Effect of Inoculum Type on the Performance of a Microbial Fuel Cell Fed with Spent Organic Extracts from Hydrogenogenic Fermentation of Organic Solid Wastes

Abstract

The objective of this work was to evaluate the effect of inoculum type on the production of electricity and general performance of a lab scale microbial fuel cell (MFC), fed with a model of extracts from spent solids generated in fermentative hydrogenogenic process using two different inocula (methanogenic and sulfate-reducing consortia). All variables showed a better performance using the sulfatereducing inoculum (SR-In) than using the methanogenic inoculum (M-In). The maximum potential of the resistance-loaded MFC was 0.26 and 0.46 V for M-In and SR-In, respectively. Average cell potentials of 0.10 (M-In) and 0.37 V (SR-In), were registered during the 50 h of operation. The anodic average power density of 12.31 mW m-2 using S-In, was about 12 times higher than the power density of a MFC loaded with M-In (1.04 mW m-2 anode). Organic matter removal was low to moderate: 25% using a M-In and 43% in the MFC loaded with a SR-In. These results were consistent with very low values of the coloumbic efficiency. Finally, the MFC seeded with SR-In outperformed the MFC loaded with a M-In. Further research is undergoing in order to look for increased performance of our MFC.

Full text

49
1. INTRODUCTION
Current energy needs of our societies are mainly met with fossil
fuels, whereas renewable energy plays only a marginal role. Fossil
fuels use present several disadvantages: increasing scarcity in the
near future, negative pollution impacts due to combustion products
and spills and leaks during exploration, production and transport
[1]. Moreover, energy experts [2] advocate a near future techno-
logical transition where fossil fuels importance will progressively
decrease and renewable energy contribution will become more
significant. Renewable energy represents an interesting alternative
because of their potential lower costs and minimum environment
negative impact [3,4]. However its availability depends on the type
and geographic localization of such systems. In this regard energy
production from biomass and organic wastes is an attractive goal.
Among the energy systems based on biomass, the biological
processes that use microorganisms display significant advantages
over the other, because they use biomass or wastes as raw material
and then attaining the double goal of waste treatment and energy
production [5]. Renewable bio-energies usually look for the most
complete conversion of waste to energy. Sometimes this is not
possible, such as in the hydrogen production from fermentation of
organic wastes. In this type of processes, there is just a partial
biodegradation of waste to hydrogen, with the consistent produc-
tion of organic metabolits remaining in the spent solids and liquors
[6]. These metabolites can be used to yield additional energy by a
methanogenic system, by phototrophic bacteria capable of produc-
ing hydrogen as a fuel or by a microbial fuel cell [7].
A Microbial Fuel Cell (MFC) is a device capable to convert
organic matter into electricity. At the anode the microorganisms
degrade the organic matter and release to the media electrons and
protons (Figure 1). The electrons flow trough an external circuit
producing electricity and finally react at the cathode with the pro-
tons and oxygen producing water [8]. There is a predominance of
works with MFCs of two chambers and only at laboratory scale
(Table 1 and Figure 2). For example Fig. 2A, B, E and F show
MFCs designs in which the anodic and the cathodic chambers are
separated by a Proton Exchange Membrane (PEM), usually a
Nafion or Ultrex CMI7000 membranes (Table 1). In contrast to
those MFCs configurations there are some designs which use an
*To whom correspondence should be addressed: Email:hectorpoggi2001@gmail.com
Phone: (5255) 5747 3800 ex.4324; Fax : (5255) 5747 3313
Effect of Inoculum Type on the Performance of a Microbial Fuel Cell Fed with
Spent Organic Extracts from Hydrogenogenic Fermentation of Organic Solid Wastes
H. M. Poggi-Varaldo*,1, A. Carmona-Martínez1, A. L. Vázquez-Larios1 and O. Solorza-Feria2
1Environmental Biotechnology and Renewable Energies R&D Group, Dept. Biotechnology & Bioengineering,
Centro de Investigación y de Estudios Avanzados del I.P.N., P.O. Box 14-740, México D.F. 07000, México.
