Applications

Overview

Oil is a finite resource and eventually, the world’s reserves will be depleted. To substitute the missing energy from the disappearing resources, one has to develop new strategies to produce sustainable energy. The recent approach to use microbial communities to generate electricity from organic matter could be a part of the solution here. Though wastewater treatment seems to be the most promising application for microbial fuel cells, they can be utilized in many other ways, for example in bioremediation systems or as power sources for environmental sensors.

Current Applications

Wastewater treatment

Figure1: Sewage treatment plant in Maryland (U.S.), where all major plants are required to upgrade to enhanced nutrient removal technologies that will remove most of the nutrients from the wastewater.
Image Credit: [http://www.chesapeakebay.net/ Chesapeake Bay Program]

Full-scale, effective MFC’s for wastewater treatment could generate constant amounts of power from a rather freely available substrate. Those cells could be implemented at suitable industrial locations, where reliable, substrate-rich effluents are present. Wastewater from food processing plants or digester effluents should be named here. Calculations show that a ten-year payback of the required investments can be achieved for a plant producing 7.5 t waste organics per day (Logan et al., 2006).
For the long term, electricity production from domestic sewage could also be made accessible through MFC’s. Currently, energy is invested to treat domestic wastewater, which contains approximately 9.3 times as much as the treatment energy itself (Logan and Regan, 2006). The usage of this contained energy could prospectively lead to a net excess of energy, where energy input is required at present.
As promising as those applications might sound, a scale-up of efficient MFC’s is currently not quite within reach. Material issues and cost-effectiveness are the key obstacles that need to be overcome by future research. Also, most of the present low-scale designs for MFC’s cannot be scaled up to turn into large wastewater treatment plants yet.

Bioremediation

A microbial fuel cell can be modified in its structure so that its purpose is no longer the production of energy, but the execution of other desired reactions. Besides the capability to provide electrons to the anode, bacterial communities can also accept electrons from the cathode to drive certain reactive pathways. Using this concept, soluble pollutants can be insolubilized and thus precipitated. Gregory et al. (2004, 2005) were able to clean up uranium-polluted groundwater by reducing uranium(VI) to uranium(IV) with microorganisms. The reduced species remained insoluble at the cathode and solubilized again, when the electrode was exposed to oxygen.

Environmental sensors

Figure2: Schematic design of a MFC with an integrated power switch. This build comes to use in sediment fuel cells with low current densities and allows them to build up current and output power periodically in bursts.([http://beweb.ucsd.edu/courses/senior-design/projects/2009/project_9/environmental.html Chris Kim et al.])

Sensors in the environment, especially looking at the ones with difficult accessibility like river or deep-water environments require power for their operation. When using batteries as power supply they have to be replaced with new ones periodically, which might be quite hard.
Microbial fuel cells could be used here to power these sensors by converting the available organic substrates in the surroundings to electric energy. Sediment fuel cells are worth mentioning at this point; they use organic matter from the sediment to generate electricity. Although they provide only comparatively low current densities, this problem can be solved by storing the energy and transmitting the sensorial data periodically in data bursts.

Hydrogen production

Similar to the above mentioned method of the bioremediation modification, a microbial fuel cell can be altered to use energy to make fuel, in contrast to the common converse operational mode.

Figure3: (a) A classic MFC-design is shown on the left, where waste organics are converted into electrical energy by bacteria. (b) An alternative setup of a MFC,where protons and electrons provided by the anodic reaction can recombine to Hydrogen gas at the cathode. Additional power must be supplied to drive this reaction.(Rozendal et al.)

An example for a targeted product in this approach would be hydrogen gas. Here, the protons and electrons generated in the anodic compartment can combine at the cathode under oxygen exclusion conditions. To activate this process however, an additional cell potential of ~0.25V must be put into the circuit.

Future Applications

Implantable biomedical devices

A often less noticed application for the MFC-technology is the generation of power within the human body. To function properly, the implanted medical devices require power which is mostly provided by batteries up to now. This is a serious issue, because they need to be replaced once drained, making another surgery inevitable. Therefore, a continuously power generating device would mean a huge break-through in patient care. MFCs could be those devices and could power cardiac pacing or glucose sensing implants.

Commercial use

Soon-to-be, microbial fuel cells could be redesigned from large scales to smaller ones with the aim to commercialize them. The broad public could potentially make good use of this mobile form of renewable energy. During camping trips into the open, an MFC could power all small electric devices like lamps. Tiny MFCs could be attached to belts and be used to charge mobile phones or music players on the go. Whole houses could be made independent of sewage treatment plants by cleaning their own wastewater with an MFC, which could be further fed with all organic wastes or compost from household and gardening.

References

• Logan, B. E., & Regan, J. M. (2006). Microbial fuel cells-challenges and applications. [http://pubs.acs.org/doi/pdf/10.1021/es0627592 Environmental science & technology, 40](17), 5172-5180.

• Gregory, K. B., Bond, D. R., & Lovley, D. R. (2004). Graphite electrodes as electron donors for anaerobic respiration. [http://onlinelibrary.wiley.com/doi/10.1111/j.1462-2920.2004.00593.x/abstract;jsessionid=A93CE35A839513E534255478956CD253.f03t01?deniedAccessCustomisedMessage=&userIsAuthenticated=false Environmental microbiology, 6](6), 596-604.

• Gregory, K. B., & Lovley, D. R. (2005). Remediation and recovery of uranium from contaminated subsurface environments with electrodes. [http://pubs.acs.org/doi/abs/10.1021/es050457e Environmental science & technology, 39](22), 8943-8947.

• Donovan, C., Dewan, A., Heo, D., & Beyenal, H. (2008). Batteryless, wireless sensor powered by a sediment microbial fuel cell. [http://pubs.acs.org/doi/abs/10.1021/es801763g Environmental science & technology, 42](22), 8591-8596.

• Kim, B. H., Chang, I. S., & Gadd, G. M. (2007). Challenges in microbial fuel cell development and operation. [http://link.springer.com/article/10.1007/s00253-007-1027-4#page-1 Applied Microbiology and Biotechnology, 76](3), 485-494.

• Logan, B. E., Hamelers, B., Rozendal, R., Schröder, U., Keller, J., Freguia, S., ... & Rabaey, K. (2006). Microbial fuel cells: methodology and technology. [http://pubs.acs.org/doi/abs/10.1021/es0605016 Environmental science & technology, 40](17), 5181-5192.

• Rozendal, R. A., Hamelers, H. V., Rabaey, K., Keller, J., & Buisman, C. J. (2008). Towards practical implementation of bioelectrochemical wastewater treatment. [http://www.sciencedirect.com/science/article/pii/S0167779908001595 Trends in biotechnology, 26](8), 450-459.