PRODUCING ENHANCED QUALITY CLASS B BIOSOLIDS THROUGH ANAEROBIC DIGESTER OPTIMIZATION

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 NH3, 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°C and 55°C and NH3 concentrations increased with temperature. At this SRT the highest CH4 yields were observed at temperatures of 42°C and 55°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°C and 62°C resulted in essentially non-detectable concentrations of fecal coliforms, E. coli. and Salmonella spp.. Operation at 42°C and 49°C resulted in incremental decreases in effluent concentrations as compared to operation at 35°C. With the exception of operation at 62°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.