Accepted Manuscript

Electricity generation in a membrane-less microbial fuel cell with down-flow

feeding onto the cathode

Feng Zhu, Wancheng Wang, Xiaoyan Zhang, Guanhong Tao

PII: S0960-8524(11)00576-1

DOI: 10.1016/j.biortech.2011.04.062

Reference: BITE 8408

To appear in: Bioresource Technology

Received Date: 20 February 2011

Revised Date: 18 April 2011

Accepted Date: 19 April 2011

Please cite this article as: Zhu, F., Wang, W., Zhang, X., Tao, G., Electricity generation in a membrane-less microbial

fuel cell with down-flow feeding onto the cathode, Bioresource Technology (2011), doi: 10.1016/j.biortech.

2011.04.062

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers

we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and

review of the resulting proof before it is published in its final form. Please note that during the production process

errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

1

Electricity generation in a membrane-less microbial fuel cell

with down-flow feeding onto the cathode

Feng Zhu, Wancheng Wang, Xiaoyan Zhang, Guanhong Tao*

College of Chemistry, Chemical Engineering and Materials Science, Soochow

University, Suzhou, Jiangsu 215123, China

* Corresponding author. Tel: +86 512 65226517; fax: +86 512 65223409; E-mail

address: taogh@suda.edu.cn (G. Tao)

2

Abstract

A novel membrane-less microbial fuel cell (MFC) with down-flow feeding was

constructed to generate electricity. Wastewater was fed directly onto the cathode

which was horizontally installed in the upper part of the MFC. Oxygen could be

utilized readily from the air. The concentration of dissolved oxygen in the influent

wastewater had little effect on the power generation. A saturation-type relationship

was observed between the initial COD and the power generation. The influent flow

rate could affect greatly the power density. Fed by the synthetic glucose wastewater

with a COD value of 3500 mg/L at a flow rate of 4.0 mL/min, the developed MFC

could produce a maximum power density of 37.4 mW/m2. Its applicability was further

evaluated by the treatment of brewery wastewater. The system could be scaled up

readily due to its simple configuration, easy operation and relatively high power

density.

Keywords: microbial fuel cell; membrane-less; down-flow; air cathode; oxygen

diffusion

3

1. Introduction

A microbial fuel cell (MFC) is a device that uses microorganisms to convert

chemical energy from biodegradable substrates to electrical energy via

microbial-catalyzed redox reactions (Logan et al., 2006; Du et al., 2007; Oh et al.,

2010). Electrons and protons are generated during the oxidization of substrates by

microorganisms. The generated electrons flow from the anode via an external circuit

to the cathode, where they typically combine with protons and oxygen to form water.

Electricity is thus produced and harnessed by inserting a load between the two

electrodes. This provides MFCs promising potentials to generate renewable electricity

while accomplishing the biodegradation of organic matters or wastes when they are

assembled and integrated in wastewater treatment process. Therefore, MFCs have

attracted great attentions in recent years.

It has been reported that the construction of the microbial reactor in MFCs is an

important factor that affects the performance and further application of MFC (He et

al., 2006; Li et al., 2009; Liu and Logan, 2004). The most commonly reported MFCs

consist of an anode chamber and a cathode chamber separated by a proton exchange

membrane (PEM) (Oh et al., 2004; You et al., 2006; Fornero et al., 2008). However,

the so-called dual-chamber MFCs possess some inherent disadvantages, such as the

high cost of PEM, the potential for bio-fouling and associated high internal resistance

(Liu and Logan, 2004; Liu et al., 2008). In order to overcome these disadvantages,

single–chamber MFCs without PEM have been developed (Min et al., 2005; Jang et

al., 2004). The omission of PEM significantly simplifies the configuration and

4

reduces the cost and the internal resistance. In the membrane-less MFCs, oxygen is

often used as the terminal electron acceptor. Compared to the other electron acceptors

such as potassium ferricyanide and potassium permanganate (You et al., 2006; Oh et

al., 2004), oxygen can be freely available in the air and readily reduced on the cathode.

Therefore, the so-called air-cathode MFCs can reduce the cost and improve

sustainability.

