In an era of climate change, alternate energy sources are desired to replace oil and
carbon resources. Subsequently, climate change effects in some areas and the increasing
production of biofuels are also putting pressure on available water resources. A microbial
fuel cell (MFC) or biological fuel cell is a bio-electrochemical system that drives
a current by using bacteria and mimicking bacterial interactions found in nature. MFCs
can be grouped into two general categories, those that use a mediator and those that are
mediator-less. The first MFCs, demonstrated in the early 20th century, used a mediator: a
chemical that transfers electrons from the bacteria in the cell to the anode. Mediator-less
MFCs are a more recent development dating to the 1970s; in this type of MFC the bacteria
typically have electrochemically active redox proteins such as cytochromes on their outer
membrane that can transfer electrons directly to the anode.Since the turn of the 21st
century MFCs have started to find a commercial use in the treatment of wastewater.
Microbial Fuel Cells have the potential to simultaneously treat wastewater for reuse and
to generate electricity; thereby producing two increasingly scarce resources. While the
Microbial Fuel Cell has generated interest in the wastewater treatment field, knowledge is
still limited and many fundamental and technical problems remain to be solved Microbial
fuel cell technology represents a new form of renewable energy by generating electricity
from what would otherwise be considered waste, such as industrial wastes or waste water
etc. A microbial fuel cell [Microbial Fuel Cell] is a biological reactor that turns chemical
energy present in the bonds of organic compounds into electric energy, through the
reactions of microorganism in aerobic conditions.It is achieved by catalytic reaction of
microorganisms.A current is produced by the interaction of bacteria.Generally the fuel cell
consist of anode and cathode .These two are separated by using a membrane which is
positively charged called cations.
Different operations takes place in both the compartments.Microorganisms oxidise the
fuel inorder to generate crbondioxide along with protons and electrons in the anode
compartment.These electrons and protons are too transferred to cathode compartment.An
external circuit is required to transfer the electrons.The membrane is required to transfer
protons.Protons and electrons in the cathode compartment mix with oxygen to produce
The idea of using microbial cells in an attempt to produce electricity was first conceived
in the early twentieth century. M. Potter was the first to perform work on the subject in
1911.A professor of botany at the University of Durham, Potter managed to generate
electricity from E. coli, but the work was not to receive any major coverage. In 1931,
however, Barnet Cohen drew more attention to the area when he created a number of
microbial half fuel cells that, when connected in series, were capable of producing over 35
volts, though only with a current of 2 milliamps.
More work on the subject came with a study by DelDuca et al. who used hydrogen
produced by the fermentation of glucose by Clostridium butyricum as the reactant at the
anode of a hydrogen and air fuel cell. Though the cell functioned, it was found to be
unreliable owing to the unstable nature of hydrogen production by the micro-
organisms. Although this issue was later resolved in work by Suzuki et al. in 1976 the
current design concept of an MFC came into existence a year later with work once again
By the time of Suzuki’s work in the late 1970s, little was understood about how microbial
fuel cells functioned; however, the idea was picked up and studied later in more detail first
by MJ Allen and then later by H. Peter Bennetto both from King's College London.
People saw the fuel cell as a possible method for the generation of electricity for
developing countries. His work, starting in the early 1980s, helped build an understanding
of how fuel cells operate, and until his retirement, he was seen by many as the foremost
authority on the subject.
It is now known that electricity can be produced directly from the degradation of organic
matter in a microbial fuel cell. Like a normal fuel cell, an MFC has both an anode and a
cathode chamber. The anoxic anode chamber is connected internally to the cathode
chamber via an ion exchange membrane with the circuit completed by an external wire.
In May 2007, the University of Queensland, Australia completed its prototype MFC as a
cooperative effort with Foster's Brewing. The prototype, a 10 L design, converts brewery
wastewater into carbon dioxide, clean water, and electricity. With the prototype proven
successful,plans are in effect to produce a 660 gallon version for the brewery, which is
estimated to produce 2 kilowatts of power.
MICROBIAL FUEL CELLS
A microbial fuel cell is a device that converts chemical energy to electrical energy by
the catalytic reaction of microorganisms.
A typical microbial fuel cell consists of anode and cathode compartments separated by a
cation (positively charged ion) specific membrane. In the anode compartment, fuel is
oxidized by microorganisms, generating CO2, electrons and protons. Electrons are
transferred to the cathode compartment through an external electric circuit, while protons
are transferred to the cathode compartment through the membrane. Electrons and protons
are consumed in the cathode compartment, combining with oxygen to form water.
3.2 Construction and working of microbial fuel cells:
Figure 3.1 Microbial fuel cell
Microbial fuel cell consists of anode and cathode, connected by an external circuit and
separated by Proton Exchange Membrane.
Anodic material must be conductive, bio compatible, and chemically stable with
substrate. Metal anodes consisting of noncorrosive stainless steel mesh can be utilized, but
copper is not useful due to the toxicity of even trace copper ions to bacteria. The simplest
materials for anode electrodes are graphite plates or rods as they are relatively
inexpensive, easy to handle, and have a defined surface area. Much larger surface areas
are achieved with graphite felt electrodes.The most versatile electrode material is carbon,
available as compact graphite plates, rods, or granules, as fibrous material (felt, cloth,
paper, fibers, foam), and as glassy carbon.Proton Exchange Membrane is usually made up
of NAFION or ULTREX.
Microbial Fuel Cells utilise microbial communities to degrade organics found within
wastewater and theoretically in any organic waste product; converting stored chemical
energy to electrical energy in a single step.
