Resolution No. 4793 - Laserfiche WebLink

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MWMC FACILITIES PLAN New hot water heat exchangers will be needed. A typical TPAD process requires undigested sludge to be heated to 54°C (130°F) prior to entering the thermophilic digester. Further evaluation of the heat exchangers is needed. New boilers may be needed to supply the necessary hot water to the existing heat exchangers. Further evaluation of the boilers is needed to determine if they can supply enough hot water to increase undigested sludge to thermophilic temperatures. New sludge/sludge heat exchangers are needed. These will serve as pre-heat treatment to the hot water heat exchangers and in recovering heat from the heated sludge after the therrnophilic retention time. · Four new 5-day SRT thermophilic tanks and one mesophilic tank would be constructed, The existing digesters could be used in the TI>AD process (they would serve as the mesophilic tank). Volume required for thermophilic digestion would need to be 1.4 MG (based on 2025 W~VMM flows), or 354,000 gallons per tank (based on four tanks). The new mesophflic digester would be the same size as the existing digesters. · I&C modifications would be required. Recommendations Table 6.5.2-2 presents a preliminary project cost and non-monetary comparisor~ between the alternatives. Costs are based on industry averages for treatment plartts of a size similar to the WPCF. TABLE 6.5.2-2 Digestion Alternabves Cost Comparison MWtt4C Facilltles Plan, EugeneoSprinCeld Project Cost Process (millions of dollars) Non-Monetary Rating(~) Mesophllic Digeshon $5-$6M 21 TPAD $11 -$13.5M 21 Pre-Pasteunzation $13,5-$16M 16 Maximum possible score ot 30 points As can be seen ~rom Table 6.5.2-2, construction of a conventional mesophilic digester has the lowest project cost and is the same as TPAD for the highest non-monetary rating. The primary reason for the high non-monetary rating is that the process performance is well- known and Eugene operational staff are accustomed to operating and maintaining digesters of this type. Pre-pasteurization has the highest project cost and ~te lowest non-monetary rat/rig. However, the thermophilic pasteurization tanks are much smaller (6,000 gallons versus 354,000 gallons) than those required for TPAD operation and siting may be easier. TPAD has the second highest project cost and is the same as mesophilic digest/on for the highest non-monetary rating. The benefits that TPAD provides, when considering the issues associated with downstream processes (VSS loading on the FSLs), makes this alternative attractive for achieving long-term goals of meeting solids stabilization requirements with MWMC_60~REV11.DOC