Civil & Environmental Engineering
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Item ENERGY-POSITIVE METHODS OF WASTEWATER TREATMENT-- AN EXAMINATION OF ANAEROBIC DIGESTION & BIO-ELECTROCHEMICAL TECHNOLOGY(2013) Gregoire, Kyla Patricia; Tender, Leonard; Torrents, Alba; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The results presented here demonstrate plausibility of a hybrid Anaerobic Digester-Microbial Fuel Cell (AD-MFC) system for anaerobic primary (AD) and secondary (MFC) treatment and resource recovery from high-strength wastewater. We empirically determine the treatment efficiencies and energy densities achieved by the AD and MFC processes, both separately and when integrated as primary and secondary unit operations. On the basis of current production, undigested wastewater yielded an stable anodic current of 131 A/m3 when continuously fed to triplicate MFCs (chronoamperometry, Ean, -0.200V vs. Ag/AgCl). Substrate limitations in digested sludge reduced anodic current--36 A/m3, 17 A/m3, and 9 A/m3 were achieved from 6d, 13d, and 21d digestate, respectively. Cathodic limitations severely limited power/energy production by the MFC, with maximum power output of 11 W/m3 (69 mW/m2). Presumably, this was due to mass transport of oxygen reduction intermediates. When AD and MFC processes are de-coupled (i.e. each fed with undigested wastewater), the energy realized from AD (as biogas) was, on average, 29.6 kJ per m3 wastewater treated (8.2 Wh/m3), whereas the MFC produced, on average, 2.1 kJ per m3 wastewater treated (0.58 Wh/m3). On the basis of COD removal, AD separately generated 9,110 kJ per kg COD removed (2,530 Wh/kg COD) whereas MFC separately generated 0.18 kJ per kg COD removed (0.05 Wh/kg COD). When combined as primary and secondary unit processes with a 6-d digestion period (reaction period which yielded the highest net energy production), the energy output from AD (as biogas) was 23.9 kJ per m3 wastewater; the energy output from MFC (as electrical power) was 2.1 kJ per m3 wastewater. MFC treatment rates exceeded 90% COD removal, 80% VS removal and 80% TS removal, likely owing to the upflow, baffled reactor design that maximized interaction between wastewater and the anodic biofilm. Results indicate an inverse logarithmic relationship between digester retention time and subsequent MFC current production, i.e. maximal MFC current production is achieved with undigested waste, and an inverse linear relationship between digester retention time and subsequent COD/VS removal in MFCs. Breakthroughs must be made to address cathodic limitations of MFCs, before scaling is practically or economically viable.Item Design and testing of a microbial fuel cell for the conversion of lignocellulosic biomass into electricity(2010) Gregoire, Kyla Patricia; Becker, Jennifer; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Previous research has demonstrated that microbial fuel cells (MFCs) have the ability to degrade soluble substrates such as wastewater; however, very few studies have attempted the conversion particulate biomass to electricity in an MFC. A single-chamber, air cathode MFC was developed using a solid, lignocellulosic substrate (corncob pellets) as the electron donor. The first trial, using a prototype reactor with a graphite rod anode, ran for 415 hours, and generated a maximum open circuit voltage and current of 0.67 V and 0.25 mA, respectively. The second trial employed graphite brush anodes and multiple microbial inocula. A pasteurized soil inoculum resulted in negligible power (P = 0.144 mW/m3). The addition of rumen fluid, which naturally contains cellulose-degrading microorganisms, and Geobacter metallireducens, resulted in Pmax values of 77 mW/m3 and 159 mW/m3, respectively. Analysis of hydrogen, methane, organic acids, and the mass of substrate consumed provided insight into the relationship between cellulose oxidation, methanogenesis, and power production.