Energy recovery from biodiesel waste : performance of microbial electrochemical systems on glycerol Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/bv73c384r

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  • Pure glycerol and the crude waste glycerin byproduct of biodiesel production were tested as substrates for electricity production in single-chamber, air-cathode microbial fuel cells (MFCs) and in single-chamber microbial electrolysis cells (MECs), using pure and mixed microbial cultures as anode biocatalyst. Current densities of 0.40 A/m² and 0.13 A/m² were generated on 50 mM glycerol in aircathode MFCs by pure cultures of Shewanella oneidensis MR-1 and Rhodopseudomonas palustris ATCC 17001, respectively, after aerobic flask culture. A mixed culture of bacteria originally derived from wastewater generated higher current and power densities than any of the pure cultures and, at 10 mM glycerol, achieved an average maximum power density of 2.70 ± 0.15 W/m² anode surface area (47.8 ± 2.6 W/m³ reactor volume) at a current density of 7.66 ± 0.21 A/m² anode surface area. At an optimal fixed external resistance of 210 Ω the mixed culture MFC followed Michaelis-Menten saturation kinetics, resulting in a Km of 2.92 mM glycerol and a theoretical Vmax of 0.437 volts. Coulombic efficiencies decreased linearly with increased glycerol concentration. Power was generated by mixed culture MFCs from raw waste glycerin byproduct of biodiesel manufacture both with and without methanol, and with and without potassium salts and soaps. Maximum volumetric current and power densities achieved on waste glycerin (147.7 A/m³ and 56.8 W/m³) were greater than those reported in previous studies, but CE values (10-17.6%) were significantly lower, likely due to losses from aerobic respiration in the micro-aerobic environment of an MFC. Decreases in maximum current density of 43.4% and 65.1% were observed over successive batches of waste glycerin with and without methanol, respectively. Decreases in performance were attributed primarily to the presence of potassium salts, soaps, FFAs, and residual catalyst in the waste glycerin, rather than to methanol. MFCs operating on waste glycerin from which these salts and soaps had been precipitated did not show the same pattern of decreasing maximum current density over multiple batches. Cathode potentiometry indicated a decrease in cathode performance after development of a thick biofilm on the cathode surface during batches of glycerol and glycerin. The best fit lines of cathode potential vs. current density before and after cathode biofilm development during batches of glycerol were used to predict a 30.2% decrease in power density, a result that corresponded well to the 25.7% decrease in power density that was actually observed. Single chamber, membrane-free mixed culture MECs were able to produce hydrogen successfully from both pure glycerol and waste glycerin byproduct from biodiesel manufacture. At an applied voltage of 0.6 V, a maximum current density of 7.5 ± 0.4 A/m² (238.6 ± 12.7 A/m³) was observed, the highest reported current density for a MEC operating on glycerol. Maximum current densities on 0.5% waste glycerin with and without methanol were 0.1-0.2 A/m² less than previously reported values. Maximum hydrogen yields from the mixed culture MEC on 50 mM glycerol were 1.8 ± 0.1 mol hydrogen/mol glycerol, at an energy efficiency of 117.7%. Hydrogen yields on waste glycerin were an order of magnitude lower, though energy efficiencies were greater than those for pure glycerol. Hydrogen production rates were highest at 50 mM glycerol, reaching 1.3 ± 0.1 m³/day/m³. Methane formation reduced hydrogen recoveries by 30.4% and 36.8% for 50 mM and 10 mM glycerol, respectively, assuming cathodic losses of electrons went to methane formation. A culture dominated by a member of the genus Citrobacter produced high current densities of ~2.0 A/m² in MECs on 50 mM glycerol, at yields of 0.93 ± 0.05 mol hydrogen/mol glycerol, energy efficiency greater than 100%, and hydrogen production rate greater than that of fermentative hydrogen production from glycerol by this species and by a genetically altered E. coli strain. The results of this study suggest that, ideally with removal of potassium salts, soaps, and catalyst, and with some method to minimize biofilm formation on the surface of MFC air-cathodes, MFCs and MECs represent promising treatment methods to generate electricity or hydrogen gas while treating the waste products of biodiesel manufacture. These results represent an important step in improving the economic viability of the ever-growing biodiesel industry as the world energy economy continues to shift from fossil fuels to renewable energy sources.
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