In the air-cathode MFCs, the power density is often constrained by the oxygen

reduction rate on the cathode. It is caused by the low solubility of oxygen in the

aqueous cathodic solution and its subsequent availability at the cathode. To enhance

the efficiency of oxygen utilization, cathodes are often treated by coating a catalyst

(Yang et al., 2009; Lefebvre et al., 2009; Zhang et al., 2009). Air aeration in the

cathodic area is also used to increase the oxygen concentration (Ghangrekar et al.,

2007; Aldrovandi et al., 2009; Luo et al., 2009). Fornero et al. (2008) constructed a

MFC with pressurized cathodic chamber to increase the solubility of oxygen. Li et al.

(2009) reported an overflow-type wetted-wall MFC with a tubular cathode chamber to

improve the oxygen utilization.

However, in the membrane-less air-cathode MFCs, oxygen can diffuse from the

cathode area to the anode chamber as removing PEM. Oxygen diffusion into the

anode chamber can result in a loss of electron donor and thus lowering overall

Coulombic efficiency (CE) (Liu and Logan, 2004). In order to restrain the oxygen

diffusion, some modifications have been made on the configuration of the

membrane-less air-cathode MFCs. Fan et al. (2007) used a cloth between the anode

5

and cathode to reduce the oxygen diffusion rate. You et al. (2007) developed a MFC

with graphite-granule anode and tubular air-cathode. Aldrovandi et al. (2009) and Hu

(2008) designed a baffle-chamber reactor to improve the CE. Li et al. (2009)

combined a tubular anode chamber reactor and a baffle-chamber reactor together. Yet,

particularly due to their reactor and electrode designs, these systems are still somehow

complicated, making it difficult to be scaled up.

In the present study, a new single-chamber membrane-less air-cathode MFC was

constructed. The top of the single-chamber MFC was open to air. Inexpensive

graphite plates were used as the electrodes without catalyst-coating treatment.

Wastewater was fed directly onto the graphite cathode which was horizontally placed

just above the water surface, and flowed out from the bottom of the MFC. It is the

most significant difference from the previously reported MFCs, in which wastewater

was fed from the bottom of the MFCs (Jang et al., 2004; Rabaey et al., 2005). The

MFC developed in this study was referred as down-flow MFC. The goal of the

present study was to develop a MFC with simple and inexpensive configuration which

could be easily scaled-up. The feasibility of continuous electricity generation using

this down-flow MFC and factors affecting its performance were investigated.

2. Materials and Methods

2.1. MFC construction

The scheme of the down-flow single-chamber MFC is shown in Fig. 1. A

6

cylindrical glass tube was used to construct the MFC. The inner diameter of the tube

was 6 cm and the height was 30 cm, with a working volume of about 850 mL. The top

of the MFC was open to air. A rectangle anode (10×4×0.5 cm) was vertically inserted

into the anaerobic activated sludge at the bottom of the MFC and a circular cathode

with a diameter of 5.6 cm and a thickness of 0.5 cm was horizontally placed on the

support in the upper part of the MFC. About half of the cathode was kept above the

liquid level. Both anode and cathode were made from graphite plates (Wangnian

Graphite Factory, Jiangsu, China) without catalyst-coating treatment. The effect of the

distance between the cathode and anode on the power production was investigated.

The graphite electrodes were connected with copper wire coming out of the glass tube

to provide the connection points for the external circuit. All the connection points

were sealed with epoxy resin. The anode and cathode were connected via an external

variable resistor. A plastic tube was connected to the conical bottom of the MFC so

that the liquid level in the chamber could be adjusted by raising or lowering the outlet

of the plastic tube. An overflow port was opened on the wall of glass chamber about

2.5 cm down from the top.

2.2. Inoculation and operation

The MFC was inoculated with anaerobic activated sludge collected from the

up-flow anaerobic sludge bed reactor in our laboratory, which has been run for over

one year to treat synthetic glucose wastewater. It was filled with 200 mL of the

anaerobic sludge, reaching a concentration of ca. 10.0 g/L for volatile suspended

7

solids. After the MFC was inoculated with synthetic wastewater for half a month in a

batch operating mode, the down-flow feeding was used. Wastewater was introduced

continuously onto the air cathode and flowed out from the bottom of the MFC at a

fixed flow-rate of 4.0 mL/min using a peristaltic pump (BT100-100M, Longer

Precision Pump, China). A water bath was used to maintain the temperature of the

MFCs at 30±1oC. The system was considered to be operating under stable conditions

when the power production reached stable for at least 10 hrs.