Oxygen is most suitable electron acceptor for an microbial fuel cell due to its high
oxidation potential, availability, sustainability and lack of chemical waste product, as the
only end product is water.
If acetate is used as substrate, following reaction takes place:
CH3COO- + H2O → 2CO2 + 2H+ +8e-
O2 + 4e- + 4 H+ → 2 H2O
Electrons produced by bacteria from these substrates are transferred to anode (negative
terminal) and flow to the cathode ( positive terminal) linked by a conductive material.
Protons move to cathodic compartment through Proton Exchange Membrane and
complete the circuit. Microbial fuel cells use inorganic mediators to tap into the electron
transport chain of cells and steal the electrons that are produced. The mediator crosses the
outer cell lipid membranes and plasma wall; it then begins to liberate electrons from the
electron transport chain that would normally be taken up by oxygen or other
The now-reduced mediator exits the cell laden with electrons that it shuttles to an
electrode where it deposits them; this electrode becomes the electro-generic anode
(negatively charged electrode). The release of the electrons means that the mediator
returns to its original oxidised state ready to repeat the process. It is important to note that
this can only happen under anaerobic conditions, if oxygen is present then it will collect
all the electrons as it has a greater electronegativity than the mediator.
Organic substrates are utilized by microbes as their energies are transferred to electron
acceptor( molecular oxygen) in absence of such electron acceptors micro-organisms
shuttle electron into anode surface with help of mediators. However few micro-organisms
are able to transfer electrons directly to electrode. This type of system is called as
Mediator Less Microbial Fuel Cell. Examples of such micro-organisms which are
currently available are : shwanella, geobacter etc. Mediator Less Microbial Fuel Cell have
more commercial potential as mediators are expensive and sometimes toxic to
3.3 Thermodynamics and the electromotive force
Electricity is generated in an Microbial Fuel Cell only if the overall reaction is
thermodynamically favorable. The reaction can be evaluated in terms of Gibbs free energy
expressed in units of Joules (J), which is a measure of the maximal work that can be
derived from the reaction calculated as,
∆G r = Gr
0 + RT(lnπ)
Where, Gr (J) is the Gibbs free energy for the specific conditions, G0
r (J) is the Gibbs free
energy under standard conditions usually defined as 298.15 K, 1 bar pressure, and 1 M
concentration for all species, R (8.31447 J mol-1 K-1) is the universal gas constant, T (K)
is the absolute temperature, and π is the reaction quotient calculated as the activities of
the products divided by those of reactants. The standard Gibbs free energy is calculated
from tabulated energies of formation for organic compounds in water.
For Microbial Fuel Cell calculations, it is more convenient to evaluate the reaction in
terms of the overall cell electromotive force (emf), Eemf (V), defined as the potential
difference between the cathode and anode. This is related to the work, W(J), produced by
W = EemfQ = ∆ Gr
Where, Q = nF is the charge transferred in the reaction, expressed in coulomb (C), which
is determined by the number of electrons exchanged in the reaction, n is the number of
electrons per reaction mole and F is Faraday’s constant(9.64853×104 C/mol). Combining
these two equations, we have,
Eemf = ∆ Gr∕ nF
If all reactions are evaluated at standard conditions, π = 1, then
emf = G0
r ∕ nF
emf (V) is the standard cell electromotive force. We can therefore use the above
equations to express the overall reaction in terms of the potential as
Eemf = E0
emf –RT∕nF ln(π)
The advantage of above equation is that it is positive for a favorable reaction , and
directly produces a value of the emf for the reaction. This calculated emf provides an
upper limit for the cell voltage; the actual potential derived from the Microbial Fuel Cell
will be lower due to various potential losses.
MICROBIAL FUEL CELL DESIGN
Many different configurations are possible for Microbial Fuel Fells. A widely used and
inexpensive design is a two chamber Microbial Fuel Fell built in a traditional “H” shape,
consisting usually of two bottles connected by a tube containing a separator which is
usually a cation exchange membrane (CEM) such as Nafion or Ultrex, or a plain salt
bridge. The key to this design is to choose a membrane that allows protons to pass
between the chambers (the CEM is also called a proton exchange membrane, PEM), but
optimally not the substrate or electron acceptor in the cathode chamber (typically oxygen).
In the H-configuration, the membrane is clamped in the middle of the tubes connecting the
bottle . However, the tube itself is not needed. As long as the two chambers are kept
separated, they can be pressed up onto either side of the membrane and clamped together
to form a large surface. An inexpensive way to join the bottles is to use a glass tube that is
heated and bent into a U-shape, filled with agar and salt (to serve the same function as a
cation exchange membrane), and inserted through the lid of each bottle . The salt bridge
Microbial Fuel Fell, however, produces little power due the high internal resistance
observed. H-shape systems are acceptable for basic parameter research, such as examining
power production using new materials, or types of microbial communities that arise
during the degradation of specific compounds, but they typically produce low power
densities. The amount of power that is generated in these systems is affected by the
surface area of the cathode relative to that of the anode and the surface of the membrane .
The power density (P) produced by these systems is typically limited by high internal
resistance and electrode-based losses. When comparing power produced by these systems,
it makes the most sense to compare them on the basis of equally sized anodes, cathodes,
and membranes. Using ferricyanide as the electron acceptor in the cathode chamber
increases the power density due to the availability of a good electron acceptor at high
concentrations. Ferricyanide increased power by 1.5 to 1.8 times compared to a Pt-catalyst
cathode and dissolved oxygen (H-design reactor with a Nafion CEM) . The highest power
densities so far reported for MFC systems have been low internal resistance systems with
ferricyanide at the cathode . While ferricyanide is an excellent catholyte in terms of
system performance, it must be chemically regenerated and its use is not sustainable in
practice. Thus, the use of ferricyanide is restricted to fundamental laboratory studies.
