Bioelectrochemical systems need an anode with a high abundance of exoelectrogenic bacteria for an optimal performance. Among all possible operational parameters for an efficient enrichment, the role of external resistance in microbial fuel cell (MFC) has gained a lot of interest since it indirectly poises an anode potential, a key parameter for biofilm distribution and morphology. Thus, this work aims at investigating and discussing whether bioanodes selected at different external resistances under MFC operation present different responses under both MFC and microbial electrolysis cell (MEC) operation. A better MEC performance (i.e. shorter start-up time, higher current intensity and higher H2 production rate) was obtained with an anode from an MFC developed under low external resistance. Quantitative real-time polymerase chain reaction (qPCR) confirmed that a low external resistance provides an MFC anodic biofilm with the highest content of Geobacter because it allows higher current intensity, which is correlated to exoelectrogenic activity. High external resistances such as 1,000 Ω led to a slower start-up time under MEC operation.

INTRODUCTION

Bioelectrochemical systems (BESs) are an emerging technology focused on converting waste into energy (microbial fuel cells, MFCs) or added-value chemical compounds (microbial electrolysis cells, MECs). BESs rely on the anode enrichment with exoelectrogenic bacteria, also called anode respiring bacteria. These bacteria have the ability to transfer the electrons from their metabolism to an external solid anode. These electrons flow through an electrical circuit to a cathode (Logan 2009), where a reductive reaction occurs. In an MFC, where oxygen reduction takes place on the cathode, the overall process is spontaneous and electricity is produced (Logan 2008). On the other hand, in an MEC, additional energy supply is required, since the reductive cathodic process is different from oxygen reduction, for example, hydrogen production (Tartakovsky et al. 2009; Ruiz et al. 2015).

Optimizing MFC performance to obtain maximum power output at affordable costs has been the research goal of many authors so far. Many different factors, from reactor configuration to medium composition, have been studied (Borole et al. 2009; Kiely et al. 2011; Velasquez-Orta et al. 2011). Among them, the role of the external resistance in MFCs has gained a lot of interest due to its significant effect on biofilm characteristics. In this frame, Jung & Regan (2011) showed that the use of different external resistances modifies the anodic potential, which is equivalent to changes in the anode's capacity to accept electrons. These changes influence the competition between exoelectrogenic and non-exoelectrogenic bacteria, as, for example, methanogens. Zhang et al. (2011) stated that the external resistance affected MFC performance and, also, the structural and morphological characteristics of anodic biofilms. Rismani-Yazdi et al. (2011) demonstrated that changes in the external resistance affected not only the MFC performance but also the microbial metabolism and the relation between planktonic and anode-attached biofilm. Conversely, Lyon et al. (2010) evaluated the effect of external resistance on power production and microbial community structure and observed that power production was independent of external resistance even though the internal biofilm structure varied. Finally, Premier et al. (2011) showed that MFC performance could be increased by using a real-time dynamic control strategy that modified on-line the external resistance to match the internal MFC resistance estimated with power curves.

These previous works show how the choice of a certain external resistance influences several key factors that affect the final MFC performance in terms of power output in MFC. The choice of an external load equal to the internal cell resistance leads to an optimum MFC operation (Premier et al. 2011). On the other hand, the external resistance indirectly poises an anode potential, a key parameter in BESs, since it determines the energy gain for the bacteria, and thus enhances the growth of different anode microbial populations and determines the biofilm morphology. In fact, many authors control the anode potential of BESs using a potentiostat (Aelterman et al. 2008; Wagner et al. 2010; Batlle-Vilanova et al. 2014), which is a rather expensive piece of equipment. In contrast, the use of external resistances is a low-cost procedure to regulate the anode potential and the current. While the effect of external resistance on MFC performance has already been described, the response of microbial communities grown with different external resistances when operated as MEC is yet to be studied.

The start-up of an MEC can be conducted from an already running MFC (the most usual scenario) or from anaerobic sludge. In the latter case, there are two options: operating with a fixed anodic potential (using a potentiostat) or with a fixed applied potential (using a power source). Again, the potentiostat is an expensive option, but the start-up with a power source may be slower and prone to failure due to the lack of biofilm and to the uncontrolled anodic potential. Inoculating an MEC with an already running MFC can avoid the use of a potentiostat due to the already developed exoelectrogenic biofilm. Hence, this work aims at investigating and discussing whether bioanodes selected at different external resistances under MFC operation present different responses when operated as MEC with a fixed applied potential.

