Resolution No. 4793 - Laserfiche WebLink

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DEVELOPMENT AND EVALUATION OF ALTERNATIVES Alternative 1 - Conventional, ~lesophilic Digester Alternative 1 consists of constructing a traditional mesophilic digester. It is likely that the anaerobic digestion process would be the typical h~gh-rate, single-stage process. Advantages and disadvantages o£ this alternaQve are listed below. Advantages: A conventional process recognLzed and £amffiar to WPCF staff Non-proprietary process Proven performance record in municipal wastewater treatment plants Operating schemes more in line with current digestion process o Most widely implemented digestion process at publicly owned treatment works (POTWs) across the U.S Disadvantages: Potential foaming problems Low volatile solids reduction compared with other alternatives May require larger retention times to achieve required volatile solids reduction · Good mixing system required to maintain active digester volume The major equipment that would be necessary under this alternative includes sludge recirculation pump(s), heat exchanger(s), a digester mixing pump(s), a digested sludge transfer pump, and hot water recircttlahon pump(s). Alternative 2 - Pasteurization Pre-pasteurization of predigested sludge involves heating raw sludge to 70°C (160°F) before transferring it to a separate holding tank, in this case one of the existing digesters. The temperature of the solids in the holding tank is maintained at a minimum of 70°C for 30 minutes. Upon completion of the pasteurization process, the temperature of the solids is decreased to 40°C (100°F) in a spiral heat exchanger and transferred to the digester. By using a sludge/sludge spiral heat exchanger, it is possible to recover greater than 50 percent of the required thermal energy prior to entering the digester. Advantages: Requires minimal solids retention time and also provides Class A biosolids that satisfy regulatory requirements Although more heat is required and higher heating costs are incurred, this process has been reported to recover over 50 percent of the thermal energy May improve dewatering characteristics · Greater flexibLlity in reverting to mesophilic operations because thermophilic organisms do not need to be developed Abil/ty to meet vector attraction reduction (VAR) requirements, when combined with anaerobic digestion · Small reactor is required ahead of the existing digestion process · Public recognizes the "pasteurization" term and, therefore, public education and acceptance may be easier IVIWMC_60_REVI 1 DOC 6-35