Full-loop operation and cathodic acidification of a microbial fuel cell operatedon domestic wastewater Olivier Lefebvrea, Yujia Shena, Zi Tana, Arnaud Uzabiagaa, In Seop Changb, How Yong
Trang 1Full-loop operation and cathodic acidification of a microbial fuel cell operated
on domestic wastewater
Olivier Lefebvrea, Yujia Shena, Zi Tana, Arnaud Uzabiagaa, In Seop Changb, How Yong Nga,⇑
a
Centre for Water Research, Department of Civil and Environmental Engineering, National University of Singapore, 1 Engineering Dr 2, Singapore 117576, Singapore
b
Department of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST), 261 Cheomdan Gwagi-ro, Buk-gu, Gwangju 500-712, Republic of Korea
a r t i c l e i n f o
Article history:
Received 3 November 2010
Received in revised form 29 January 2011
Accepted 23 February 2011
Available online xxxx
Keywords:
Microbial fuel cell
Loop
Overflow
Selemion
Wastewater treatment
a b s t r a c t
The present study emphasizes the importance of overcoming proton limitation in a microbial fuel cell operated on domestic wastewater When the anode-treated effluent was allowed to trickle into the cathodic compartment (full-loop operation), high COD and suspended solids removal efficiencies over 75% and 84%, respectively, were achieved while ensuring substantial and sustainable power generation Lower removal efficiencies resulted in decreased cell electromotive force caused by excess substrate crossover By decreasing the pH in the cathodic compartment to values below 2, we were able to further increase the maximum power generation by 180% in batch mode and 380% in continuous mode as com-pared to a negative control (tap water of pH 7.6) Under the optimized conditions, the internal resistance and electromotive force were 11Xand 0.6 V, respectively, which correspond to the state of the art
Ó 2011 Elsevier Ltd All rights reserved
1 Introduction
Providing the world’s population with adequate sanitation is
among the major development challenges of this century and
recovering energy from wastewater seems the only way to allow
sanitation programs to maintain their development without
dras-tically increasing energy consumption Among the different types
of wastewater, management of domestic wastewater is
particu-larly crucial in fast growing and land-limited Singapore In the
re-cent years, there has been growing interest in microbial fuel cell
(MFC), a technology derived from chemical fuel cell that allows
simultaneous wastewater treatment and energy recovery directly
in the form of electricity However, the technology is still in its
in-fancy due to scale-up limitations and large applications in
waste-water treatment plants are hindered by the inner limitations of
the technology One of the major limitations of MFCs nowadays
is related to stacking issues Even though parallel stacking can be
efficient – at least on a limited number of fuel cells (Aelterman
et al., 2006) – series connections of MFCs rapidly lead to energy
losses and reduced stacking efficiencies (Wang and Han, 2009)
Furthermore, series stacking has been shown to result in cell
polar-ity reversal (Aelterman et al., 2006) This raises the question of whether the power should be maximized on a volumetric basis –
as is the consensus nowadays – or on a ‘‘per cell’’ basis, in order
to reduce the number of cells to be connected together
At first glance, the main difference between conventional chem-ical fuel cells and MFCs is that MFC anodes are ‘‘alive’’ and rely on microbial metabolism However, another major difference be-tween the two technologies lies in the extremely different environ-mental conditions applied to both systems In an MFC, the temperature is ambient and the pH circum-neutral to allow bacte-rial growth on the anode On the other hand, in a conventional hydrogen fuel cell the temperature is above 80 °C and the cathode
is kept under a pressure of 2 bars of oxygen and hydrogen (Barbir,
2005) This makes it possible in hydrogen fuel cells to achieve elec-tromotive force higher than in MFCs On top of it, the pH in the cat-ion exchange membrane of a hydrogen fuel cell is around 3, which implies that there are much more protons available to contribute
to the charge transfer between the electrodes, and this transfer is further facilitated by high temperatures that dramatically increase proton conductivity (Barbir, 2005) Kinetics are also much faster in chemical fuel cells, which helps to maintain their internal resis-tance at low levels, in the order of a few mX
Because of the disadvantageous environmental conditions ap-plied to MFC systems and described above, the anode is seldom the limiting factor in a well constructed modern MFC design and the highest anodic power density achieved so far was obtained in
an MFC where the cathode was 14 times larger than the anode
0960-8524/$ - see front matter Ó 2011 Elsevier Ltd All rights reserved.
doi: 10.1016/j.biortech.2011.02.098
Abbreviations: HRT, hydraulic retention time; HCl, hydrochloric acid; MEA,
membrane electrode assembly; MFC, microbial fuel cell; OCV, open circuit voltage;
PTFE, polytetrafluoroethylene; SS, suspended solids; VSS, volatile suspended solids.
⇑ Corresponding author Tel.: +65 6516 4777; fax: +65 6774 4202.
E-mail address: esenghy@nus.edu.sg (H.Y Ng).
