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


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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.
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The Mexican pulp and paper industry and municipal authorities are facing increasing regulatory and cost-related pressures regarding the handling, treatment and disposal of waste sludge and solid wastes. The dry anaerobic digestion (DASS) is a promising alternative for the co-stabilization of waste sludge, municipal and industrial solid wastes. However, appropriate and fast process start-up is a bottleneck for the dissemination of DASS technology in developing countries. This work aimed at determining a reliable and fast DASS start-up procedure from non-anaerobic inocula for the digestion of a mixture of paper sludge, waste sludge and municipal solid waste. Three types of inoculum were used: cattle manure (CM), soil (S) and waste activated sludge (WAS). Results were analyzed in terms of the stabilization time Te (the time required to develop a full methanogenic regime) and the overall start-up time To (time required to reach at least 25% TS inside the reactor since the inoculation). A factorial experiment was implemented; factors were the inoculum type (five combinations of CM, S and WAS), temperature (35 and 55°C) and loading rate (4.5 and 8.2 g VS/kg.d). Results showed that the fastest start-up was obtained with reactors using inoculum I3 (33% CM, 33% S and 33% WAS) at 55°C and 8.2 g VS/ loading rate. Interestingly, thermophilic regime favoured shorter stabilization times, in spite of the fact that the inocula used were meso- or psychrophilic. Results from DASS reactors that reached steady state after start-up (I3 and I4) showed typical performance responses of 60% removal efficiency (% total VS basis), biogas productivity between 2.7-3.5 NL/kg, a in the range 0.3-1.3 and pH around 8. This suggested that the DASS process is a feasible alternative for co-digesting paper-mill sludge, MSW and biosolids.
In a recent communication we demonstrated that electrodes consisting of a platinum electrocatalyst covered by a conductive polymer like polyaniline are well suited to serve as anodes harvesting electricity in microbial fuel cells [Angew. Chem. Int. Edn. 42 (2003) 2880]. In this communication we show that the fluorinated polyanilines poly(2-fluoroaniline) and poly(2,3,5,6-tetrafluoroaniline) outstrip the parent compound polyaniline in their performance as an electrode modifier. Similar to polyaniline they improve the catalytic activity of platinum towards the oxidation of hydrogen, product of the anaerobic microbial metabolism. Compared to polyaniline the fluorinated polymers are superior in the protection of platinum from becoming poisoned by metabolic by-products. The high stability of poly(2,3,5,6-tetrafluoroaniline) towards microbial and chemical degradation makes this compound a highly promising candidate for applications in microbially aggressive environments like sewage or sewage sludge.
Alkalinity measurements to pH endpoints of 5.75 and 4.3 were used to evaluate process stability of anaerobic digesters that treat a high-strength waste of various HRTs and feed concentrations. Total volatile acids concentrations, which varied between 500 mg/l as HAc during steady-state periods and nearly 10,000 mg/l as HAc during stress periods, correlated strongly with intermediate alkalinity (from pH 5.75 to 4.3). Partial alkalinity (to pH 5.75) was used to approximate bicarbonate concentration. Combination of both alkalinites into an IA: PA ratio greatly increased the sensitivity of the alkalinity test, and provided a simple, inexpensive parameter to detect process upset and recovery.