5707r 2009 American Chemical Society pubs.acs.org/EF

Energy Fuels 2009, 23, 5707–5716 : DOI:10.1021/ef9005934Published on Web 10/19/2009

Continuous FeedMicrobial Fuel Cell Using An Air Cathode and ADisc Anode Stack for

Wastewater Treatment

Mirella Di Lorenzo,*,‡ Keith Scott,‡ Tom P. Curtis,† Krishna P. Katuri,‡ and Ian M. Head†

†School of Civil Engineering and Geosciences, ‡School of Chemical Engineering and Advanced Materials, Newcastle University,Newcastle upon Tyne, NE1 7RU United Kingdom

Received June 10, 2009. Revised Manuscript Received September 16, 2009

The performance of a single-chamber microbial fuel cell (SCMFC), with air cathodes surrounding a seriesof graphite discs anodes, is reported. The materials used in the MFC are low-cost options for itsconstruction. The air cathode, made from an activated carbon, was supported on a low-cost polypropy-lene/silica composite membrane. The system was operated in continuous mode and fed with wastewaterfrom the primary clarifier of a wastewater treatment plant. The optimal external load was 100 Ω, with acurrent output of 2.2mAand a power density of 50mWm-2, definedwith regard to the anode surface area.In the treatment of thewastewater, the chemical oxygen demand (COD) loading ratewas a key factor in theperformance of the SCMFC: lower organic loadings gave higher Coulombic efficiency with amaximumof63.4( 4.2%when the systemwas fed with 0.055 kg CODm-3 d-1, meaning that the electro-active biofilmwas responsible for the majority of the COD removal. The performance was compared with an alternativeanode configuration made of graphite granules. The use of granules led to a better treatment of thewastewater but did not improve the performance of the SCMFC with regard to the current and poweroutput. In particular, the power output was approximately three times lower compared to the graphite diskanode (0.17 mW vs 0.484 mW), and the maximum Coulombic efficiency was only 28%.

Introduction

Industrial processes, as well as municipal wastewater col-lection, almost always result in waste or side-products thatrequire further treatment. Environmentally benign low-costprocesses are needed by the end users of water pollutioncontrol equipment. Microbe-based biological methods con-tinue to be the prime choice for efficient and sustainablewastewater processing in conjunction with chemical andphysical treatment. In particular, biological treatment meth-ods clearly dominate the secondary wastewater treatmentsector as it is the most effective and eco-friendly optioncurrently available.

In recent years, a combination of tighter restrictions onsludge disposal site location, air pollution, hazardous wastedisposal, and odor control, has had substantial impact on theapplicability of aerobic treatment of industrial wastewater.1

Moreover, the urgent global demand for alternative energysources, less energy intensive processes, and the emergingdrive toward a more sustainable society, has led to anincreasing interest in technologies that consider wastewateras a renewable source of energy.

In this context, anaerobic wastewater treatment (AWT)is becoming increasingly popular worldwide and up tonow represents the only technology proven to be capableof extracting energy from wastewater on a commercialscale.2

In comparison with conventional aerobic treatment, themain advantages of AWT are: lower treatment costs; highflexibility; high loading rate operation; smaller volume ofwaste sludge; and finally, anaerobic organisms can be pre-served unfed for long periods of time.1

The UASB (upflow anaerobic sludge blanket) technology,developed about 30 year ago, is the most common among allthe anaerobic treatment systems, and approximately 60% ofthe anaerobic full-scale treatment facilities worldwide are nowbased on the UASB design.2

Recently, in addition to anaerobic digestion, microbial fuelcell (MFC) technologyhas attracted increasingattention fromboth the academics and the public as a novel biotechnology totreat wastewater and harvest energy from dissolved organiccarbon.

MFCs rely on the ability of certain species of microorgan-isms to transfer electrons from the inside of the cell to anelectrode (anode) while they are oxidizing, and thereforeremoving, the organic materials in wastewater.3 In contrastto chemical fuel cells, in an MFC microorganisms act asbiocatalyst at the anode.

A recent review compared MFC performance to conven-tional AWT.4 MFCs have the great advantages of producingelectricity from organic waste in a direct way, and thereforeuse energy much more efficiently than standard combustionengines that are limited by the Carnot cycle. Also, there is noneed for gas treatment, which is required with traditionalanaerobic digestion.

