Electricity Generation from Wastewater Using an Anaerobic Fluidized Bed Microbial Fuel Cell

ARTICLE

https://www.sodocs.net/doc/ab8062512.html,/IECR Electricity Generation from Wastewater Using an Anaerobic Fluidized Bed Microbial Fuel Cell

Weifang Kong,Qingjie Guo,*Xuyun Wang,and Xuehai Yue

Key Laboratory of Clean Chemical Processing Engineering in Universities of Shandong,Qingdao University of Science and Technology, Shandong Province,266042,People’s Republic of China

ABSTRACT:The anaerobic?uidized bed microbial fuel cell(AFBMFC)was developed to generate electricity while simultaneously treating wastewater.During a complete cycle,the AFBMFC continuously generated electricity with a maximum power density of 1100mW/m2and removal of total chemical oxygen demand(COD)of89%.To achieve this power density,the arti?cial electron-mediator neutral red(NR)was employed in the anode chamber.Granular biological electrodes,?uidization behavior,electron mediators,and temperature were evaluated to improve power production and wastewater treatment e?ciency.The results showed that the maximum power density production of granule-graphite AFBMFC was530mW/m2,much higher than410mW/m2using a granular activated carbon AFBMFC in the same reactor.Fluidization behaviors enhance the mass transfer and momentum transfer between activated carbon and wastewater.The power density increased with increasing methylene blue(MB)and NR concentration. Furthermore,power density reveals a slight increase as MB and NR concentrations exceed0.5and1.7mmol/L.The optimum temperature ranges from23to40°C.The Coulombic e?ciency was9.3%under the best operating conditions.

1.INTRODUCTION

A microbial fuel cell(MFC)is a device that can directly produce electricity from the bacterial oxidation of organic matter such as glucose or acetate1or inorganic species such as sul?des.2 In comparison with the power consumption in a conventional biological process,MFCs can generate energy from biodegrad-able organics using bacteria as catalyst,demonstrating its poten-tial as a promising and competitive technology for wastewater treatment.3,4The electrons generated at the anode can then be passed through an external circuit to generate power.These electrons enter the cathode to combine with the protons(H+) that transfer through a PEM and bind with externally provided oxygen to yield water.

In the past2decades,numerous investigations have focused on considerably increasing the power densities to a maximum of 4310mW/m2and Coulombic e?ciencies up to89%,respectively.5à8 This maximum power density and high Coulombic e?ciency were reached in a dual-chamber MFC with a well-acclimated,mixed-culture bio?lm in the anode chamber and ferricyanide in the cathode chamber while using plain graphite electrodes.For the above-men-tioned microbial fuel,high power densities and Coulumbic e?ciencies were achieved,however,at low volumetric loading rates7,8and high cost.MFCs need to be converted to practical and economical wastewater treatment processes.Low volumetric loading rates require reactors too large to be practical.Scaleup will only be economical if volumetric loading rates can be increased without a decrease in Coulombic e?ciency.The membranes in MFCs are very expensive, which also restricts system performance.The high costs make the use of membranes prohibitive in large-scale application of MFCs (i.e.,wastewater treatment).The other adverse e?ect of the membrane on performance is attributed to increased internal resistance.If the solution conductance or e?ective di?usivity of a proton,or chemical species carrying a proton,is reduced by the presence of the membrane,then the internal resistance of the system will increase with the power production reduced.8It is clear that ferricyanide and other chemical catholytes can be used to increase power production in MFCs.However,much of this power density is derived from the chemical energy in the catholyte rather than the organic substrate,and unlike oxygen,the use of these chemical catholytes is di?cult to sustain.9

