Microbial fuel cells for the treatment of waste streams with energy recovery

Aerobic wastewater treatment is energy and resource intensive. As a consequence, the accessibility of clean water is closely related to the availability of energy. Interestingly, wastewater itself is a source of chemical energy, indeed 1 kg of carbohydrates corresponds to roughly 4 kWh of energy. The question is how to convert the chemical energy into a practical energy source which can be used on the spot. Microbial fuel cells (MFCs) enable to recover the chemical energy during the treatment of various organic (waste)streams as electricity. A typical MFC consists of an anode and a cathode compartment which are separated by a proton exchange membrane. In the anode compartment a substrate is microbially oxidized. During this microbial oxidation, not only protons and oxidized products are formed but remarkably electrons are transferred from the bacteria towards a solid electrode. The electrons flow through an electrical circuit towards the cathode where a final electron acceptor is reduced. The flow of electrons together with the positive potential difference between the anode and cathode, give rise to the generation of electrical power. The use of microorganisms to catalyze the electrochemical oxidation of organics is highly attractive as it allows to take advantage of the versatility and resilience which bacteria exert. However, to date no firm microbial characterization has been performed and there is a need for practical operational parameters to stimulate the growth and activity of bacteria. In addition, powerful and sustainable MFC reactors are needed for the direct conversion of wastewater into electricity at a practical energy level. The presence of an electrochemical active microbial community is essential to generate high power outputs using MFCs. Moreover, during this research, the evolution of the electrochemical and microbial features of MFCs have been related. During a continuous operation of about 3 months, a notable shift of both the microbial community and the maximum power output was noted. Whereas 6 MFCs had initially 6 different microbial communities, they all evolved to a specific consortium which was dominated by a Gram positive Brevibacillus species. The enrichment accompanied a decrease of the internal losses of the reactors, a shift in the polarization curves and an overall increase of the power outputs. However as this selection occurred autonomous, the question was raised how this selection could be steered, resulting in the faster growth of a highly active microbial community. Therefore, the operational parameters determining the activity of the microbial catalyst have been investigated. The anode potential is associated with the energy metabolism of the electrochemical active microorganisms. It is hypothesized that the higher the anode potential is, the more metabolic energy there is available for the bacteria, but the lower the power output for the consumer will be. During this research three MFCs were poised at an anode potential of respectively 0 (R0), -200 (R-200) and -400 (R-400) mV versus a Ag/AgCl reference electrode. However, R-200 had overall the highest performance. Indeed, during the 31 day test period, R 200 produced 15% more charge compared to R0 and R-400. In addition, R-200 had the highest maximal power density (up to 199 W.m-3 total anode compartment (TAC) during polarization). Conversely, the reactor poised at -400 mV had overall the lowest current and power generation. Moreover, during polarization, a sharp levelling of the current at an anode potential of -300 mV versus Ag/AgCl was noted. This was associated with a lower availability of energy to invest in electron conducting mechanisms. The maximum respiration rate of the bacteria during batch tests was also considerably lower for R-400. The specific biomass activity however, was the highest for R-400 (6.93 g COD.g-1 biomass-VSS.d-1 on day 14). However, this value lowered during the course of the experiment due to an increase of the biomass concentration to an average level of 578 +- 106 mg biomass-VSS L-1 graphite granules for the three reactors. This research indicated that an optimal anode potential of 200 mV versus Ag/AgCl exists, regulating the activity of bacteria to sustain an enhanced current and power generation. In spite of the importance of the anode potential, it might be a difficult to control parameter in future industrial MFCs. Closely related to the anode potential, is the applied external resistance. The latter controls the ratio of the MFC cell voltage and the current generation and as a consequence also determines the maximum power output. During this research, the influence of the external resistance and the loading rate on the electricity generation was investigated. During the experiments it was found that a doubling of the loading rate to 3.3 g COD.L-1 TAC.d-1 resulted only in an increased current generation when the external resistance was low (10.5 – 25 Ω) or during polarization. It was hypothesized that the amount of energy which was available for growth and maintenance of the bacteria, as determined by the external resistance, was probably too low to sustain a higher metabolic activity. Therefore, an increase of the loading rate needs to be accompanied by a decrease of the external resistance in order to increase the continuous current generation. Conversely, a lowering of the external resistance resulted in a concise and steady increase of both the kinetic capacities of the biocatalyst and the continuous current generation