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M.M. Ghangrekar and V.B. Shinde

Wastewater Treatment in Microbial Fuel Cell and Electricity Generation:

A Sustainable Approach

M.M. Ghangrekar*

and V.B. Shinde

Assistant Professor, Department of Civil Engineering, Indian Institute of Technology, Kharagpur 721302. India.

(E-mail: [email protected])Research Student, Department of Civil Engineering, Indian Institute of Technology, Kharagpur 721302. India.

(E-mail: [email protected])

ABSTRACT

Application of microbial fuel cell (MFC) for wastewater treatment could be an attractive

alternative to reduce the cost of treatment and generate electricity. Studies were conducted in

the laboratory scale membrane less MFCs for treatment of synthetic wastewater. These MFCs

performed well for COD and BOD removal from the wastewater, demonstrating the

effectiveness of this device for wastewater treatment with COD and BOD removal efficiencyabout 90%. Using graphite electrodes, the electricity generation in these MFCs was observed

under different MFC configuration and influent COD concentration. The maximum power

density observed was 6.73 mW/m2. Thus, power can be produced from membrane less MFC

using organic matter from wastewater as source of energy. However, the electricity

generation is lower, and further investigations are necessary to optimize power production.

Keywords: MFC, Membrane-less MFC, Electricity generation, Wastewater treatment

INTRODUCTION

The high energy requirement of conventional sewage treatment systems are demanding for

the alternative treatment technology which will require less energy for its efficient operation

and recover useful energy to make this operation sustainable. In past two decade high rate

anaerobic processes are finding increasing application for the treatment of domestic as well

as industrial wastewaters. Although, energy can be recovered in the form of methane gas

during anaerobic treatment of the wastewater, but utilization of methane is not attractive

while treating small quantity of low strength wastewater and usually it is flared. In addition,

due to global environmental concerns and energy insecurity there is emergent interest to find

out sustainable and clean energy source with minimal or zero use of hydrocarbons. Microbial

fuel cells, used as biosensors, if used for wastewater treatment, are capable to provide clean

energy, apart from effective treatment of wastewater. The enriched microbial culture in theseMFCs have capabilities to use organic matter present in the wastewater as energy source and

produce electrons and protons, through which electricity can be recovered.

Microbial fuel cell (MFC) is a device which converts chemical energy to electrical energy

during substrate oxidation with the help of microorganisms [Allen and Bennetto, 1993; Kim

et al., 1999a,b, Park and Zeikus, 2000, Bond and Lovely, 2003, Gil et al., 2003, Liu et al.,

2004]. Microbial fuel cell is made up of two compartments, anode and cathode, separated

with proton/cation exchange membrane. Microorganisms oxidize the substrate and produce

electrons and protons in the anode chamber of MFC. Electrons collected on the anode are

transported to cathode by external circuit and protons are transferred through the membrane

internally. Thus, potential difference is produced between anode and cathode chamber due to

* Author for correspondence; Tel. No. +91 3222 283440 (O); Fax No. +91 3222 282254

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Paper presented in the 12th

international sustainable development research conference. April 6-8, 2006, Hong Kong

dissimilar liquid solutions. Electrons and protons are consumed in the cathode compartment

by utilizing oxygen from water. Most studies have used electrodes of solid graphite,

graphite-felt, carbon cloth and platinum coated graphite cathode electrode.

Most of the bacterial species used in MFC are inactive for transport of electron; hence for

intervention, synthetic and natural compounds called redox mediators are required. Dyemediators such as neutral red, methylene blue, thionine, humic acid are used as a mediator

[Delaney et al., 1984; Lithgow et al., 1986; Park and Zeikus, 2000]. Commercial application

of MFCs for wastewater treatment is difficult, because most of the mediators are expensive

and toxic in nature. Hence, today emphasize is being given on development of mediator less

MFC, enhancing its power production, and reduction of its operational cost, to increase its

acceptance as wastewater treatment process. More recently it has been demonstrated that an

iron-reducer, bacteria such as shewanella putrefaciens which is electrochemically active is

used as a catalyst in a mediator-less microbial fuel cell [Kim et al., 1999a,b, 2002]. Also, the

family Geobacteraceae can directly transfer electrons to electrodes using electrochemically

active redox enzymes, such as cytochromes on their outer membrane [Kaufmann and Lovely,

2001, Magnuson et al., 2000].