The approaches to increasing power densities can be grouped into three major categories:9(1)kinetic limitation,which occurs when energy is consumed to proceed reactions in anode and cathode chambers;(2)ohmic limitation,which is caused by the ionic resistance of electrolytes and the electric resistance of the electrodes and connection materials;and(3)transport limita-tion,which is due to the ine?cient mass transport of substrates to the reaction sites within the bacterial cell.Practical applications of MFCs will require that a developed design will not only produce high power and Coulombic e?ciencies but also is economical to mass produce based on the reactor architecture.In the open literature,a wide range of system designs have been used in MFCs to improve the reactor performance in terms of voltage output,Coulombic e?ciency,stability,and longevity.In the early stage,cells have been designed in a tubular shape.The outer surface of the tube represents the anode being exposed to the fuel compartment,and the inner surface is in contact with air.This design concept guarantees the separation of fuel and air compart-ments,but it leads to relatively low power densities and originates high costs,because of the high speci?c amount of expensive cathode material being used.10You et al.11developed a graphite-granule anode,tubular air-cathode MFC(GTMFC),and the power produced with glucose in the original tests was12.5W/m3. An unusually small reactor was developed by Ringeisen et al.,12 Received:April8,2011

Accepted:September21,2011

Revised:September2,2011

Published:September21,2011

consisting of two chambers.The cathode contained ferricyanide as a catholyte,with the solution exposed to air on one side of the reactor.Min and Logan13developed a?at-plate MFC with a CEM(Na?on)sandwiched in between the anode and cathode. However,power densities produced by these reactors were low. He et al.14used an up?ow microbial fuel cell(UMFC)to treat arti?cial wastewater of high chemical oxygen demand(COD) concentration by combining the advantage of the UASB system. Power densities were increased for enhanced mass transfer,but they are far from satisfactory.Liang et al.15designed a kind of packed bed reactors,and the power densities was increased by28%. The increased performance can be attributed to high biomass levels.Nevertheless,the power loss of organic is great because of nonuniform contact between microorganisms and organic material in the packed bed reactor.

In addition,Power density was increased to115mW/m2while using an ammonia-treated electrode.Ammonia gas treatment of a carbon cloth anode increased substantially the surface charge of the electrode and improved MFC performance.16The applica-

tion of a pulse plating technique provides the opportunity for preparation of new biocompatible nickel-nanomodi?ed carbon felt materials possessing high electrocatalytic activity as anodes in a yeast-biofuel cell.17Decreasing the electrode spacing from4to 2cm increased power output from720to1210mW/m2with acetate as a substrate,due to a reduction in internal resistance of the reactor.18Accordingly,it is valuable in modifying con?gura-tion,enhancing mass transfer,increasing surface charge of the electrode,and decreasing the electrode spacing to improve the performance of MFC.

In particular,most studies were based on arti?cial wastewater, for example,glucose,acetate,and sucrose.7,19à21There were only a few researchers focusing on actual wastewaters.Cheese whey was used without any pretreatment and it was seen that electricity production was possible but at a reduced e?ciency.22 In addition,the cheese whey was?lter-sterilized before being used in the MFC,and it was found that the substrate removal e?ciency and the maximum power output of the MFC seem not to be a?ected by wastewater strength.Filter sterilization of the cheese whey leads to much higher power densities.23

We developed a MFC system architecture called anaerobic ?uidized bed membraneless microbial fuel cell(AFBMFC), combining the?uidized bed system with a single-chamber air cathode MFC.This design can reduce internal resistance by strengthening mass transfer and increasing the surface charge of the electrode(addition of activated carbon in anode chamber). These advantages accomplished the high-rate treatment of waste-water,and the system is more convenient to scale up than other single chamber MFCs.This study demonstrates the feasibility and performance of this combined technology for wastewater treat-ment.Key factors like?uidization behavior,granular electrode, temperature,and electron mediators were assessed for treatment e?ciency and power production.

2.MATERIALS AND METHODS

2.1.Experimental Apparatus.As illustrated in Figure1,an AFBMFC was constructed of a Plexiglas vessel with the internal diameter of4cm and fluidized bed anode chambers with a height of60cm.It consists of a graphite rod embedded in anaerobic sludge and a cathode,which is made of a carbon cloth(

3.14cm2, 0.35mg/cm2Pt,30%wet-proofing;E-TEK)and coated with four diffusion layers,fixed onto one side of the chamber wall.A100g portion of fresh particles(active carbon/graphite)with an average diameter of0.2à0.9mm,which were used both as the carrier media for biofilm and granular electrode,was fed in the fluidized bed.The specific surface areas of activated carbon and graphite particles were276.54and0.81m2/g,respectively.The distributor was a porous glass plate with a thickness of2mm and a pore size of 2mm,which has a fractional perforated area of20%.

