Microbial fuel cells (MFCs) are devices that exploit microbial catabolic activities to generate electricity from a variety of materials, including complex organic waste and renewable biomass. These sources provide MFCs with a great advantage over chemical fuel cells that can utilize only purified reactive fuels (e.g., hydrogen). A developing primary application of MFCs is its use in wastewater treatment coupled with electricity generation, although further technical developments are necessary for its practical use.
The aim of this study was to test a novel MFC design for wastewater treatment and simultaneous electricity production. Two identical MFCs of the design were used with high concentration substrate synthetic dairy wastewater with 4g of COD (chemical oxygen demand) per litre. The MFCs were run continuously at sub-ambient temperatures (15oC) for 35 days from the 19th of January-22nd of February.
The hypothesis of the design was to bring MFC s closer to large scale industrialisation. As such the materials and mechanics of the design were conceived with the consideration of the needs of large scale wastewater treatment plants.
Many tests were carried out to assess the condition for the bacteria within the two MFCs such as VFA, SMA as well as the continuous recording of voltage and COD removal efficiency. SEM and confocal microscopy was also carried out to describe the bacteria being used. Published works andrecent advances in MFC technologies that can become fundamentals for future practical MFC developments are reviewed and combined with the results of this experiment for evaluation and discussion.
The MFC design proved successful in that it achieved 60% removal efficiency, the predicted target whilst producing relatively high values of electricity compared to other MFCs in the literature with the max power per volume value for both MFCs 8.4W m-3. The coulombic efficiency for the two MFCs however was very low at only 2% due to the high concentration of COD in the substrate. This means only 2% of the wastewater treated was converted into energy.
As of 31/10/2011, there are over 7 billion people on the planet (1). Exploitation of the energy stored in fossil fuels has supported global industrialization and economic growth during the past one hundred and fifty years but it is obvious that this practice cannot be sustained. Oil will not actually run out for at least another 100 years or more but demand for oil is expected to exceed production capabilities from known and anticipated oil reserves within the 2015 to 2025 time frame (2).
The use of carbon fuels is increasing the concentration of carbon dioxide in the atmosphere. Without substantial changes to our energy production methods, we will greatly exceeded any historic level of CO2 concentrations in the atmosphere. Global mean temperatures have already risen to above pre-historic levels resulting in melting of glaciers and rising sea levels (3).
Whether you believe there is a link between the two or not, to believe that rapid production of a particular gas (CO2) and release of it into the atmosphere will not have some effect on our atmosphere, global climate and hence biosphere is idiotic. The earth’s collect ive biodiversity is experiencing its sixth major extinction event since multicellular life first evolved. Our population is increasing, our climate is changing rapidly and we are losing our main energy sources. I understand that “nothing endures but change”(2), however if change continues along this trend, we, most definitely will not endure. It sounds bad, but remember our one great virtue as humans, at least in theory, we have a choice (4).
One choice we can make is to meet, head on, our greatest environmental challenge; to simultaneously solve energy production and reduce CO2 emissions. There is no silver bullet per se; it will most likely involve many ideas and methods of sustainable energy production, one of which may be the development and use of microbial fuel cell technologies (3).
The purpose of this literary review is to organise relevant information and apply the principles of a feasibility study to MFC technology. MFCs (Microbial Fuel Cells) Electricity is our main source of power and is nothing more than a flow of electrons. A 5
voltaic cell uses a spontaneous oxidation coupled to reduction reaction to generate electricity.
The cell is divided into two compartments where the oxidation (loss of electrons) and
reduction (gaining of electrons) occur. Each compartment has an electrode where the reaction
occurs. The electrode where oxidation occurs is called the anode (Negative -) and the
electrode where reduction occurs is called the cathode (Positive +). Attracted by the positive
cathode the negative electrons (opposites attract) flow from the anode along an external
circuit where they must get through a resistor forcing them to carry out electrical work (heat,
Microbial fuel cells works similarly to a voltaic cell, except they use the catalytic reaction of
microorganisms such as bacteria to convert virtually any organic material into electricity.
Some common substrates utilised by MFCs include glucose, acetate and wastewater. In an
MFC, microorganisms degrade (oxidise: take electrons from) organic matter, producing
electrons that travel through a series of respiratory enzymes in the cell resulting in energy for
the cell in the form Adenosine triphosphate (ATP). The electrons are then released to a
terminal electron acceptor (TEA) which accepts the electrons and becomes reduced (loses
electrons)(3,5). Many TEAs such as oxygen, nitrate, sulphate, and others readily diffuse into
the cell where they accept electrons forming products that can then diffuse out of the cell.