METHODS

MFC and MEC operation

Three cube-shaped air-cathode MFCs were used in this work. Each MFC was a 28 mL methacrylate cylindrical vessel provided with a lateral aperture (3.8 cm diameter), where the cathode was fitted. The cathode consisted of a graphite fiber cloth (3.8 cm diameter, 7 cm2 total exposed area) coated with platinum (5 mg Pt/cm2, ElectroChem Inc.) in the catalytic layer and a polytetrafluoroethylene diffusion layer which permitted oxygen diffusion into the cell while preventing water leakage (Cheng et al. 2006). It was placed 2.5 cm apart from the anode. The anode was a graphite fiber brush (20 mm diameter × 30 mm length; 0.18 m2) made with fibers (diameter 7.2 μm, PANEX33 160 K, MillRose Company, Mentor, OH, USA) connected with a titanium wire core.

The anode was inoculated by mixing (in a volume ratio 1:1) fresh medium and media from an already working Inoculum-MFC (with an external resistance of 12 Ω), built as described in Ribot-Llobet et al. (2014) using a 400 mL glass vessel with a lateral aperture (6.3 cm diameter) for the air-cathode assembling. For each cube-shaped air-cathode MFC, the external resistance to connect both electrodes was different: 12 Ω (MFC12), 220 Ω (MFC220) and 1,000 Ω (MFC1000).

The MFCs achieved stable operation around day 60, when similar performance was observed in consecutive cycles. Subsequently, the configuration was changed to MEC by transferring the MFCs, anodes to three different MECs as detailed in Montpart et al. (2014) and with the application of a constant potential of 0.8 V. Both MFCs and MECs were operated in batch mode. The fresh medium was a 100 mM phosphate buffer with the following components in 1 L of deionized water: NH4Cl (0.41 g), mineral media (5 mL), 1 mL of 4 g L−1 FeCl2 stock solution, and 0.5 mL of 37.2 g L−1 Na2S·9H2O stock solution, and fed with 30 mM of acetate. The mineral media had the composition previously described in Parameswaran et al. (2009).

Chemical and electrochemical analysis

Acetate was analyzed by gas chromatography (Agilent Technologies, 7820-A, Santa Clara, CA, USA) using a flame ionization detector with helium as carrier gas. H2 and CH4 were also analyzed with the same gas chromatograph equipment but using a thermal conductivity detector with argon as carrier gas, as described in Ruiz et al. (2013). Chronoamperometric analyses were performed to evaluate MEC performance using a Multi Autolab system (Ecochemie, Utrecht, The Netherlands). The anode was set as the working electrode and the cathode was used as both the auxiliary and the reference electrode. The anode potential (vs. cathode potential) was set at 13 levels ranging between 250 mV and 850 mV in steps of 50 mV. Each potential was set for 300 s to allow current intensity to stabilize. The last data point corresponding to each potential was used to build the curve Current intensity vs. Electrode potential. Similar chronoamperometric measurements were used to build polarization curves for MFCs (Cell potential vs. Current intensity). In this case, the anode potential (vs. cathode potential) was set at 17 levels from −675 mV (anode open circuit potential) to −20 mV. The applied potentials were negative, because it is equivalent to using different external resistances. Finally, power curves (Power vs. Current intensity) were calculated from the polarization curves as the product between potential and current intensity.

Electrochemical calculations

Coulombic efficiency (CE) was calculated as in Equation (1):
formula
1
where t0 and tF are the initial and final time of an experiment, Δc is the change in acetate concentration during the experiment (g acetate·L−1 cell), M is the molecular weight of acetate (59 g·mol−1), bAc is the number of e transferred per mole of acetate (8 mol e mol−1 acetate), F is Faraday's constant (96,485 C mol−1 e), I is the current intensity and VL is the volume of liquid in the reactor (L).
Energy recovery of the MEC was calculated as the amount of energy contained in the produced hydrogen with respect to the electrical input (rE) (Equation (2)) and to the electrical input and the energy content of the substrate (rE+S) (Equation (3)).
formula
2
where nH2 are the moles of produced hydrogen, ΔHH2 is hydrogen heat of combustion (−285.83 kJ·mol−1), Eap is the applied voltage (V) and Rext is the external resistance used for monitoring (Ω).
formula
3
where nS are the moles of consumed acetate and ΔHS is the heat of combustion of acetate (−870.28 kJ·mol−1).