In the operation of mediator-less MFC several factors are limiting steps for electricity

generation, such as, fuel oxidation at the anode, electron transfer from microorganism to

anode hence, presence of electrochemically active redox enzymes, external resistance of the

circuit, proton transfer through the membrane to the cathode, and oxygen reduction at the

cathode. Transfer of protons through the membrane to the cathode compartment can be a

limiting factor when proton permeability of the membrane is poor. Under the proton transfer

limited conditions, the microbial activity and the electron transfer to the electrode in the

anode compartment can be reduced due to the pH change, in addition to the slow cathode

reaction due to limited supply of proton [Gil et al., 2003]. Also, application of MFC in large

scale, for wastewater treatment containing suspended solids, will be limited due to high initial

cost of the membrane and fouling of the membrane requiring replacement.

The acceptability of MFC for wastewater treatment would increase, if use of membrane can

be eliminated using some alternative. A membrane-less microbial fuel cell (ML-MFC) was

used successfully [Jang et al., 2004] that converted organic contaminants to electricity. Since,

wastewater flow through anaerobic and aerobic zone, proper operation of this device can

achieve treatment of wastewater containing organic contaminants. Thus, a membrane-less

MFC can improve the economic feasibility. Hence, the objective of the study was to explore

the possibility of using membrane less MFC for the wastewater treatment. It was decided to

evaluate effectiveness this device for COD, BOD and nitrogen removal from syntheticwastewater, and to study electricity production potential using graphite electrode.

MATERIALS AND METHODS

Membrane-less microbial fuel cell

Figure 1shows the schematic diagram of the membrane-less microbial fuel cell used in this

study. Two such MFCs, made of polyacrylic cylinder with effective height 60 cm were used,

having anode compartment at bottom and cathode compartment at top. For MFC-1, the

internal diameter of the cylinder was 10 cm. Glass wool (4 cm depth) and glass bead (4 cm

depth) were placed at the upper portion of the anode in both the MFCs and graphite rod, as

roll form, was used for both anode and cathode. The distance between the anode and cathode

electrode was 20 cm including glass wool and glass bead in the MFC-1, with apparent surface

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area of each anode and cathode of 46.65 cm2. For MFC-2, cylinder with 15 cm internal

diameter was used with three graphite electrodes in each anode and cathode compartments.

The distances between the respective anode and cathode electrodes were 20 cm, 24 cm, and

28 cm. The total apparent surface area of each anode and cathode electrodes was 210.64 cm2.

The fuel was supplied from the bottom of the anode compartment and the effluent left

through the cathode compartment at top for both the MFC. The electrodes were connectedwith copper wire through a resistance ranging from 10 to 100 , including the resistance of

copper wire,and a multimeter. The internal resistance was ranging from 3 M to 5 M for

MFC-1 and 10 M to 15 M for MFC-2.

Figure 1 Schematic diagram of membrane less microbial fuel cell used in the study

Wastewater composition and inoculation of MFC

The wastewater was applied at the rate of 5.011 L/d to both the ML-MFCs making hydraulic

retention time (HRT) in the MFC-1 of 24 h and in MFC-2 as 50.78 h. The cathode

compartment was aerated at rate of 60 ml/min in both. A synthetic wastewater containing

sucrose as a carbon source was used throughout the study. The composition of the synthetic

wastewater used is provided in Table 1. The COD of synthetic wastewater used was in the

range 312 to 446 mg/l for MFC-1 and 325 mg/L for MFC-2. Influent pH was maintained in

the range 7.2 to 7.6 by suitable alkalinity addition in both the ML-MFCs. The ML-MFCs

were inoculated with anaerobic sludge (1.0 L) collected from septic tank bottom. The

inoculum sludge was preheated at 100oC for 15 minutes to suppress the methanogens, cooled

at room temperature and 1.0 L volume of sludge was added to the anode compartment. No

microbial addition was carried out in cathode compartment in both the cases. All experiments

were conducted at room temperature ranging from 29 to 33oC.

