Session C5
Paper #197
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MICROBIAL FUEL CELLS: AN ALTERNATE SOURCE OF ENERGY
Sara Kenes, [email protected], Sanchez 5:00, Salonee Saraiya, [email protected], Vidic 2:00
Abstract— There are instances in the world where
resources are dwindling and polluted wastewater is in
abundance. With the use of microbial fuel cells (MFCs),
wastewater can be treated and converted to electricity
simultaneously., without the use of dwindling fossil fuels.
This technology will reduce the number of toxins in the
water and protect beneficial resources currently being used
to generate electricity. This process is achieved through
oxidation-reduction reactions. Bacteria and microbes from
the wastewater are oxidized in the anode and break into
electrons and protons. Both of these travel to the cathode to
complete the reaction and generate electricity in the
formation of water. Using MFCs to transform energy and
treat wastewater prevents the need to use other energy
sources, while thoroughly decontaminating the water. When
comparing the two-chambered microbial cells to other kinds
of fuel cells and comparing municipal wastewater to other
kinds of water, the low power density output of the
wastewater conversion was clear. Along with the low
generation rate, the cost of maintaining a two-chambered
microbial fuel cell is questionable. However, there are ways
to modify the MFC to increase the power output and make
them more efficient. them more efficient. Over the years,
Scientists have termed MFCs as “energy of the future”.
Even today MFCs are expensive and this primary barrier
prevents further research in the production and distribution
of microbial fuel cells.
Key Words—Cellular respiration—electricity—Microbial
fuel cells—oxidation—reduction—wastewater
THE NECESSITY FOR AN ALTERNATE
SOURCE OF ENERGY
Water demand in the USA far exceeds the supply of
available freshwater. Approximately 355,000 Mgal/d of
freshwater is utilized every year, and the average American
family uses 300 gallons per annum [1]. Due to this, droughts
are extremely common in many states. Rainwater harvesting
is not applicable due to the scarcity of rains in those dry
regions. Population growth and dwindling supplies of
groundwater due to overuse of groundwater for agricultural
purposes and irrigation has led to scarcity as well. Irrigation
potentials are not utilized efficiently. Moreover, collection
University of Pittsburgh Swanson School of Engineering 1
Submission Date 03.03. 2017
efficiency of surface water is quite low with majority of it
being carried away to rivers [2]. The next major factor is the
inefficient treatment of wastewater. Globally more than 330
km3/year of municipal wastewater alone is produced [3].
Industries discharge untreated sewage and effluent into
rivers. Anaerobic treatment is the most commonly used
treatment for companies that do treat their wastewater.
Energy is produced through this process, but almost all is
released into the atmosphere in the form of methane. This
can be problematic, since methane is one of the most potent
greenhouse gases [4].
In the USA, approximately 38.354 million m3 of
sewage is generated per day [5]. That comes to about 1.4 x
1010 m3 of sewage per year out of which only about 30 % is
treated. The majority of wastewater produced goes untreated
and causes problems for the environment and those living
near the pollution. About 2 × 106 joules electrical energy
treating per m3 wastewater is consumed for treatment using
the traditional aerobic activated sludge treatment and
anaerobic sludge digestion techniques (AD) [5]. The former
treatment uses the mixing of water and oxygen (aeration) to
separate bacteria from the water, while the latter uses microorganisms to decompose the waste in the absence of oxygen.
These processes succeed at cleansing wastewater, but in the
treatment process, they produce carbon dioxide, oxygen, and
methane. The treatment of sewage water by these processes
is itself beneficial, but are counterproductive with the
formation of methane.
Reducing matters such as carbohydrates and ammonia
are present in the sewage, and allow a place to store
chemical energy. With the right treatment process, water is
able to be cleaned, while also harnessing the stored energy.
The higher the concentration of reducing matter, the more
energy contained in the water. In standard wastewater,
roughly 0.5 kg/m3 of oxygen is required to oxidize the
reducing matter. The energy density of the same standard
wastewater is roughly 107 J/m3 [5]. For comparison, this is
roughly 50 million times less energy than is stored in a
lithium battery. The energy density of wastewater changes
slightly when mixing with other water, such as domestic or
industrial. Nevertheless, treating wastewater produces five
times more energy than the process consumes.
Sara Kenes
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With the use of microbial fuel cells, energy from
sewage water is able to be converted into electricity at no
harmful risks to the environment.
electricity was first thought of in 1911 by M.C. Potter, and
the technology has been evolving over the last century [9].
