Date of Award


Document Type


Degree Name

Master of Science (MS)


Environmental Engineering and Science

Committee Member

Sudeep Popat

Committee Member

Kevin T. Finneran

Committee Member

David L. Freedman


Two-phase anaerobic digestion (AD) of municipal wastewater separates the hydrolytic-acidogenic phase from the methanogenic phase, resulting in optimization of the respective microbial communities, increased stability, shorter residence times, and increased methane production rates compared to conventional single-phase treatment. Anaerobic co-digestion of fats, oils, and grease (FOG) with municipal wastewater also offers the potential for increased methane production rates; however, FOG can lead to system upsets and overall digester failure if not managed properly. Anaerobic co-digestion in a two-phase system could offer the potential of alleviating many of these upsets, including more efficient conversion of long chain fatty acids (LCFAs) and volatile fatty acids (VFAs) to acetate. This study employed six lab-scale semi-continuously fed reactors to further understand the growth dynamics involved in the two-phase and co-digestion processes by monitoring intermediates and products in the AD pathway and microbial community changes throughout the life of the reactors. Three reactors were operated without FOG: one as a single-phase reactor and the others as a two-phase system. Changes in solids residence time (SRT) were implemented in these reactors to study differences in performance under high and low organic loading rates (OLR). The final three reactors were operated using a 10% FOG loading rate by volume: one as a single-phase co-digesting reactor and the others as a two-phase co-digesting system. The two-phase system without FOG achieved 56.3+/-4.0% CODR at the lowest SRT, while the single-phase reactor achieved 43.6+/-10.0% CODR, indicating similar conversion independent of phased systems. Similar results were also evident with volatile solids (VS) destruction as 53.8+/-6.2% in the single-phase system and 64.4+/-10.4% in the two-phase system at the lowest SRT. Microbial communities remained consistent in the single and two-phase system without FOG, with high abundances of Dechloromonas, Methansaeta, and Methanobrevibacter during periods of high methane production, strongly indicating these organisms play important roles in conversion of influent organics to methane in a stable digester community. In the co-digesting reactors, buffering of feed for the single-phase reactor stimulated the growth of methanogenic bacteria and the subsequent balance with the acidogenic communities during startup but was not required once a stable community was established. This system maintained stable methane production, VS destruction, and carbonaceous oxygen demand reduction (CODR). Propionate, palmitic, and stearic acid conversion lagged during startup, but eventually degraded after Clostridium and Syntrophomonas genera became dominant members of the community, strongly indicating they play a key role in LCFA and VFA conversion. Methanesarcina was the dominant methanogenic genus in the single-phase co-digester, suggesting its importance for conversion when FOG is present. The two-phase co-digesting system never achieved stable methane production, VS destruction, or CODR. Palmitic and stearic acid did not degrade in the acidogenic phase and accumulated in the methanogenic reactor, stabilizing at a concentration of 5 mM, leading to a lag in methane production.



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