DNA extraction

Total DNA was extracted from approximately 0.15 g of samples using a PowerSoil DNA Isolation Kit (MoBio Laboratories, Inc., Carlsbad, CA) according to the manufacturer's instructions. The Inoculum sample was obtained by centrifuging at 10,000 g (Beckmann Coulter TM, Avanti J20XP; USA) 350 mL of the media obtained from the running Inoculum-MFC, which was used to inoculate the cube MFCs (MFC12, MFC220, MFC1000). The MFC samples (after 60 days of operation) and the MEC samples (after 30 days of operation) were obtained from the anodes of each cell. The anode graphite fibers were cut and combined for DNA extraction. Previously, the fibers were rinsed with 1 mL of sterile MilliQ water to remove residues from growth medium or biofilm. Quality and quantity of the DNA were measured using a NanoDrop® spectrophotometer (ThermoScientific, Wilmington, DE, USA). Moreover, DNA was visualized under UV in a 0.7% gel electrophoresis with TBE 0.5X (Tris-Borate 50 mM; EDTA 0.1 mM; pH 7.5–8).

Quantitative real-time polymerase chain reaction

Quantitative hydrolysis probe based real-time polymerase chain reaction (qPCR) was used to quantify exoelectrogenic proteobacteria Geobacter as a member of the Fe(III)-reducing Geobacteraceae family. qPCR was performed in a LightCycler 480 instrument (LC480; Roche, Basel, Switzerland) using the corresponding primers GEO561F (5′-GCCATGCACCWCCTCT-3′) and GEO825R (5′-GCGTGTAGGCGGTTTCTTAA-3′) and the Gbc1 Taqman probe (5′-AGCACCACAACGCGTGGA-3′) previously described (Stults et al. 2001; Cummings et al. 2003). Each reaction mixture of 20 μL was prepared using the LightCycler 480 Probe Master kit (Roche Diagnostics), primers (final concentration 300 nM), hydrolysis probes (final concentration 100 nM), 2X LC480 Probe Master and 2 μL of template DNA. Geobacter was quantified as described by Rago et al. (2015). All DNA templates were analyzed in duplicate. A detailed description of the quantitative standard curves generation can be found in Rago et al. (2015).

RESULTS AND DISCUSSION

MFC operation with different external resistances

Three MFCs were inoculated by mixing (1:1) the effluent from an already working MFC (with an external resistance of 12 Ω) and fresh medium. Each MFC was operated in batch cycles (5 days per cycle) using a different external resistance: 12 Ω (MFC12), 220 Ω (MFC220) and 1,000 Ω (MFC1000). Current intensity and power values obtained from the first five batch cycles for the three MFCs are shown in Figure 1.
Figure 1

Evolution of current intensity (left) and power output (right) over time in three MFCs with different external resistance (MFC12-solid; MFC220-dotted; MFC1000-dashed).

Figure 1

Evolution of current intensity (left) and power output (right) over time in three MFCs with different external resistance (MFC12-solid; MFC220-dotted; MFC1000-dashed).

A higher current intensity was obtained under lower external resistance, in agreement with previous literature reports (Jung & Regan 2011; Rismani-Yazdi et al. 2011). Regarding CE, higher values were obtained for lower external resistances: MFC12 (74%), MFC220 (53%) and MFC1000 (23%). Finally, the lowest power output was obtained at lower external resistances despite its high current intensity. This observation was previously discussed in the literature. Zhang et al. (2011) stated that this lower power output might be due to the significant ohmic losses resulting from void spaces in the interior of the biofilm. Zhang et al. (2011) also demonstrated that the use of an external resistance lower than the optimum value generates a reduction of the electrical conductivity within the biofilm, which has a higher active biomass but also a higher exopolysaccharide content. On the other hand, the highest power output was obtained from MFC220 (MFC220: 718 μW, MFC12: 86 μW and MFC1000: 279 μW) due to the closeness between the external resistance and the cell internal resistance. The internal resistances of the MFCs were estimated through polarization curves, obtaining values around 100 Ω for MFC250 and MFC1000, and slightly higher for MFC12 (118 Ω for MFC12; 101 Ω for MFC250 and 95 Ω for MFC1000). Therefore, the experimental data in this work corroborate that an optimal external resistance has to be used to obtain the highest sustainable current generation and power output. In this sense, many efforts in on-line optimization of the external resistance have been reported in the literature (Logan 2008; Pinto et al. 2011; Molognoni et al. 2014).