MFC technology represents a new way to use bacteria
for generation of bioelectricity by oxidation of organic waste
and renewable biomass [10]. Since the most inexpensive
and abundant sources of carbon available are often industrial
wastewaters, MFC’s have great potential for improving the
efficiency of bioremediation for industrial waste by allowing
energy to be recovered during effluent processing.
WHAT ARE MICROBIAL FUEL CELLS?
Microbial Fuel Cells (MFCs) have been described as
“bioreactors that convert the energy in the chemical bonds of
organic compounds into electrical energy through catalytic
activity of micro-organisms under anaerobic conditions” [6].
In more simple terms, a fuel cell is a voltaic cell that uses
chemical reactions to convert some kind of reactant to
electricity; in this case, microbial fuel cells convert
wastewater to electrical energy. A typical MFC (see Fig. 2)
consists of two compartments - the anodic and cathodic halfcells - which are separated by a selectively permeable,
cation-specific membrane or a salt-bridge. The cell uses
oxidation- reduction reactions to convert the chemical
energy of wastewater into electrical energy.
CHEMICAL REACTIONS INVOLVED IN A
MICROBIAL FUEL CELL
In order to go through chemical reactions to convert
chemical energy to electrical energy, a microbial fuel cell
needs chemical energy. As long as there are microbes in the
wastewater to power the microbial fuel cell, chemical
reactions will continue to occur and electricity will be
generated.
To convert wastewater to electricity, a two-step,
oxidation-reduction reaction occurs. In the first step,
electrons are removed (oxidized) from some source of
organic matter in the absence of oxygen. In the second step,
these electrons are given to some source which can accept
them (reduction) such as oxygen [10]. Thus, there are two
electrodes: an anode, where oxidation takes place, and a
cathode, where reduction takes place. The anodic chamber
consists of microbes suspended under anaerobic conditions
in the anolyte and the cathodic chamber contains the electron
acceptor (oxygen). In essence, the electron donor is
physically separated from the terminal electron acceptor
across the two chambers.
More specifically, the reducing matters from the water
powering an MFC first go through the chemical reaction of
oxidation. Microbes in the reducing matter become attracted
to the anode, coated in some kind of metal. The anode
breaks down the microbe into electrons and protons. The
electrons are driven by the oxygen in the other half-cell and
move to the cathode through the external circuit, while the
protons diffuse through the electrolyte in the cell, both
ending up in the cathode. For every electron oxidized, a
proton is transported across the membrane to the cathode to
complete the reaction. The electricity is generated when both
the electrons and the protons react with oxygen to create
water [11]. Not only is electricity generated when going
through the oxidation-reduction reaction, but the water is
also being purified.
In this situation, carbon dioxide flows into and out of
the anode. The wastewater, including pure water and
microbes in the waste, and oxygen go into the reaction, and
water and a maximum of 6.9 W/m2 of power is produced
[12].
Electrons are transferred from 1 electrode to the other
either by using an external circuit, either electron mediators,
membranes, or directly using nanowires generated by
FIGURE 1 [24]
A diagram showing the oxidation-reduction
reaction
The process of a microbial fuel cell is similar in theory
to that of a chemical fuel cell, and even more similar to that
of other fuel cells. Chemical fuel cell reactions take place in
batteries, while all fuel cell reactions, including microbial
fuel cells, take place in a cathode. MFC and standard fuel
cells differ only in that MFCs, in this specific case, are
fueled by wastewater and microorganisms mediate
oxidation, while standard fuel cells are powered by hydrogen
and have a metal catalyst mediate. Essentially, microbial
fuel cells use anaerobic digestion along with cellular
respiration, while other types of fuel cells use a variety of
different reactions (aerobic digestion with cellular
respiration, reverse cellular respiration, etc.) to obtain their
products [8].
Fuel cells are a novel addition to the inventory of
alternate energy sources having minimal or no net-CO2
emission. The concept of using microbes to produce
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bacteria [12]. In order to optimize the electricity generated,
both the electrons and protons need to get to the cathode in
the most efficient way. With regards to the three different
external circuits, membranes are the best choice for protons
to diffuse through, while electron mediators and nanowires
generated by bacteria are the only options for electrons to
travel through [13].
Different kinds of microbial fuel cells have different
ranges of power output. Between each individual type, only
a few changes are going to affect the power, however,
changing the pH, temperature, ionic strength, or the substrate
concentration of the solution will affect the power
generation. Oftentimes, a low power output will be due to
high internal resistance, caused by anode and cathode
overpotentials, substrate concentration, or resistance in the
membranes [12].
fossil fuels, more specifically, natural gas [14]. Hydrogen
production by modified MFCs operating on organic waste
may be an interesting alternative to using such a large
amount of fossil fuels. Microbial fuel cells and fuel cells in
general are so diverse in the kinds of reactants necessary to
create a product.