The anode and the cathode were connected via an external resistor. The voltage

(V) on the external resistor was monitored with a data acquisition system and

converted to the power density, P (W/m2) according to P=V2/(RA), where R is the

external load and A is the submerged area of the cathode, which is 26.0 cm2 in this

study. The current density (Id) was calculated as Id=I/A, where I is the current and A is

the submerged area of the cathode.

In order to compare the present system with the previously reported MFC system

with up-flow feeding (Ghangrekar et al., 2007; Jang et al., 2004; Rabaey et al., 2005),

another MFC with the same configuration was operated in parallel using the same

conditions except that the wastewater was introduced from the bottom of the MFC

and flowed out from the top.

Synthetic glucose wastewater was used to investigate the effect of affecting

factors on the performance of the developed MFC because of its simple and constant

composition. It contained (per liter of tap water) 3.95 g glucose, 0.38 g NH4Cl, 0.015g

FeSO4•7H2O, 0.026g MgSO4, 0.019 g CaCl2, 0.76 mg NiSO4•6H2O and 9.36 g NaCl.

8

A phosphate buffer solution (50 mM) was used to adjust its pH to 7.0. Wastewater

collected from a local brewery was used to evaluate the applicability of the developed

system. The characteristics of the brewery wastewater were: pH, 4.65; COD, 2850

mg/L; NH3-N, 45.3 mg/L; TN, 67.2 mg/L; TP, 22.7 mg/L; SS, 725 mg/L. Its pH was

adjusted to 7.0 with sodium carbonate powder and phosphate buffer. Same amounts of

trace elements required by microorganism growth and NaCl were added as did in the

synthetic wastewater. During the application test, the MFC was first fed with brewery

wastewater diluted with the synthetic glucose wastewater (1:1) and run for 7 days.

Then the brewery wastewater was introduced until the MFC reached stable.

3. Results and discussion

3.1. Electrical power production and polarization

As shown in Fig. 2, the down-flow MFC could successfully produce a stable

power density of 30.0 mW/m2, i.e. an output voltage of 153 mV at a fixed external

load of 300 �, 11 hrs after its operation. The power density was twice higher than that

achieved by the MFC using up-flow feeding, which was 14.6 mW/m2. This

comparison was made under the same conditions that the DO concentration in the

feeding wastewater was about 3.0 mg/L and air aeration was not used in the cathode

area for both feeding modes.

For the down-flow MFC, the DO concentration in the feeding wastewater had

little effect on power output, which will be discussed later in Section 3.2. However,

9

for the up-flow feeding, as shown in Fig. 2, the power density was increased from

14.6 to 18.7 mW/m2 after the feed was deoxygenated with nitrogen purging. This was

because that the DO in the feed could lead to the non-productive redox reaction of

organic carbon oxidation coupled with oxygen reduction in the anode biofilm (Liu et

al., 2004). The reaction was minimized when the feed was deoxygenated, resulting

higher power output.

In the air cathode MFCs, DO concentration in the cathode area played a

significant role in power production (Fornero et al., 2008; An et al., 2009). When the

up-flow feeding was used, the DO concentration in the cathode area became very low

(0 to 1.2 mg/L) after it passed through the MFC from the bottom. The low DO

concentration restrained the oxygen reduction rate on the cathode, resulting in low

power density (An et al., 2009). Several measures were reported to improve the

efficiency of oxygen utilization (Li et al., 2009; Ghangrekar et al., 2007; Aldrovandi

et al., 2009). Air aeration in the cathode area that was commonly used in the previous

studies (Aldrovandi et al., 2009; Luo et al., 2009) was adopted in the present study

with up-flow feeding. As shown in Fig. 2, the power density was increased to 33.2

and 24.0 mW/m2 when aeration was used in the up-flow MFC with and without

deoxygenation, respectively.

As shown in Fig. 2, the down-flow MFC could achieve higher power output than

the up-flow MFC without the deoxygenation of the feed or aeration in the cathode

area, and almost the same power output as the up-flow MFC with deoxygenation and

aeration. It showed the superiority of the down-flow MFC to its up-flow counterpart

10

thanks to its simple configuration and easy operation.