Transference of MFC anodes to MEC

After 60 days of MFC operation, the anodes of MFC12, MFC220 and MFC1000 were transferred to three different MECs (MEC12, MEC220 and MEC1000) with 0.8 V of applied voltage between anode and cathode provided by a power source. The current intensity values obtained from the three MECs during 4 weeks are shown in Figure 2. The anodes obtained from MFC12 and MFC220 (i.e. inoculated with lower external resistances) were immediately adapted to the new configuration. The maximum current intensity was achieved from the first cycle in MEC12 and from the second cycle in MEC220. The cycle-length of each batch was lower in MEC mode than in MFC mode (2 vs. 5 days) since the intensity in MEC was higher and the cycles had the same initial amount of acetate. On the other hand, the anode from MFC1000 needed more than 1 week (five MEC batch cycles) to obtain its best performance. Although the initial current intensity produced by MEC12 was the highest, it decreased around 15% throughout the 25 days of MEC operation, whereas the current intensity produced by MEC220 and MEC1000 did not decrease after the start-up period. Despite the 15% decrease in current intensity for MEC12, this cell provided the best performance and faster adaptation to the MEC operation. The volume of hydrogen produced was measured for each MEC at the end of each of the last two batch cycles. Hydrogen production rate in MEC12 (1.54 ± 0.13 LH2·L−1REACTOR·d−1, n = 2) was slightly higher than that in MEC220 (1.34 ± 0.02 LH2·L−1REACTOR·d−1, n = 2) or MEC1000 (1.1 ± 0.3 LH2·L−1REACTOR·d−1, n = 2) in agreement with the trend in current intensity values. Moreover, in all cases, the CE was around 90%.
Figure 2

Current intensity versus time measured in the three MECs (MEC12-solid; MEC220-dotted; MEC1000-dashed).

Figure 2

Current intensity versus time measured in the three MECs (MEC12-solid; MEC220-dotted; MEC1000-dashed).

Common MEC performance indexes were calculated for the three MECs. The values obtained are in agreement with those found in the literature for similar systems (Ruiz et al. 2015) and indicate that a positive energy recovery is obtained when considering the electrical input. The energy recovery (rE) was 143 ± 9% for MEC12, 138 ± 13% for MEC250 and 153 ± 27% for MEC1000. The energy recovery obtained considering the substrate internal energy and the electrical input (rE+S) was 54 ± 7% for MEC12, 55 ± 9% for MEC250 and 54 ± 6% for MEC1000. These results do not show a clear trend among the different MECs in steady-state operation.

Supplementary Figure S1 (available with the online version of this paper) shows the chronoamperometric measurements from the three MECs after 4 weeks of operation. For a certain applied potential, higher intensity values were obtained from the anode operated with lower external resistance in MFC. For example, at an applied potential of 0.8 V, the highest current intensity was obtained from MEC12 (7.7 mA) and from MEC220 (6.3 mA), while in MEC1000 (4.5 mA) it was 41% lower than that obtained for MEC12.

Regarding the microbiological analysis, Figure 3 compares the Geobacter quantification by qPCR from the inoculum and the anodes from the three MFCs and MECs. The concentration of planktonic Geobacter in the inoculum was 6.98 × 105 gene copies/mg. The concentration of Geobacter in the anode was higher in MFCs and was further increased in MECs. The presence of Geobacter in the anodic biofilm was almost similar for MFC12 (1.69 × 107 gene copies/mg) and MFC220 (1.17 × 107 gene copies/mg), but only 1.48 × 106 gene copies/mg were obtained from MFC1000. Planktonic Geobacter concentration in the inoculum sample was lower compared with anodic biofilm samples obtained from the MFCs (around 20 times in the case of MFC12 and MFC220, and two times for MFC1000). Then, the development of a biofilm with a high content of Geobacter was possible in all the MFC anodes, although the best development was observed at the lowest external resistances. This higher Geobacter concentration under lower external resistances agrees with the fact that the CE under those resistances was higher.