2.2.MFC Inoculation and Operation.The MFC was inocu-lated with150mL of anaerobic sludge collected from the Waste-water Treatment Plant,Qingdao,China.Mixed cultures are more suitable for the use of complex fuels such as wastewater,as single organisms generally metabolize quite a limited range of organic compounds.Wastewater,collected from Qingdao sanitary waste-water,was used as the fluidization liquid supplied with a peristaltic pump without any additions of nutrient and buffer solution for the anode chamber.The pH of the solution in anode chamber was initially adjusted to around7.During the investigations,the feed solution was replaced when the voltage dropped below20mV, representing one complete cycle of operation.A water bath(Hetto HMT-200)was used for maintaining at the temperature of30°C. MFC experiments were generally performed at30°C unless otherwise specified.

2.3.Graphite-Granule Electrode Pretreatment.The exis-tence of oil and metals in graphite granules often reduces efficiency of the biological processes because such compounds influence fluidization and resist microbiological attack;thus we have to remove these impurities before use.The detailed process can be described as follows:

(1)Graphite granules were immersed in a solution of HCl

(1mmol/L)for2h.

(2)The acid-washed graphite granules were heated for about

10h and then cleaned several times with distilled water.

(3)The graphite granules treated above were immersed into

NaOH solution(1mmol/L),and the alkali-washed graphite

granules were heated for4h and afterward cleaned several

times with distilled water.

(4)The graphite granules were stored in deionized water

before use.

2.4.Analytical Measurements and Calculations.2.4.1. Mechanical Properties of Granular Electrodes.Granular elec-trodes should have excellent wearability because interparticle collisions occur in the process

of particles’fluidizing.In this Figure1.Schematic diagram of the anaerobic?uidized bed MFC.

test,the wearability was determined with abrasive wearing equipment.The same weight (M 1)of two granular electrodes was put into the water (50mL)and stirred for 10h by using a magnetic mixer.Then the granular electrode was dried and weighed (M 2)after removing the residue.The wear rate was defined as δ?

eM 1àM 2TM 1?100%

e1T

2.4.2.Electrochemical Analysis.MFC has characteristics simi-lar to anaerobic reactors,which can be described by electrochem-ical parameters such as power density,electrical current output,and cell voltage,while the substrate (COD)removal efficiency 7is also considered seriously.

The electronical potential across the resistor was recorded every 2min using a multimeter with a data acquisition system (USB1608FS,Measurement Computing Corp.).Polarization data were collected by changing the external resistance (varied from 100Ωto 90k Ω)with a variable resistor box during the stable power production stage of each batch experiment.The current was calculated according to Ohm ’s law:I ?V =R

e2T

Power density (mW/m 2

)was calculated by P ?UI A ?U 2

RA

e3T

Coulombic e ?ciency was calculated by η?

C a

C t

?100%e4Twhere C a is the total Coulombs calculated by integrating the current over time

C a ?Z

I d t e5Tand C t is the theoretical amount of Coulombs available from the

oxidation of acetate

C t ?Fb ?ΔCO

D ?V =M

e6T

where U (mV)is the voltage,P (mW/m 2)is power density,R (Ω)is the external resistance,A (cm 2)is the geometric surface area of the anode electrode,t is reaction time (s),V is the liquid volume in the reactor (L),ΔCOD is the change in COD (kg/L),M is the molecular weight of oxygen (32,g/mol),I is current (A),F is Faraday ’s constant (96485,C/mol),b is the number of moles of electrons exchanged per mole of oxygen (4mol e à/mol).24

3.RESULTS AND DISCUSSION

3.1.Effect of Granular Electrode on MFC Performance.3.1.1.Mechanical Properties of Granular Electrodes.The experimental data

are collected in Table 1.

As shown in Table 1,the wear rate of granular graphite and activated carbon were 7.3%and 3.6%,respectively.Activated carbon is more suitable as a biological carrier in a ?uidized bed due to its better wearability.