2Dept. of Chemistry, Centro de Investigación y de Estudios Avanzados del I.P.N. Mexico.
Received: January 05, 2009, Accepted: January 31, 2009
Abstract: The objective of this work was to evaluate the effect of inoculum type on the production of electricity and general performance
of a lab scale microbial fuel cell (MFC), fed with a model of extracts from spent solids generated in fermentative hydrogenogenic process
using two different inocula (methanogenic and sulfate-reducing consortia). All variables showed a better performance using the sulfate-
reducing inoculum (SR-In) than using the methanogenic inoculum (M-In). The maximum potential of the resistance-loaded MFC was 0.26
and 0.46 V for M-In and SR-In, respectively. Average cell potentials of 0.10 (M-In) and 0.37 V (SR-In), were registered during the 50 h of
operation. The anodic average power density of 12.31 mW m-2 using S-In, was about 12 times higher than the power density of a MFC
loaded with M-In (1.04 mW m-2 anode). Organic matter removal was low to moderate: 25% using a M-In and 43% in the MFC loaded
with a SR-In. These results were consistent with very low values of the coloumbic efficiency. Finally, the MFC seeded with SR-In outperfor-
med the MFC loaded with a M-In. Further research is undergoing in order to look for increased performance of our MFC.
Keywords: Microbial fuel cell; inoculum influence; batch tests; electricity production; leachate, solid wastes
Journal of New Materials for Electrochemical Systems 12, 049-054 (2009)
© J. New Mat. Electrochem. Systems

50
H. M. Poggi-Varaldo et al. / J. New Mat. Electrochem. Systems
aerobic cathode that is exposed to the environment to provide the
oxygen necessary to overcome the production of water (Figure 2C
and H). Additionally there are some MFCs configurations in which
the search focus is to apply this type of systems to the treatment of
wastewater. For example, the MFC in Figure 2E could be seen as a
conventional upflow anaerobic sludge blanket (UASB) reactor due
to the vertical configuration and the type of operation. Finally there
are some outstanding designs in which phototrophic microorgan-
isms are exploited for electricity production (Figure 2D and G),
demonstrating another great opportunity for producing direct en-
ergy as electricity from a photosynthetic catalysis.
2. MATERIALS AND METHODS
2.1. Microbial fuel cell architecture
The MFC consisted of a horizontal cylinder built in Plexiglas 78
mm long (between electrodes) and 48 mm internal diameter (Figure
3). The cylinder was fitted with a circular anode made of stainless
steel plate 1 mm thickness and a cathode made of a sandwich of
three circular layers (from inside to outside): proton exchange
membrane (Nafion 117), flexible carbon-cloth containing 0.5
mg/cm2 platinum catalyst (Pt 10 wt%/C-ETEK), and a perforated
plate of stainless steel 1 mm thickness [10]. The cathode was in
direct contact with atmospheric air on the metallic plate side.
Figure 3. Configuration of the microbial fuel cell in this work.
Figure 2. Different Configurations of MFCs. References: A. Min
et al. (2005) [11]. B. Rabaey et al. (2005) [12]. C. Carmona-
Martínez et al. (2007) [13]. D. Rosenbaum et al. (2005) [14]. E.
He et al. (2005) [15]. F. Min y Logan (2004) [16]. G. Rosenbaum
et al. (2005) [17]. H. Liu et al. (2004) [18].
Figure 1. Microbial Fuel Cell Process. Adapted from Schröder,
(2007) [9].