In such devices, anaerobic conditions are maintained
in the cathode chamber and an additional voltage of around
0.25 V (roughly 7 times smaller than that of a AA battery) is
applied to the bacteria in the anode. The addition of the
voltage insures the possibility of makes pure hydrogen gas at
the cathode, and creates a spontaneous reaction [15]. Under
these changes, protons are reduced to hydrogen on the
cathode, and hydrogen is stored instead of generating
electricity. Protons and electrons produced by the bacteria go
through the same process as the wastewater does, except
hydrogen gas is a product in a process called the hydrogen
evolution reaction (HER).
When applying a voltage of 0.6V, hydrogen gas was
produced at a rate of 1.1 m3-H2/m3 of reactor per day [15].
Modified MFCs are termed bio-electrochemically
assisted microbial reactors (BEAMR), microbial electrolysis
cells (MECs), and biocatalyzed electrolysis cells (BECs)
[16]. The only difference between an MFC and a BEAMR is
the presence of oxygen in the MFC cathode, and the
presence of a current in the BEAMR anode.
BOD Sensors
Biochemical oxygen demand (BOD) sensors are
widely used tools that test for the amount of dissolved
oxygen needed by aerobic organisms in a body of water to
break down organic materials. BOD sensors are not always
the most reliable or efficient, since taking one measurement
requires an immediate test of the dissolved oxygen levels,
and another dissolved oxygen test after the sample was
incubated for 5 days. The BOD sensor is also difficult to use,
and often results in errors, even with the most experienced
user.
MFCs have the ability to act as a BOD sensor, since
the amount of charge produced by the MFC is proportional
to the concentration of microbes used. MFC-type BOD
sensors were able to function for nearly 5 years with
minimal maintenance required. However, the signal from the
MFC fluctuated when near electron acceptors of higher
redox potential, such as nitrate and oxygen [17]. This can be
assumed is happening due to the redox reactions taking place
while the MFC is function as a converter.
FIGURE 2 [25]
An example of an electrolytic cell inside a Microbial
fuel cell
APPLICATIONS OF MICROBIAL FUEL
CELLS
With the ability of a highly beneficial energy generator,
microbial fuel cells have a multitude of uses, from treating
brewery and domestic wastewater to desalination, hydrogen
production, and remote sensing, to pollution remediation,
and as a remote power source. Due to the changeability of a
microbial fuel cell, adjustments can be made to the
technology to adapt to different needs or generate different
products.
Hydrogen Production
Power supply to remote sensors
Hydrogen production is the process of generating
hydrogen, oftentimes with steam reforming from
hydrocarbons, hydrolysis, or thermolysis. In the United
States, hydrogen production is more than a $100 billion
industry, however 95% of the hydrogen used comes from
Remote sensors are often used by major companies, the
government, or the military, to gather specific data over a
period of time. For longer-term investigations, the charge in
the sensors don’t last, and a significant amount of money
could be spent replacing batteries in the sensors so as not to
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lose the data gathered. The ability of an MFC to create
electricity with a wide variety of substrates makes it’s the
perfect technology to power remote sensors, especially when
the sensors are underwater.
Although the power density slightly increased over the
period of one month, MFCs struggled in a real-life
experiment testing this situation. There are many different
kinds of microbial fuel cells, and each one performs
differently. In the tests done by Trophos Energy, MFC
performances between different kinds of fuel cells were
varied. The study concluded that the environment for an
MFC to thrive in is very particular, but overall the fuel cells
did better as a long-term power source than standard
batteries [19]. Microbial fuel cells are capable of being longterm power sources, but only as a last resort.
way to produce energy, in this scenario, more energy is
created from nonrenewable resources and other kinds of
alternative power (solar, wind).
Within the spectrum of microbial fuel cells, there are
better options than the two-chambered cell. Air-cathode
MFCs are single-chambered cells that generate 28 mW/m2
when converting wastewater to electricity, which is nearly
10 times the amount of electricity produced by doublechambered MFCs [20].
OUR FOCUS: CONVERTING
WASTEWATER TO ELECTRICITY USING
MFCs
With 2 million tons of human waste alone being
dumped into water sources every day, and the dwindling
amount of fossil fuels remaining, using an abundant resource
in place of those in danger of running out is beneficial in
multiple ways. Most of this wastewater contains almost 10
times more energy than is needed for treatment in a modern
waste plant. Microbial fuel cells thrive on the organic
materials especially in wastewater, and use that to fuel
oxidation-reduction reactions, generating electrical energy.