3.1.2.Acclimation Time.The startup of MFC is the process of bacterial cells adapting to the environment and decomposing organic substances to generate stable and sustainable electricity.To reduce the influence of substrate concentration on electricity generation,10mL of fresh wastewater was injected into anolyte everyday.

Voltage was produced over time when wastewater was pumped into the anode chamber.The cell voltage was recorded using a data acquisition system controlled by a computer (Figure 2).For granular activated carbon AFBMFC (GACMFC),just after the start-up,the voltage was very low,which may be explained by the fact that little bacteria cells were immobilized on the electrode at the beginning.The voltage is mainly produced by suspended bacteria.With time elapsed,a steady increase in open-circuit voltage was observed and remained at a maximum of 0.63V after 97h.When the anode compartment was re ?lled with fresh waste-water,the voltage immediately increased and reached stability.The power generation without lag phase was primarily due to the immobilized biomass on the anode electrode,indicating that the start-up of MFC had ?nished.The experimental data demon-strated the feasibility of bioelectricity from domestic wastewater using AFBMFC.

After inoculation,GACMFC,graphite-granule AFBMFC (GGMFC),and AFBMFC required 97,105,and 150h before reaching the ?rst maximum power production,respectively,as shown in Figure 2.Activated carbon and graphite particles added to the anode chamber shortened start-up time obviously.This is because the surface of activated carbon and graphite particles are

Table 1.Wear Rate of Two Granular Electrodes

granular electrodes M 1/g M 2/g δ/%activated carbon

3028.92 3.6graphite

30

27.81

7.3

Figure 2.Voltage generation during the startup period

of the GACMFC,GGMFC,and AFBMFC.

Figure 3.Polarization curve and power density curve in GACMFC,GGMFC,and AFBMFC.

rough,which o ?er lots of surface area for bacteria to grow.However,due to their special pore structures,compared with GGMFC,the start-up time of GACMFC was reduced because the speci ?c surface area of activated carbon was greater than that of graphite particles.It was easier for bacterial cells to attach on the surfaces of activated carbon particles and form a mature biomembrane.

3.1.3.Effect of Granular Electrodes on Treatment Efficiency and Power Production.The performance of a microbial fuel cell was investigated with different fluidization granule electrodes.The maximum power densities of GGMFC and GACMFC determined from the polarization curve,in Figure 3,were 540and 397mW/m 2,much greater than 244mW/m 2in AFBMFC without granular electrode.One possible reason is that both graphite and activated carbon provided an excellent medium for bacteria to grow,caused a long biomass retention time and a high biomass level,and then improved the electrogenesis capacity of MFC.As illustrated in Figures 4and 5,the removal of COD of GGMFC and GACMFC was 79%and 86%,respectively,whereas the removal of COD reaches 50%without granular electrode addition.Both GGMFC and GACMFC exhibited high removal of COD.This can be explained by the fact that graphite and activated carbon can adsorb organic matter from wastewater.This process removes organic carbon compounds,depending on a combination of adsorption and biodegradation.The biomass growth was pro-moted by the biodegradable matter in the water.Adsorption of nutrients,which leads to retention of them on the carbon surfaces,extends contact time between the contaminant and the biomass.This process promotes biodegradation and improves the removal of COD.

Compared with GACMFC,GGMFC improved power pro-duction.This can be explained by the fact that the resistivity of graphite particulates (0.01Ω3cm)was lower than that of activated carbon (1Ω3cm),which results in a lower internal resistance.It was well-documented that ?uidization particles

conductivity was an important factor that determines power output of MFC.

The relationships between time and removal of COD are described in Figures 4and 5.An increase in operating time led to an increase in removal of COD,from 34%for 2days to 86%for 10days using GACMFC,and from 30%for 2days to 81%for 10days using https://www.sodocs.net/doc/ab8062512.html,pared with the GGMFC,GACMFC improved the removal of COD.It may be because activated carbon has a great speci ?c surface area,which provides a high adherent area for microorganism and increases microbial aggregates,resulting in high removal of COD.Interestingly,the removal rate of COD in GACMFC changed greatly from startup to 6days,about 73%,but it increased slowly from 73%to 86%in the last 4days.The same trend appeared in GGMFC.It was concluded that a decrease in wastewater concentration limited the activity of microorganism,which decreased the speed of removal of COD.