METABOLISM IN MICROBIAL FUEL CELLS
To assess bacterial electricity generation, metabolic pathways governing microbial
electron and proton ﬂows must be determined. In addition to the inﬂuence of the substrate
the potential of the anode will also determine the bacterial metabolism. Increasing MFC
current will decrease the potential of the anode, forcing the bacteria to deliver the
electrons through more-reduced complexes. The potential of the anode will therefore
determine the redox potential of the ﬁnal bacterial electron shuttle, and therefore, the
metabolism. Several different metabolism routes can be distinguished based on the anode
potential: high redox oxidative metabolism; medium to low redox oxidative metabolism;
and fermentation. Hence, the organisms reported to date in MFCs vary from aerobes and
facultative anaerobes towards strict anaerobes. At high anodic potentials, bacteria can use
the respiratory chain in an oxidative metabolism. Electrons and, concomitantly, protons
can be transported through the NADH dehydrogenase, ubiquinone, coenzyme Q or
cytochrome. The use of this pathway was investigated. They observed that the generation
of electrical current from an MFC was inhibited by various inhibitors of the respiratory
chain. The electron transport system in their MFC used NADH dehydrogenase, Fe/S
(iron/sulphur) proteins and quinines as electron carriers, but does not use site 2 of the
electron transport chain or the terminal oxidase. Processes using oxidative
phosphorylation have regularly been observed in MFCs, yielding high energy efﬁciencies
of up to 65%. Examples are consortia containing Pseudomonas aeruginosa, Enterococcus
faecium and Rhodoferax ferrireducens.If the anode potential decreases in the presence of
alternative electron acceptors such as sulphate, the electrons are likely to be deposited
onto these components. Methane production has repeatedly been observed when the
inoculum was anaerobic sludge , indicating that the bacteria do not use the anode. If no
sulphate, nitrate or other electron acceptors are present, fermentation will be the main
process when the anode potential remains low. For example, during fermentation of
glucose, possible reactions can be:
C6H12O6 + 2 H2O →4H2 + 2CO2 + 2C2H4O2
C6H12O6 → 2 H2 + 2CO2 + C4H8O2
This shows that a maximum of one-third of a hexose substrate electrons can theoretically
be used to generate current, whereas two thirds remain in the produced fermentation
product such as acetate and butyrate.The one-third of the total electrons are possibly
available for electricity generation because the hydrogenases, which generally use the
electrons to produce hydrogen gas, are often situated at places on the membrane surface
that are accessible from outside by mobile electron shuttles or that connect directly to the
electrode. As repeatedly observed, this metabolic type can imply a high acetate or butyrate
production. This pathway is further substantiated by the signiﬁcant hydrogen production
observed when MFC enriched cultures are incubated anaerobically in a separate
MICRO-ORGANISMS USED IN MICROBIAL FUEL
6.1 Axenic bacterial cultures
Some bacterial species in MFCs, of which metal-reducing bacterial are the most
important, have recently been reported to directly transfer electrons to the anode. Metal-
reducing bacteria are commonly found in sediments, where they use insoluble electron
acceptors such as Fe (III) and Mn (IV). Specific cytochromes at the outside of the cell
membrane make Shewanella putrefaciens electrochemically active in case it is grown
under anaerobic conditions. The same holds true for bacteria of the family
Geobacteraceae, which have been reported to form a biofilm on the anode surface in
MFCs and to transfer the electrons from acetate with high efficiency.
Rhodoferax species isolated from an anoxic sediment were able to efficiently transfer
electrons to a graphite anode using glucose as a sole carbon source. Remarkably, this
bacterium is the first reported strain that can completely mineralize glucose to CO2 while
concomitantly generating electricity at 90% efficiency. In terms of performance, current
densities in the order of 0.2-0.6mA and a total power density of 1-17 W/m 2 graphite have
reported for Shewanella putrefaciens, Geobacter sulfurreducens and Rhodoferax
ferrireducens at conventional (woven) graphite electrodes. However, in case woven
graphite in the Rhodoferax study was replaced by highly porous graphite electrodes, the
current and power output was increased up to 74 mA/m2
and 33 mW/m2
Although these bacteria generally show high electron transfer efficiency, they have a
slow growth rate, a high substrate specificity (mostly acetate or lactate) and relatively low
energy transfer efficiency compared to mixed cultures. Furthermore, the use of a pure
culture implies a continuous risk of contamination of the MFCs with undesired bacteria.
6.2 Mixed bacterial cultures
MFCs that make use of mixed bacterial cultures have some important advantages over
MFCs driven by axenic cultures: higher resistance against process disturbances, higher
substrate consumption rates, smaller substrate specificity and higher power output.
Mostly, the electrochemically active mixed cultures are enriched either from sediment
(both marine and lake sediment) or activated sludge from wastewater treatment plants.
By means of molecular analysis, electrochemically active species of Geobacteraceae,
Desulfuromonas, Alcaligenes faecalis, Enterococcus faecium, Pseudomonas aeruginosa,
Proteobacteria, Clostridia, Bacteroides and Aeromonas species were detected in the
before-mentioned studies.Also reorted the presence of nitrogen fixing bacteria (e.g.,
Azoarcus and Azospirillum) amongst the electrochemically active bacterial populations.
