IJERD – International Journal of Environmental and Rural Development (2020) 11-2

Research article
erd

Methods for Improving the Performance of Paddy Soil-Used
Sediment Microbial Fuel Cells
NARONG TOUCH*
Faculty of Regional Environment Science, Tokyo University of Agriculture, Tokyo, Japan
Email: nt207118@nodai.ac.jp

TAKAHIKO NAKAMURA
Faculty of Regional Environment Science, Tokyo University of Agriculture, Tokyo, Japan
Received 5 December 2019

Accepted 1 July 2020(*Corresponding Author)

Abstract Sediment microbial fuel cells (SMFCs) have received considerable attention for
remediating sediment and wastewater while providing electricity. This technology offers
many benefits for rural regions, particularly for those in developing counties. SMFCs cantreat agricultural wastewater, household sewage, and organic waste while providing
electricity to households. However, the performance of SMFCs remains low and must be
improved. It has reported that mixing steelmaking slag (SS) with littoral sediment improves
SMFC performance. The present study examines the use of SS to potentially improve theperformance of a SMFC that uses paddy soil as its fuel (PS-SMFC). Moreover, we examinechanges in the performance of a SS-SMFC (the PS-SMFC that the anode chamber is filledwith SS) that uses lactic acid bacteria (LAB) and BN strain of Bacillus subtilis (BNB)
solution-mixed paddy soil as its fuel. In laboratory experiments, paddy soil was poured into
the SS layer (the anode chamber) of a SS-SMFC. LAB or BNB were added to the SS-SMFC
anode by mixing a bacteria solution with the paddy soil. The SMFC performance was
evaluated by measuring the polarization (current–voltage relation) and power density. The
power density of the SS-SMFC was observed to be 152 mW.m−2, which is approximately
twice than that observed without using SS (80 mW.m−2, as reported in previous studies). Thisshows that the performance of a paddy soil-used SMFC increases after mixing SS with the
paddy soil. Furthermore, the SS-SMFC power density increased from 66 to 191 mW.m−2
after adding BNB and to 247 mW.m−2 after adding LAB. Thus, adding LAB or BNB to
paddy soil is shown to improve SMFC performance. In comparison with BNB, LAB has
greater potential for increasing SMFC performance.
Keywords sediment microbial fuel cell, performance, paddy soil, steelmaking slag,
lactic acid bacteria, BN strain B. subtilis

INTRODUCTION
Recently, sediment microbial fuel cells (SMFCs) have received considerable attention for improving
sediment and water quality. Numerous studies have reported that when using SMFCs, the amountand variations in the state of the organic matter present in sediment-applied SMFCs decrease and theredox potential of the sediment increases (Sacco et al., 2012; Sajana et al., 2013; Touch et al., 2014).
Furthermore, SMFCs have been proven to be effective in preventing the deterioration of water quality
due to ion diffusion from sediment (Touch et al., 2017a) and improving the benthic habitat
environment in highly reduced sediment (Touch et al., 2018a). Moreover, Touch et al. (2017b)
showed that using SMFCs is effective for removing hydrogen sulfide from sediment and fixing
phosphate in them; they showed that the oxidation of reduced substances at the SMFC anode plays
an important role in improving the sediment quality and the benthic habitat environment. As such,
SMFCs were employed into the sediment deposited near a sewage outlet (Touch et al., 2018b) andan oyster farm (Touch et al., 2018c) to improve the quality of the sediment and water.

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IJERD – International Journal of Environmental and Rural Development (2020) 11-2

Households

Farmlands
Agricultural wastes➢ water➢ biomass➢ Sewage➢ kitchen waste

SMFC system
Electricity

Resources➢ nutrients➢ treated water➢ treated sludge

Fig. 1 Benefits of using SMFCs in rural regions
The achievements reported in previous studies suggested that SMFCs offer many benefits forrural regions, particularly those in developing counties. For example, SMFCs can treat agricultural
wastewater, household sewage, and organic waste while providing electricity to households andrecycled resources to farmlands (Fig. 1). However, the performance of SMFCs remains low and must
be improved. Kouzuma et al. (2014) reported the maximum power and short-circuit current densities
of SMFCs in rice paddy fields are only 80 mW.m−2 and 550 mA.m−2, respectively (based on the
projected anode area). Many methods have been proposed for improving the SMFC performance,such as adding mediators to the sediment (Lovley et al., 2004) and the activation of organisms onelectrode (Wang et al., 2009). It has been showed that adding steelmaking slag (SS) to littoral
sediment could improve the SMFC performance (Nishimura et al., 2018). In the present study, we
focus on using SS to improve the performance of a SMFC that uses paddy soil as its fuel (PS-SMFC);
further, we examine the applicability of lactic acid bacteria (LAB) and the BN strain of Bacillus
subtilis (BNB) to improve the performance of a SS-SMFC (the PS-SMFC that the anode chamber isfilled with SS).
OBJECTIVE
The first objective of this study was to examine the changes in the performance (current and power)of a PS-SMFC after adding SS to the anode chamber, i.e., the performance of SS-SMFC. The second
objective of this study was to examine the applicability of LAB and BNB to improve the performanceof SS-SMFC.
METHODOLOGY
Experimental Device and Materials
The experimental device comprised a cylindrical bottle with an inner diameter and height of 120 mm
and 150 mm, respectively. The bottle was filled with SS (diameter range of 5–40 mm) to a depth of
20 mm, and the anode electrode was placed on the SS layer. Then, 20 mm of the SS layer was placedon the anode electrode. The wet paddy soil was mixed with deionized water in a soil-to-water weightratio of 1:2; subsequently, the mixture was slowly poured into the bottle until it reached a height of
50 mm from the bottom of the bottle. Finally, tap water was poured over the soil layer, and thecathode electrode was submerged near the surface of the water (Fig. 2a).
The paddy soil was collected from a rice filed (Ebina, Kanagawa, Japan) during the agriculturaloff-season. Approximately 150 mm of the surface soil was collected and transported to the laboratory.
The paddy soil was mixed with deionized water to facilitate its easy pouring into the SS layer.
Solutions of LAB or BNB were used instead of the aforementioned deionized water to examine theireffects. Either 50-g Natto or 75-g yogurt (commercial products) was mixed with 3-L tap water andⒸ ISERD