3.2.Effect of Operating Conditions on Reactor Perfor-mance.3.2.1.Effect of Fluidization Behavior on MFC Perfor-mance.When a fluid passed through the packed bed,the bed pressure drop of the fluid is approximately proportional to the fluid ’s superficial velocity.When the velocity of fluid exceeds a critical velocity,known as the minimum fluidization velocity,u mf ,the bed ’s mass is suspended directly by the flow of the fluid stream and the pressure drop almost keeps constant as velocity is raised.A packed bed shifts to a fluidized bed as the liquid velocity exceeds the minimum fluidization velocity.The minimum fluidization velocity of AFBMFC was determined using decreasing way (3.39mm/s).

Liquid velocity was kept below 3.39mm/s operated in packed bed regime.Liquid velocity was changed to investigate its e ?ect on power production.Power density production rises from 281to 327mW/m 2with initial wastewater velocity ranging from 0.82to 2.41mm/s,as shown in Figure 6.As the velocity was increased,the internal resistance of MFC determined from the polarization curve was 920Ωat 2.41mm/s,much lower than the 1100Ωat 0.82mm/s.

As shown in Figure 7,the maximum power density at 3.69,4.03,and 6.11mm/s was 364,396,410mW/m 2,respectively.An increase in the initial wastewater velocity increased voltage and power density.This ?nding was consistent with that in packed bed MFC.It demonstrated that the velocity had a great in ?uence on MFC https://www.sodocs.net/doc/ab8062512.html,pared with packed bed MFC,the max-imum voltage and power density both increased as a result of increased liquid velocity,when transitioning from a packed bed to a ?uidized bed.This may be due to an increase in the liquid velocity strengthened mass transport of substrate to the bacterial cell.25The contact of the solid particles with the ?uidization medium in ?

uidized bed (liquid)was greatly enhanced,enabling

Figure 4.Curve of

COD versus operation time in GACMFC.

Figure 5.Curve of

COD versus operation time in GGMFC.

Figure 6.Polarization curve and power density curve under di ?erent velocities in the packed bed MFC.

excellent mass transfer in the ?uidized bed.Similar to the good mass transfer for a well-mixed liquid,the bed can have a signi ?cant heat capacity while a homogeneous temperature ?eld is main-tained.To a certain extent,power output produced from waste-water was positively correlated with the velocity.After running 10days,the removal of COD reached 87%,higher than 82%in the packed bed MFC,which implied that the use of ?uidized bed enhanced the power generation,as well as reinforced wastewater treatment.

The maximum power production at 6.94,7.96,and 8.35mm/s was 254,180,and 124mW/m 2,determined from the polariza-tion curve (Figure 8).It reveals that the electrogenesis capacity was decreased with an increase of wasterwater velocity higher than 6.11mm/s,which may be assigned to the excessive liquid velocity reducing bio ?lm thickness.Suitable velocity is bene ?cial for maintaining a balance between growth rate and shed rate of bio ?lm.So the velocity should be operated within the operation range.3.2.2.Effect of Temperature on MFC Performance.MFC operated generally at temperatures ranging from 20to 35°C.However,no articles were published to clarify the influence of temperature on the efficiency of electricity production from an MFC.In this study,the maximum power density and removal of COD at different temperatures highlighted the effect of tem-perature (18,23,30,40,and 50°C)on the performance of MFC.The power generation and removal of COD were both increased at temperatures from 18to 30°C,as indicated in Figures 9à11.The maximum power density and the removal of COD at 30°C were 396mW/m 2and 86%,respectively.Such trends can be attributed to the following reasons.First,biochemical reaction rates for biomass growth and substrate utilization were increased with increased temperature.26Much microbial growth would also help bacterial attachment to an electrode.Second,conductivity of the