LIST OF SUBSTRATES USED IN MICROBIAL FUEL
Substrates not only provides energy for the bacterial cell but also influences the
economic viability and overall performance such as power density and coloumbic
efficiency of MFCs.The composition , concentration and type of the substrate also effects
the microbial community and power production.Many organic substrates including
carbohydrates, proteins volatile acids, cellulose and waste water have been used as a feed
in MFC studies.It can range from simple pure low molecular sugar to complex organic
matter containing waste water to generate electricity.In most of the MFCs, acetate is
commonly used as a substrate due to its inertness towards alternative microbial
conversions(fermentations and methenogenisis) that lead to high coloumbic efficiency and
power output.Power generated to b higher when compared with other substrate.Following
table presents list of substrate used in MFCs.
Substrate type Concentrations Current density
Acetate 1g/L 0.8
Lactate 18mM 0.005
Glucose 6.7Mm 0.7
Sucrose 2674mg/L 0.19
Glucaronic acid 6.7mM 1.18
Phenol 400mg/L 0.1
Sodium fumerate 25mM 2.05
Starch 10g/L 1.3
Cellulosic particles 4g/L 0.02
Xylose 6.7mM 0.74
Domestic wastewater 600mg/L 0.06
Brewery wastewater 2240mg/L 0.2
Table 7.1 Substrates and current density
TYPES OF MICROBIAL FUEL CELLS
More broadly, there are two types of microbial fuel cell: mediator and mediator-less
microbial fuel cells.
8.1 Mediator microbial fuel cell
Most of the microbial cells are electrochemically inactive. The electron transfer from
microbial cells to the electrode is facilitated by mediators such as thionine, methyl
viologen, methyl blue, humic acid, and neutral red. Most of the mediators available
are expensive and toxic.
8.2 Mediator free microbial fuel cell
Figure 8.1 A plant microbial fuel cell [PMFC]
Mediator-free microbial fuel cells do not require a mediator but use electrochemically
active bacteria to transfer electrons to the electrode (electrons are carried directly from the
bacterial respiratory enzyme to the electrode). Among the electrochemically active
bacteria are, Shewanella putrefaciens, Aeromonas hydrophila, and others. Some
bacteria, which have pili on their external membrane, are able to transfer their electron
production via these pili. Mediator-less MFCs are a more recent area of research and, due
to this, factors that affect optimum efficiency, such as the strain of bacteria used in the
system, type of ion-exchange membrane, and system conditions (temperature, pH, etc.)
are not particularly well understood.Mediator-less microbial fuel cells can, besides
running on wastewater, also derive energy directly from certain plants. This configuration
is known as a plant microbial fuel cell.
Possible plants include reed sweetgrass, cordgrass, rice, tomatoes, lupines, and
algae. Given that the power is thus derived from living plants (in situ-energy production),
this variant can provide additional ecological advantages.
8.3 Microbial electrolysis cell
A variation of the mediator-less MFC is the microbial electrolysis cells (MEC). Whilst
MFC's produce electric current by the bacterial decomposition of organic compounds in
water, MECs partially reverse the process to generate hydrogen or methane by applying a
voltage to bacteria to supplement the voltage generated by the microbial decomposition of
organics sufficiently lead to the electrolysis of water or the production of methane.A
complete reversal of the MFC principle is found in microbial electrosynthesis, in which
carbon dioxide is reduced by bacteria using an external electric current to form multi-
carbon organic compounds.
8.4 Soil-based microbial fuel cell
Figure 8.2 A soil based MFC
Soil-based microbial fuel cells adhere to the same basic MFC principles as described
above, whereby soil acts as the nutrient-rich anodic media, the inoculum, and the proton-
exchange membrane (PEM). The anode is placed at a certain depth within the soil, while
the cathode rests on top the soil and is exposed to the oxygen in the air above it.
Soils are naturally teeming with a diverse consortium of microbes, including the
electrogenic microbes needed for MFCs, and are full of complex sugars and other
nutrients that have accumulated over millions of years of plant and animal material decay.
Moreover, the aerobic (oxygen consuming) microbes present in the soil act as an oxygen
filter, much like the expensive PEM materials used in laboratory MFC systems, which
cause the redox potential of the soil to decrease with greater depth. Soil-based MFCs are
becoming popular educational tools for science classrooms.
8.5 Phototrophic biofilm microbial fuel cell
Phototrophic biofilm MFCs (PBMFCs) are the ones that make use of anode with a
phototrophicbiofilm containing photosynthetic microorganism like chlorophyta,
cyanophyta etc., since they could carry out photosynthesis and thus they act as both
producers of organic metabolites and also as electron donors
A study conducted by Strik et al. reveals that PBMFCs yield one of the highest power
densities and, therefore, show promise in practical applications. Researchers face
difficulties in increasing their power density and long-term performance so as to obtain a
The sub-category of phototrophic microbial fuel cells that use purely oxygenic
photosynthetic material at the anode are sometimes called biological
8.6 Nanoporous membrane microbial fuel cells
The United States Naval Research Laboratory (NRL) developed the nanoporous
membrane microbial fuel cells which operate the same as most MFCs, but use a non-PEM
to generate passive diffusion within the cell.The membrane used instead is a nonporous
polymer filter (nylon, cellulose, or polycarbonate) which generates comparable power
densities as Nafion (a well-know PEM) while remaining more durable than Nafion.
Porous membranes allow passive diffusion thereby reducing the necessary power supplied
to the MFC in order to keep the PEM active and increasing the total output of energy from
MFCs that do not use a membrane can deploy anaerobic bacteria in aerobic
environments however, membrane-less MFCs will experience cathode contamination by
the indigenous bacteria and the power-supplying microbe. The novel passive diffusion of
nanoporous membranes can achieve the benefits of a membrane-less MFC without worry
0f cathode contaminatio.Nanoporous membranes are also ten times cheaper than Nafion
(Nafion-117, $0.22/cm2 vs. polycarbonate, <$0.02/cm2).