from 77 (50 ohm) up to 253 (10.5 ohm) A.m-3 TAC. Interestingly, lowering the external resistance will not necessarily result in a lower power output as our result showed that both the continuous current generation, power generation and the coulombic efficiency increased at lower external resistances. In general, when the external resistance approaches the internal resistance of the MFC, the increased current will be accompanied with a higher power output. Next, it is important to attune the external resistance and the volumetric loading rate. In a second line, improvements of the MFC technology were proposed to boost the power output to practical level and to improve the sustainability of MFCs. A new MFC design was developed consisting of 1.5 cm thin anode and cathode compartments which were filled with a three dimensional electrode structure. As graphite and carbon electrodes have many properties which render them highly suited for the application in MFCs, different structures (felt, granules and wool) of these materials were tested. When operated at an external resistance of 10.5 ohm and at a loading rate of 3.3 g COD L-1.d-1, the use of a three-dimensional graphite felt electrode yielded the highest power output amounting up to 386 W.m-3 TAC which was a factor 1.5 higher compared to a graphite granules electrode. The increase was probably due to a decrease of the contact losses. Conversely, the use of smaller graphite granules, resulted in a lower power and current generation. A possible issue remains the clogging of the anode compartment when wastewater containing particulates is used. Moreover, when the anode compartment thickness of an open air cathode MFC was increased from 1.5 to 7.5 cm, this resulted in a lowering of the power and current density. Therefore, a clever solution to enable a proportional increase of current while increasing the anode thickness will be needed. Due to the microbial boundaries and physical constraints, the absolute voltage and current of MFCs is limited. To increase the voltage and current to practical levels, the connection of multiple MFCs in series or parallel will be needed. However, the effect on the microbial activity of the series or parallel connection has not been investigated yet and was a subject of this research. The connection of the 6 MFC units in series and parallel enabled an increase of the voltages (2.02 V at 88 W.m-3 TAC) and the currents (255 mA at 95 W.m-3 TAC), while retaining high power outputs. The fact that the maximum power outputs were unaffected by the series or parallel connection is important for future application. However, during the connection in series, it was noted that the individual MFC voltages diverged at increasing currents, moreover some MFCs even switched polarity. As a consequence, an important drawback of the series connection was revealed: cell reversal. This phenomenon typically occurred when the current delivered by the in series connected MFC was higher than the current generation of the subunits. Ensuring an equal feed and - as we suggest - an equal catalyst (microbial and chemical) activity in the anode and cathode compartments, is crucial to prevent cell reversal. However, more research is needed to elucidate the exact reasons. The use of non-noble metal based cathodes can enhance the sustainability of MFCs. We demonstrated that an iron chelated complex could effectively be used as a catholyte or as an iron chelated open air cathode to generate current with the use of MFCs. An iron ethylenediaminetetraacetic acid (Fe-EDTA) catholyte generated a maximum current of 34.4 mA and a maximum power density of 22.9 W.m-3 total anode compartment (TAC). Compared to a MFC with a hexacyanoferrate catholyte, the maximum current was similar but the maximum power was 50% lower. However, no replenishment of the Fe-EDTA catholyte was needed. The creation of an activated carbon cloth open air cathode with Fe-EDTA- polytetrafluoroethylene (PTFE) applied to it increased the maximum power density to 40.3 W.m-³ TAC and generated a stable current of 12.9 mA (at 300 mV). It was observed that the ohmic loss of an open air cathode MFC was dependent on the type of membrane used. The development of specifc membranes for MFCs and the search for highly active cathodes are crucial for the future application of MBCs. The outcome of this research was fuelled by the quest towards the ultimate goal of MFC technology: the implementation as an energy efficient wastewater treatment. Therefore, in a third line, the use of MFCs as a technology for the treatment of wastewaters was explored. The treatment of a potato processing wastewaters and a hospital wastewater effluent with a MFC resulted in a power generation of up to 22 W.m-3 TAC. However, the power generation was dependent upon the buffer capacity of the wastewater. Moreover, the COD removal efficiency needs to be improved. Finally, a set of process configurations in which MFCs could be useful to treat wastewaters is schematized. To conclude, this research has resulted in new insights which are helpful for the future application of MFCs in the domain of wastewater treatment. This research provided a further characterization of the microbial catalysts, the important operational parameters for MFCs have been investigated and new technological advances have been proposed. The study of microorganisms in a “energetically controlled environment” is unique and allows to explore the microbial metabolism at a new level. It is expected that, next to the recovery of energy out of wastewater, many new applications for MFCs will emerge