anolyte and catholyte is improved with increasing temperature,as shown by Arrhenius laws.27Third,electrochemical reactions on the electrodes are enhanced at high temperature based on the Bulter àVolmer equation.28However,a decrease in power gen-eration and removal of COD were observed when the tempera-ture was higher than 30°C,as depicted in Figures 9à11.These changes are considered to be denaturation of the enzyme.In the case of enzymatic reactions,the reaction rate increases with temperature to a maximum level and then abruptly declines with further increase of temperature.Because most biological enzymes rapidly become denatured at temperatures above 40°C,most enzyme determinations are carried out below that tem-perature.Consequently,it is crucial to keep operation tempera-tures at a certain range,23à40°C in this test,to achieve a high power production and removal of COD.

3.2.3.MFC Operations with Different Media in the Anode Chamber.In the microbial fuel cell (MFC),the rate of electron transfer to anode electrode is a key intrinsic limiting factor on the power density of MFCs.Microorganisms can transfer electrons to the anode electrode in three ways:using exogenous mediators (ones external to the bacterial cell)such as potassium ferricya-nide,anthraquinone-2,6-disulfonic acid,cobalt sepulchrate,and thionine;using endogenous mediators produced by bacteria;or by direct transfer of electrons from the respiratory enzymes (i.e.,cytochromes)to the electrode.29Many bacteria cannot produce electricity without a mediator present in the MFC.It has been found that direct electron transfer from microbial cells to electrodes occurs only at very low efficiency.Exogenous media-tors can divert electrons from the respiratory chain by entering the outer cell membrane,becoming reduced,and then leaving in a reduced state to shuttle the

electrons to the electrode.In this

Figure 8.Polarization curve and power density curve under di ?erent velocities (above 6.11

mm/s)in the ?uidized bed MFC.

Figure 9.Polarization curve under di ?erent temperatures

in the ?uidized bed MFC.

Figure 10.Power density curve under di ?erent

temperatures in the ?uidized bed MFC.

Figure 7.Polarization curve and power density curve under di ?erent velocities in the ?uidized bed MFC.

study,MB/NR were used in wastewater with different strengths in order to exclude the effect of redox mediator on power production.

MB and NR were added at the same concentration to waste-water in the anode chamber to determine the e ?ect of media category and concentration on power generation.Figure 12exhibits the cell voltage outputs as a function of mediator concentration of MFC-1(MB as mediator)and MFC-2(NR as mediator).An increase in the MB concentration from 0to 0.5mmol/L increased the maximum output voltage of MFC from 610mV to 706mV ?rst,nearly 100mV more voltage than with the control (wastewater without any addition).However,the maximum output voltage decreased to 440mV when MB concentration further increased to 1.5mmol/L.Furthermore,the maximum power density was increased as a result of increased media concentration,and the maximum power density signi ?cantly increased up to 470mW/m 2for MFC-1along with 0.5mmol/L MB as electron mediator,which was much higher than the control (Figure 13).Conversely,a further increase in MB concentra-tion decreased power density.The MFC with MB concentration of 1.5mmol/L produced a very low power density (280mW/m 2),30%lower than the power density of the control.This may be due to strong adsorption of MB in the bio ?lm inhibiting the electron transfer and thus decreasing the power output.

For MFC-2,a signi ?cant increased voltage output was ob-served with an increase of NR,and a maximum power output of 900mV was approached when the NR concentration was 1.7mmol/L (Figure 12).Also,the increase of NR concentration led to increased power density.With the addition of 1.7mmol/L NR,MFC achieved a maximum power density of 1100mW/m 2,almost three times as high as the control.However,no further increase in voltage and power generation was observed when

NR concentration was more than 1.7mmol/L (Figures 12and Figure14b).This phenomenon re ?ected that there existed ?xed and limited carrier binding points in bio ?lm.The carrier binding points were all occupied when the mediator concentration was high enough.A further increase in the concentration of mediator cannot increase transport rate any more.

The presence of MB and NR promote the transfer of electrons from microbes to the surface of electrode,consequently resulting in decreasing internal resistance.Low internal resistance could result in the observed increase in potential and power density.To a certain extent,the electron transfer e ?ciencies in microbial fuel cells could be

improved if more suitable electron mediators were

Figure 12.Voltage of MFC at di

?erent methylene blue/neutral red concentrations.