*To whom correspondence should be addressed. Phone: þ390832298120. Fax:þ39 0832298146. E-mail: Mirella.DiLorenzo@unisa-lento.it.(1) Lema, J. M.; Omil, F. Water Sci. Technol. 2001, 44 (8), 133–140.(2) Angenent, L. T.; Karim, K.; Al-Dahlam, M.; Wrenn, B. A.;

Domiguez-Espinosa, R. Trends Biotechnol. 2004, 22 (9), 477–485.

(3) Rozendal, R. A.; Hamelers, H. V. M.; Rabaey, K.; Keller, J.;Buisman, C. J. N. Trends Biotechnol. 2008, 26 (8), 450–459.

(4) Pham, T. H.; Rabaey, K.; Aelterman, P.; Clauwaert, P.;De Schamphelaire, L.; Boon, N.; Verstraete, W. Eng. Life Sci. 2006, 6(3), 285–292.

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Theoretically, MFCs can convert 1 kg of chemical oxygendemand (COD) to 4 kW, whereas with anaerobic digestion1 kg of COD could be converted to roughly 1 kW.11

MFCs can operate at low temperatures of operation (below20 �C) and low COD concentration, where AWT generallywould fail due to low reaction rates. Nitrogen removal inMFCs has also been demonstrated.5,6

In the past few years, the configuration of MFCs has beencontinuously improved, and the performance increased10-fold since the first application on a laboratory-scale withtwo-chamber MFCs.7

However, the maximum current generated with MFCs isonly around 0.1 A, and the average power density ofMFCs isaround 40 W m-3.8 These peak values refer strictly to theutilization of a synthetic effluent, characterized by easilybiodegradable compounds, and to operation in batch mode.The electrical performances in general decrease when realwastewater is used and/or the MFC is operated with contin-uous flow.8

The biggest barriers for practical applications of this noveltechnology in a wastewater treatment plant, are related todifficulties in scaling-up the process and to their capital costs.So far,MFCs have been operated only at a small-scale from afew milliliters to several liters at most.

It has been recently estimated that the capital costs of a full-scale MFC system would be orders of magnitude higher thanthose of conventional wastewater treatment systems. Thesecosts are particularly affected by the price of the cathodecatalyst, which is platinum in most cases, and the price of theproton exchange membrane (PEM), with an impact in thetotal price compared to conventional aerobic treatment sys-tems of approximately 47 and 38% respectively.3

Recently,many studies have focused on alternative cathodecatalysts. Cotetra-methyl phenylporphyrin (CoTMPP), ironphthalocyanine (FePc), and manganese oxide have recentlybeen shown to be suitable alternatives toPt inMFCs, howeverthey still need further investigation.9-12 System using bacteriaas catalyst at the cathode (biocathodes) have been alsoinvestigated.13,14

To overcome the high price of the PEM, membranelessMFCs have been tested. However, due to substantial oxygendiffusion into the anode chamber, the Coulombic efficienciesdecreased.15

In this study we analyzed the performance of a single-chamber microbial fuel cell (SCMFC) with activated carbonair cathodes surrounding a series of graphite discs anodes. The

air cathode was supported on a low-cost polyethylene/silicacomposite membrane. The performance was compared withan alternative anode configuration made of graphite granulesand characterized by a higher surface area. In order to bettersimulate the operating condition in a treatment plant, thereactor was operated in continuous mode and fed with realwastewater.

Material and Methods

Microbial fuel cell. The microbial fuel cell (SCMFC) usedin the experiments was of square cross-section and wasplaced between two glass tubes (QVF) as shown in Figure 1.The material of construction was polyacrylate. The MFCcontained four cathodes (2 cm�8 cm), one on each face of thequadrangle. The cathodes were exposed to air on one sideand to the anodic compartment on the other.