Contents lists available atScienceDirect
Bioresource Technology
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / b i o r t e c h
Trang 2MFC generally combine with oxygen to form water and in this
sense, proton availability and mobility are of prime importance
and most of the time lacking in MFCs, where electro neutrality is
achieved by transport of other more abundant cation species such
as Na+, K+, NHþ
4, Ca2+and Mg2+(Rozendal et al., 2006) Bringing
pro-tons directly to the cathode could be a way to improve the
electri-cal performance of the system One way is by operating the MFC in
a full-loop mode where the anode-treated effluent is allowed to
enter the cathode compartment and, recently, some publications
have presented the advantages of such method (Clauwaert et al.,
2009; Freguia et al., 2008a; Li et al., 2009) From the point of view
of electrochemistry, this helps counterbalancing pH variations in
two-chamber MFCs, in which otherwise cathode alkalinization
and anode acidification with time are observed (Gil et al., 2003)
Furthermore, from the point of view of wastewater treatment
engi-neering, the cathodic compartment occupies a large footprint that
is not directly used for wastewater treatment in most cases With
domestic wastewater, MFC is known to produce an effluent of
insufficient quality for discharge after passing through the anodic
compartment (Cheng et al., 2006; Lefebvre et al., 2008) The
situa-tion even worsens if the MFC is optimized for electricity
genera-tion, because the power output increases when the HRT
decreases but the trend is opposite for COD removal (Ahn and
Logan, 2010; Liu et al., 2004) This means that effluent polishing
will be required in an MFC-based wastewater treatment plant If
the anolyte is introduced into the cathodic compartment, the latter
has the potential to provide aerobic post-treatment for the
anode-treated effluent, while protons are transported directly by the
ano-lyte to the cathode of the MFC Another way to overcome proton
limitation at the cathode is by bringing them intentionally into
the cathode compartment, which is the main focus of the present
study
In this paper, we disclose a prototype MFC that is suitable for
continuous treatment of domestic wastewater, according to the
re-search objectives of our laboratory This implies that the MFC has
to be of a reasonable size (a few litres) In a larger reactor, power
generation is further maximized per cell used and not on a
volu-metric basis, which – as explained above – might prove useful until
the stacking technology matures In order to overcome proton
lim-itation at the cathode, two strategies were employed –
introduc-tion of the anolyte into the cathodic compartment of the MFC,
and acidification of the cathode using HCl For that purpose, the
MFC made use of a membrane electrode assembly (MEA)
incorpo-rating a proton selective membrane, in order to prevent accidental
acidification of the anodic compartment
2 Methods
2.1 MEA-MFC design
The reactor vessel was a vertical cylinder (length = 90 cm,
diam-eter = 7 cm) made of transparent polyacrylic plastic
(Thermoplas-tics, Singapore) and the MEA was incorporated in the middle The
MEA (length = 90 cm, diameter = 3 cm)was wrapped around a
stainless steel grid acting as the cathodic current collector and a
stainless steel mesh was tightened over the anode acting as the
anodic current collector The anodic compartment had a volume
of 2.9 L and the cathodic compartment capacity was of 0.6 L The
experimental design is shown inFig 1
The MEA consisted of an anode and a cathode wrapped on
opposite sides of an ion exchange membrane A proton-selective
Selemion ion exchange membrane (model HSF, Asahi, Japan) was
selected over the comparable Nafion membrane due to its more
competitive price Selemion HSF membrane, originally designed
burst strength of 0.2 MPa and a resistivity of 0.3Xcm2 in 0.5 mol L1HCl or H2SO4 (manufacturer data) Two layers of re-verse osmosis spacer were incorporated, between the anode and the Selemion membrane, and between the cathode and the mem-brane The purpose of such spacer was to prevent partial short-circuit between the anode and the cathode in the MEA In the absence of spacer indeed, the resistance between the anode and the cathode was only of around 200Xafter tightening the MEA, but with the use of spacer this value could be increased to over
1 MX, indicating that the partial short-circuit was largely overcome This emphasizes that, even though the anode and the cathode should be maintained as close as possible to one another
in an MFC system (Cheng et al., 2006), there is a limitation in MEA designs due to the risk of short-circuits Both the anode and the cathode were made of carbon cloth (designation B, E-Tek, USA) and the cathode was further coated with Pt (0.5 mg cm2)
on one side A detail of the MEA is shown inFig 1c
2.2 Experimental conditions
The fuel (domestic wastewater) was circulated continuously in
an upflow mode into the anodic compartment at a flow rate of
20 mL min1 (HRT = 2.4 h).Unless specified otherwise, aeration was provided by actively bubbling air into the cathodic compart-ment using an air compressor and regulating the airflow with a valve at 5 L min1 In early experiments, the anode-treated effluent collected from the top of the reactor was introduced into the top of the cathodic compartment where it was allowed to trickle along the cathodic wall (seeFig 1a) Domestic wastewater was used simultaneously as the fuel and the inoculum for the reactor, which was collected from the primary decantation basin of the Ulu Pan-dan Reclamation Plant in Singapore The pH, COD, suspended solids (SS) and volatile suspended solids (VSS) content ranged from 7.3 to 7.9240 to 459 mg L1, 152 to 288mg L1and 138 to 426 mg L1, respectively The VSS to SS ratio was ranged between 0.6 and 0.9, indicating good biodegradability The reactor was set-up at ambi-ent temperature (25 °C) and wrapped with aluminum foil to pre-vent algae growth