8.7 Sediment microbial fuel cells
A likely application of microbial fuel cell (MFC) technology is in remote bodies of
water where electric energy can be extracted from organic-rich aquatic sediments. For this
purpose, researchers have developed sediment MFCs that consist of an anode electrode
embedded in the anaerobic sediment and a cathode electrode suspended in the aerobic
water column above the anode electrode. Electricigenic bacteria in the sediment transfer
electrons produced during the oxidation of organic or inorganic matter to the anode
electrode; while oxygen is reduced in the water column by accepting electrons from the
cathode electrode. As a result, an electric current is generated. Classically, H-type MFCs
have been used to study microbial respiration in the anode. Such MFCs contain a cation
exchange membrane to separate the anaerobic anode from the aerobic cathode. A cation
exchange membrane is not necessary in sediment MFCs, because the decreasing oxygen
gradient over the depth of water and sediment columns creates the necessary potential
difference naturally By placing one electrode into a marine sediment rich in organic
matter and sulfides, and the other in the overlying oxic water, electricity can be generated
at sufficient levels to power some marine devices. Protons conducted by the seawater can
produce a power density of up to 28 mW/m2. Graphite disks can be used for the
electrodes, although platinum mesh electrodes have also been used. “Bottle brush”
cathodes used for seawater batteries may hold the most promise for long-term operation of
unattended systems as these electrodes provide a high surface area and are made of
noncorrosive materials. Sediments have also been placed into H-tube configured two-
chamber systems to allow investigation of the bacterial community.
8.8 Permanganate cathodic electron acceptor MFC
Permanganate has been used as an environment-friendly oxidant in industries for many
years. Its high redox potential offers the possibility of its application in a fuel cell system
to establish a high potential difference between the anode and the cathode. Five-fold more
power density can be achieved in a permanganate two-chamber MFC than with other
electron acceptors such as hexacynoferrate and oxygen; In a MFC, also a three-fold
maximum power density can be produced when using permanganate as the electron
acceptor as compared to using hexacynoferrate . It is the outstanding redox potential of the
permanganate that enhanced the power output of a MFC. The similar mechanism also
applies to the other high redox potential electron acceptors such as hexacynoferrate which
generates higher power by higher redox potentials than dissolved oxygen.
Moreover, it is worth pointing out that thispermanganate method has no need for a
catalyst, which makes this process simple and economical. But on the other hand, it
should be noted that like the other liquid-state electron acceptors this permanganate MFC
also requires liquid replacements to compensate its depletion.
8.9 Membrane less MFC
In a meditor less MFC, the membrane separates the anode from the cathode as in other
MFCs, and the membrane functions as an electrolyte that plays the role of an electric
insulator and allows protons to move through. However, the use of membrane can limit
the application of MFC to wastewater treatment. Proton transfer through the membrane
can be a rate limiting factor especially with fouling expected due to suspended solids and
soluble contaminants in a large scale wastewater treatment process. In addition,
membranes are expensive and hence may limit its application. A membrane-less microbial
fuel cell (ML-MFC) was developed and used successfully to enrich electrochemically
active microbes that converted organic contaminants to electricity. The COD (Chemical
Oxygen Demnd) removal rate of 526.67 g/m3 day was reported with maximum power
production 1.3 mW/m2 and current density 6.9 mA/m2. The design used in the study
showed poor cathode reaction allowing a large quantity of oxygen to diffuse toward the
anode. Further studies are required to improve the design of ML-MFC to improve current
yield and COD removal efficiency.
APPLICATIONS OF MICROBIAL FUEL CELLS
9.1 Waste water treatment
Due to unique metabolic assets of microbes, variety of microorganisms are used in
Microbial Fuel Cells either single species or consortia. Some substrates (sanitary wastes,
food processing waste water,swine waste water and com stovers) are exceptionally loaded
with organic matter that itself feed wide range of microbes used in Microbial Fuel Cells.
Microbial Fuel Cells using certain microbes have a special ability to remove sulfides as
required in waste water treatment. Microbial Fuel Cell substrates have huge content of
growth promoters that can enhance growth of bio-electrochemically active microbes
during waste water treatment. This simultaneous operation not only reduces energy
demand on treatment plant but also reduces amount of unfeasible sludge produced by
existing anaerobic production. Microbial Fuel Cells connected in series have high level of
removal efficiency to treat leachate with supplementary benefit of generating electricity.
Consider a conventional Waste Water Treatment Plant designed for 30000 IE, receiving
a daily influent of 5400m3. At a biodegradable chemical oxygen demand (bCOD)
concentration 0f 500mg/L, this represents an influx of organic matter of 2700kg dry
weight per day. The amount of sludge formed,at a nominal yield of 0.4g cell dry weight
per g bCOD converted will be 1080 kg per day. This needs to be disposed off at a cost
which can rise up to €5 00 per ton dry matter. The other costs contained in the operational
cost are the aeration costs and pump costs for recirculation and processing.
If a Microbial Fuel Cell is used with an open air cathode, no aeration is needed. The
putative energy of the input organic matter amounts to 8950kWH/day. The costs for
sludge processing will be lower, since no aerobic cell yields can be attained . for
methanogenesis, the cell yield is about 0.05g CDW/g substrate; for Microbial Fuel Cell
the yield can be estimated somewhere in between aerobic and methanogenic conditions.
