the phasing chart for the FSLs based on the design criteria of 25 lbs VSS/1000 sf-day. As seen in Figure 6.5.3-1, additional FSL surface area is needed in year 2015. This also accotmts for the current operational procedures of taking one FSL offline for approximately 6 months during the FSL harvesting season. The additional FSL volume added in 2015 assumed either construction of a new 5- acre FSL, or using the existing 14-acre lagoon at the SIWF. FIGURE 6.5.3-1 Facultative Sludge Lagoon Phasing MWMC Facilities Plan, Eugene- Spdnfield (Annual Average Conditions, 25 lbs VSS/IO00 st-day loading) 27.00 30000 25.00 25000 23.00 20000 21.00 15000 4 Existing Lagoons + SIW (14 acres) 19.00 10000 17.00 5000 15.00 0 2005 2010 2015 2020 2025 Year Increased VSS loading to the FSLs may result in increased odors and may also affect the FSLs' ability to achieve the regulatory requirement of an overall plant VSS reduction of 38 percent. However, MWMC has consistently exceeded the 38 percent VSS reduction MWMC_60 REV11.DOC 6-39