Cathodic acidification was performed in batch and in continu-ous modes Batch acidification used 500 mL of hydrochloric acid (HCl) solutions diluted in tap water at different pH, ranging from
6 to 1, and pumped into the cathodic compartment, which had been previously closed (Fig 1b) Air was allowed to bubble into the acidic solution at a flow rate of 5 L min1 The voltage was re-corded every 10 s across an external resistance of 40 O and, finally, the cathodic compartment was reopened to collect the catholyte and measure its pH Tap water with a pH of 7.6 was used as a neg-ative control Continuous acidification used HCl solution diluted in tap water at different pH, ranging from 1.2 to 6, and pumped at a flowrate of 250 mL min1into the cathodic compartment Airflow rate was similarly fixed at 5 L min1 The experiment began under the open circuit voltage (OCV) conditions After OCV stabilization, the external load was progressively decreased in order to record polarization curves Samples of the cathodic outlet were collected regularly during the experimental period Tap water (pH 7.6) and phosphate buffer solutions (pH 7) having ionic strengths of 102
and 104M, respectively, were used as negative controls 2.3 Analytical methods and calculations
COD, SS, VSS and pH were analyzed according to the standard methods (APHA, 2005) The cell voltage was monitored with a mul-timeter (M3500A, Array Electronic, Taiwan) connected to a com-puter by a data acquisition system (PC1604, TTi, RS, Singapore) Starting from the OCV, polarization curves were obtained by
Trang 3decreasing the applied external resistance and recording the
pseu-do steady-state voltage The current was then determined using
the Ohm’s law and the Coulombic efficiency was calculated based
on the current and COD removal followingLogan et al (2006) The
cell electromotive force (Eemf, V) and internal resistance (Rint, O)
were determined using a linear regression (least squares method)
on the linear part of the polarization curve that corresponds to
the Ohmic zone The electromotive force was estimated as the
intercept of the regression with the Y-axis whereas the internal
resistance was the opposite of its slope The maximum power
(Pmax, W) supplied by the MFC was calculated according to Eq 1
Pmax¼ðEemfÞ
2
Data analyzes were assessed statistically by Student’s t-tests for
un-paired samples Two sets of data were considered significantly
dif-ferent when the P value was inferior than 0.05
2.4 Performance assessment as compared to the literature
The performance of our MFC set-up was compared to the
liter-ature available using the following criteria: OCV, Eemf, Rint, Pmax,
and the volumetric power (Pvmax) Eemfand Rintwere determined
using the polarization curves provided in the selected articles
and Pmax, was calculated using Eq 1 Furthermore, in an effort of
standardization, Pvmaxwas calculated for each study taking into
ac-count the total working volume of the MFCs, which corresponds to
the sum of the volume of the anodic and cathodic compartments
In our opinion, this shall reflect more accurately the impact of
the footprint of the treatment system
3 Results and discussion
The experiments were conducted over a period of 140 d Four
distinct stages could be identified Stages 1 to 3 (65 d) correspond
to the MFC operation in a full-loop mode and are described in
Sec-tion 3.1 Stage 4 (75 d) corresponds to the MFC operaSec-tion in a sin-gle-chamber mode with an open-air cathode (i.e., full-loop operation was ceased) Cathodic acidification experiments were carried out during stage 4 between days 95 and 120 Details of operation during stage 4 are detailed in Section 3.2
3.1 Operation in a full-loop mode (stages 1–3)
The full-loop experiments were carried over a period of 65 d and the experimental protocol is shown inFig 1a Three distinct stages could be identified Stage 1 (7 d) corresponded to the start-up of the reactor followed by stage 2 (20 d) characterized
by a deterioration of the reactor performance Finally, in Stage 3 (38 d), the anodic compartment was filled with graphite granules resulting in improved performance The detail of the three stages
is given below and the electrochemical characteristics of the reac-tor and of the treatment performance at each stage are summa-rized inFigs 2 and 3
3.1.1 Stage 1 The MEA-MFC was operated initially on domestic wastewater in
a full-loop mode in which the effluent flowed upward into the ano-dic compartment then trickled into the inner cathoano-dic compart-ment (seeFig 1a) The power rose constantly over the first week
of operation and attained 1.74 ± 0.16 mW (Fig 2) The OCV at that time was higher than 0.7 V, which indicates proper functioning of the system as an electricity generation device The cell electromo-tive force averaged 0.59 ± 0.08 V and the internal resistance was estimated at 52 ± 19X After passing through the anodic compart-ment, only 39 ± 11% of the COD and 57 ± 10% of the SS were re-moved (Fig 3) However, the treated effluent collected from the outlet of the cathodic compartment appeared much clearer in color and the COD removal averaged 71 ± 5% In addition, 78 ± 2% of the
SS were removed in the process This confirms the potential of using the aerobic cathode compartment for effluent polishing in the MFC technology The Coulombic efficiency was estimated to
air
effluent out
effluent in
effluent in
effluent out
current collector
current collector
cathode
anode
spacer
spacer Selemion
lumen
HCl in
anode compartment
cathode compartment
HCl out
(c)
Fig 1 Experimental design (a) operation in full-loop where the effluent from the anodic compartment is introduced into the cathodic compartment, (b) operation with acidification of the cathodic compartment by HCl, (c) detail of the membrane electrode assembly.