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IJERD – International Journal of Environmental and Rural Development (2020) 11-2

fermented for 7 days at ambient temperature. Then, the supernatant was extracted and mixed withthe paddy soil.
<Cathode electrode>(100*100) mm2 –brush type
120 mmExternal resistance

100 mmTap water

20 mm

50 mm

Paddy soil

R

V

External resistance

CathodeCathodeFor a fixedcathode potentialof 0.3 V

R

V

Anode

Anode

<Anode electrode>
Steelmaking slag
(100*100) mm2 –brush type(b) Current(a) SS-SMFC
measurements

Potentiostat(c) Polarization measurements

Fig. 2 Experimental device and measurement methods
The electrode material used was carbon cloth (News Company, PL200-E), which was heated at
500 °C for 1 h to improve its performance before being used as the electrode material according to
Nagatsu et al. (2014). The heated carbon cloth, with a surface area of 10000 mm2 (100-mm widthand 100-mm long), was separated into carbon fibers to make the brush-type anode or cathode (Fig.
2a). Plastic-coated copper wire was used for all the connections in the SS-SMFC and operating
circuits.
Operations and Measurements
The experiments were conducted by following the operations and measurements as shown in Fig. 3.
All SS-SMFCs were put into an open circuit (without electrical current flow) mode until their
voltages stabilized (~7 days), after which the polarization was measured using the circuit shown in
Fig. 2b. An external resistance was loaded between the anode and cathode and varied over 2.2 Ω–10
kΩ. The cell voltage was recorded 1 min after each external resistance was loaded. The recorded
voltage was used to calculate the current according to the Ohm’s law, namely, I = U/Rex, where U[V] is the voltage, I [A] is the current, and Rex [Ω] is the external resistance; further, the power P was
calculated according to P = IU. The current and power densities were obtained by dividing thesevalues by the surface area of the electrode, i.e., 0.01 m2. To understand the long-term performanceof SS-SMFCs, an external resistance of 20 Ω was loaded between the anode and the cathodeelectrodes (Fig. 2b), after the polarization measurement.

SS-SMFCs
creation

Anode potential➢Bacterial effects

Open circuit(no current flow)

Stable voltage

Performance➢Current density➢Power density

Polarization measurement➢0.3 V of cathode potential➢2.2 Ω to 10 kΩ of external
resistance
20 Ω-external resistance(current generation)

Performance➢Current density➢Power density

Polarization measurement➢2.2 Ω to 10 kΩ of external
resistanceFig. 3 Operation conditions and measurements during experimentsⒸ ISERD

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IJERD – International Journal of Environmental and Rural Development (2020) 11-2

To calculate the circuit current, the voltage across the external resistance was measured every
30 min using a voltmeter (Graphtec, midi LOGGER GL840-M). After long-term current generation,
the SS-SMFCs were put into an open circuit mode again until their voltages stabilized. To evaluate
the effects of LAB or BNB in the SS-SMFCs, the polarization was measured by fixing the cathodepotential at 0.3 V versus Ag/AgCl using a potentiostat (Hokuto, HB-151B) (Fig. 2c). This value wassourced from the study conducted by Nagama et al. (2018), who noted that the potential of sea water
is approximately 0.3 V. After fixing the cathode potential, an external resistance was loaded between
the anode and cathode and varied over 2.2 Ω–10 kΩ. The cell voltage was recorded 1 min after
loading each external resistance.
RESULTS AND DISCUSSION
Improved Performance of Paddy Soil-Used SMFCs by Steelmaking Slag
Fig. 4 shows the polarization curves of the SS-SMFCs before they generate electrical current. As
shown in Fig. 4a, the maximum power and current densities of the SS-SMFCs were approximately
150 mW.m-2 and 1600 mA.m-2, respectively. Kouzuma et al. (2014) reported the maximum powerand short-circuit current densities of SMFCs in rice paddy fields were only 80 mW.m−2 and 550 mA.
m−2, respectively. Comparison with the two latter shows that the performance of a PS-SMFC
increases when SS is used in the anode chamber of the PS-SMFC.