At an energetic efficiency of 35%, which should be attainable on large scale,
approximately 3150 kWh/day of useful energy will be produced. This comparison does
not take into account the capital cost of both systems. However, if the capital cost is of
same order, the comparison illustrates a significant difference in operational costs. Hence,
if large scale Microbial Fuel Cells can be built at an acceptable price, this will be a viable
Under present investigation, the membrane less MFC was used effectively for synthetic
wastewater treatment with COD and BOD removal about 90%.
9.2 Power generation
Microbial fuel cells have a number of potential uses. The most readily apparent is
harvesting electricity produced for use as a power source. The use of MFCs is attractive
for applications that require only low power but where replacing batteries may be time-
consuming and expensive such as wireless sensor networks. Virtually any organic
material could be used to feed the fuel cell, including coupling cells to wastewater
Bacteria would consume waste material from the water and produce supplementary
power for the plant. The gains to be made from doing this are that MFCs are a very clean
and efficient method of energy production. Chemical processing wastewater and designed
synthetic wastewater have been used to produce bioelectricity in dual- and single-
chamber mediatorless MFCs (non-coated graphite electrodes) apart from wastewater
Higher power production was observed with biofilm covered anode (graphite).A fuel
cell’s emissions are well below regulations.MFCs also use energy much more efficiently
than standard combustion engines, which are limited by theCarnot Cycle. In theory, an
MFC is capable of energy efficiency far beyond 50%. According to new research
conducted by René Rozendal, using the new microbial fuel cells, conversion of the energy
to hydrogen is 8 times as high as conventional hydrogen production technologies.
However, MFCs do not have to be used on a large scale, as the electrodes in some cases
need only be 7 μm thick by 2 cm long. The advantages to using an MFC in this situation
as opposed to a normal battery is that it uses a renewable form of energy and would not
need to be recharged like a standard battery would. In addition to this, they could operate
well in mild conditions, 20 °C to 40 °C and also at pH of around 7. Although more
powerful than metal catalysts, they are currently too unstable for long-term medical
applications such as in pacemakers (Biotech/Life Sciences Portal).
Besides wastewater power plants, as mentioned before, energy can also be derived
directly from crops. This allows the set-up of power stations based on algae platforms or
other plants incorporating a large field of aquatic plants. According to Bert Hamelers, the
fields are best set-up in synergy with existing renewable plants (e.g., offshore wind
turbines). This reduces costs as the microbial fuel cell plant can then make use of the same
electricity lines as the wind turbines.
9.3 Secondary fuel production
With minor modifications,Microbial Fuel Cells can be employed to produce secondary
fuels like hydrogen (H2) as an alternative of electricity. Under standard experimental
conditions, proton and electron produced in anodic chamber get transferred to cathode,
which then combines with oxygen to form water. H2 generation is thermodynamically not
favored or it is a harsh process for a cell to convert aproton into H2. Increase in external
potential applied at cathode can be competent to overcome thermodynamic barrier in
reaction and used for H2 generation. As a result, proton and electron produced in anodic
reaction chamber combine at cathode to form H2. Microbial Fuel Cells can probably
produce extra H2 as compared to quantity that pull off from classical glucose fermentation
method. Single-chamber membrane-free MECs were designed and successfully produced
hydrogen from organic matter using one mixed culture and one pure culture:Shewanella
oneidensis MR-1. At an applied voltage of 0.6 V, a hydrogen production rate of
0.53m3/day/m3 was obtained using a mixed bacterial culture by the single-chamber MECs
operated at pH 7.0. Higher hydrogen production rate (0.69m3 /day/m3 ) was obtained
when the MECs were operated at pH 5.8. High current densities of 9.3 A/m2 (pH 7) and
14 A/m2 Were achieved with the mixed culture in the single-chamber MEC system,
attributing to the reduced potential losses associated with membrane. Applied voltages
exerted signiﬁcant inﬂuences on MEC’s performance. The performances at 0.6 V were
more than two times higher than those at 0.4 V in terms of hydrogen production rate,
overall energy efﬁciency, hydrogen yield, Coulombic efﬁciency and current density.
While 0.3 V was the minimum applied voltage to achieve measurable hydrogen
production rate in the MEC system. Hydrogenotrophic methanogens in the mixed culture
systems adversely affected hydrogen production. Methanogenesis was avoided by using
the pure bacterial culture S. oneidensis in this MEC system. However, the current
hydrogen production rates were much lower than those with the mixed culture systems.
The current density and volumetric hydrogen production rate of this system have potential
to increase signiﬁcantly by further reducing the electrode spacing and increasing the ratio
of electrode surface area/cell volume.
Figure 9.1 H2 Generation in MFC
Since the current generated from a microbial fuel cell is directly proportional to the
energy content of wastewater used as the fuel, an MFC can be used to measure the solute
concentration of wastewater i.e as a biosensor system. The strength of wastewater is
commonly evaluated as biochemical oxygen demand (BOD) value.BOD values are
determined incubating samples for 5 days with proper source of microbes, usually activate
sludge collected from sewage works. When BOD values are used as a real-time control
parameter, 5 days' incubation is too long.
An MFC-type BOD sensor can be used to measure real-time BOD values. Oxygen and
nitrate are preferred electron acceptors over the electrode reducing current generation
from an MFC. MFC-type BOD sensors underestimate BOD values in the presence of
these electron acceptors. This can be avoided by inhibiting aerobic and nitrate respirations
in the MFC using terminal oxidase inhibitors such as cyanide and azide.This type of BOD
sensor is commercially available.