Trang 4average 0.2 ± 0.1%, which reflects that most of the COD was
removed by ways that did not contribute to electricity generation
Mostly, there was a tendency to observe sludge accumulation at
the bottom of the anodic compartment, where fermentation was
probably taking place
3.1.2 Stage 2
After 14 d, the power generation was found to be much lower
and the OCV was reduced to below 0.4 V The electromotive force
dropped drastically to 0.13 ± 0.04 V while the internal resistance
was affected to a lesser extent, averaging 70 ± 13X(Fig 2)
Conse-quently, the maximum power output (0.06 ± 0.04 mW) was found
to be significantly lower than that during stage 1 (P = 0.005;
Stu-dent’s t-test) This drop in performance was accompanied by a
poorer quality of the treated effluent that looked darker in color
and the COD and SS removal efficiencies dropped to 54 ± 11 and
32 ± 30% (Fig 3), respectively, resulting in unchanged Coulombic
efficiency of 0.2 ± 0.1% A probable reason for the observed drop
of electromotive force could be attributed to excess substrate
crossover from the anode to the cathode, resulting in a cathodic
potential mixed between that of O2 and that of the
above-mentioned substrate, as explained byHarnisch et al (2009)
Fur-thermore, another explanation could be related to aerobic bacteria
growing on the cathode due to excess substrate and further
limit-ing oxygen access for the cathodic reaction Similar problem was
also noted by Freguia et al (2008a)who used acetate as a
sub-strate When they increased the loading rate in their MFC,
over-loading of the cathodic compartment and a drop in power
occurred However, for practical reasons, it was not possible to
ac-cess the MFC cathode in the course of our experiment to evaluate
biofilm growth
3.1.3 Stage 3
In order to improve the quality of the anode-treated effluent,
the anodic compartment was filled with graphite granules having
an estimated projected surface of between 500 and 3000 m2m3
(Carbone Lorraine, Belgium) In addition, the cathode compartment was washed with plenty of water As a result, the COD and SS re-moval efficiencies after passing through the anodic compartment increased to 75 ± 21% and 84 ± 19%, respectively (Fig 3) The high variability of COD and SS removal efficiencies could be attributed
to the variability of the influent itself depending on the weather conditions (i.e., rainfall) For instance, during stage 3 the influent
SS dropped and this was accompanied by a drop in the SS removal efficiency that followed the same pattern Introducing the anolyte into the cathode further improved the COD removal to 92 ± 4%, whereas the SS removal remained unchanged In terms of electro-chemical performance, the fuel cell electromotive force rose to 0.62 ± 0.06 V while the internal resistance remained unaffected at
70 ± 9 O (Fig 2) These results confirm the hypothesis that the drop
in power observed in stage 2 could be attributed to substrate crossover as explained byHarnisch et al (2009), since only the electromotive force seemed to be affected by the quality of the anode-treated effluent The maximum power was then of 1.41 ± 0.21 mW (an increase by 2250% as compared to stage 2;
P = 0.00003 using Student’s t-test) and remained stable over the next 37 d The Coulombic efficiency was also considerably en-hanced, averaging 11 ± 1% (an increase by 5400% from stage 2).These results confirm the benefits of letting the anolyte flow into the cathode compartment to increase proton availability at the cathode However, this should be done with care to avoid overload-ing the cathode with excess substrate
3.2 Effect of cathodic acidification (stage 4) The cathodic acidification experiments were carried out with-out granules in the anode compartment This phase corresponds
to stage 4 inFigs 2 and 3 In the first place and before adding HCl, full-loop operation and active aeration were stopped so the cathode became an open-air cathode Under steady state
Fig 2 Evolution of the electromotive force (E emf ), maximum power (P max ), internal resistance (R int ) and Coulombic efficiency (CE) of the microbial fuel cell during the four stages of the experimental period Stage 1 corresponds to the start-up of the reactor Stage 2 is a period during which the reactor performance deteriorated Stage 3 corresponds to the operation of the anodic compartment with graphite granules Stage 4 corresponds to the operation in a single-chamber mode with an open air–cathode Cathodic acidification experiments were carried out during stage 4.
