| Since organic wastewater treatment is an energy intensive technology, high energy consumption is one of main problems for wastewater treatment. Traditional wastewater treatment processes not only needed large amount of fossil energy, but also discharged CO2 into atmosphere, incurring new environmental problems. Microbial fuel cell (MFC) is a new environmental technology for wastewater treatment with simultaneous energy recovery in 21st century. In MFCs, chemical energy from organic pollutant can be directly converted into electrical energy by anodic exoelectrogenic bacteria. However, the mechanism of electron transfer in MFCs is still not clear. Considering for its future applications, high cost and the mechanisms of environmental factors affected the performance of MFCs still need to be investigated. In present thesis, research works were performed focusing on the multiple biomass conversion, development of new electrode materials and low-cost operation of MFCs. The aim is to decrease the cost of MFCs for future applications. Beer brewery wastewater was demonstrated for the first time as substrate in MFCs, and the maximum power density was 205mW·m-2 (30°C) in an air-cathode MFC using aboriginal bacteria as inoculum and full-strength wastewater as substrate. Decrease of temperature resulted in the decline of cathode potential and power density (170 mW·m-2). However, the COD removal efficiency and Coulombic efficiency (CE) were not affected by the temperature. Addition of buffer with concentrations of 50 and 200mmol·L-1 increased power densities. Power densities increased with increase of wastewater concentration, accompanied with a decrease of CE. Corn stover hydrolysate was also demonstrated to be suitable substrate for electricity generation in MFCs. The maximum power density was 644mW·m-2. Conductivity affected the anode potential, resulting in an increase of power density, followed by a decrease to 351mW·m-2. When the PBS concentration increased, power density possiblely due to high buffer capacity provided a neutral surface condition for cathode. By the bioaugmentation of exoelectrogenic bacteria and celluosic saccharificating bacteria (H-C), it was demonstrated for the first time that natural corn stover can be used for electricity generation in MFCs. The maximum power density was 331mW·m-2. Power can be increased into 406mW·m-2 when substrate was switched into steam exploded corn stover residual solids. Community analysis showed that the potential exoelectrogene in this system was Rhodopseudomonas palustris.New inexpensive anode material, carbon mesh, and Nafion/PTFE binder for cathodes can decrease both the unit volume cost and unit power cost by more than 70%, When using carbon mesh instead of E-TEK carbon cloth, the cost of anode materials reduced by 75%, and the maximum power density increased by 7%. The performance of carbon mesh without any pretreatment was low. However, after heating in furnace (450°C, 30min), the maximum power density was 3% higher than that using carbon mesh only cleaned by acetone, or 7% lower than ammonia treated carbon cloth. The mechanisms of performance enhancement after pretreatments were as follow: Acetone cleaning and heat pretreatment removed the surface contaminations that affected electron transfer, provided a larger electrochemical active area for electron transfer. Besides surface cleaning, N/C atomic ratio was one time higher after high temperature ammonia treatment than untreated samples, showing that nitrogen related functional groups (e.g. amine group) that facilitated electron transfer from bacteria to electrodes were formed. Based on the hypothesis, functionalization was performed using diazonium salt to connect amine group on the surface of carbon cloth anode. Power densities were not significantly increased when the amine group content is low. However, when the functional group content increased to 0.4% and 0.9% (w/w), the maximum power densities were similar with that obtained using ammonia treated anode, and the surface protein content were also similar. Over fictionalization inhibited bacterial growth, resulted in a decrease of power density. Novel inexpensive Nafion-PTFE mixed binders were developed to decrease the total cost of air-cathode. The maximum power densities linearly increased with percentage of Nafion in binders. The cost of binder will be decreased by 50 % with only a decrease of 25 % in power. During 25 cycles, the CEs of MFCs with Nafion binder was 20-29%, addition of PTFE into binders slightly decreased CEs to 17-26 %. No distinct change in maximum voltages was observed.The start-up time was decreased by poising anode potential at +200mV versus Ag/AgCl, which decreased the operational costs. High current generated in poised potential system was due to the high driving force of substrate oxidation. Similar performances were observed in MFCs using different start-up methods when systems were well acclimated. Further tests showed that higher anode potential over a range from -400 to +200mV resulted in a higher and earlier current peak. However, when anode potential increased further to +400mV, the peak current decreased to 6.9mA. CEs increased with anode potentials, and the low CE at low anode potentials (-400 to 0mV) were due to H2 and CH4 generation. Increasing anode potentials enhanced driving force of substrate oxidation and therefore inhibited the production of fermentation gas. The fraction of electron for bacterial growth decreased at high anode potential, indicating that the growth of bacteria was inhibited.To remove the energy consumable cathodic aeration, microbial carbon capture cells (MCCs) were constructed by growing Chlorella vulgaris in catholyte for in situ carbon capture and oxygen generation. The maximum power density was 5.6W·m-3. Cyclic voltammetry and dissolved oxygen tests showed that the cathode reaction was oxygen reduction. All the gaseous CO2 was sequestrated. Carbon balance showed that the carbon capture rate of MCC was 94±1%. MCC provided a new method for wastewater treatment with zero carbon emission.
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