Table 1 The composition of the synthetic wastewater

Component Sucrose NaHCO3 NH4Cl K2HPO4 KH2PO4 CaCl2.2H2O MgSO4.7H2O

mg/ L 300-450 480 95.5 10.5 5.25 63.1 19.2Trace metals were added as FeSO4.7H2O = 10 mg/L, NiSO4.6H2O = 0.526 mg/l, MnSO4.H2O = 0.526 mg/l, ZnSO4.7H2O =

0.106 mg/l, H3BO3 = 0.106 mg/l, CoCl2.6H2O = 52.6 g/L, CuSO4.5H2O = 4.5 g/L, and (NH4)6Mo7O24.4H2O = 52.6 g/L.

Analyses

The potential was measured using a digital multimeter (MASTECH M-830B, Russia) and

converted to power according to P= iV, where P = power (W), i = current (A), and V =

voltage (V). Influent and effluent characteristics such as, COD, pH, dissolve oxygen (DO),suspended solids (SS), volatile suspended solids (VSS), ammonical nitrogen and total

kjeldahl nitrogen (TKN) were monitored according to standard methods [APHA, 1998].

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Biochemical oxygen demand (BOD) was determined for three days at 27oC. The nitrate

nitrogen was measured using electrode (Orion make, USA). The microstructure of the biofilm

developed on the electrodes was examined using scanning electron microscopy (SEM,

Stereoscan 360, Cambridge, MA). The samples preparation procedure followed was as per

Fang et al. [1994].

RESULTS AND DISCUSSION

COD removal efficiency

The MFC-1 was operated at influent COD concentration ranging from 312 mg/L to 446 mg/L

as presented in Table 2. After 55 days of operation, when steady state condition for COD

removal was observed, the COD and BOD removal efficiency of MFC-1 was 90.86% and

90.67%, respectively. Volumetric COD removal rate of this MFC was 0.41 kg COD/ m3.d.

After achieving steady state, the effluent COD was 40.8 mg/L and BOD3 at 27 C was at 25

mg/L. The COD and BOD values reported are for unsettled effluent containing average SS

and VSS as 40 mg/L and 6.2 mg/L, respectively. Further improvement in the effluent qualityis expected by using settling tank after MFC. The COD removal efficiency observed is on the

higher side of the maximum efficiency reported as 80 to 90% [Liu et al., 2004; Jang et al.,

2004]. Further studies are required to explore maximum loading capacity of the MFC.

In MFC-2, the COD removal efficiency after two weeks of continuous operation was greater

than 80% with average COD removal efficiency of 86.5% at organic loading rate 0.15 kg

COD /m3.d. The COD removal for 70 days of operation of this MFC is presented in Figure 2.

The effluent COD concentration under steady state condition was 38.13 mg/L with SS

concentration 43.35 mg/L and VSS 12.30 mg/L. The effluent BOD concentration was 26.04

mg/L, making BOD removal 88.46 percent. The COD and BOD removal efficiencies

observed in both the MFCs demonstrate its ability to be used as the effective wastewatertreatment process. The cathode compartment of both the MFCs was aerated by supplying

compressed air. The DO in the effluent observed was in the range 3.75 to 4.88 mg/L. Even

with continuous aeration, lower values of DO in the cathode compartment and in the effluent

were due to utilization of DO for the cathode reaction, where oxygen is reduced, and might

be consumed for nitrification in small amount.