Bacteria show lower metabolic activity when inhibited by toxic compounds. This will
cause a lower electron transfer towards an electrode. Bio-sensors could be constructed, in
which bacteria are immobilized onto an electrode and protected behind a membrane. If a
toxic component diffuses through the membrane, this can be measured by the change in
potential over the sensor. Such sensors could be extremely useful as indicators of
toxicants in rivers, at the entrance of wastewater treatment plants, to detect pollution or
illegal dumping, or to perform research on polluted site.
MFCs with replaceable anaerobic consortium could be used as a biosensor for online
monitoring of organic matter. Though diverse conventional methods are used to calculate
organic content in terms of Biological Oxygen Demand(BOD) in waste water, most of
them are unsuitable for on line monitoring and control of biological waste water treatment
process. A linear correlation between coulombic yield and strength of organic matter in
waste water makes MFC a possible BOD sensor. Coulombic yield of MFC provides an
idea about BOD of liquid stream that proves to be an accurate method to measure BOD
value at quite wide concentration range of organic matter in waste water.
A mediator-less microbial fuel cell was tested as a continuous BOD sensor. At a feeding
rate of 0.35ml/min (HRT = 1.05 h), BOD values of up to 100mg/l could be measured
based on a linear relation. Higher BOD values were then measured using either a model
ﬁtting method or a lower feeding rate. About 60min was required to reach a new steady-
state current after changing the strength of the AW. When the MFC was starved, the
original current value was regained with varying recovery periods depending on the length
of the starvation. During starvation, the MFC generated a background level current,
probably through an endogenous metabolism.
The United States Navy is looking into microbial fuel cells particularly for
environmental sensors. The use of microbial fuel cells to power environmental sensors
would be beneficial because they would be able to sustain power for a longer amount of
time and enable the collection and retrieval of undersea data without using a wire
infrastructure. The energy created by these fuel cells was enough to sustain sensors after
an initial startup time in research to demonstrate the effectiveness of the fuel cell as a
power source for such sensors.Due to undersea conditions (high salt concentrations,
fluctuating temperatures, and limited nutrient supply), the U.S. Navy is looking to deploy
their MFCs with a mixture of salt-tolerant microorganisms. A mixture would also allow
for a more complete utilization of available nutrients to be converted into electricity.
Currently, Shewanella oneidensis is their primary microorganism for electrical generation,
but their mixture might also include Shewanella spp. as it is very heat- and cold-tolerant.
Desalination of sea water and brackish water for use as drinking water has always
presented significant problems because of the amount of energy required to remove the
dissolved salts from the water.By using an adapted microbial fuel cells this process could
proceed with no external energy output.
By adding a third chamber in etween the two electrodes of a standard MFC and filling it
with sea water,the cell’s positive and negative electrodes attract the positive and negative
salt ions in the water and using semi-permeable membrane,filters out the salt from the sea
Figure 9.2 A Desalination microbial fuel cell
9.6 Educational tool
Soil-based microbial fuel cells are popular educational tools, as they employ a range of
scientific disciplines (microbiology, geochemistry, electrical engineering, etc.), and can be
made using commonly available materials, such as soils and items from the refrigerator.
There are also kits available for classrooms and hobbyists.and research-grade kits for
scientific laboratories and corporations.
ADVANTAGES OF MICROBIAL FUEL CELLS
Microbial fuel cells present several advantages, both operational and functional, in
comparison to the currently used technologies for generation of energy out of organic
matter or treatment of waste streams:
10.1 Generation of energy out of biowaste/organic matter
This feature is certainly the most ‘green’ aspect of microbial fuel cells. Electricity is
being generated in a direct way from biowastes and organic matter. This energy can be
used for operation of the waste treatment plant, or sold to the energy market. Furthermore,
the generated current can be used to produce hydrogen gas. Since waste flows are often
variable, a temporary storage of the energy in the form of hydrogen, as a buffer, can be
10.2 Direct conversion of substrate energy to electricity
As previously reported, in anaerobic processes the yield of high value electrical energy
is only one third of the input energy during the thermal combustion of the biogas. While
recuperation of energy can be obtained by heat exchange, the overall effective yield still
remains of the order of 30%.
A microbial fuel cell has no substantial intermediary processes. This means that if the
efficiency of the MFC equals at best 30% conversion, it is the most efficient biological
electricity producing process at this moment. However, this power comes at potentials of
approximately 0.5 Volts per biofuel cell. Hence, significant amounts of MFCs will be
needed, either in stack or separated in series, in order to reach acceptable voltages. If this
is not possible, transformation will be needed, entailing additional investments and an
energy loss of approximately 5 %.
Another important aspect is the fact that a fuel cell does not –as is the case for a
conventional battery- need to be charged during several hours before being operational,
but can operate within a very short time after feeding, unless the starvation period before
use was too long too sustain active biomass.
10.3 Omission of gas treatment
Generally, off-gases of anaerobic processes contain high concentrations of nitrogen gas,
hydrogen sulphide and carbon dioxide next to the desired hydrogen or methane gas. The
off-gases of MFCs have generally no economic value, since the energy contained in the
substrate was prior directed towards the anode. The separation has been done by the
bacteria, draining off the energy of the compounds towards the anode in the form of
electrons. The gas generated by the anode compartment can hence b discharged provided
no larger quantities of H2S and other odorous compounds are present in the gas and no
aerosol with undesired bacteria are liberated into the environment.
The cathode can be installed as a ‘membrane electrode assembly’, in which the cathode
is precipitated on top of the proton exchange membrane or conductive support, and is
exposed to the open air. This omits the necessity for aeration, thereby largely decreasing
electricity costs. However, from a technical point of view, several aspects need additional
consideration when open air cathodes are used.
