Title page for ETD etd-12202009-211917


Type of Document Dissertation
Author Wilson, Christopher Allen
Author's Email Address wilsonca@vt.edu
URN etd-12202009-211917
Title Mechanisms of Methanogenic Inhibition in Advanced Anaerobic Digestion
Degree PhD
Department Civil Engineering
Advisory Committee
Advisor Name Title
Novak, John T. Committee Chair
Boardman, Gregory D. Committee Member
Chen, Jiann-Shin Committee Member
Higgins, Matthew J. Committee Member
Murthy, Sudhir N. Committee Member
Keywords
  • acetic acid
  • advanced anaerobic digestion
  • ammonia
  • thermal hydrolysis
  • methanogenic inhibition
  • themophilic digestion
Date of Defense 2009-12-07
Availability restricted
Abstract
A series of lab-scaled digestion studies including conventional mesophilic anaerobic digestion(MAD), thermophilic anaerobic digestion (TAD) at a range of treatment temperatures, and mesophilic high solids digestion of thermally pretreated wastewater sludge (THD) were carried out. Enhanced digestion performance in terms of solids destruction and methane generation by THD relative to MAD was achieved, and was largely attributable to the solubilization and subsequent biodegradation of energy-rich substrates within blended primary and secondary sludge. TAD was observed to underperform MAD, especially at elevated temperatures as methanogenic inhibition resulted in the accumulation of headspace hydrogen, thus resulting in poor removal of volatile fatty acids. The thermodynamics of fatty acid metabolism was favorable at each digestion temperature, thus it was concluded that microbial inhibition was the controlling factor in poor thermophilic performance.

Inhibition by free unionized ammonia (NH3) was characterized for THD and MAD biomass. Acetic acid degradation was equally affected over a range of NH3 concentrations; however, methane generation by THD was less sensitive to ammonia inhibition, thus suggesting that methanogenesis by THD was less dependent on the NH3-sensitive process of aceticlastic methanogenesis. Total ammonia nitrogen (TAN) and bicarbonate alkalinity were stoichiometrically produced from proteinaceous material during thermal hydrolytic pretreatment and subsequent high solids anaerobic digestion. Combined effects of TAN and high pH resulted in NH3-inhibition during THD. Kinetic evaluations suggested that a growth rate reduction of approximately 65% was associated with in-situ NH3 concentrations of the THD reactor.

NH3-inhibition was apparently responsible for a shift in dominant methanogenic community of the aceticlastic Methanosarcina barkeri in MAD to the hydrogenotrophic Methanoculleus bourgensis in THD. A similar shift in methanogenic community was observed between low temperature thermophilic digestion at 47°C, where the dominant order was Methanosarcinales, to high temperature thermophilic digestion at 59°C where the dominant order was Methanobacteriales. These findings support a process-driven pathway shift from aceticlastic to non-aceticlastic methanogenesis between 180 and 290 mg/L NH3-N. Such a threshold is supported by previous literature related to ammonia tolerance of pure cultures of methanogens and has significant implications for the kinetic design of advanced anaerobic digestion processes.

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