When the organic compounds present in the waste become
oxidized, electrons are released yielding a steady source of
electrical current that is present for as long as organic
compounds are present.
Microbes have the ability to simultaneously treat
sewage water and generate electricity. As one of the only
types of energy sources that can achieve that level of
productivity, while also being beneficial to the environment,
microbial fuel cells can be the future of energy generation.
Flexible with its reactants, MFCs can treat multiple types of
water, as well as a variety of different substrates and
compounds.
FIGURE 3 [26]
Schematic of a simple two-chambered Microbial Fuel
Cell
MAJOR PROS AND CONS OF A
MICROBIAL FUEL CELL
Benefits to Microbial Fuel Cell Use
Using microbial fuel cells to convert wastewater to
energy allows a safer method of reducing the toxins in
wastewater compared to more standard ways of treating
water. Standard methods of treating water involve adding
chemicals that don’t get completely filtered out. MFC
reactions treat wastewater with the breakdown of sewage by
electron transports and chemical reactions, and
decontaminate 99% of the bacteria in the process [20]. The
chemical reaction takes place in an anaerobic cathode, which
causes a decrease in the amount of sludge produced, since
there is no oxygen for the bacteria to survive on. MFCs
allow for a more efficient way to produce electricity, while
being more eco-friendly and using less fossil fuels than other
sources of energy. Since there is no carbon dioxide produced
from the oxidation-reduction reactions, MFCs have a nearly
nonexistent carbon footprint.
Efficiency of Conversion
Having an effective MFC would entail having high
efficient sustainability, high power density, and low cost.
The maximum power output of a standard-sized
microbial fuel cell treating municipal water is roughly 0.17
Wm-3 [19]. Comparing the municipal wastewater supply to
other kinds of water, not as much energy is created. Treating
brewery wastewater generated .830 W/m3, and treating
synthetic wastewater generated .00673 W/m2 [20]. These
differences are likely due to the excess organic substances in
the water and their tendency to oxidize more. Although
using the two-chambered MFCs allows for an alternative
Drawbacks to Microbial Fuel Cell Use
Although microbial fuel cells produce a comparable
amount of energy for their size, other forms of fuel cells are
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generating more electricity, and from better water sources.
The use of municipal wastewater paired with the doublecambered cathode doesn’t have as many organic substances
to be oxidized and produce energy, and the size compared to
other fuel cells hinders its output. Because of the importance
of chemical reactions in the conversion process, MFCs are
extremely sensitive to temperature changes. Within a 10degree C difference, the MFC reduced the power density
output by nearly 40% [20]. Since MFCs are a relatively new
technology, these fuel cells aren’t researched as thoroughly
as other technologies, leading to some uncertainties about
the technology after lengthened use. Also due to its novice
presence, the cost of making and maintaining the MFC is
more expensive than nearly every other kind of energy
source.
Compared to other forms of technology, and even other
forms of fuel cells, MFCs are sub-par. For the price of
maintaining an MFC, the power density output should be
higher than those of fossil fuels, however this isn’t the case.
The use of other kinds of microbial fuel cells, and fuel cells
in general would offer the same benefits, but produce a
higher power density, being more efficient.
research in microbial fuel cells. MFCs have been known for
their sustainability and the only obstacle to overcome is cost.
THE FUTURE OF MICROBIAL FUEL
CELLS
Currently, most research done on microbial fuel cells is
done at higher level Universities and through government
organizations. Microbial fuel cells are being adapted by
government organizations to fit the needs of the military. In
one instance, MFCs were adapted to retain and generate
energy for long periods of time in order to power marine
sensors [22]. Many large corporations were thoroughly
involved in the production and implementation in the early
2010s, but stopped the concentration as the budget became
too constricting.
Though it might be too much to expect an MFC to
produce power density equivalent to a hydrogen fuel cell,
there is scope for further improvement. Current research is
mainly focus to abate these problems. Once solved, MFCs
can be used as an alternate source of power while treating
wastewater at the same time. If power generation in these
systems can be increased, MFCs may provide a new method
to offset operating costs of waste water treatment plants,
making advanced waste water treatment more affordable in
both developing and industrialized nations. MFC power can
be scaled up on its own, however, this seems unlikely
currently. Increasing the reactor size, as well as the treatment
capacity to a comparable size to other energy sources would
help the MFC achieve levels of useful energy [2]. Along
with this, optimizing the internal resistance, pH buffering,
cost, and cultures would raise the power density to a
practical level.