In order to investigate the performance of the down-flow MFC, the output

voltages under various external loads from 0 to 15000 � were recorded and the

polarization curve was obtained. As shown in Fig. 3, the power density increased with

the decrease of external resistance and a maximum power density of 30.0 mW/m2 was

reached with the external resistance of 300 �, corresponding to a current density of

196 mA/m2. The maximum power density was higher than that obtained by using

up-flow feeding mode in this study (14.6 mW/m2) and that achieved in a similar MFC

configuration reported by Ghangrekar et al (2007) (10.13 mW/m2). It is also shown in

Fig. 3 that the power density was declined with the decrease of external resistance

lower than 300 �. The results suggested that the electron transfer through the external

circuit to the cathode be the limiting factor for power production at higher external

loads.

3.2. Effect of dissolved oxygen concentration on power production

It was reported that DO concentration in the cathode area played a great role in

power density in the air cathode MFCs (Fornero et al., 2008). In the present study

with down-flow feeding, the effect of DO concentration in wastewater on power

production was investigated and the results are shown Fig. 4. Yet, the initial DO

concentration in wastewater did not affect the power production significantly in the

present study. It was revealed in Fig. 4 that the DO concentration in the cathode area

had become almost the same after the wastewater was fed dropwise onto the cathode.

11

Oxygen in air could be dissolved readily into the wastewater while it was fed

dropwise onto the cathode. The DO concentration reached to 6.0 to 6.3 mg/L at room

temperature of 25 oC. The oxygen-saturated wastewater (8.25 mg/L) also reached to

that DO level by releasing oxygen to air when it entered the MFC. Therefore the

initial DO concentration in wastewater had little effect on the power production in the

present study. It made the present system simpler than the previous systems due to the

omission of air-aeration while high power density could still be achieved.

3.3. Effect of distance between anode and cathode on power generation

The position of cathode was adjusted to investigate the effect of the distance

between the cathode and the top of the anode on power generation. Three different

distances, i.e. 5, 10 and 15 cm were evaluated. The highest power density of 37.4

mW/m2 was obtained with the distance of 10 cm. The maximum power densities were

22.3 and 30.0 mW/m2 for the distances of 5 and 15 cm. The internal resistances were

135, 210 and 300 � for the distances of 5, 10 and 15 cm, respectively. For the

distance of 15 cm, the internal resistance was increased, resulting in a lower power

output. Although shorter electrode distance reduced the internal resistance, the

diffusion of oxygen to the anode area increased. It was observed that the DO

concentration of effluent was 2.5±0.3 mg/L for the distance of 5 cm, which was

higher than those (1.5±0.2 and 1.2±0.2 mg/L) for the distances of 10 and 15 cm. The

decline of power generation for the distance of 5 cm would be attributed to the

diffusion of oxygen to the anode. Therefore the spacing of 10 cm was used for the

12

further study.

3.4. Effect of influent flow rate on power production

To examine the effect of wastewater feeding rate on power production, the flow

rate was increased from 0.8 to 8.0 mL/min. An external resistor of 210 � was used for

the investigation. As shown in Fig. 5, when the flow rate was increased from 0.8 to

4.0 mL/min, the power density increased from 19.4 to 37.4 mW/m2 and the internal

resistance decreased from 395 to 210 �. With the increase in flow rate, the boundary

layer over the electrode surface became thinner and thus the mass transfer to the

electrodes was enhanced, resulting in lower internal resistance and therefore higher

power output. Another possible explanation for this increase might be the

enhancement of electrochemical activity of the bacteria by an increase in the rate of

mass transport. (Di Lorenzo et al., 2009). Yet, further increasing the flow rate to 8.0

mL/min resulted in the decrease of the maximum power density to 25.9 mW/min. The

possible explanation might be that oxygen diffusion to the anode area at higher flow

rates became more straightforwardly, causing the loss of electron donor and thus

lowering the power density.

3.5. Effect of COD on power generation

The effect of COD value in wastewater on power generation was investigated in

the range of 875 to 4000 mg/L. As shown in Fig. 6, power density increased with the

increase in COD concentration. The power generation as a function of initial COD

13

concentration showed a saturation-type relationship in the investigated range. The

power density was modeled as a function of COD concentration using the

Monod-type equation (Kovar and Egli, 1998). The maximum power density in the

equation for this hyperbola was 45.9 mW/m2 with a half-saturation constant of 1638

mg/L. The observation was consistent with the results reported in previous studies

regardless of the cell configuration and substrates (Min et al., 2005; Li et al., 2009; Li

et al., 2008).

