Investigating and employing microbial functions in bioelectrochemical systems for bioenergy production
Open Access
- Author:
- Yan, Hengjing
- Graduate Program:
- Environmental Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 03, 2013
- Committee Members:
- John Michael Regan, Dissertation Advisor/Co-Advisor
John Michael Regan, Committee Chair/Co-Chair
Bruce Ernest Logan, Committee Member
William D Burgos, Committee Member
Jason Philip Kaye, Committee Member - Keywords:
- Bioelectrochemical systems
anode potentials
microbial community
Clostridium acetobutylicum
metabolic flux
nitrogen removal - Abstract:
- Microbial fuel cells (MFCs) rely on microbes serving as the biocatalysts for bioenergy recovery from organic feedstocks, such as wastewater. Microbes can work on both the anode and cathode electrodes for electron transfer, or contribute distinct functions combined with bioelectrochemical systems (BESs). For a better understanding and application of microbial functions in BESs, investigations were conducted on anode community behaviors corresponding to different electrochemical conditions, the potential application of clostridia biocathodes for biofuels production, and combined nutrient treatment in MFCs with a nitrifier-enriched mixed community. Serving as biocatalysts in anode electron transfer, exoelectrogenic bacteria and their co-colonizers on the anode electrode are subject to various changes in the system operating conditions. Anode potentials in BESs were found to affect both the electrochemical performance and the microbial community structures of the systems. The performance of MFCs under both potentiostatic operation and fixed external resistance has been previously studied. To investigate whether setting anode potentials will result in distinct microbial communities from dynamic anode potentials, fixed anode potentials of -250 mV and -119 mV vs SHE (throughout) were picked to match with the negative peak anode potentials obtained from MFCs operated with a fixed external resistance of 1 kΩ and 47 Ω, respectively. Pyrosequence data from two-month time series samples showed hindered enrichment of Geobacter spp. in a more diverse anode bacterial community at the lower fixed anode potential (-250 mV) compared to 1 kΩ, though more comparable Geobacter abundances in anode biofilms were found between the -119 mV and 47 Ω reactors. Setting the anode potential at the negative peak values results in less energy extraction by the microorganisms for growth, which might have slowed down the development of the whole anode microbial community as well as the enrichment of exoelectrogenic bacteria. In addition, a possible limitation of potentiostatic operation with a volumetric anode is uncertainty in the actual anode potential due to variable proximity to the reference electrode. This study indicated that a balance between screening exoelectrogenic bacteria and encouraging microbial growth needs to be considered when setting a low anode potential in BESs. Exoelectrotrophic bacteria contribute to cathode electron transfer and are helpful in converting electrical energy to other useful energy carriers or products. Altered electron flow has previously been induced in Clostridium acetobutylicum fermentation using electrochemical energy as a source of reducing equivalents in the presence of the electron mediator methyl viologen. Also, C. acetobutylicum was demonstrated to produce exclusively hydrogen using cathode-derived electrons under organic-free conditions in the presence of methyl viologen. Recently, C. acetobutylicum was demonstrated to generate current in MFC anodes without the addition of redox mediators. The study here was aimed at testing the possibility of using electrical energy without exogenous mediator addition to influence the distribution of valuable extracellular products, such as hydrogen, butanol, and ethanol, using C. acetobutylicum as the cathode biocatalyst. C. acetobutylicum was demonstrated to be exoelectrotrophic with a fixed cathode potential at -400 mV and organic carbon sources. A current uptake of up to 83 mA/m2 was achieved by C. acetobutylicum, with a decreasing trend over time that electrochemical impedance spectroscopy showed was probably due to an increased cathode charge transfer resistance. Control experiments and cyclic voltammetry tests ruled out abiotic electrochemical hydrogen evolution and electron shuttles for electron transfer, suggesting that the cells derived electrons directly from the electrode. Increased yields of more energy dense products (hydrogen and butanol) and decreased yields of butyrate, acetate, and ethanol were found with current uptake by C. acetobutylicum. The electron balance indicated that current uptake might have also reduced the biomass production of C. acetobutylicum. This study demonstrated the non-mediated exoelectrotrophic phenotype of a solventogenic bacterium. The metabolic flux shift with current uptake found in this study provides a primary guidance for the potential utilization of a C. acetobutylicum biocathode in a BES. Single-chamber MFCs with nitrifiers pre-enriched at the air cathodes have previously been demonstrated as a passive strategy for integrating nitrogen removal into current-generating BESs. To further define system design parameters for this strategy, the effects of oxygen diffusion area and COD/N ratio in continuous-flow reactors were investigated. Doubling the gas diffusion area by adding an additional air cathode or a diffusion cloth significantly increased the ammonia and COD removal rates (by up to 115% and 39%), ammonia removal efficiency (by up to 134%), the cell voltage and cathode potentials, and the power densities (by up to 124%). When the COD/N ratio was lowered from 13 to 3, up to 244% higher ammonia removal rate but at least 19% lower ammonia removal efficiency were detected. An increase of COD removal rate by up to 27% was also found when the COD/N ratio was lowered from 11 to 3. The Coulombic efficiency (CE) was not affected by the additional air cathode, but decreased an average of 11% with the addition of a diffusion cloth. Ammonia removal by assimilation was also estimated to understand the ammonia removal mechanism in these systems. These results show that the doubling of gas diffusion area enhanced N and COD removal rates without compromising electrochemical performance.