Power generated by MFCs can be increased simply by
integrating another energy source. Combining MFC
reactions with anaerobic digestion processes would allow for
a broader use with complementing processes. Microbial fuel
cells treat medium and low-strength waters efficiently, while
AD treats high-strength waters the same. Working closer
together, substrates going through AD are broken down in a
more feasible way to go through the MFC and create
stronger electricity.
SUSTAINABILITY
Sustainable wastewater treatment is a fascinating
concept that promises to partially address the multiple
challenges of energy shortage, resource depletion and
environmental pollution. It is widely accepted that a
sustainable treatment process should strive for: neuralenergy operation, balanced investment and economic output,
stable treatment performance, high effluent quality to meet
water reclamation and reuse requirement, less resource
consumption, a low environmental footprint, and good social
equity. MFCs have been conceived and intensively studied
as a promising technology to achieve sustainable wastewater
treatment. One of the most commonly-quoted advantages of
MFCs is their capability to directly extract electric energy
from organic matters in wastewater. Unlike other energy
products such as CH4 or H2 produced in AD processes,
electricity is a cleaner and more widely utilizable form of
energy [21]. Moreover, MFCs can work well at ambient
temperature and thus consume less energy for temperature
maintenance than AD reactors. It has been reported that
MFCs could produce normalized energy recovery (NER) of
0.026 kWh/m3 wastewater, or 0.080 kWh/kg chemical
oxygen demand (COD) from municipal wastewater.
Along with this, environmental sustainability is a high
priority today. Currently, MFCs are also known as an
energy-saving technology resulting from reducing aeration
(for air-breathing cathode MFC) and less sludge production
than the conventional activated sludge processes [21].
breathing cathode MFC) and less sludge production than the
conventional activated sludge processes [11]. This is most
certainly a step in the right direction for the ongoing
ECONOMIC FEASIBILITY
Costs associated with microbial fuel cells include:
initial capital investment, operation/maintenance of the cells,
and energy, chemical, and material consumption fees. To
counteract this cost, subsidizes might be given by the
government, and you may obtain a fee from the contaminant
discharge. Additional revenue can also be accounted for
from
generated
electricity,
nitrogen/
excess
wastewater/fertilizer, and treated water [2].
Making a fully functional microbial fuel cell requires
quality materials. The more complex or in depth your MFC
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[2] W. Li, et al. “Towards sustainable wastewater treatment
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is, the higher the cost is for just materials. Not taking into
account size of the MFC, the total cost, including operational
fees, capital cost, and materials, ranges from $4.2-25.5
million, depending on how long the MFC will run [23].
Considering the net income, MFCs that run for shorter
amount of time are going to have a higher net income,
especially compared to the MFCs that run for significant
time. The net income could range from a $4.6 million gain to
a $15 million loss [23].
This price might be economically feasible for a major
energy corporation or sewage plant, but doesn’t seem very
realistic for a hobby. However, due to the large cost and the
relatively low power density, questions arise on whether this
specific kind of MFC would be a good investment,
especially for commercial production. Since there is such a
low power density, a large amount of MFCs would be
required to generate a significant amount of electricity,
which would end up costing hundreds of trillions of dollars.
This primary barrier prevents further research to perfect a
microbial fuel cell.
CONSENSUS OF MICROBIAL FUEL CELLS
Although the use of microbial fuel cells to convert
wastewater to electricity appears to be a good idea, there are
many flaws. The MFC we were focused on was the twochamber fuel cell powered by microbes found in municipal
wastewater, which was effective and semi-efficient for the
sole purpose of converting microbes to electrical energy.
The conversion to electricity did work, but to generate a
significant amount electricity would require a large amount
of MFCs. The cost of generating a significant amount of
energy from the microbial fuel cells could easily be spent on
another energy source with better power outcome. While the
MFC is beneficial to the environment in protecting the use
of fossil fuels, the use of more powerful MFCs can also be
used at the same benefit to the environment.
Within the span of different microbial fuel cells, other
cells are more efficient and have a higher power density than
the two-chambered cell. Not only is the fuel cell not as
efficient, but the type of water being treated does not allow
for large amounts of energy to be produced due to the
amount of organic matter it contains.
Microbial fuel cells do their job and treat water while
simultaneously generating energy, but at a level that isn’t
realistic. There are plenty of other options for energy sources
that would cost less and produce more electricity.
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ACKNOWLEDGMENTS
We would like to thank our conference co-chair Harrison
Lawson for helping us proof read our paper and providing a
resourceful feedback for improving its content. We would
also like you acknowledge the writing center for providing
us with a neutral opinion on our previous writing proposal.
This gave us a ground for the outline of the paper.
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