3.6. Effect of ionic strength on power generation

Ohmic loss was one of the main causes for the decrease of power generation in a

MFC system (Liu et al., 2008; Liu et al., 2005). It could be reduced by increasing the

electrical conductivity in the cell. In this study, NaCl was added to adjust the ionic

strength, and thus the electrical conductivity. With the increase of NaCl concentration

from 0 to 0.16 M, the power density increased rapidly. Further increase of ionic

strength from 0.16 to 0.32 M had a negligible effect on the power density, indicating

that the ionic strength was not a limiting factor beyond 0.16 M. Therefore 0.16 M of

NaCl was added in the synthetic wastewater in the present study.

3.7. Application of the developed system

The developed MFC system was applied to the treatment of the wastewater from

a local brewery. A maximum power density of 19.8 mV/m2, i.e. 104 mV was achieved

after it was run for three weeks. It showed the feasibility for the application of the

14

system to real wastewater treatment. The study on the scaling up of the system was

currently conducting on a reactor with a volume of 25 L. in which wastewater was fed

onto the cathode with a shower nozzle-like distributor.

4. Conclusions

The feasibility of the down-flow MFC with air cathode in electricity generation

was demonstrated. Oxygen could be utilized directly from the air with the downward

feeding of wastewater onto the cathode. Factors such as electrode spacing, influent

flow rate and COD concentration affected significantly the power production while

DO concentration didn’t. A maximum power density of 37.4 mW/m2 was produced

from the MFC fed by synthetic wastewater with a COD value of 3500 mg/L. The

system was characterized by simple configuration, easy operation and relatively high

power density. It was successfully applied to the treatment of brewery wastewater.

Acknowledgements

This research is partially supported by National Natural Science Foundation of

China (Project Nos. 20345006 and 20575043).

15

References

Aldrovandi, A., Marsili, E., Stante, L., Paganin, P., Tabacchioni, S., Giordano, A.,

2009. Sustainable power production in a membrane-less and mediator-less

synthetic wastewater microbial fuel cell. Bioresour. Technol. 100, 3252-3260.

An, J., Kim, D., Chun, Y., Lee, S.J., NG, H.Y., Chang, I.S., 2009. Floating-type

microbial fuel cell (FT-MFC) for treating organic-contaminated water. Environ.

Sci. Technol. 43, 1642-1647.

Di Lorenzo, M., Curtis, T.P., Head, I.M., Scott, K., 2009. A single-chamber microbial

fuel cell as a biosensor for wastewaters. Water Res. 43, 3145-3154.

Du, Z., Li, H., Gu, T., 2007. A state of the art review on microbial fuel cells: A

promising technology for wastewater treatment and bioenergy. Biotechnol. Adv.

25, 464-482.

Fan, Y., Hu, H., Liu, H., 2007. Enhanced Coulombic efficiency and power density of

air-cathode microbial fuel cells with an improved cell configuration. J. Power

Sources 171, 348-354.

Fornero, J.J., Rosenbaum, M., Cotta, M.A., Angenent, L.T., 2008. Microbial Fuel Cell

Performance with a Pressurized Cathode Chamber. Environ. Sci. Technol. 42,

8578-8584.

Ghangrekar, M.M., Shinde, V.B., 2007. Performance of membrane-less microbial fuel

cell treating wastewater and effect of electrode distance and area on electricity

production. Bioresour. Technol. 98, 2879-2885.

16

He, Z., Wangner, N., Minteer, S.D., Amgenent, L.T., 2006. An upflow microbial fuel

cell with an interior cathode: assessment of the internal resistance by impedance

spectroscopy. Environ. Sci. Technol. 40, 5212-5217.

Hu, Z., 2008. Electricity generation by a baffle-chamber membraneless microbial fuel

cell. J. Power Sources 179, 27-33.

Jang, J.K., Pham, T.H., Chang, I.S., Kang, K.H., Moon, H.S., Cho, K.S., Kim, B.H.,

2004. Construction and operation of a novel mediator- and membrane-less

microbial fuel cell. Process Biochem. 39, 1007-1012.

Kovar, K.K., Egli, T., 1998. Growth kinetics of suspended microbial cell: From single

substrate-controlled growth to mixed-substrate kinetics. Microbiol. Mol. Biol.

Rev. 62, 646-666.