200
Cell voltage (V)Power density (mW.m-2)

250

150
100
50

(a)
0
0

200 400 600 800 100012001400160018002000
Current density (mA.m-2)

1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0

SS-SMFC
SS-SMFC with BNB
SS-SMFC with LAB(b)0

200 400 600 800 100012001400160018002000
Current density (mA.m-2)

Fig. 4 Polarization curves of SS-SMFCs before generating electrical current
The results of the present study agree well with those of Nishimura et al. (2018), who suggested
that mixing SS with littoral sediment increases SMFC performance. They argued that the reductionof iron compounds in the SS and its dissolution could make sediment into a higher reduced condition.
Furthermore, the SS might play an important role in supporting electron transfer in the sediment. Our
results suggest that mixing SS with either littoral sediment or paddy soil can improve SMFC
performance.
Effects of Lactic Acid Bacteria (LAB) and BN stain of Bacillus subtilis (BNB) in SS-SMFCs
Fig. 4 (the polarization without fixing the cathode potential) shows that the differences in the powerand current densities of a SS-SMFC with and without BNB were unclear; this occurs because the
SMFC performance depends on the performance of the cathode and anode electrodes. Thus, we must
fix the cathode electrode at the same potential to obtain a better understanding the anode electrode
performance, i.e., the effects of either LAB or BNB in the SS-SMFC anode.
Fig. 5 depicts the polarization curves for a fixed cathode potential of 0.3 V. The maximum
power density was 66 mW.m−2 in the case of using paddy soil alone; it increased to 190 mW.m−2 (athreefold increase) after adding BNB to the paddy soil and to 247 mW.m−2 (an increase of 3.74 times)Ⓒ ISERD

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after adding LAB (Fig. 5a). This suggests that adding either BNB or LAB improves SS-SMFC
performance, with LAB improving the performance more. A Similar conclusion is reached bycomparing the current density of each SS-SMFC (Fig. 5).
300
250
Cell voltage (V)Power density (mW.m-2)

(a)
200
150

100
50
0

0

200

1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0

400 600 800 1000 1200 1400
Current density (mA.m-2)(b)

SS-SMFC
SS-SMFC with BNB
SS-SMFC with LAB

200

400 600 800 1000 1200 1400
Current density (mA.m-2)

Fig. 5 Polarization curves for a fixed cathode potential of 0.3 V
40

(a)

35
30

(b)

400

SS-SMFC with BNB

Current density (mA.m-2)

Power density (mW.m-2)

450

SS-SMFC

SS-SMFC with LAB

25
20

15
10
5

350
300

250
200
150
100

Cathodeexposed to air

50

0

0
0

10

20
30
Opertation time (day)

40

50

0

10

20
30
Opertation time (day)40

50

Fig. 6 Long-term variations of power and current densities of SS-SMFCs
A SMFC experiences various potential losses that decrease the cell voltage during electricity
generation (Touch et al., 2019). In other words, the cell voltage of a SMFC with higher performanceis less reduced for a given generated current. For a current density of 400 mA.m-2, Fig. 5b shows thatthe cell voltage decreased by approximately 0.70, 0.45, and 0.35 V for the basic SS-SMFC, SSSMFC with BNB, and SS-SMFC with LAB, respectively. From this, it can be said that adding either
BNB or LAB can improve the SS-SMFC performance. The improvement may be partly due to theenhanced electron transfer to the anode and the accelerated decomposition of organic matter by thebacteria; unfortunately, those reasons could not be examined in the present study.
In addition, having continuously generated electrical current for 40 days, the power densities
were 7, 16, and 25 mW.m-2 for the basic SS-SMFC, SS-SMFC with BNB, and SS-SMFC with LAB,
respectively (Fig. 6a), and the current densities were 180, 280, and 350 mA.m -2, respectively (Fig.
6b). These results suggest that the bacteria will continue to affect the SS-SMFCs during long-term
operations.
CONCLUSION
In this study, laboratory experiments were conducted to examine changes in the performance of the
PS-SMFC when using SS in the anode chamber and the applicability of LAB and BNB for improving
SS-SMFC performance. A twofold increase in the power density was observed, suggesting that theutilization of SS in the anode chamber improves the PS-SMFC performance. Furthermore, the powerⒸ ISERD

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density increased by 2.9 times after adding BNB and 3.7 times after adding LAB to paddy soil,
thereby indicating that adding either LAB or BNB improves the SS-SMFC performance; further,adding LAB was shown to increase the SS-SMFC performance more. It was considered that thesebacteria enhance the electron transfer to the anode and accelerate the decomposition of organic matter.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge the students of the Rural Environmental Engineering Laboratory,
Tokyo University of Agriculture, for their efforts in collecting data.
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