Table 2 Performance of the ML-MFCs for COD and BOD removal

Days MFC

used

External

Resista-

nce ()

COD

(Inlet)

(mg/l)

COD

(middle)

(mg/l)

COD

(outlet)

(mg/l)

Effici

-ency

(%)

BOD

(Inlet)

(mg/l)

BOD

(outlet)

(mg/l)

DO

(Inlet)

(mg/l)

DO

(outlet)

(mg/l)0-15 MFC-1 100 329.17

(45.4)

248.50

(19.1)

162.50

(24.7)

50.63 ND ND ND ND

16-35 MFC-1 10 399.90

(34.3)

312.30

(22.6)

72.4

(20.9)

81.89 280 48 5.4

(1.0)

4.33

(0.25)

36-55 MFC-1 25 312.33

(16.7)

208.17

(55.5)

56.30

(11)

81.97 240 35 5.5

(1.0)

3.75

(0.35)

56-78 MFC-1 50 446.50

(36.1)

239.50

(17. 7)

40.8

(3.3)

90.86 268 25 5.4

(1.0)

4.14

(0.39)

0-15 MFC-2 100 314.37

(21.3)

199.57

(41.5)

115.13

(57.2)

55.89 ND ND ND 4.79

(0.2)

16-70 MFC-2 100 323.61

(10.3)

173.13

(11.81)

38.13

(5.85)

86.48 225.7

(8.1)

26.04

(7.3)

5.0

(1.0)

4.88

(0.16)ND Not Determined

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0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 10 20 30 40 50 60 70 80

Time (days)

Current(mA)

0

20

40

60

80

100

CODremovale

fficiency,

%

Current production

COD Removel Efficiency (%)

Figure 2 COD removal efficiency and current production observed in MFC-2. (On 19th day

lactose and on 20

th

day dextrose was used as a substrate instead of sucrose.)

Current Production

After inoculation and feed application in continuous mode, slow increase in current was

observed in both the MFCs with duration of operation. The MFC-1 took two weeks to get

acclimatized with necessary microbial culture development to produce steady current. The

maximum current production of 0.175 mA was observed in MFC-1 at external load of 10

with voltage 0.19 V (Table 3). The open circuit potential observed was about 0.3 V after

fifteen days of operation with maximum power density of 6.73 mW/m2 for MFC-1. The

MFC-2 required two weeks to produce the stable current and the maximum current

production was 0.81 mA at external resistance of 50 . At the continuous external load of

100 , the current production in MFC-2 reached to the maximum value of 0.56 mA on 12 th

day and dropped to 0.48 mA on 15th day, with fairly constant current production on later

days. The open circuit voltage potential of 0.5 V was observed in this, with closed circuit

voltage drop across the resistance of 43 mV. The power density in MFC-2 with three

electrodes in each compartment was observed to be 1.12 mW/m2. The power density

observed in MFC-1 was higher than the reported value in the literature as 1.3 mW/m2 and it is

comparable with the observed value of MFC-2 using three electrodes for anode, increasing

total surface area of anode [Jang et al., 2004].

Table 3 Power production observed in the ML-MFCsMFC

used

Days Current (mA) Voltage

(V)

Resistance

(ohm)

Power

(mW/m2)

Jule

(J/d)0-15 0.091 (0.011) 0.116 (0.007) 100 2.29 (0.39) 0.92

16-35 0.175 (0.007) 0.188 (0.003) 10 6.73 (0.44) 2.72

36-55 0.148 (0.007) 0.175 (0.007) 25 5.46 (0.39) 2.20

MFC-1

56-78 0.121 (0.008) 0.151 (0.005) 50 3.96 (0.46) 1.596

0 15 0.344 (0.20) 0.035 (0.020) 100 0.83 (0.51) 1.030MFC-216-70 0.489 (0.14) 0.051 (0.014) 100 1.26 (0.74) 2.144

The current production at different external resistance in the ML-MFCs is presented in the

Table 3. Higher current production was observed in MFC-2 as compare to MFC-1. Thetypical current production observed in the MFC-2 is presented in the Figure 2 with duration

of operation. Stable electric current production was observed in both the MFCs after

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achieving steady state. From these observations it can be said that, production of electricity

form ML-MFC is possible and it can become an attractive option to recover power during

wastewater treatment to compensate the cost of treatment.

It was observed that production of current varies with the type of substrate. In the MFC-2,

three different substrates such as, sucrose, lactose and dextrose were used as carbon source.The lactose (COD = 340 mg/L) was used as a substrate on day 19 and dextrose (COD = 310

mg/L) was used as a substrate on 20th day and for remaining days sucrose was used. As

shown in Figure 2, when all conditions were maintained same except the change in carbon

source, increase in current production was observed, and it was 0.72 mA for lactose and 0.66

mA for dextrose, as compared to 0.48 mA for sucrose at external load of 100 . Increase in

current production during lactose as substrate was due to more electron production during

lactose oxidation than sucrose.

