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6. DEVELOPMENT AND EVALUATION OF ALTERNATIVES Alternative 3 - Temperature-Phased Anaerobic Digestion (TPAD) TPAD is a two-step process in which a small thermophilic digester is followed by a large mesophilic digester. Typical retention time requ. irements are 5-day thermophilic and 10-day mesophilic. The majority of pathogen destruction and solids treatment occurs in the first digester, including volatile solids destruction reported to be 80 to 90 percent, and greater methane gas production. Additional polishing, deodorizing, and sludge conditioning for subsequent dewatering occurs in the second digester. Advantages: · Digester volume reduction can be achieved through higher loading (higher concentration and lower retention time). · Less capital expense (lower digestion capacity required) than other advanced digestion processes. · Currently being used in the U.S. Four operating plants that have converted to TPAD from conventional digesters include Newton, IA; Sturgeon Bay, WI; Omaha, NE; and Mobile, AL. Additional plants in Waterloo, IA; Mason Farm, NC; Neenah-Menasha, WI; and Independence; IA have recently been constructed. · Relatively simple to modify an existing mesophilic digestion process. · Increased volatile solids reduction and greater methane gas production. Disadvantages: · The main disadvantage of the TPAD process is that it is not a Process to Further Reduce Pathogens (PFRP) (Class A process) and will require Class A verification by EPA · Process is patented (patent held by Iowa State University) · Sludge-sludge heat exchangers are required · Increased production of ammonia in sidestream (from higher VSS destruction) · Holding tank for undigested sludge requires odor control The major components associated with the TPAD process include thermophilic (5-day minimum SRT, typical) and mesophilic (10-day minimttm SRT, typical.) digesters, sludge- sludge heat exchangers for pre-heating undigested sludge and for recovering thennophilically heated digested sludge, hot water heat exchangers for primary sludge heating, and recycle and transfer pumps. To convert TPAD to a Class A process, the draw/fill cycles need to be discontinuous for the thermophilic reactor (between ~2 ~ and 1-day gap in feed and withdrawal cycles). A holding tank (1-day retention time) prior to digestion is required to achieve batch feeding. To improve destruction of pathogens, Infilco Degremont, Inc. (IDI) has developed and pilot- tested a variation of the TlPAD process at the Belmont wastewater treatment plant (WWTP) in Indianapolis, IN. This variation is in the operation of a thermophilic first-phase reactor in a draw/fill mode with four feedings per day. Site-specific issues associated with the implementation of TPAD as a Class A treatment option at the WPCF are as follows: MWMC_6.0_REV11 DOC 6-37