ABBREVIATIONS
bCOD Moles of electrons harvested from the COD
CE Coulombic efficiency
COD Chemical oxygen demand
CRS Produced electrons in reality from the substrate
CTS Electrons that could be produced from the substrate
CODi Initial COD
CODf Final COD
EMFc MFC voltage
Fi Faraday’s constant
IAn Current density
IMFc Current intensity
MCOD COD’s molecular weight
M-In Methanogenic inoculum
MFC Microbial fuel cell
OFMSW Organic fraction of municipal solid waste
Ot Operation time
PAn-ave Average power density
PAn-max Maximum power density
PMFC MFC power
PV-ave Average volumetric power
PV-max Maximum volumetric power
Rext External resistance
SR-In Sulphate reducer inoculum
VMFC MFC operation volume
Greek characters
ηCOD Chemical oxygen demand removal
ηCoul Columbic efficiency

51
Effect of Inoculum Type on the Performance of a Microbial Fuel Cell Fed with Spent Organic Extracts
from Hydrogenogenic Fermentation of Organic Solid Wastes
/ J. New Mat. Electrochem. Systems
2.2. Model Extract and Inocula
The MFC was loaded with 7 ml from a model extract similar to
the produced metabolites profile found in the biological hydrogen
production from the organic fraction of the municipal solid wastes
[5,19]. The model extract was concocted with a mixture of the next
compounds (in g/L): acetic, propionic and butyric acids (4 each) as
well as acetone and ethanol (4 each) and mineral salts like NaHCO3
and Na2CO3 (3 each) and K2HPO4 and NH4Cl (0.6 each). Organic
matter concentration of model extract was ca. 16 g COD/L. Initial
COD of cell liquor was ca. 700 mg/L. The MFC was inoculated
with either a methanogenic (M-In) or sulphate-reducing (SR-In)
inoculum from complete mix reactors. The biomass concentration
in the inoculum was ca. 200 mg VSS/L. Both reactors had an op-
eration volume of 3 L. The reactors were operated at 37°C in a
constant temperature room. A medium containing sucrose was fed
at a flow rate of 120 mL/d. Its composition was (in g/L): sucrose
(5.0), Acetic acid (1.5), NaHCO3 (3.0), K2HPO4 (0.6), Na2CO3
(3.0), NH4Cl (0.6). The sulphate-reducing one also received a con-
venient concentration of sodium sulphate.
2.3. Monitoring and analyses
MFCs were batch-operated for 50 h at 37oC. The circuit of each
MFC was fitted either with a 33 or 10 kΩ external resistance in
order to be consistent with the Theorem of Jacobi [20]. Voltage cell
(EMFC), Current intensity cell (IMFC) and Power cell (PMFC) were
measured against time.
The voltage was determined with a Multimeter ESCORT 3146A.
The current was calculated by the Ohm’s Law:
The delivered power was obtained as the product of the Current
times Voltage, that is:
With the purpose to get a value comparable with the works al-
ready published, the power was calculated as a function of the an-
ode superficial area, as:
where AAn is the anode superficial area in m2 and Rext is the exter-
nal resistance in ohms. The performance of an MFC can be meas-
ured by two essential parameters, the first one is the Chemical oxy-
gen demand removal (ηCOD) and the second one is the Coulombic
efficiency (ηCoul). The ηCOD it is a method widely distributed to
analyze the organic matter removal from a specific system (Robles-
González et al¸ 2006).
The Coulombic efficiency is the ratio between the produced
electrons in reality (CRS) and the electrons that could be produced
from the substrate (CTS), as it follows:
ext
MFC
MFC R
E
I=(1)
MFCMFCMFC EIP =(2)
extAn
2
An RA
E
PMFC
=(3)
100
COD
CODCOD
ηCOD(%)
initial
finalinitial ×
−
=(4)
Table 1. Results from published works with MFCs.
