M.Sc. Thesis: Electricity production in a microbial fuel cell fed with spent organic extracts from hydrogenogenic fermentation of organic solid wastes

Abstract

Coupling of hydrogen production from the anaerobic fermentation of organic solid wastes with post-treatment of its spent extracts in microbial fuel cells, can substantially increase the energy efficiency from municipal organic wastes. Therefore, the aim of this work was to evaluate the electricity production and the general performance of a lab scale microbial fuel cell (MFC) during the batch treatment of extracts from depleted solids generated in fermentative hydrogenogenic process.
As part of the main objective of this work, the microbial fuel cell was characterized by the construction of a polarization curve testing two different inocula, methanogenic and sulfate-reducing. The polarization curve showed that the internal resistance of the microbial the cell load with a methanogenic inoculum was four times higher than the cell load with a sulfate-reducing inoculum (33 y 8 k, respectably). The high internal resistance of both inocula affected the current and the power from the microbial fuel cell (methanogenic inoculum: 5x10-7 and sulfate-reducing inoculum: 1x10-5 W).
After the microbial fuel cell was characterized with two types of inocula, batch essays were conducted with each type of inocula: methanogenic and sulfate-reducing. The voltage at open circuit was very similar to those reported in literature (0.4-0.6 V). The power density produced by the sulfate-reducing inoculum was similar also in comparison to other research groups with microbial fuel cells under different conditions of inocula and substrate (12.31 mW/m2), whereas with a methanogenic inoculum the power density was low (1.96 mW/m2). the performance in batch of the microbial fuel cells was measured as a function of the chemical oxygen demand decrease and as a function of the Coulombic efficiency. The decrease of the chemical oxygen demand was no greater than the 50% for the batch essays (methanogenic inoculum: ηCOD=25% and sulfate-reducing inoculum: ηCOD=43%).
Because of the high values for the internal resistance for the microbial fuel cells, a procedure was carried out in order to diminish this resistance and in that way to improve the general performance of the cell. The sulfate-reducing inoculum was used because it has proven a better performance at the batch tests, however the concentration of inoculum load to the cell was greater by modifying the feed for the sulfate-reducing reactor adding activated sludge. During the operation of the cell in repeated batch electricity was produced constantly for more than 504 h (ECCM =0.2239-0.2284 V). The operation of the cell was carried out under an external resistance equal to 1000 , this was used to corroborate that the inoculation protocol was successful because the internal resistance of the cell decrease from 10 000 to 1000  and to improve the cell performance (from PAn=12.31 mW/m2 in batch test to 27.34 mW/m2 in repeated batch test). The electrochemical performance was better when the concentration of the feed was lower, showing an inhibitory effect. In other way, the biochemical performance (as decrease of the chemical oxygen demand) was better when the feed cell concentration was higher, indicating the additional capacity of the microbial consortium for the treatment of polluted effluents (batch: ηCOD=43% and repeated bacth: ηCOD=62%).
Finally to corroborate the presence of fixed microorganisms over the surface of the anode, an analysis by scanning electron microscopy was carried out. On the flexible carbon-cloth Toray used as a control microbial growth was not found. On the anode used in the repeated batch test different microbial shapes were found (coccus, sarcina, resembling methanosarcina and methanosaeta), not only for what could be sulfate-reducing bacteria but also for metahanogenic archae. It is necessary to mention that there was not enough experimental evidence to corroborate this statement.