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6. DEVELOPMENT AND EVALUATION OF ALTERNATIVES Alternative 1 - Conventional, Mesophilic Digester Alternative 1 consists of constructing a traditional mesophilic digester. It is likely that the anaerobic digestion process would be the typical high-rate, single-stage process. Advantages and disadvantages of this alternative are listed below. Advantages: · A conventional process recognized and familiar to WPCF staff · Non-proprietary process · Proven performance record in municipal wastewater treatment plants · Operating schemes more in line with current digestion process · 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 recirculation 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 flexibility in reverting to mesophilic operations because thermophilic organisms do not need to be developed · Ability 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 MWMC_6.0_REV11.DOC 6-35