Trang 5conditions, the maximum power averaged 2.29 ± 0.32 mW, an
in-crease by 62% as compared to stage 3 (P = 0.0002; Student’s t-test)
The internal resistance was reduced to 48 ± 11 O and the
electro-motive force reached 0.66 ± 0.04 V However, the COD and SS
removal efficiencies were reduced to 50 ± 9% and 61 ± 12%,
respec-tively, as compared to stage 3 (Fig 3), and the Coulombic efficiency
dropped to 0.9 ± 0.4%, due to the absence of granules The
increased electrochemical performance of the MFC reactor in
open-air cathode conditions comes as no surprise, knowing that
this mode of operation results in increased oxygen availability at
the cathode and reduced internal resistance, as it has largely been
documented in the literature (Liu et al., 2004) After assessing the performance without HCl, the actual cathodic acidification experi-ments were carried out between days 95 and 120 subsequently in batch mode and in continuous manner
3.2.1 Batch experiments
In the beginning, cathodic acidification was conducted over a period of 14 d under different pH conditions in a batch mode by pumping 500 mL of diluted HCl solution into the cathodic com-partment The power response is shown in Fig 4, from which it can be seen that the injection of diluted HCl solution resulted in
(a)
(b)
Fig 3 Evolution of the (a) COD and (b) suspended solids (SS) concentrations in the influent, anodic outlet, cathodic outlet and corresponding total removal efficiencies during the four stages of the experimental period.
Trang 6an immediate power increase However, the improvement caused
by the decreasing pH was not steady and the power rapidly
de-creased again after the protons were consumed As a result the
catholyte had a pH around 8 at the end of the experimental time,
regardless of the initial pH applied It should be noted that both
the maximum power and the time taken to stabilize increased
when the pH was decreased Ultimately, the power went back to
the steady state conditions under neutral pH after a period of time
varying from 20 min to 8 h depending on the initial pH applied
According toFig 4, the optimal power production was attained
when the cathodic pH was decreased to 2 Under these conditions,
a maximum power of 4.5 mW (Rext= 40X) was achieved after
2.5 h, an increase by 180% as compared to a negative control (tap
water of pH 7.6) In a comparable test, also in batch mode but at
a cathodic pH of 1,Erable et al (2009)found an increase of power
by 250% in batch mode as compared to neutral conditions,
achiev-ing 3.5 mW in their 700 mL MFC system consistachiev-ing of two
cham-bers with an open-air cathode However, in our case when the
catholyte pH was further decreased to 1, the power production
reached a maximum of 3.6 mW after 2.3 min, followed by a rapid
decrease The catholyte was then characterized by a green color
and an acidic pH of 2, which indicated that the protons had not
been consumed in their entirety by the reaction at the cathode
The resistance to HCl of the stainless steel current collector used
at the cathode and that of the platinum coated carbon cloth
cath-ode were tested by exposing samples at a pH of 1 and, within two
days, stainless steel was completely degraded resulting in a green
color solution, whereas the carbon cloth was unaffected The green
compound was identified to be FeCl2 – a greenish tetrahydrate
responsible for the green colored solution – and it can be
hypoth-esized that Fe and HCl combined to form FeCl2, a reaction that is
shown in Eq 2
Such redox reaction was detrimental to the cathodic reaction
and resulted in decreased power production at pH 1 It should be
noted here that in the study ofErable et al (2009), this problem
was not encountered as they utilized brass as current collector
3.2.2 Continuous experiments
Continuous acidification experiments were carried out over a
period of 10 d between day 110 and 120 Under continuous
acidi-fication, stable power could be generated Power curves at different
catholyte pH confirmed the trend of increasing power generation
with decreasing pH (Fig 5a) Continuous cathodic acidification at
pH = 1.2 allowed the generation of power supply 380% as high as
what could be obtained with tap water (pH 7.6) Under such
con-ditions, the reactor generated a maximum power of 7.4 mW
(Rext= 15X), the maximum observed in this study Two phosphate buffer solutions with ionic concentrations of 102and 104M to cover the entire range of our acidic solutions were used as negative controls to demonstrate that the improved concentration is due to increased proton access and not simply due to increased ionic strength It appears clearly from Fig 5 that the effect of ionic strength was limited as compared to that of pH
The polarization curves provide a deeper insight into the impact
of acidification on the electrical performance of the MFC (Fig 5b) First, the internal resistance decreased with the pH from 58 O at pH 7.6 to 11Xat pH 1.2 The effect on the electromotive force was more complex Eemffirst increased when pH was decreased and reached an optimum value of 0.7 V at a pH of between 3 and 2 The Nernstian effects, whereby the cathodic potential increases with the proton concentration, can explain this However, further decrease in pH resulted in a decreasing electromotive force that reached 0.58 V at a pH of 1.2 and this was related to the corrosion reaction previously described that affected the potential of the cathode when the pH became too acidic However, in terms of power generation, higher values were obtained at pH 1.2 than 2, which shows that, overall, the impact of acid on the internal resis-tance was stronger than that on the electromotive force
Finally, we analyzed the pH of the solution in the outlet of the cathodic compartment and it appeared that the pH was slightly higher than that of the feed to the cathodic compartment for feed
pH higher than 2 For a solution of pH 2 or less, the pH in the outlet was similar to that of the inlet, indicating that the proton limita-tion might be overcome under these condilimita-tions in our MFC system
At pH 2, the maximum power was achieved at a current of 15 mA (Fig 5a) and, since 1 mol of electron reacts with 1 mol of protons in the cathodic reaction, the quantity of pH 2 solution to maintain the
0
1
2
3
4
5
Time (min)
pH 7.6 pH 6
pH 4 pH 3
pH 2 pH 1 01
2 3 4
Time (min)
Fig 4 Power response after batch acidification of the cathode (R ext = 40 O) Insert:
details of the first 30 min.