The amount of Geobacter in MECs was higher than in MFCs. No clear trend was detected between MEC anodic biofilm samples, in contrast to MFCs: after 4 weeks of operation all MECs had almost the same amount of Geobacter, between 4.38 × 107 (MEC12) and 4.92 × 107 gene copies/mg (MEC1000), regardless of the external resistance used in MFC operation. Furthermore, it was demonstrated that the applied potential and the anoxic conditions of MEC reactors favored Geobacter growth in the anodic biofilm, which resulted in a concentration more than double with respect to the MFCs' anodic biofilm. The improved operational conditions with respect to MFC (total absence of oxygen and applied voltage) also justified the increased CE around 90% in all three MECs.
Figure 3

qPRC results of Geobacter (mean of triplicate values ± standard deviation) in the inoculum and anodes of MFC and MEC. Black: 12 Ω; light grey: 220 Ω; dark grey: 1,000 Ω.

Figure 3

qPRC results of Geobacter (mean of triplicate values ± standard deviation) in the inoculum and anodes of MFC and MEC. Black: 12 Ω; light grey: 220 Ω; dark grey: 1,000 Ω.

Considering all the results presented, this work shows that the shortest start-up time for an MEC is obtained using a very low external resistance in MFC (12 Ω). Although MFC12 shows the lowest power output, it also shows the highest current intensity, which results in a better development of the anodic biofilm (Figure 3). This allows a very fast adaptation to MEC operation: MEC12 is able to achieve a current intensity very close to its maximum value in just one cycle. In addition, after the start-up period, MEC12 achieves the highest current intensity compared with MEC220 and MEC1000 (Figure 2) and also the maximum current intensity at different applied potentials (Supplementary Figure S1). This observation is related to published reports aiming at maximizing current intensity in MFC by optimizing external resistance (Aelterman et al. 2008; Pinto et al. 2011; Molognoni et al. 2014). An MFC operating at high current intensity will result in better results when moved to MEC mode.

The utilization of a 220 Ω resistance also provides good performance, although the current intensities are slightly lower in both MFC and MEC. High resistances such as 1,000 Ω are not recommended because Geobacter growth is more limited (lower current intensity), which leads to a slower start-up time under MEC operation. Nevertheless, under steady-state conditions, any of the three resistances led to an anodic biofilm with a high content of Geobacter, near 5 × 107 gene copies/mg.

CONCLUSIONS

An anode inoculated under low external resistance in MFC mode (12 Ω) showed better performance when moved to MEC mode (i.e. it gave higher current intensity and showed higher H2 production rate) than other MFCs inoculated under higher external resistances (220 and 1,000 Ω). qPCR confirmed that the MFC under lowest external resistance had the highest content of Geobacter. Low external resistances resulted in higher current intensity, which is correlated with exoelectrogenic activity. Lower external resistances also resulted in a faster transition to stable MEC operation, since the anodic biofilm was able to drive higher current intensities. However, long-term MEC operation resulted in anodic biofilms with similar Geobacter content of around 5 × 107 gene copies/mg in spite of external resistance used in the previous MFC mode.

ACKNOWLEDGEMENTS

Laura Rago, Núria Montpart, Juan A. Baeza and Albert Guisasola are members of the GENOCOV group (Grup de Recerca Consolidat de la Generalitat de Catalunya, 2014 SGR 1255). Pilar Cortés is a member of the group of Microbiologia Molecular (Grup de Recerca Consolidat de la Generalitat de Catalunya, 2014 SGR 572). Laura Rago and Núria Montpart are grateful for the grants received from the Spanish government (FPI) and from the Universitat Autònoma de Barcelona (PIF).