Cell Type Inoculum Substrate Conditions Performance Membrane Reference
Dual G. sulfurreducens Acetate [5mM] T=30ºC, Ot=960h,
pH=6.8 PAn=16 mW m-2, without Membrane [34]
Dual G. fermentans Acetate, propionate,
malate, lactate and
succionate [5mM] Ot=960h N.S. Nafion 117 [35]
Concentric Cath-
ode G. metallireducens Wastewater T=30ºC; HRT=33d PAn=26 mW m-2; ηCoul=12% Nafion 117 [36]
Dual R. ferrireducens Glucose [2mM] 25ºC, Ot=1000h Ian=30 mA m-2 Nafion 117 [37]
Upflow Anaerobic sludge Sucrose T=35ºC; TRH=1d PAn=170 mW m-2;
ηCoul=8,1% CMI-7000 [28]
Dual Anaerobic sludge Modified wastewater T=30ºC; Ot=50h PAn=8 mW m-2; ηCoul=40% Nafion 117 [29]
Air-Cathode Anaerobic sludge
Modified wastewater
and glucose T=30ºC; Ot=120h PAn=262 mW m-2;
ηCoul=55% Nafion 117 [38]
Air-Cathode Wastewater Acetate modified
wastewater T=32-20ºC PAn=1200 mW m-2;
ηCoul=61,4% without Membrana [39]
Air-Cathode Wastewater Acetate or butirate
modified wastewater Ot=60h PAn=500 mW m-2;
ηCoul=30% without Membrana [40]
Dual Marine sediment Acetate [0.1mM] 22ºC, pH=6.8,
Ot=1920h N.S. Nafion 118 [41]
Dual Geobacter sulfurre-
ducens Acetate [5 mM]or H2 30oC, pH = 6.8
Ot = 168 h PAn=111 mW m-2 Nafion 117 [42]
Air-Cathode Methanogenic con-
sortium Mixture of organic
acids and solvents T=37°C, Ot=50h PAn=1.04 mW m-2;
ηCoul=0.12% Nafion 117 This work
Air-Cathode Sulphate reducer
consortium Mixture of organic
acids and solvents T=37°C, Ot=50h PAn=12.3 mW m-2;
ηCoul=1.22% Nafion 117 This work
Notes: PAn: Power density; IAn= Current density; ηCoul: Coulombic efficiency; N.S.: Ot=Operation time; HRT: Hydraulic retention time; N.S.=Not shown.

52
H. M. Poggi-Varaldo et al. / J. New Mat. Electrochem. Systems
where:
Fi: Faraday’s constant (Coulombs mol/e-)
bCOD: Moles of electrons harvested from the COD (mol e-)
CODi: Initial COD (g/L)
CODf: Final COD (g/L)
VMFC: MFC operation volume (L)
MCOD: COD’s molecular weight (g/mol)
3. RESULTS AND DISCUSSION
Figure 4 shows the time course of voltage of the two MFCs dur-
ing the 50 h of operation. The MFC loaded with M-In showed a
higher internal resistance value (33 kΩ) whereas the MFC loaded
with SR-In had only 10 kΩ of internal resistance, both found by a
Polarization Curve [21]. All response variables showed a better
performance using SR-In than using M-In. In Figure 4 the gray
area shows that the maximum, open circuit potential (the two first
hours and without a resistance in the external circuit) of the MFC
loaded with M-In was 0.4 V, whereas the one loaded with SR-In
was 0.7 V. On the other hand the maximum potential of the resis-
tance-loaded MFC was 0.26 and 0.46 V with an M-In and SR-In,
respectively. Voltage decrease under loading conditions was a con-
sequence of the high internal resistance value our MFCs compared
with other currently published data where they report small resis-
tances [22-25]. Average cell potentials of 0.10 (M-In) and 0.37 V
(SR-In), were registered during the 50 h of operation (Figure 4),
with the best result for the MFC loaded with SR-In.
Table 2 displays the best results and average values for each
MFC during the 50 h of operation. The anodic average power den-
sity of 12.31 mW m-2 using S-In, was about 12 times higher than
the power density of a MFC loaded with M-In (1.04 mW m-2 an-
ode). Yet, power densities recorded were lower than selected val-
ues reported recently in the open literature ([26], Table 1). Mean-
while, values of EMFC were comparable to those found in the litera-
ture for other MFCs, while IMFC resulted lower than those observed
for MFCs treating dilute organic wastewaters. The volumetric
power values were in the order of the results published lately; in
fact, the volumetric power exhibited by the MFC loaded with SR-
In was considerably high compared with literature reports [27].