0 2 4 6 8
0 0.2 0.4 0.6 0.8
Current ( A)
Current ( A)
Buffer 10 M Buffer 10 M
(a)
(b)
Fig 5 (a) Power curves under continuous acidification of the cathode at different pH; (b) polarization curves under continuous acidification of the cathode at different pH.
Trang 7current at 15 mA was estimated at 1.3 L d1 That corresponds
roughly to as little as 2 mL of 37% HCl solution, as commonly
avail-able for sale Considering the HRT of the reactor (2.4 h), this
corre-sponds roughly to 0.07 mL of 37% HCl solution used per litre of
wastewater treated
3.2.3 Reversibility of the cathodic acidification experiments
With the use of the Selemion membrane that allows protons to
be transferred only in one direction from the anode towards the
cathode, the pH at the anode was never affected by the drastic
change of pH in the cathode compartment The MFC operation in
a reversible mode from cathodic acidification back to open-air
cathode operation was further assessed at the end of stage 4
(day 120–140) As can be seen fromFig 2, the MFC immediately
re-verted to performance similar to what was observed before the
cathodic acidification experiments The internal resistance and
electromotive force averaged 47 ± 8 O and 0.63 ± 0.05 V,
respec-tively, resulting in a maximum power of 2.17 ± 0.01 mW, not
significantly different from the values obtained before the
acidifi-cation experiments (P = 0.6; Student’s t-test) FromFig 3, it can
be further seen that the COD and SS removal efficiencies were also
largely unaffected by the cathodic acidification experiments, with
mean values of 53 ± 7% and 62 ± 2%, respectively As a result the
Coulombic efficiency also remained stable at 0.4 ± 0.2% This
further shows that bacteria possibly present as a biofilm on the
cathode played little to no role in power generation as compared
to abiotic processes catalyzed by platinum This is because any
bio-film on the cathode would have been damaged by the acidic
treat-ment but nevertheless, the electrical performance remained stable
3.3 Performance assessment as compared to the literature
Table 1compares the performance of our MFC set-up to the
lit-erature available
Under the optimized conditions, the internal resistance of our
MFC system was 11Xand the electromotive force was 0.6 V These
values correspond to the state of the art as showcased inTable
1.The maximum power attained was 7.4 mW, which corresponded
to a volumetric power of 2 W m3by considering the total volume
of our MFC being of 3.5 L (anodic compartment of 2.9 L and catho-dic compartment of 0.6 L) This can appear low considering the state of the art presented inTable 1 However, in terms of power generated on a per cell basis, our MFC was in the highest range This is because the volumetric power is a decreasing function of the geometrical parameters However, by reducing the size too much, the raw power output is also expected to decrease and this mathematically increases the number of cells that would have to
be stacked together in order to provide enough power to be of use Considering the very low maturity of MFC-stacking, this is a serious obstacle to MFC application Furthermore, the electrical resistivity and the volume of all the connectors that will be used
to perform stacking will also affect the electrical performance of the system and its total volume The impact on a volumetric power based on the total volume and not on the working volume may be-come negative
Moreover, the substrate used to operate an MFC – ranging from acetate or glucose to complex industrial wastewater – is known to impact widely on its performance and the best perfor-mance from the electrical as well as from the wastewater treat-ment point of views is usually obtained with synthetic and simple substrates (see Table 1) However, in view of optimizing MFCs for wastewater treatment, it is also useful to assess their performance with domestic wastewater, but this does not play
in favor of our system It is interesting to see that our system could remove in average above 50% of the COD and 56% of the
SS at a HRT of 2.4 h With domestic wastewater, Ahn and Logan (2010) found COD removal higher than 88% in batch mode However, in that same study, during continuous operation max-imizing power generation and at ambient temperature, COD re-moval was reduced to 19%, with a Coulombic efficiency of 0.7%
at a HRT of 4.2 min In our study, the COD removal efficiency was higher but Coulombic efficiency also remained below 1%,
Table 1
State of the art in microbial fuel cell research.
Substrate OCV (V) E emf (V) R int (X) P max (mW cell 1
) V (mL) Pv max (W m 3
) Ref.
Two chamber design, ferricyanide catholyte
Two chamber design, aerated cathode
Two chamber design, open-air cathode
Single chamber design, open-air cathode
NA, not available.