REFERENCES

REFERENCES
Aelterman
P.
Freguia
S.
Keller
J.
Verstraete
W.
Rabaey
K.
2008
The anode potential regulates bacterial activity in microbial fuel cells
.
Applied Microbiology and Biotechnology
78
(
3
),
409
418
.
Batlle-Vilanova
P.
Puig
S.
Gonzalez-Olmos
R.
Vilajeliu-Pons
A.
Bañeras
L.
Balaguer
M. D.
Colprim
J.
2014
Assessment of biotic and abiotic graphite cathodes for hydrogen production in microbial electrolysis cells
.
International Journal of Hydrogen Energy
39
(
3
),
1297
1305
.
Borole
A. P.
Hamilton
C. Y.
Vishnivetskaya
T.
Leak
D.
Andras
C.
2009
Improving power production in acetate-fed microbial fuel cells via enrichment of exoelectrogenic organisms in flow-through systems
.
Biochemical Engineering Journal
48
(
1
),
71
80
.
Cheng
S.
Liu
H.
Logan
B. E.
2006
Increased performance of single-chamber microbial fuel cells using an improved cathode structure
.
Electrochemistry Communications
8
(
3
),
489
494
.
Cummings
D. E.
Snoeyenbos-West
O. L.
Newby
D. T.
Niggemyer
A. M.
Lovley
D. R.
Achenbach
L. A.
Rosenzweig
R. F.
2003
Diversity of Geobacteraceae species inhabiting metal-polluted freshwater lake sediments ascertained by 16S rDNA analyses
.
Microbial Ecology
46
(
2
),
257
269
.
Logan
B. E.
2008
Microbial Fuel Cells
.
John Wiley & Sons
,
New York
.
Logan
B. E.
2009
Exoelectrogenic bacteria that power microbial fuel cells
.
Nature Reviews Microbiology
7
(
5
),
375
381
.
Molognoni
D.
Puig
S.
Balaguer
M. D.
Liberale
A.
Capodaglio
A. G.
Callegari
A.
Colprim
J.
2014
Reducing start-up time and minimizing energy losses of microbial fuel cells using maximum power point tracking strategy
.
Journal of Power Sources
269
,
403
411
.
Montpart
N.
Ribot-Llobet
E.
Garlapati
V. K.
Rago
L.
Baeza
J. A.
Guisasola
A.
2014
Methanol opportunities for electricity and hydrogen production in bioelectrochemical systems
.
International Journal of Hydrogen Energy
39
(
2
),
770
777
.
Parameswaran
P.
Torres
C. I.
Lee
H. S.
Krajmalnik-Brown
R.
Rittmann
B. E.
2009
Syntrophic interactions among anode respiring bacteria (ARB) and non-ARB in a biofilm anode: electron balances
.
Biotechnology and Bioengineering
103
(
3
),
513
523
.
Pinto
R. P.
Srinivasan
B.
Uiot
S. R.
Tartakovsky
B.
2011
The effect of real-time external resistance optimization on microbial fuel cell performance
.
Water Research
45
(
4
),
1571
1578
.
Premier
G. C.
Kim
J. R.
Michie
I.
Dinsdale
R. M.
Guwy
A. J.
2011
Automatic control of load increases power and efficiency in a microbial fuel cell
.
Journal of Power Sources
196
(
4
),
2013
2019
.
Ribot-Llobet
E.
Montpart
N.
Ruiz-Franco
Y.
Rago
L.
Lafuente
J.
Baeza
J. A.
Guisasola
A.
2014
Obtaining microbial communities with exoelectrogenic activity from anaerobic sludge using a simplified procedure
.
Journal of Chemical Technology & Biotechnology
89
(
11
),
1727
1732
.
Rismani-Yazdi
H.
Christy
A. D.
Carver
S. M.
Yu
Z.
Dehority
B. A.
Tuovinen
O. H.
2011
Effect of external resistance on bacterial diversity and metabolism in cellulose-fed microbial fuel cells
.
Bioresource Technology
102
(
1
),
278
283
.
Ruiz
Y.
Baeza
J. A.
Guisasola
A.
2013
Revealing the proliferation of hydrogen scavengers in a single-chamber microbial electrolysis cell using electron balances
.
International Journal of Hydrogen Energy
38
(
36
),
15917
15927
.
Stults
J. R.
Snoeyenbos-West
O.
Methe
B.
Lovley
D. R.
Chandler
D. P.
2001
Application of the 5' fluorogenic exonuclease assay (TaqMan) for quantitative ribosomal DNA and rRNA analysis in sediments
.
Applied and Environmental Microbiology
67
(
6
),
2781
2789
.
Tartakovsky
B.
Manuel
M.
Wang
H.
Guiot
S.
2009
High rate membrane-less microbial electrolysis cell for continuous hydrogen production
.
International Journal of Hydrogen Energy
34
(
2
),
672
677
.
Velasquez-Orta
S. B.
Head
I. M.
Curtis
T. P.
Scott
K.
2011
Factors affecting current production in microbial fuel cells using different industrial wastewaters
.
Bioresource Technology
102
(
8
),
5105
5112
.
Wagner
R. C.
Call
D. F.
Logan
B. E.
2010
Optimal set anode potentials vary in bioelectrochemical systems
.
Environmental Science and Technology
44
(
16
),
6036
6041
.

Supplementary data