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Microbes are dynamic organisms that are associated with building various innovative technologies. One such developing technology is microbial fuel cells (MFC) that can produce electricity from organic waste and reduce carbon footprint and environmental pollution. Many industries have adopted this technology for various applications such as wastewater treatment, bioenergy production, and biosensors.
One of the potential microorganisms that produce electric current is Sporomusa ovata, which is an anaerobic Gram-negative bacterium that converts hydrogen and carbon dioxide to acetate through fermentation.
In comparison to conventional methods, MFC significantly decreases the energy requirement of wastewater treatment plants. MFC realizes this by engaging electroactive bacteria capable of oxidation of organic substances and transferring the electrons released to a solid electron acceptor, i.e., an electrode-anode.
The understanding of biofilm dynamics (pure or mixed culture microbes, extracellular electron transfer, and interface characteristics) and the development of materials such as catalysts, electrodes, and membranes have helped reduce production costs and improve the technology’s efficiency.
How do Microbial Fuel Cells Generate Electricity?
Electron-reducing organisms produce energy by donating electrons to the anode. In other words, MFC utilizes several bacteria that can catalyze electrochemical oxidations or oxidation/reduction reactions to harvest electric current.
Energy production by microbes via an electron transfer system occurs by adopting different pathways. Some of these mechanisms are described below:
- Electrons transferred to the anode through a soluble mediator in the solution covering the anode.
- Electrons transferred directly to the anode via proteins present on the bacterial outer membrane. For example, Shewanella oneidensis utilizes cytochrome c to transfer electrons. However, it requires an anaerobic environment for the conversion of lactate to acetate.
- Electrons transferred to the anode by the bacteria that form a thick biofilm on the cathode, the bacterial pili or nanowires. This reaction involves electrons from the cathode to reduce substances in the cathode chamber. The bacteria convert oxygen to water (aerobic chambers), nitrate to nitrite, nitrogen or sulfate sulfur ions (anaerobic environments), or carbon dioxide to methane or acetate. For example, we can consider the case of Geobacter sulfurreducens that converts fumarate to succinate with electrons obtained from the cathode.
Mathematical Model
Researchers have published a new mathematical model in the May 2020 edition of Nature for the calculation of microbial fuel cell density.
Scientists believe this model can predict the amount of energy produced by MFC. lt is based on the dimensional analysis approach and correlates eight important system variables (functional dependencies) to predict power density such as COD concentration, solution conductivity, and an anode's projected surface area. MFC researchers can use this model to assess initial power density and modify their optimum outcome resources.
Industrial Applications of Microbial Fuel Cells
An increase in MFC's industrial usage, for example, in the production of biosensors, wastewater treatment, and robotics has been observed.
Sensors and biosensors developed using conventional processes are often equipped with electrochemical batteries, such as lithium batteries. These batteries have a limited life that needs a periodical recharge or replacement.
The self-renewable MFC offers a long-standing power supply for biosensors and remote monitoring sensors. This technology also lowers operating costs and environmental risks.
It is essential to assess the quality of water for humans, animals, and plant consumption. The water quality depends on several crucial parameters. MFC acts as a fitting sensitive system (biosensor) since the presence of any toxic substance would adversely affect the metabolic activity of microbes. However, the selection of a specific type of electroactive microbe increases the sensitivity to particular toxicants. Therefore, it is crucial to design biosensors keeping in mind the required monitoring needs.
MFC is also used for the determination of biochemical oxygen demand (BOD) levels. BOD control is essential for monitoring the level of pollutants and microbial activities in groundwater.
MFC-based BOD sensors offer a simple, rapid, non-invasive, and cost-effective screening system. There is considerable demand for MFC for wastewater treatment application because several microbes can eliminate sulfides as required in wastewater treatment.
Some of the companies that utilize MFC technologies are discussed below.
Cambrian Innovation Inc
Cambrian Innovation is associated with developing and marketing environmental products using MFC. It has developed an advanced bioelectrochemical wastewater treatment system used in municipalities and several industrial facilities. It utilizes exoelectrogens, i.e., microbes directly in contact with electrodes, to produce electricity.
The company has also developed real-time, low-cost, and long-lasting bioelectrochemical nitrate sensors using MFC for monitoring surface water, which is essential across different industries such as precision farming.
Cambrian Innovation has a denitrification system with electrically active, nitrate-reducing bacteria in a proprietary cell. This system removes nitrates from discharge water, which helps to tackle a serious problem faced by various industries and agricultural sectors.
Through funding by the National Aeronautics and Space Administration (NASA), Cambrian Innovation developed crewed life support using MFC. It demonstrated the production of electricity from the water for reuse. This energy can be utilized for multiple functions.
Cambrian Innovation has also developed a superior air revitalization system for life support onboard the International Space Station, decreasing the costs associated with carbon dioxide reduction.
MICROrganic Technologies
MICROrganic Technologies has established a novel method for wastewater treatment. Its VIVA module, based on MFC, is an advanced system that provides 24/7 real-time monitoring, a significant reduction in sludge, production of DC power, and treatment of organic waste to the discharge phase.
Prongineer
Prongineer claims its unique open-air MFC design is cost-effective and highly efficient. It increases the cathode-air interface surface area and maximizes electrical potential. The device is robust, versatile, and can be installed in various sectors with residential septic tanks and large municipal wastewater treatment plants.
References and Further Readings
Gadkari, S., Sadhukhan, J. (2020) A robust correlation based on dimensional analysis to characterize microbial fuel cells. Science Report. 10, 8407. https://doi.org/10.1038/s41598-020-65375-5
Ivars- Barcelo, F., et al. (2018) Novel Applications of Microbial Fuel Cells in Sensors and Biosensors. Applied Science. 8(7), 1184; https://doi.org/10.3390/app8071184
Prongineer. [Online] Available at: https://prongineer.com/localhost/prongineer.com/index.html
Cambrian Innovation [Online] Available at: https://cambrianinnovation.com/
MICROrganic Technologies [Online] https://www.microrganictech.com/
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