Trang 8granules In the latter case, Coulombic efficiency was much
high-er at 11%
Low Coulombic efficiency has to be related to the complex
nat-ure of domestic wastewater allowing fermentation and other
biological processes to interfere with electricity generation
Fur-thermore, it seems that electrochemically active bacteria can only
use a limited range of products For example,Freguia et al (2008b)
showed that glucose had to be converted into acetate before it can
be utilized for electricity generation It is consequently not
surpris-ing that butyrate and acetate, respectively, generated 62% and 82%
more power than domestic wastewater in the study of Ahn and
Logan (2010) Even though the low Coulombic efficiency could be
seen as a problem, it is merely the result of the MFC having reached
its maximum capacity as an electricity generation device The
elec-tromotive force cannot be expected to go much beyond the values
observed in the present study – the theoretical maximum voltage
achievable in open circuit being approximately of 1.1 V in typical
MFC conditions (Logan et al., 2006) – and our values of internal
resistance are also competitive As a result, any organic matter in
excess is further removed by other means The vertical
configura-tion of the reactor allowed sludge accumulaconfigura-tion at the bottom of
the anodic compartment, where fermentation and even probably
methane production were allowed to take place, even though our
design did not allow for any gas collection and analysis to verify
that hypothesis This made our MFC a hybrid system that overall
contributed to improve the elimination of organic matter, which
is the primary goal of a wastewater treatment plant Overall, it
seems more important to optimize the effluent COD and SS
removal efficiencies in the anodic compartment rather than the
Coulombic efficiency in wastewater treatment application, which
further renders possible the introduction of the anolyte inside
the cathode compartment without reducing the performance of
the reactor, as shown in this study
4 Conclusion
The present study shows the importance of overcoming proton
limitation in an MFC system and this was achieved first by letting
the anolyte flow into the cathode compartment (full-loop
opera-tion) and second by acidifying the cathode The latter experiment
led to the best electrochemical performance and it appeared
clearly that the anolyte quality must be good enough before being
allowed to enter the cathodic compartment We believe that the
future of the MFC technology as a treatment plant lies in the design
of hybrid systems that will harvest part of the energy in the form of
electricity and the rest in the form of biogas
Acknowledgements
This work was supported by a Grant from the Environment &
Water and Industry Development Council, Singapore (MEWR
651/06/159)
References
Aelterman, P., Rabaey, K., Pham, H.T., Boon, N., Verstraete, W., 2006 Continuous
electricity generation at high voltages and currents using stacked microbial fuel
cells Environ Sci Technol 40, 3388–3394.
Ahn, Y., Logan, B.E., 2010 Effectiveness of domestic wastewater treatment using
microbial fuel cells at ambient and mesophilic temperatures Bioresour.
Technol 101, 469–475.
APHA., 2005 Standard Methods for Water and Wastewater Examination 21 ed,
Washington.
Barbir, F., 2005 PEM Fuel Cells: Theory and Practice Elsevier Academic, Amsterdam.
Cheng, S., Dempsey, B.A., Logan, B.E., 2007 Electricity generation from synthetic
acid-mine drainage (AMD) water using fuel cell technologies Environ Sci.
Technol 41, 8149–8153.
MFC with advective flow through the porous anode and reduced electrode spacing Environ Sci Technol 40, 2426–2432.
Cheng, S.A., Logan, B.E., 2007 Ammonia treatment of carbon cloth anodes to enhance power generation of microbial fuel cells Electrochem Commun 9, 492–496.
Clauwaert, P., Mulenga, S., Aelterman, P., Verstraete, W., 2009 Litre-scale microbial fuel cells operated in a complete loop Appl Microbiol Biotechnol 83, 241–247 Deng, Q., Li, X.Y., Zuo, J.E., Ling, A., Logan, B.E., 2010 Power generation using an activated carbon fiber felt cathode in an upflow microbial fuel cell J Power Sources 195, 1130–1135.
Erable, B., Etcheverry, L., Bergel, A., 2009 Increased power from a two-chamber microbial fuel cell with a low-pH air-cathode compartment Electrochem Commun 11, 619–622.
Fan, Y.Z., Hu, H.Q., Liu, H., 2007 Sustainable power generation in microbial fuel cells using bicarbonate buffer and proton transfer mechanisms Environ Sci Technol.
41, 8154–8158.
Fan, Y.Z., Sharbrough, E., Liu, H., 2008 Quantification of the internal resistance distribution of microbial fuel cells Environ Sci Technol 42, 8101–8107 Feng, Y.J., Lee, H., Wang, X., Liu, Y.L., He, W.H., 2010 Continuous electricity generation by a graphite granule baffled air-cathode microbial fuel cell Bioresour Technol 101, 632–638.
Freguia, S., Rabaey, K., Yuan, Z., Keller, J., 2007 Non-catalyzed cathodic oxygen reduction at graphite granules in microbial fuel cells Electrochim Acta 53, 598– 603.