Organic matter removal was low to moderate: 25% using a M-In
and 43% in the MFC loaded with a SR-In. These results were con-
sistent with very low values of the coulombic efficiency (CE). Both
parameters could be increased by increasing the time of operation
(because at the end of the run the majority of the organic substrate
was still available) and by lowering the internal resistance of the
cell.
Initial biomass concentrations were 350 and 265 mg VSS/L for
the methanogenic and the sulfate-reducing MFCs, respectively. The
final VSS concentrations were slightly higher than the initial ones,
that is, 370 and 330 mg/L for the methanogenic and the sulfate-
reducing MFCs, respectively. The increase in biomass concentra-
tion might have also contributed to a loss of electron flow for elec-
tricity generation (low coulombic efficiency), because a portion of
substrate and substrate electrons was used for biomass synthesis.
Adequacy of inocula could have also had an influence on low
power density and CE efficiency values. He et al. [28] used granu-
lar methanogenic sludge (previously crushed) from a bioreactor
treating a brewery wastewater for inoculating a lab scale upflow
microbial fuel cell (UMFC). After an extensive time of cell opera-
tion where the inoculum was acclimated to a feed where sucrose
was the main electron donor, it was found a maximum power den-
sity of 170 mW m-2 and high removal efficiency of organic matter
(ca. 90% COD basis). Yet, low CE (in the range 0.7 to 8%) was
determined that suggested that electron-transfer microorganisms
were not able to convert all of the available carbon source into
100(%) ⋅= CTS
CRS
Coul
η
(5)
∫
=
t
CCM dtICRS
0
(6)
(
)
COD
MFCfiCODi
M
VCODCODbF
CTS ⋅−⋅⋅
=(7)
Figure 4. Electricity generation by an MFC using two types of
inocula.
Table 2. Average performance of Microbial Fuel Cells in this
work.
Parameter Methanogenic
inoculum
Sulphate reducing
inoculum
PAn-max (mW m-2) 1.04 12.31
PV-max (mW m-3) 13.37 157.81
ECCM-max (V) 0.26 0.48
ICCM-max (mA) 7.86×10-3 4.91×10-2
PCCM-max (mW) 2.01×10-3 2.37×10-2
PAn-ave (mW m-2) 0.15 ± 0.06 7.13 ± 0.71
PV-ave (mW m-3) 1.96 ± 0.82 91.35 ± 9.04
ECCM-ave (V) 0.10 ± 0.02 0.37 ± 0.02
ICCM-ave (mA) 2.93×10-3 ± 5.01×10-4 3.73×10-2 ± 1.72×10-3
PCCM-ave (mW) 2.95×10-4 ± 2.93×10-3 1.37×10-2 ± 3.73×10-2
ηCOD (%) 25.00 42.86
ηCoul (%) 0.13 1.22
Notes: PAn-max: Maximum power density; PAn-ave: Average power density; PV-max:
Maximum volumetric power; PV-ave: Average volumetric power; ηCOD: Chemical
oxygen demand removal; ηCoul: Columbic efficiency.

53
Effect of Inoculum Type on the Performance of a Microbial Fuel Cell Fed with Spent Organic Extracts
from Hydrogenogenic Fermentation of Organic Solid Wastes
/ J. New Mat. Electrochem. Systems
electrical energy. In this regard, the authors also found that between
1/3 to half of the removal efficiency of organic matter in their
UFMC was due to active methanogenesis, thus also explaining the
low CE. Kim et al. [29] studied various techniques to enrich elec-
trochemically active bacteria on an electrode using anaerobic sew-
age sludge in a two-chambered MFC. As part of their experiments,
when a porous carbon paper anode electrode was used, a power
density up to 8 mW m-2 was obtained within 50 h with a CE of
40%. When an iron oxide-coated electrode was used, the power and
the CE reached 30 mW m-2 and 80%, respectively. Spiking with a
specific methanogen inhibitor (2-bromoethanesulfonate) increased
the CE to 70%. This result strongly suggested that methanogenic
activity of the inoculum is not desirable and is linked to lower per-
formance of the MFC. Also, this result agrees with the observations
reported by He et al. [28] above.