Fig.1: Schematic of the basic components of an MFC (6).
However we now know that some bacteria can transfer electrons exogenously (outside of the
cell) to a TEA such as a metal oxide like iron oxide. It is these bacteria that can exogenously
transfer electrons, called exoelectrogens that can be used in an MFC. The bacteria grow on
the anode and oxidise organic matter, releasing electrons to the anode, and protons to the
solution. The two electrodes are connected by a wire containing a load (i.e. the device being
powered). The protons (H+) or other ions, filter through the membrane ensuring
electroneutrality. Protons may combine with the electrons transferred via the wire and
oxygen, present in the water, forming water at the cathode. Thus by feeding the bacteria
organic matter we get electrons and water. (3)
History of Microbial Fuel Cells
Microbial fuel cells are the newest approach for generating electricity-bioelectricity
generation from biomass using bacteria (3). Their history albeit brief, (only becoming of real
interest in the early 1990’s), is an interest ing one.
The first MFC concept was demonstrated by MC Potter in 1910. Using platinum electrodes
electricity was produced from living cultures of Escherichia coli and Saccharomyces. In the
1980’s it was discovered that current density and power output could be greatly enhanced by
the addition of electron transfer mediators, or electron shuttles which can carry electrons from
inside the cell to exogenous electrodes, the ‘middlemen’ in other words(3,7). Only
anodophiles (exoelectrogens) can directly transfer electrons directly to the anode. This is
because the outer layers of the majority of microbial species are composed of non-
conductive lipid membrane, peptidoglycans and lipopolysaccharides that hinder the direct
electron transfer to the anode. The problem with synthetic mediators in relation to MFCs is
that they are unstable and toxic to most bacteria. A breakthrough was made however in 1999
when microbes were found to transfer electrons directly to the anode (our exoelectrogens) (7-
9). The growth of the bacteria is supported by the very power production process we utilise
(electron transfer) resulting in long term, stable power production (8,10). Shewanella
putrefaciens, Geobacteraceae sulfurreducens, Geobacter metallireducens and Rhodoferax
ferrireducens are all bioelectrochemically active species and can form a biofilm on the anode
surface and transfer electrons directly by conductance through their cell membrane(7). Here
the anode acts as the final electron acceptor in the dissimilatory respiratory chain of the
microbes in the biofilm. Biofilms also form on the cathode surface and may also play an
important role in electron transfer between the microbes. They can also serve as electron
donors for Thiobacillus ferrooxidans suspended in a catholyte (11) for an MFC system that
contains microbes in both its anodic and cathodic chambers. G. metallireducens and G.
sulfurreducens (12) or other seawater-induced biofilms(13) may all act as final electron
acceptors by grabbing the electrons from cathode as electron donors. Since the cost of a
mediator is eliminated, mediator-less MFCs are advantageous in wastewater treatment and
Modern MFCs can therefore be considered to have only emerged in 1999 with the finding of
electricity generation without the need for exogenous mediators. (3, 7)
Since MFC’s can convert practically any organic material into fuel there are obviously many
possible applications for the technology.
The most obvious use of MFCs is as a source of electricity. MFCs are capable of converting
the chemical energy stored in the chemical compounds in a biomass to electricity. As
chemical energy from the oxidization of fuel molecules is converted directly into electricity
instead of heat, the Carnot cycle with a limited thermal efficiency is avoided and theoretically
a much higher conversion efficiency can be achieved, as high as 80% (7,8). Higher electron
recovery again as electricity of up to 89% was also reported (15), Coulombic efficiency of
97% was reported during the oxidation of formate with the catalysis of Pt black (16).
However the problem with MFCs is that their power generation( the rate of electron
abstraction) is still very low. This is what is currently holding MFCs back from
commercialisation. One way to solve this problem is to store the electricity in rechargeable
devices and then distribute the electricity to end-users (17). On the other hand it is MFCs
high electron efficiency and low power output that make them ideal for powering small
telemetry systems and wireless sensors that have only low power requirements to transmit
signals such as temperature to receivers in remote locations (18). MFCs themselves can serve
as distributed power systems for local uses, especially in underdeveloped regions of the world
(7). MFCs may also be the perfect supply candidate for Gastrobots (a class of intelligent
machines that derive their operational power by exploiting the digestion of real food by self-
feeding the biomass collected by themselves)(18) which can “directly convert various food
substrates into electricity.”(19). MFCs may someday even be used on spaceships since they
can supply electricity while degrading wastes generated on board.(7) Some scientists envision
that in the future a miniature MFC can be implanted in a human body to power an
implantable medical device with the nutrients supplied by the human body(20). However it is
more likely that enzyme-based biofuel cells would instead be used for this purpose as they
can directly utilise glucose produced by the body without the need for microorganisms which
would require many health and safety issues to be thoroughly solved before they could be
applied for this service.
