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This study evaluated the efficiency of three stage operation of anaerobic digestion of municipal sewage sludge. In this study five sets of three stage anaerobic digesters operating in series were compared against a conventional single stage digester. Various combinations of conventional (35 o C) and non-conventional (42 and 49 o C) temperatures were evaluated in this study. The results showed that three stage operation was able to achieve enhanced Volatile Solids (VS) reduction as compared to single stage operation. Operation of the first stage of the three stage system at 42ºC and 49ºC resulted in an 8-12% increase in VS destruction. Stable operation of three stage configurations was indicated by low concentrations of total volatile fatty acids (TVFA) and substantial alkalinity generation. An increase in temperature to 42ºC in the second stage of the investigated system showed no significant change in final effluent quality of the digesters. For all of the indicators examined three stage operation resulted in a greater extent of removal when the first stage of the 3 stage system was operated at either 42 o C or 49 o C as compared to when the 3 stage system was operated at 35 o C. Clostridium perfringens were more resistant to inactivation in the digesters as compared to Fecal coliform, E. Coli. and Salmonella spp.

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The operation of anaerobic digesters over a range of temperatures and SRT's for the treatment of municipal wastewater treatment plant sludges was evaluated at bench scale. The digesters were characterized with respect to conventional digestion parameters as well as bacterial pathogen indicators. Successful anaerobic digestion of mixed primary and secondary sludges was achieved over a range of conventional and extreme mesophilic and thermophilic temperatures. Substantially different responses were observed between operation at SRT's of 15 and 20 days. Operation at a 20 day SRT generally had higher VSR, reduced accumulation of TVFA's, an increased release of NH 3 , higher methane yields and decreased dewaterability as compared to operation at a 15 day SRT. At the 20 day SRT there was little impact of temperature on VS reduction, however TVFA accumulation increased with temperature and methane yield decreased with temperature. At the 15 day SRT, TVFA concentrations were lowest at 42 o C and 55 o C and NH 3 concentrations increased with temperature. At this SRT the highest CH 4 yields were observed at temperatures of 42 o C and 55 o C. With the exception of Clostridium perfringens the concentrations of the microbial species in the digester effluents decreased as the temperature of operation increased. Operation at 55 o C and 62 o C resulted in essentially non-detectable concentrations of fecal coliforms, E. coli. and Salmonella spp.. Operation at 42 o C and 49 o C resulted in incremental decreases in effluent concentrations as compared to operation at 35 o C. With the exception of operation at 62 o C, the effluent concentrations of Clostridium perfringens were observed to increase relative to the feed concentrations. The results suggest that Clostridium perfringens may not be a useful indicator organism for the fate of pathogens in anaerobic digestion.