Lefebvre, O., Ooi, W.K., Tang, Z., Abdullah-Al-Mamum, M., Chua, D.H., 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., Yao, L., Kong, L., Liu, H., 2008. Electricity generation using a baffled

microbial fuel cell convenient for stacking. Bioresour. Technol. 99, 1650-1655.

Li, Z, Zhang, X, Zeng, Y, Lei, L., 2009. Electricity production by an overflow-type

wetted-wall microbial fuel cell. Bioresour. Technol. 100, 2551-2555.

Liu, H., Cheng, S.A., Logan, B.E., 2005. Power generation in fed-batch microbial fuel

cells as a function of ionic strength, temperature, and reactor configuration.

Environ. Sci. Technol. 39, 5488-5493.

Liu, H., Cheng, S.A., Huang, L.P., Logan, B.E., 2008. Scale-up of membrane-free

17

single-chamber microbial fuel cells. J. Power Sources 179, 274-279.

Liu, H., Logan, B.E., 2004. Electricity generation using an air-cathode single chamber

microbial fuel cell in the presence and absence of a proton exchange membrane.

Environ. Sci. Technol. 38, 4040-4046.

Logan, B.E., Hamelers, B., Rozendal, R., Schroder, U., Keller, J., Freguia, S.,

Aelterman, P., Verstraete, W., Rabaey, K., 2006. Microbial fuel cells:

Methodology and Technology. Environ. Sci. Technol. 40, 5181-5192.

Luo, H., Liu, G., Zhang, R, Jin, S., 2009. Phenol degradation in microbial fuel cells.

Chem. Eng. J. 147, 259-264.

Min, B., Kim J.R., Oh, S.E., Regan, J.M., Logan, B.E., 2005. Electricity generation

from swine wastewater using microbial fuel cells. Water Res., 39, 4961-4968.

Oh, S.E., Min, B., Logan, B.E., 2004. Cathode performance as a factor in electricity

generation in microbial fuel cells. Environ. Sci. Technol. 38, 4900-4904.

Oh, S.T., Kim, J.R., Premier, G.C., Lee, T.H., Kim, C., Sloan, W.T., 2010. Sustainable

wastewater treatment: How might microbial fuel cells contribute. Biotechnol.

Adv. 28, 871-881.

Rabaey, K., Clauwert, P., Aelterman, P., Verstraete, W., 2005. Tubular microbial fuel

cells for efficient electricity generation. Environ. Sci. Technol. 39, 8077-8022.

Yang, S., Jia, B., Liu, H., 2009. Effects of the Pt loading side and cathode-biofilm on

the performance of a membrane-less and single-chamber microbial fuel cell.

Bioresour. Technol. 100, 1197-1202.

You, S., Zhao, Q., Zhang, J., Jiang, J., Wan, C., Du, M., Zhao, S., 2007. A

18

graphite-granule membrane-less tubular air-cathode microbial fuel cell for power

generation under continuously operational conditions. J. Power Sources 173,

172-177.

You, S., Zhao, Q., Zhang, J., Jiang, J., Zhao S., 2006. A microbial fuel cell using

permanganate as the cathodic electron acceptor. J. Power Sources 162,

1409-1415.

Zhang, L., Liu, C., Zhuang, L., Li, W., Zhou, S., Zhang, J., 2009. Manganese dioxide

as an alternative cathodic catalyst to platinum in microbial fuel cells. Biosens.

Bioelectron. 24, 2825-2829.

19

Figure Captions

Fig. 1. Schematic diagram of the down-flow MFC used in the study.

Fig. 2. MFC performance using different feeding mode. The distance between the two

electrodes was 15 cm. The external load was 300 �. Other conditions are specified in

Section 2.

Fig. 3. Power density and voltage as function of current density. Same operating

conditions were used as shown in Fig. 2.

Fig. 4. Effect of DO in influent wastewater on power density. Same operating

conditions were used as shown in Fig. 2.

Fig. 5. Effect of feeding rate on power production and internal resistance. The

distance between the two electrodes was 10 cm. Other conditions are specified in

Section 2.

Fig. 6. Effect of influent COD concentration on power density. Same operating

conditions were used as shown in Fig. 5.

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Oxygen was utilized readily from air while wastewater feeding onto the cathode.

DO did not affect power generation while COD, flow rate and electrode spacing

did.

Maximum power density was twice as high as that obtained by up-flow feeding.

Readily to be scaled up due to its simple configuration and easy operation.