Figure 13.Polarization curve and power density curve of MFC

at di ?erent MB concentrations.

Figure 14.Polarization curve and power density curve of

MFC at di ?erent NR concentrations.

Figure 11.The removal of COD under di ?erent temperatures in the ?uidized bed MFC.

used.After 10days,removal of COD and Coulombic e ?ciency (CE)of MFC-1and MFC-2reached 86.7%,6.0%and 88.2%,9.3%,respectively,as shown in Figure 15.The reasons for the low CE in some studies are presented in the following.First,oxygen transfer into the system through the cathode may lower the CE.Oxygen is the terminal electron acceptor in the cathode compart-ment,and therefore,the presence of it in the anode compartment would disrupt the electron ?ow through the external circuit.30,31Second,extracellular biopolymers,cell storage,aerobic and di ?usion losses,other electron consumption processes such as sulfate reduc-tion and heterotrophic denitri ?cation,and other unidenti ?ed pro-cesses may therefore contribute to the low Coulombic e ?ciency.32Third,substrate is used for bacterial growth and production of biomass.Fourth,substrate loss is also possible due to methanogenesis.The high concentrations of acetate and anaerobic conditions favor methane production in the anode chamber.It follows that higher Coulombic e ?ciency and more power density were generated when NR was the electron mediator,suggesting that NR was a better electron mediator than MB for enhancing electricity production.

4.CONCLUSIONS

After running a complete cycle of feeding wastewater as the electron donor,the AFBMFC continuously generated electricity with a maximum power density of 1100mW/m 2and removal rate of COD of 88.2%.The AFBMFC is a promising technology for wastewater treatment while simultaneously generating elec-tricity.On the basis of the present investigation,the following conclusions can be drawn:

(1)The use of activated carbon and graphite-granule particles

can shorten start-up time,as well as enhance power density and removal of COD.The start-up time was shorter and removal of COD was higher when activated carbon was applied in the AFBMFC because of its higher speci ?c surface area.Besides,the wearability of activated carbon is better comparatively.However,the resistivity of graphite granules is lower and the use of graphite granules can achieve higher power production.

(2)Power production was raised when ?uid super ?cial veloc-ity was raised in packed bed MFC and AFBMFC.Higher power density and removal of COD are reached in AFBMFC because the mass transfer was greatly enhanced.Neverthe-less,the velocity should be limited in a proper range because an excessive liquid velocity would reduce bio ?lm thickness.(3)The power density and removal of COD are increased

with the increase of temperature less than 30°C.How-ever,a decrease in power generation and removal of COD

was observed when the temperature was higher than 30°C.The optimal temperature range for the operation of AFBMFC is 23à40°C,during which the power density and removal of COD achieved the maximum value.

(4)Both MB and NR can improve power density of AFBMFC

signi ?cantly.NR is more suitable,for the power density and Coulombic e ?ciency were higher with NR used as mediator.Further research is needed to investigate the reasons for the low Coulombic e ?ciency in AFBMFC.

’AUTHOR INFORMATION

Corresponding Author

*E-mail:qj_guo@https://www.sodocs.net/doc/ab8062512.html,.

’ACKNOWLEDGMENT

The authors gratefully acknowledge the kind support of the National Natural Science Foundation of China (20876079),Science Foundation for Distinguished Young Scholars of Shan-dong Province (JQ200904),and Key Scienti ?c and Technolog-ical Projects Shandong Province (2008GG10006010).’NOMENCLATURE

A (cm 2)=geometric surface area of the anode electrode

b (4mol e à/mol)=number of moles of electrons exchanged per

mole of oxygen

ΔCOD (kg/L)=the change in COD F (96485C/mol)=Faraday ’s constant I (A)=current

M (32,g/mol)=molecular weight of oxygen P (mW/m 2)=power density R (Ω)=external resistance T (s)=reaction time U (mV)=voltage

V (L)=liquid volume in the reactor ’REFERENCES

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