First, the cathode needs to remain sufficiently moist to ensure electrical contact.
Preliminary experiments by sceintists indicated that the water formation through oxygen
reduction is insufficient to keep the cathode moist. Therefore, a water recirculation needs
to be installed, possibly entailing extra energy costs. Secondly, the cathode needs to
contain a non-soluble redox mediator to efficiently transfer the electrons from the
electrode to oxygen. Generally, platinum is being used as a catalyst, at concentrations up
to 40% w/w, representing considerable costs. However, new catalysts need to be
developed, which would compensate their possible lower efficiency by a significantly
reduced cost and higher sustainability.
LIMITATIONS OF MICROBIAL FUEL CELLS
11.1 Low power density
The major limitations to implementation of MFCs for are their power density is still
relatively low and the technology is only in the laboratory phase. Based on the potential
difference, ΔE, between the electron donor and acceptor, a maximum potential of nearly
1V can be expected in MFCs, which is not much greater than the 0.7 V that is currently
being produced. However, by linking several MFCs together, the voltage can be
increased. Current and power densities are lower than what is theoretically possible, and
system performance varies considerably. The maximum power density reported in the
literature, 3600mW/m2, was observed in a dual-chamber fuel cell treating glucose with an
adapted anaerobic consortium in the anode chamber and a continuously aerated cathode
chamber containing an electrolyte solution that was formulated to improve oxygen
transfer to cathode
11.2 High initial cost
A limiting factor to general MFC use is the high cost of materials, such as the nafion
membrane commonly used in laboratories as a proton permeable membrane. Attempts are
currently underway to produce low cost MFCs constructed from earthen pots for use in
India. By removing the proton permeable membrane, utilizing locally produced 400 ml
earthen pots, stainless steel mesh cathodes and a graphite plate anode, each MFC unit
could be produced for US $1. The earthen pot MFCs used sewerage sludge as an initial
inoculum and experiments were conducted using acetate as a carbon source. While
producing low levels of power, these devices could potentially be incorporated in large
numbers into oxidation ponds for the treatment of concentrated wastewater while
generating power. In areas where off grid applications are required, even low power MFC
devices may prove useful. The World Bank has provided funding to a company named
Lebone (http://www.lebone.org/) to start trials with MFC technology to provide energy to
isolated communities. Initial trials will be based in Tanzania and attempt to provide power
for high efficiency LEDs and battery powered devices. Current applications are all limited
to low power level devices. If power can be increased, or cells engineered for specific
applications, then a large range of potential applications have been speculated to be
11.3 Activation losses
Due to the activation energy needed for an oxidation/reduction reaction, activation losses
(or activation polarization) occur during the transfer of electrons from or to a compound
reacting at the electrode surface. This compound can be present at the bacterial surface, as
a mediator in the solution, or as the final electron acceptor reacting at the cathode.
Activation losses often show a strong increase at low currents and steadily increase when
current density increases. Low activation losses can be achieved by increasing the
electrode surface area, improving electrode catalysis, increasing the operating
temperature, and through the establishment of an enriched biofilm on the electrode(s).
11.4 Ohmic losses
The ohmic losses (or ohmic polarization) in an MFC include both the resistance to the
flow of electrons through the electrodes and interconnections, and the resistance to the
flow of ions through the CEM (if present) and the anodic and cathodic electrolytes. Ohmic
losses can be reduced by minimizing the electrode spacing, using a membrane with a low
resistivity, checking thoroughly all contacts, and (if practical) increasing solution
conductivity to the maximum tolerated by the bacteria.
11.5 Bacterial metabolic losses
To generate metabolic energy, bacteria transport electrons from a substrate at a low
potential through the electron transport chain to the final electron acceptor (such as
oxygen or nitrate) at a higher potential. In an MFC, the anode is the final electron
acceptorandits potential determines the energy gain for the bacteria. The higher the
difference between the redox potential of the substrate and the anode potential, the higher
the possible metabolic energy gain for the bacteria, but the lower the maximum attainable
MFC voltage. To maximize the MFC voltage, therefore, the potential of the anode should
be kept as low (negative) as possible. However, if the anode potential becomes too low,
electron transport will be inhibited and fermentation of the substrate (if possible) may
provide greater energy for the microorganisms. The impact of a low anode potential, and
its possible impact on the stability of power generation, should be addressed in future
Development of MFCs was triggered by USA space program in 1960s as a possible
technology for a waste disposal system for space flights that would also generate power.
MFC technology has been extensively reviewed focusing on recent improvement,
practical implementation, anode performance, cathodic limitations, different substrates
etc. MFCs have been explored as a new source of electricity generation during operational
waste water treatment. In addition, some of the recent modification in MFCs (MEC), in
which anoxic cathode is used increased external potential at cathode. Phototropic MFCs
and solar powered MFC also represent an exceptional attempt in the progress of MFCs
technology for electricity production.
MFC is an ideal way of generating electricity since it not only as a renewable source but
also it can be used to treat waste. It can also be used for production of secondary fuel as
well as in bioremediation of toxic compounds. However, this technology is only in
research stage and more research is required before domestic MFCs can be made available
Microbial fuel cells are evolving to become a simple, robust technology. Certainly in
the ﬁeld of wastewater treatment, middle term application can be foreseen at market value
prices. However, to increase the power output towards a stable 1kW per m3 of reactor,
many technological improvements are needed. Provided the biological understanding
increases, the electrochemical technology advances and the overall electrode prices
decrease, this technology might qualify as a new core technology for conversion of
carbohydrates to electricity in years to come.
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