In our experiments, the methanogenic inoculum found a cell
liquor rich in acetate and bicarbonate and carbonate (CO2 or bicar-
bonate are recognized electron acceptors of methanogenesis [30]).
Such conditions could favour the deviation of the electron flow
from electricity to generation of methane, thus lowering both the
power density output and the CE efficiency of our MFC. On the
other hand, the sulfate-reducing inoculum in a cell whose liquor
lacks sulfate such as in our work, will have less chance to deviate
electrons from organic matter to anaerobic respiration. This is con-
sistent with higher power density and CE efficiency found for the
SR-In compared with the M-In one in our work (Table 1). Our
results suggest that methanogenic inocula are less adequate than
sulfate reducing inocula for MFC operation, although more re-
search is needed to generalize such a conclusion.
Table 1 shows a compilation of other works on MFC. Studies
with pure cultures (members of the proteobacterial families Geo-
bacteraceae) seem to predominate, although there are some recent
studies with microbial consortia. It is remarkable the use of Pt as a
catalyst for the composition of electrodes and wire material to con-
nect the external circuit. Besides, studies with synthetic or semi-
synthetic feed predominate. In this study it was preferred the use of
a microbial consortium instead of a pure culture or a mixed culture.
Our MFC architecture relied on a single chamber design with a
relative large volume compared with other designs [31-33]. In our
study Pt as a catalyst was used only at the cathode to overcome the
final reaction to produce water, the external circuit lacked of plati-
num. Another possible factor contributing to low average power
densities in this work could be the change of substrate faced by the
inocula. In effect, microbial consortia used in our experiments were
acclimated to a feed rich in sucrose and acetic acid in their inocu-
lating bioreactors. After transfer to the MFC, the substrate fed was
a model extract that lacked sucrose, and was concocted with acetic,
propionic and butyric acids as well as acetone and ethanol and
mineral salts. The absence of acclimation to the new substrate
could have played a negative effect on MFC performance. With
this it is understandable the low values for Power density and CE
compared with values obtained in other research that use platinum
in all electrodes, wires and connections, extended acclimation pro-
cedures, etc.
4. CONCLUSIONS
The MFC seeded with SR-In outperformed the MFC loaded with
a M-In. The open circuit potentials were comparable to those pub-
lished in the literature (0.4-0.6 V).
The power density produced by the sulphate reducer inoculum
was in the order of values currently published by other groups with
MFCs under different inocula and substrate conditions, whereas the
use of methanogenic inoculum leads to results in the low side of
currently published data. The COD removal was between low and
moderate, and it was higher with sulphate reducer inoculum, show-
ing the MFC capacity to diminish the organic matter from the stud-
ied system.
It was found a very high value of internal resistance for the MFC
in the order of kilo-ohms. The latter could be responsible for low
values of current intensity and coulombic efficiency. In agreement
to the results from the polarization curve and the batch tests, the
selection inoculum shall be the sulphate reducing one. Further
experiments are in course to increase the power and power density
of our MFC. Our results seem to indicate that MFC could be used
to further tapping energy from leachates generated in solid waste
disposal, thus increasing electricity yields using an easily available
and cheap resource.
5. ACKNOWLEDGEMENTS
The authors wish to thank CINVESTAV-IPN, Mexico, for par-
tial financial support to their research. AC-M and ALV-L gratefully
acknowledge Master scholarships from CONACYT, Mexico. Fi-
nally, the authors appreciate the insightful comments and sugges-
tions of the Associate Editor and anonymous referees of the Journal
of New Materials and Electrochemical Systems.
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