International Journal of Advancements in Research & Technology, Volume 2, Issue 5, M ay-2013
ISSN 2278-7763
326
Performance of salt-bridge microbial fuel cell at various agarose
Concentrations using hostel sewage waste as substrate
Ramya Nair1, Renganathan. K2, S. Barathi3, Venkatraman. K4
1 4
, Department of Biotechnology, SRM University, Chennai, India; 2 Department of Civil Engineering, SRM University, Chennai, India; 3 Department of Biotechnology, SRM University, Chennai, India;
Email: ramya.nair3@gmail.com ; ramyanair.r@ktr.srmuniv.ac.in
ABSTRACT
Microbial fuel cell (MFC) is the advancement of science that aims at utilising the oxidizing potential of bacteria for wastewater
treatment and production of bio-hydrogen and bio-electricity. Salt-bridge is the economic alternative to highly priced protonexchange membrane in the construction of a microbial fuel cell. By altering the concentration of agarose in the fabrication of saltbridge, performance of various double chambered microbial fuel cells were observed using hostel sewage wastewater as the
substrate. Agarose concentration ranging from 7% to 12% was used for the study and optimum concentration was observed to
be 10% as it showed maximum current production of 0.97mA at a voltage of 0.95 V after 22 days of operation from the time of
initial experimental setup. The maximum power density obtained was 78.21mW/m2 and the corresponding current density was
82.37 mA/m2. 75.5% reduction in COD and 91.7% BOD reduction was obtained at 10% agarose concentration salt-bridge MFC.
All experiments were performed in triplicates and the mean values are presented here.
Key words: Microbial Fuel cell, Salt-bridge, Bacteria, Electricity, Power output
1 INTRODUCTION
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In the recent years the biggest challenge the human society is
facing is to meet the ever increasing demand for different energy sources, which are getting exhausted at an alarming rate.
Microbial fuel cell is one of the best alternative sources of energy production which add wastewater to the list of renewable
resources of energy. Wastewater contains a lot of easily degradable organic materials which are metabolised by the active bacterial species present in the wastewater itself and produces electricity during the course [1]. In a double chambered
salt- bridge microbial fuel cell, the sludge organic matter (fuel)
is oxidised into simpler compounds in the anode chamber (
aerobic conditions) [2]. The electrons released are taken up by
the anode and the protons are transferred across the saltbridge to the cathode chamber.
Oxidation half- reaction (Anode chamber)
Biodegradable matter + bacteria———————› CO 2 + H+ + e(anaerobic condition)
The electrons travel across the external circuit connected with
an external resistance and reaches the aerated cathode
[3],[4],[5] where the electrons and the protons along with the
molecular oxygen produces water completing the reduction
half-reaction [6],[7] .
Reduction half-reaction (Cathode chamber):
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H+ + e- + Oxygen———————› H 2 O (Aerobic condition)
The MFC’s constructed for this work used wastewater collected from the primary clarifier of the wastewater treatment
plant at SRM University, Chennai, Tamil Nadu. Waste water
showed an initial BOD of 252 and COD of 424. Chemical oxygen demand (COD chromate) was measured by closed reflux
titrimetric method using chromate as the oxidant [8]. The
wastewater also contained different salts that were involved in
substrate conversion by microbial metabolism [9], [10]. The
microbial consortia feeds upon the organic matter for its
growth and starts releasing protons and electrons during the
exponential or log phase of the growth curve and this corresponds to the increase in the current production after the initial acclimatization of the microbes in the anaerobic chamber
[11], [12]. Some distinct micro-organisms have the unique
property of transferring the electrons produced inside the
body towards the anode through the extensions on the outersurface called as pilli that have been named as nanowires as
they are of nanometre dimensions and assist in electron transfer[13]. In some other species of micro-organisms redox enzymes like cytochromes act as electron mediators or shuttles
[14], [15], [16]. This study aims to understand the effect of
various concentration of agarose during the fabrication of saltbridge on the power production and reduction in the organic
matter in a batch-mode process. The salt-bridge acts a proton
transferring channel and this study aims to optimise the concentration of agarose that facilitates the maximum movement
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International Journal of Advancements in Research & Technology, Volume 2, Issue 5, May-2013
ISSN: 2278-7763
of the protons towards the cathode that corresponds with
higher current generation efficiency of the MFC.
.
