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MWMC FACILITIES PLAN requirement in the anaerobic digesters alone. Providing additional volume through either construction of a new FSL or using the SIWF lagoon is based on meeting the VSS loading design criteria at the current projected flows and VSS loads to the existing FSLs. If the fourth digester uses an advanced digestion process, the VSS loading to the FSLs will be less than the current projections. At this time it appears most likely that a fifth FSL will not be needed within the design period. There are several reasons for this assumption. The first is that overall plant VSS reduction requirements are likely to be achieved even if VSS loading to the existing FSLs exceeds design criteria. The second is that digester mixing improvements will increase active digester volume and further improve VSS destruction. The third is that the fourth digester (constructed between 2010 and 2012) may use an advanced digestion process that will improve reduction in VSS loads to the FSLs. Based on the phasing shown in Figure 6.5.3-1, there will be 3 to 5 years between construction of the fourth digester and when a fifth FSL is required. Solids Inventory Currently, the solids in the FSLs are processed through mechanical and passive dewatering processes. Mechanical dewatering is accomplished through three 2-meter BFPs. Passive dewatering is accomplished using thirteen air-drying beds. Mechanical dewatering is the primary processing method. The FSLs are harvested seasonally, typically from March through September. Beginning in February, one FSL is taken offiine to provide a minimum of thirty days residence time before harvest. Beginning in March a dredge is used to pump solids from the FSLs into two 370,000-gallon mix tanks that then feed the BFPs. Dewatered cake is then placed in windrow piles in the air drying beds where it is further dewatered to approximately 40 to 50 percent solids. In mid-summer the windrowed piles are trucked to local cooperative farms for land application. Solids (digested sludge) sent to the FSLs year- round exceed the seasonal capacity of the dewatering processes, based on current operation, thus leading to increased inventory. As the solids inventory in the FSLs has increased to levels above typical operating conditions, the increase in the FSL solids blanket has decreased the water cap on the FSLs. The water cap helps to control odors in the FSLs by decreasing the potential for turn over of the solids blanket. Turn over occurs when the upper layers of the solids blanket become warmer than the bottom layer and the organic material in the bottom layer flips with the warmer upper layer. This flip (turn over) brings anaerobic organic material from the bottom to the top and results in the release of odorous fumes. Maintaining a water cap depth greater than the solids depth helps to control the temperature of the solids layer beneath and, therefore, odors. On-site staff have recognized the increased solids inventory and they have anticipated that the solids inventory would be reduced once the BF Phase 1 came online in the summer of 2004. Recommendations for future operating strategies to reduce the solids inventory in the FSLs are discussed below. Recommendations At this time the need to construct a fifth FSL by a specific year is not recommended, nor is it expected that a fifth FSL will be needed within the design period. It is recommended that 6-40 MWMC_6.0_REV11.DOC