Sustainability Analysis and Microbial Community Dynamics in Ambient Temperature Anaerobic Digesters

There is an ongoing need to develop more sustainable systems for producing energy and treating wastes. Anaerobic digestion is one type of technology that can meet this need. The sustainability and microbial community dynamics of ambient temperature anaerobic digesters was evaluated by this research. An emergy analysis of the EARTH University Taiwanese model anaerobic digesters was performed, and the results were compared to other forms of energy production. The results of this analysis indicate that the EARTH University digester was a more sustainable system than traditional sources of energy production such as oil and natural gas. The success of the EARTH University digesters was due, in part, to the tropical climate of Costa Rica that provides favorable ambient temperature year round for anaerobic digestion.;The remainder of this dissertation focused on the analysis of the microbial community dynamics in variable temperature anaerobic digesters. The results of the microbial community analysis of the Waterman fixed-dome digester showed that there are significant shifts in the structure of the both the Archaeal and Bacterial communities that coincided with digester re-inoculation as well as temperature and loading rate changes. Following re-inoculation, the predominant Archaea shifted from the hydrogenotrophic Methanobrevibacter to the acetoclastic Methanosarcina which was the most abundant Archaea in the inoculum. Methonosarcina was replaced by another acetoclastic methanogen, Methanosaetae after the resumption of digester loading in the summer of 2010. Methanosaetae began to decline in abundance as the digester temperature cooled in the fall of 2010 and Methanobrevibacter increased in abundance through the end of the study period.;The third chapter of this dissertation was a laboratory experiment that investigated the relationship between temperature, organic loading rate and the structure of the microbial communities in the laboratory digesters. At loading rates of 1.8 and 0.8 kg VS/(m3 day) the digesters soured as a result of a decrease in temperature to 10 C°. The sour digesters were characterized by a shift in the dominant Archaea from the acetoclastic to the hydrogenotrophic methanogens. At a loading rate of 0.3 kg VS/(m 3day) the digesters did not sour, but they also did not develop any significant amount of acetoclastic methanogens which are characteristic of stable operating anaerobic digesters. The results of both the Waterman digester and the laboratory digesters indicate that it is difficult to maintain an adequate amount of biogas production at temperature 0.3 kg VS/m3 day. Recommendations for operating an ambient temperature digester year round in a temperate climate would be to “idle” the digester in the colder temperatures by reducing the organic loading rate to at least 0.3 kg VS/m3 day in order to prevent a shift to the microbial community associated with the sour digester. Another option would be to invest additional resources to heat the digester in the winter so an abundant population of acetoclastic methanogens could be maintained year round.