2. Materials and Methods:
2.1 MFC construction:
Dual chambered MFC was constructed using air-tight food
grade plastic containers of 1.3 litres volume each (anode and
cathode chamber). A side opening of 1.25 cm radius was made
at a height of 6 cm from the bottom of the container( approximately at the centre) on each container and was connected
with a PVC pipe( length=12cm; diameter=2.5cm). Agarose of
concentrations ranging from 7% to 12% along with 4% Potassium chloride(KCl) salt was prepared by heating it in a waterbath and the molten agarose was allowed to cool down and
poured into the PVC pipe and sealed at one end using cellotape. The agarose was left undisturbed to solidify. The PVC
pipe containing the salt-agarose mixture was fixed between
the two containers using epoxy material and behaved like the
salt-bridge assisting in the proton transfer mechanism during
the MFC operation. Carbon rods (height = 9cm; diameter = 0.5
cm) were used as electrodes. The distance between the two
electrodes was maintained at distance of 15 cm in all the MFC
setups. Copper wires were used to connect the electrodes to
the circuit. An external resistance (R) of 10Ω was connected
and the readings were measured using a digital multimeter
(Kusam-Meco model 603). Constructed salt bridge MFC is
shown in Fig 1
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Suspended Solids (TSS) was found to be 512mg/L. The cathode
chamber was filled with tap water and air was sparged into it
with the help of an aquarium air sparger. The anode chamber
was completely sealed using epoxy material and the setup was
placed in a room at a temp of 27oC to 35oC.
2.2 Data analysis:
Current (I) and Voltage (V) was measured using digital multimeter (Kusam-Meco model 603) and the readings were plotted on a graph. The maximum power (P) generated was calculated by using the formula P (maximum) = V (maximum) × I
Where V is the maximum voltage obtained and I is the corresponding current produced in m.Amp against an external resistance (R) of 10 Ω.
Current density was calculated by the formulaeCurrent density (C.D) = I (max) / A and
Power density was calculated by the formulae – Power density (P.D) = P (max) / A;
Where A is the surface area of the anode which is obtained by
the following calculation
Surface area of the anode (A) = 2(π r 2) + (2π r) h.
Where r = radius of the electrode and h = height of the electrode.
Area of the anode:
Radius = 0.5 cm
Height = 12 cm
Therefore according to the formulae: A = 2 (22/7 × 0.52) + (22/7
×0.5) ×12
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3. Results and discussion:
3.1 Comparative study of voltage developed at different agarose concentration
Fig 1: Constructed Salt bridge MFC
The anode chamber of the MFC was inoculated with the waste
water collected from the primary clarifier of the wastewater
treatment plant, SRM University. Before inoculation the physiochemical analysis of the sample was performed and the BOD
was observed to be 138mg/ L after 5 days at 27oC, COD was
418mg/L. Total Dissolved Solids (TDS) was 680mg/L and Total
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Voltage production is measured for a period of 32 days. During this period of time, 10% agarose produced the maximum
voltage and 12% agarose produced the minimum power. From
Fig 2 it is very clear that maximum voltage is produced at 10%
agarose at the end of 21 days. The graph represents the voltage
production corresponding to the growth curve of the microbial consortia in the anode chamber depicting the initial increase
of voltage during the exponential phase of growth curve but
enters a standing voltage phase and decreases as the setup
enters the decline stage due to death of microbes attributing to
the exhaustion of nutrients present in the chamber. A number
of factors affect the working efficiency of the MFC’s like physical parameters, effect of microbial growth etc [17]. The voltage
developed shows a comparative hike from 7% agarose concentration to 10% concentration, and this can be due to the reason
that as the concentration of agarose increases, the gel is highly
polymerized, inhibiting the inter mixing probability of the two
separated chamber fluids. Highly polymerized gel also preIJOART
International Journal of Advancements in Research & Technology, Volume 2, Issue 5, May-2013
ISSN: 2278-7763
vents the entry of native and molecular oxygen from the aerobic chamber to the anaerobic chamber through the salt- bridge
passage, maintaining the anaerobic conditions of the anode
chamber. The transfer of protons is of critical importance during the entire working period of the microbial fuel cells
[18],[19],[20]. But a decrease in voltage production is observed
for 11% and 12% agarose concentration, as the salt-bridge is
highly polymerized reducing the pore size, hindering the
movement of proton across the bridge.
Fig 2: Voltage production (volts) during a period of 32 days in
various double –chambered salt bridge microbial fuel cell fabricated with various agarose concentrations.