Probably the most important application of MFCs will be in wastewater treatment.
Wastewater contains energy, in the form of biodegradable organic matter, that we expend
energy to remove rather than trying to recover it. At a conventional wastewater treatment
plant in Toronto, Canada, it was estimated that there was 9.3 times the energy in the
wastewater, than was used to treat it. (3)
MFCs have been considered for treating waste water as early as 1991 (21). Municipal
wastewater contains a multitude of organic compounds that can fuel MFCs. Furthermore,
organic molecules such as acetate, propionate, butyrate can be thoroughly broken down to
CO2 and H2O. MFCs using certain microbes have a special ability to remove sulfides as
required in wastewater treatment (22). MFCs can enhance the growth of bioelectrochemically
active microbes during wastewater treatment thus they have good operational stabilities. Up
to 80% of the COD can be removed in some cases and a Coulombic efficiency as high as
80% has been recorded (22-24)
Although the energy that could be captured from wastewater is not enough to power a city, it
is large enough to someday power a treatment plant. With advances, capturing this power
could achieve energy sustainability for the water infrastructure.
There are four main advantages to using an MFC instead of one of the present conventional
bioreactors such as the Activated Sludge (AS) process or the Trickling Filter (TF) process
used for wastewater treatment.
1: Production of a useful product in the form of electricity. The current generated is
dependent on the wastewater strength and as mentioned before Coulombic efficiency.
2: Lack of a need for aeration. Aeration in AS can consume 50% of the electricity used at a
treatment plant and is costly in many other conventional treatment processes. No aeration
however is needed for an air cathode MFC and very little or none is needed for most other
3: Reduced solids production. The MFC is an anaerobic process, and thus bacterial biomass
production will be reduced compared to that of an aerobic system such as TF or AS. Solids
treatment is expensive, and using an MFC may substantially reduce solids production.
4: Potential for odour control. High surface areas needed in TFs exposed to air, and the flow
of large amounts of air through the aeration basin in an AS process greatly increases the
potential for odour generation to a surrounding community. MFCs require neither and should
theoretically produce fewer odours. However this area is the least well studied aspect of MFC
treatment performance (3).
As such it is clear there is great potential for MFCs in wastewater treatment.
Electrohydrogenesis is a recently developed electrolysis method for directly converting
biodegradable material into hydrogen using modiﬁed microbial fuel cells. A microbial
electrolysis cell (MEC) operates in a manner similar to an MFC except that the cathode is
sealed to exclude oxygen, and an additional voltage is added to the circuit. Under normal
operating conditions, protons released by the anodic reaction migrate to the cathode to
combine with oxygen to form water. Hydrogen generation from the protons and the electrons
produced by the metabolism of microbes in an MFC is thermodynamically unfavourable
(25). Applying an external potential to increase the cathode potential in a MFC circuit can
overcame the thermodynamic barrier. In this mode, protons and electrons produced by the
anodic reaction are combined at the cathode to form hydrogen (7, 26). The concentration of
wastewater is usually evaluated on the basis of the amount of oxygen used to oxidise organic
matter, in terms of biochemical oxygen demand (BOD) in a five-day biodegradation test, or
via chemical oxygen demand (COD) in a chemical test that fully oxidises all substances,
organic and inorganic. On the basis of COD, it is easy to determine the potential for hydrogen
production as one mole of COD indicates that one mole of O2 is needed for the reaction.
Thus, each mole of COD (or O2) oxidised produces 4 electrons, or the potential for 2 moles
of H2 (1 mol-COD=2 mol-H2). As oxygen has a molecular weight of 32g/mol and H2 has a
molecular weight of 2 g/mol, this means that one gram of COD produces 0.125 g-H2 (3).
MECs can produce H2 with a minimum external potential of only 110mV at neutral pH, much
lower than the 1210mV required for the electrolysis of water at the same pH. This is because
some of the MECs energy comes from the biomass oxidation process in the anode chamber.
Also unlike MFCs, MECs don’t require oxygen in the cathode chamber and so have greater
generation(3)) is no longer an issue. Also importantly for MECs and MFCs alike, hydrogen
can be accumulated and stored for later usage to counteract the inherent low power feature of
MFCs. Thus, MECs provide a renewable hydrogen source that can contribute to the
overall hydrogen demand in a hydrogen economy (27).