The maximum power density of 6.73 mW/m2 was observed in this experiment. Further

investigation is necessary to optimize power production in the mediator less ML-MFC.

Power production can be enhanced if majority of the substrate oxidation is achieved in theanode compartment. At present oxidation of fuel i.e. incoming substrate is limited to only

about 42% to 46% and remaining substrate is getting oxidized in cathode compartment.

Nitrogen removal in MFC

The nitrogen was supplemented in the form of ammonical nitrogen to fulfil the nutrient

requirement in the synthetic wastewater. During performance evaluation of the MFCs it was

observed that, these are capable to remove nitrogen from the wastewater to some extent

(Table 4). The TKN removal in MFC-1 and MFC-2 was 38.11% and 57.46%, respectively.

Majority of the TKN removal was observed in the cathode compartment. This could be

attributed to the ammonia stripping in the cathode compartment due to aeration orsimultaneous nitrification and denitrification occurring in the cathode compartment. The

nitrogen removal capacity of the MFCs is not reported in the literature and further

investigations are necessary to confirm the mechanism behind the nitrogen removal and its

effect on overall power production in the MFC. During aerobic nitrification and

denitrification by heterotrophic Bacillus strains 33% of nitrogen removal was reported

without formation of nitrous oxide during batch studies under aerobic condition by Kim et al.,

[2005]. The nitrogen removal observed is in agreement with reported value, indicating that

the dominant nitrogen removal mechanism might be due to simultaneous nitrification and

denitrification in cathode compartment, because similarBacillus strains are expected to be

existing in the MFC. The nitrogen assimilation in the biomass is expected to be less because

of low biomass production in the MFCs compared to other biological treatment processes.Production of sludge from MFC is very less as compared to any other biological treatment

process [Jang et al., 2004] because the major fraction of the energy produced from oxidation

of the organic matter is converted to electricity, and the remaining energy is only available

for microbial growth.

Effect of external resistance

It was observed that at lower external resistance the current production is higher and vice

versa. For same wastewater COD concentration the current production decreases with

increase in resistance as shown in Figure 3. When resistance was increased to 25 and 50

in MFC-1 (Table 3), the production of current decreased to 0.148 mA and 0.121 mA,respectively, from 0.175 mA at 10 resistance. These results show that the resistance

becomes the rate limiting step. Even at lower resistance, low current production could be

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attributed to the lower electron consumption rate at the cathode than the transfer rate from the

external circuit. This might be due to limited supply of proton or oxygen. The lower current

production indicates some electrons are consumed by mechanism(s) other than the expected

cathode reaction. It is plausible that under the conditions of limited electrons disposal through

the circuit with a high resistance, the electrons are consumed in the anode to reduce other

electron acceptors such as sulphate and nitrate [Gil et al., 2003], or oxygen diffused fromcathode compartment or dissolved oxygen present in the influent, or methane production in

the anode compartment. Higher fuel oxidation by the microbes is expected at a low external

resistance to remove organic contaminants at a high rate [Jang et al. 2004].

Table 4 Different form of nitrogen observed in ML-MFCs

Days TKN

(Inlet)

(mg/l)

TKN

(middle)

(mg/l)

TKN

(outlet)

(mg/l)

NH4-N

(Inlet)

(mg/l)

NH4-N

(middle)

(mg/l)

NH4-N

(outlet)

(mg/l)

NO3(Inlet)

(mg/l)

NO3(middle)

(mg/l)

NO3(outlet)

(mg/l)

16-35* 44.04

(16.01)

40.55

(12.32)

30.35

(10.05)

39.45

(5.15)

30.03

(8.39)

9.76

(3.76) ND ND ND36-55* 49.00

(1.98)

49.84

(11.87)

48.04

(7.78)

39.45

(1.06)

24.25

(0.63)

5.65

(0.63) ND ND ND

56-78* 42.27

(3.42)