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ternal resistance of the setup thus minimizing the current production [23],[24] as we know from Ohm’s law that current (I) is
inversely proportional to resistance (R). Ohm’s law: V = IR,
thus I α 1/R (Mohan et al., 2008). From fig.2 we can observe
that though there is a decrease in voltage production after
10% concentration, but the decrease is not tremendous; as the
voltage (V) is directly proportional to resistance (R) ( by
Ohm’s law V = IR).
Fig 3: Current production (m.A) during a period of 32 days in
various double –chambered salt bridge microbial fuel cell fabricated with various agarose concentration
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3.2 Comparative study of current developed at different agarose concentration
The current produced in the double chambered MFC’s were
recorded for a period of 32 days. The mA units of current produced during this period when plotted on a graph followed
the pattern of growth curve of the microbial consortia showing
an exponential increase for a period of 21-22 days and reaching the maximum value and gradually decreasing due to decreased levels of electrons produced during the microbial oxidation [21]. This decrease in electrons release is due to decreasing population of microbes as the nutrients gets used up
and finally resulting in lower current production. From fig.3 it
is observed that there is an increase in current production as
the concentration of agarose increases from 7% to 10%. This is
due to effective proton transfer and as the gels is highly polymerized; it prevents the diffusion of oxygen from the cathode chamber to anode chamber through the salt-bridge, thus
maintaining a better anaerobic environment in the anode
chamber encouraging the growth of anaerobic bacteria for
increased electrons release [22]. But there is a decrease in current production for 11% and 12% agarose conc as the extremely polymerized gel prevents the effective movement of protons, increasing the concentration of protons in the anode
chamber, reducing the pH, making the anodic environment
highly acidic for the microbes to survive. The increased
polymerization of agarose gel also results in elevating the inCopyright © 2013 SciResPub.
3.3 Comparative analysis of the efficiencies of different
MFC’s
The overall efficiency of the different MFC’s were studied by
considering various performance parameters like Voltage,
Current produced, Power, Power density, Current density.
From the table1; 10% agarose conc salt-bridge MFC is showing
maximum efficiency with a maximum power density of 78.21
mW/ m2 corresponding to a current density of 82.37 mA/ m2
calculated against the area of the anode electrodes.
Table1: Various parameters showing the performance of MFC
at different agarose concentrations.
Parameters
Voltage (V)
Current (m.A)
Power (mW)
Power
densi2
ty(mW/m )
Current density
(mA/m2)
Concentration of Agarose
7%
8%
9%
10%
11%
12%
0.75
0.79
0.592
50.32
0.83
0.87
0.722
61.32
0.91
0.93
0.846
71.87
0.95
0.97
0.921
78.21
0.57
0.62
0.353
30.01
0.29
0.44
0.127
10.83
67.09
73.88
78.98
82.37
52.65
37.36
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International Journal of Advancements in Research & Technology, Volume 2, Issue 5, May-2013
ISSN: 2278-7763
3.4 BOD Degradation rate:
The BOD (Biological Oxygen Demand) of the anode chamber
organic waste was measured at regular intervals and a 91.7 %
decrease was observed after a period of 32 days by following
the method of Dilution method.
3.5 COD Reduction rate:
The Chemical Oxygen Demand (COD) showed a reduction of
75.5% from the initial day of setup to the end of 32 days by the
closed reflux titrimetric method using chromate as the oxidant
[8]. This decrease in COD indicates bioremediation of the sewage waste supporting the concept of microbes utilizing the
organic waste for microbial oxidation.
4. Conclusion:
The study performed here confirmed the effect of agarose concentration on the performance of the microbial fuel cell as the
agarose concentration indicates the permeability of the gel
facilitating the transfer of proton. The optimal concentration of
agarose for the fabrication of salt-bridge was found to be 10%
concentration as it showed maximum power density of 78.21
mW/m2. Internal resistance hinders the production of electricity as higher the grade of polymerization of the gel, internal
resistance builds up inside the cell. Thus an optimal concentration of agarose is preferred while constructing a salt-bridge as
it enhances the voltage production as well as production of
bioelectricity. Microbial fuel cells also assist in bioremediation
and hence are of utmost concern over the recent years to clean
up the environment and add wastewater to the list of new renewable resource of bio-energy.
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Acknowledgment:
The authors wish to thank Dr. Winkins Santosh and Dr.
S.Barathi for their constant support and motivation.
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