Freguia, S., Rabaey, K., Yuan, Z.G., Keller, J., 2008a Sequential anode-cathode configuration improves cathodic oxygen reduction and effluent quality of microbial fuel cells Water Res 42, 1387–1396.
Freguia, S., Rabaey, K., Yuan, Z.G., Keller, J., 2008b Syntrophic processes drive the conversion of glucose in microbial fuel cell anodes Environ Sci Technol 42, 7937–7943.
Gil, G.C., Chang, I.S., Kim, B.H., Kim, M., Jang, J.K., Park, H.S., Kim, H.J., 2003 Operational parameters affecting the performance of a mediator-less microbial fuel cell Biosens Bioelectron 18, 327–334.
Harnisch, F., Wirth, S., Schroder, U., 2009 Effects of substrate and metabolite crossover on the cathodic oxygen reduction reaction in microbial fuel cells: Platinum vs iron(II) phthalocyanine based electrodes Electrochem Commun.
11, 2253–2256.
Jiang, J.Q., Zhao, Q.L., Zhang, J.N., Zhang, G.D., Lee, D.J., 2009 Electricity generation from bio-treatment of sewage sludge with microbial fuel cell Bioresour Technol 100, 5808–5812.
Kim, J.R., Premier, G.C., Hawkes, F.R., Dinsdale, R.M., Guwy, A.J., 2009 Development
of a tubular microbial fuel cell (MFC) employing a membrane electrode assembly cathode J Power Sources 187, 393–399.
Lefebvre, O., Al-Mamun, A., Ooi, W.K., Tang, Z., Chua, D.H.C., Ng, H.Y., 2008 An insight into cathode options for microbial fuel cells Water Sci Technol 57, 2031–2037.
Lefebvre, O., Ooi, W.K., Tang, Z., Abdullah-Al-Mamun, M., Chua, D.H.C., Ng, H.Y.,
2009 Optimization of a Pt-free cathode suitable for practical applications of microbial fuel cells Bioresour Technol 100, 4907–4910.
Li, Z.J., Zhang, X.W., Zeng, Y.X., Lei, L.C., 2009 Electricity production by an overflow-type wetted-wall microbial fuel cell Bioresour Technol 100, 2551–2555 Liu, H., Ramnarayanan, R., Logan, B.E., 2004 Production of electricity during wastewater treatment using a single chamber microbial fuel cell Environ Sci Technol 38, 2281–2285.
Logan, B., Cheng, S., Watson, V., Estadt, G., 2007 Graphite fiber brush anodes for increased power production in air-cathode microbial fuel cells Environ Sci Technol 41, 3341–3346.
Logan, B.E., Hamelers, B., Rozendal, R., Schrorder, U., Keller, J., Freguia, S., Aelterman, P., Verstraete, W., Rabaey, K., 2006 Microbial fuel cells: methodology and technology Environ Sci Technol 40, 5181–5192.
Qian, F., Baum, M., Gu, Q., Morse, D.E., 2009 A 1.5 mu L microbial fuel cell for on-chip bioelectricity generation Lab on a Chip 9, 3076–3081.
Rabaey, K., Read, S.T., Clauwaert, P., Freguia, S., Bond, P.L., Blackall, L.L., Keller, J.,
2008 Cathodic oxygen reduction catalyzed by bacteria in microbial fuel cells Isme J 2, 519–527.
Ringeisen, B.R., Henderson, E., Wu, P.K., Pietron, J., Ray, R., Little, B., Biffinger, J.C., Jones-Meehan, J.M., 2006 High power density from a miniature microbial fuel cell using Shewanella oneidensis DSP10 Environ Sci Technol 40, 2629–2634 Rozendal, R.A., Hamelers, H.V.M., Buisman, C.J.N., 2006 Effects of membrane cation transport on pH and microbial fuel cell performance Environ Sci Technol 40, 5206–5211.
Sun, J., Hu, Y.Y., Bi, Z., Cao, Y.Q., 2009 Improved performance of air-cathode single-chamber microbial fuel cell for wastewater treatment using microfiltration membranes and multiple sludge inoculation J Power Sources 187, 471–479 Wang, B., Han, J.I., 2009 A single chamber stackable microbial fuel cell with air cathode Biotechnol Lett 31, 387–393.
Wang, X., Cheng, S.A., Feng, Y.J., Merrill, M.D., Saito, T., Logan, B.E., 2009 Use of carbon mesh anodes and the effect of different pretreatment methods on power production in microbial fuel cells Environ Sci Technol 43, 6870–6874 Zhang, B.G., Zhao, H.Z., Zhou, S.G., Shi, C.H., Wang, C., Ni, J.R., 2009 A novel UASB-MFC-BAF integrated system for high strength molasses wastewater treatment and bioelectricity generation Bioresour Technol 100, 5687–5693.
Zhuang, L., Zhou, S.G., Wang, Y.Q., Liu, C.S., Geng, S., 2009 Membrane-less cloth cathode assembly (CCA) for scalable microbial fuel cells Biosens Bioelectron.
24, 3652–3656.