44.03

(5.07)

26.16

(2.52)

39.83

(1.59)

23.67

(1.90)

5.90

(0.72)

0.92

(1.06)

1.37

(0.77)

1.52

(0.85)

0-15** 45.56

(4.39)

40.25

(2.07)

24.71

(2.96)

40.40

(5.51)

35.01

(2.97)

19.82

(0.88)

1.66

(0.41)

1.75

(0.19)

2.24

(0.20)

16-70** 51.44

(2.21)

38.50

(1.276)

21.88

(1.96)

45.67

(2.59)

31.60

(0.47)

12.13

(2.80)

1.43

(0.07)

1.56

(0.04)

2.42

(0.37)

* - MFC-1, ** - MFC-2, ND Not determined

Effect of Spacing between the Electrodes on Electricity Production

The voltage and current production for different distance between the anode and cathode

electrodes was evaluated in MFC-2. Figure 3 shows the voltage and current production

observed at different external resistance and for different spacing between the anode and

cathode electrode. Increase in voltage and decrease in current was observed with increase in

resistance. The observed current and voltage production showed that, both these values

increase with decrease in the distance between the electrodes at different external resistance.

These results suggest that the mass transfer between two electrodes could be the limiting

factor, probably reducing proton transfer from anode to the cathode. Hence, the ML- MFC

should be constructed to place the electrodes as close as possible keeping the internal

resistance high enough to avoid an electrical leak [Jang et al., 2004].

0

40

80

120

160

0 50 100 150 200 250 300 350 400

Resistance (ohm)

Voltage(mV)

20 cm

24 cm

28 cm

0

0.2

0.4

0.6

0.8

0 100 200 300 400

Resistance (ohm)

Current(mA)

20 cm

24 cm

28 cm

Figure 3 Voltage and current production observed in MFC-2 at different external resistance

and for different spacing between the anode and cathode electrode.

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Microscopic examination

Figure 4 shows the SEM images of typical bacteria growth observed on the surface of anode

and cathode electrode in MFC-1. A close examination revealed that there were two

predominant bacterial morphologies. On the anode electrode small rod shape (3-5 m in the

length and 1-2 m in width) were dominating, resembling either to Fusiform Bacilli as

reported by Fang et al. [2002] during hydrogen production from sucrose containing

wastewater or family ofGeobacteraceae [Bond et al., 2002; Chaudhuri et al., 2003]. On the

cathode, rod shape bacteria (greater than 50 m in the length and 5-7 m in the width) were

dominating. This rod shaped bacteria and other coccal types bacterial morphotypes were

observed on cathode. These were similar to bacteria found in aerobic granular sludge

cultivated on glucose based synthetic wastewater [Wang et al., 2004]. On cathode, bacteria

similar to those involved in the cycling of nitrogen and sulphur compounds were also

observed. These bacteria are capable of oxidation of ammonia to nitrate, and oxidation of

hydrogen sulphide by sulphur oxidizing bacteria in their natural habitats.

Figure 4 Scanning Electron Micrograph of the biofilm growth observed on the surface of

electrodes. (A) Anode compartment showing short rod shaped bacteria, (B) Cathode

compartment showing long rod shaped bacteria.

CONCLUSIONS

Performance of the mediator less and membrane less microbial fuel cell demonstrated its

effectiveness for the treatment of wastewater with COD and BOD removal about 90%. The

electricity can be recovered from the ML-MFC during treatment of wastewater. Increase in

current and voltage production was observed with decrease in distance between the

electrodes, reducing the substrate diffusion limitations.

The production of current in the MFC depends on several factors. Further studies are

necessary to understand the effect of different operational parameters and optimize the

electricity production from the MFC. With continuous improvements in microbial fuel cell, it

may be possible to increase power generation and reduce production and operating cost of

MFCs. Thus, the combination of wastewater treatment along with electricity production may

help in compensating the cost of wastewater treatment, making it sustainable.

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ACKNOWLEDGEMENT

The grants provided by University Grants Commission, New Delhi, India (F. No. 14-10/2003

(SR)) are duly acknowledged.

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