| Summary: | 碩士 === 國立中興大學 === 環境工程學系所 === 107 === Microbial fuel cells (MFCs) are a novel energy technology, which could treat wastewater and transfer energy from organic compounds simultaneously. Compared to traditional wastewater treatment processes, MFCs are characteristic of lower energy demand and could produce extra energy for other units in wastewater treatment. Thus, they have become more popular in academic research recently. Moreover, Photomicrobial Fuel Cells (PMFCs) and Microbial Solar Cells (MSCs) are developed by combining light illumination and MFCs. It has been proved that the light-related MFCs could capture light energy and convert it into electricity.
Purple Non-Sulfur Bacteria (PNSB), Rhodopseudomonas palustris G11, were used in this study. They had been isolated from the enhanced biological phosphorus removal (EBPR) process in NCNU by Prof. Liang, which are capable of capturing light energy and storing energy as a polyphosphate, as the exoelectrogen in PMFCs and investigating its potential of power generation. The reaction system used single-chamber air-cathode microbial fuel cell with a rectangular cuboid shape and its volume was 250 mL. After operating for a period of time, the two reactors were placed from the original type to the horizontal lying type, due to the consideration of light illumination. It was hoped that horizontal lying type can increase the light illumination on the top receiving area, thereby increasing the electricity generation. As a result, the electricity generation situation of this two reactors was different. By observing the internal carbon papers in two reactors, the anodes of carbon paper, one was flat at the bottom, and the other set was upright. Furthermore, as the result of gravity , a most of the bacteria tend to precipitate in the reactors, which could increase the potentiality of contacting with the surface of anode, thereby increasing electricity generation.
In the first batch of experiments, the maximum voltage of the anode, which is vertical in center of reactor, was 2.9 mV. The maximum voltage of the anode which is horizontal at bottom of reactor was 13.6 mV. The values of voltage of the new anode reactor was higher than tranditional anode reactor about 4.69 times. In the experimental operation for 370 hours, the trend of voltage continued to decrease gradually and the TOC removal of both reactors were above 82%. In the second batch of experiments, the maximum voltage of the anode which is vertical in center of reactor was 6 mV. The maximum voltage of the anode which is horizontal at bottom of reactor was 31.03 mV. The values of voltage of the new anode reactor was 5.17 times higher than tranditional anode reactor. According to the experimental results, the design of the anode which is horizontal at bottom of reactor could really improve the electricity production of the microbial fuel cell effectively. To investigate the reason why voltage increased in the second batch, the solution in the reactors was sampled. The cell dry weight of the anode which is vertical in center of reactor with smaller electricity production was 1516.6±102.74 mg/L; the cell dry weight of the anode which is horizontal at bottom of reactor with larger electricity generation was 766.6±23.57 mg/L. The initial cell dry weight of both reactors was 725 ± 82.92 mg/L. Therefore, the result is completely different from the original inference that "the enhancing electricity is result of the increasing amount of microorganisms in the reactors".
Furthermore, this study designed another experiment to investigate whether the difference in position of anode in microbial fuel cells would affect the amount of microorganisms in reactors. Three identical reactors were used to operate in this experiment, and there are a control group without an anode in reactor, an anode which is vertical in center of reactor, and an anode which is horizontal at bottom of reactor. After being placed in microbial culture for a period of time, sampling from the three reactors separately, and the cell dry weight of the microorganisms in each reactors was measured. The result showed that the microbial biomass is indeed related to the position of anode in the same type of reactors. In addition, the result also showed that after microorganisms degraded the organic matter to produce extracellular electrons, the microorganisms would move to adhere to the surface of anode if the anode was placed in the reactor, and then the extracellular electrons would be transferred to the uncharged anode carbon cloth. Because the anode carbon cloth would be an electron acceptor, the electrons generated by the microorganisms were not used for self-cell synthesis. Therefore, the cell dry weight of anode placed in reactor was smaller than non-anode placed in reactor. Moreover, the utilization rate of total organic carbon was above 82.3% in the reators with anodes, which is higher than 71.8% of the reactor without anode. The result showed that the rate of microorganisms degrade organic matter could be accelerated if there were anodes placed in reactors. In another experiment about other anaerobic microorganisms, the change of amount of microorganisms which was caused by the position of anode occured only in the photosynthetic bacteria culture. Other non-photosynthetic anaerobic microorganisms or non-exoelectrogens were not significantly affected.
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