The cathode was produced by directly spraying an ultra-sonically mixed carbon black (Ketjen black 300, AkzoChemicals Ltd.), acetone, and 10% PTFE suspension ontoa polypropylene/silicate composite membrane (RhinoHideEntek International, UK) to a loading of 0.1 mg cm-2

The anode, consisted of 10 graphite discs (2.6 cmdiameter,0.2 cm thickness) having 40 small holes (0.2 cm diameter) forflow ofwastewater through the cell. The surface area per diskwas 8.9 cm2, resulting in a total anode surface area of 89 cm2

(Figure 1). The graphite discs were supported on a titaniumrod (0.5 cm diameter). The empty volume of the cube cell was350 cm3. The total fluid volume (Vf) in the cell and connect-ing glassware was 1100 cm3, and a head space of 500 cm3 wasmaintained by using a U-tube for the outlet.

To determine the effect of anode surface area on perfor-mance, an alternative anode configuration, using graphitegranules (diameters between 0.2 and 0.6 cm, Carbon Inter-national, London, UK) was used. The granules were con-nected to the cell using the titanium rod with two graphitediscs at the top of the rod, one disk at the bottom, andanother one in the middle. The granules were packed in thespaces between the graphite discs and held in place with atitanium net to occupy the cylindrical space in between thefour cathodes in the cube cell. The empty volume of the cubecell was reduced to 280 cm3, while the total fluid volume (Vf)was, in this case, 900 cm3.

The total external surface area of the graphite granules(Sp) was calculated by approximating the area of a singlepellet with the surface of a sphere having an average diameterof 0.4 cm and bymultiplying the surface of one pellet into thetotal number of pellets introduced in the reactor. Thisapproximation did not consider the contribution of the pelletroughness to the Sp. The total anode surface area (Satot) wasestimated as 1496 cm2, by adding the surface areas of the fourgraphite discs to the pellet surface.

Throughout the study the anode and cathode were con-nected through a voltmeter (Pico data logger) and an externalresistance to polarize the cell and monitor the current varia-tion under closed circuit conditions. The external resistancewas controlled by utilizing a resistor substitution box (RS 500,1% accuracy, Elenco electronics).

In operation of the cell, wastewater was fed through theinjection port, in up-flow mode, at flow rates in the range of0.18-0.52 cm3 min-1 using a peristaltic pump (Watson-Marlow, 520S) equipped with Marprene II tubing (0.5 cminternal diameter). The effluent was collected from the outlet(E) for analysis. TheMFCwas operated at room temperature,

(5) Jia, Y.-H.; Tran, H.-T.; Kim, D.-H.; Oh, S.-J.; Park, D.-H.;Zhang, R.-H.; Ahn, D.-H. Bioprocess Biosyst. Eng. 2008, 31 (4), 315–321.(6) Clauwaert, P.; Rabaey, K.; Aelterman, P.; De Schamphelaire, L.;

Pham,T.H.; Boeckx, P.; Boon,N.; Verstraete,W.Environ. Sci. Technol.2007, 41, 3354–3360.(7) Logan, B. E.; Regan, J. M. Trends Microbiol. 2006, 14 (12), 512–

518.(8) Logan, B. E. Microbial Fuel Cells; Wiley: New York, 2008.(9) Roche, I.; Katuri, K.; Scott, K. J. Appl. Electrochem. 2009, 39,

197–204.(10) Cheng, S.; Liu, H.; Logan, B. E. Environ. Sci. Technol. 2006, 40,

364–369.(11) Zhao, F.; Harnisch, F.; Schroder, U.; Scholz, F.; Bogdanoff, P.;

Herrman, I. Electrochem. Commun. 2005, 7, 1405–1410.(12) Hao, Yu, E.; Cheng, S.; Scott, K.; Logan, B. E. J. Power Sources

2007, 171, 275–281.(13) Clauwaert, P.; Van der Ha, N.; Boon, N.; Verbeken, K.; Ver-

haege, M.; Rabaey, K.; Verstraete, W. Environ. Sci. Technol. 2007, 41,3354–3360.(14) He, Z.; Angenent, L. T. Electroanalysis 2006, 18, 2009–2015.(15) Liu, H.; Logan, B. E. Environ. Sci. Technol. 2004, 38, 4040–4046.

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approximately 21 ( 2 �C. The feed solution was maintainedat a temperature of 7 ( 2 �C in order to minimize bacterialgrowth. Nitrogen was purged into the feeding tank.

Wastewater. The wastewater (WW) utilized during theexperiments was collected from the primary clarifier of thetreatment plant located in Cramlington (NorthumbriaWater, UK). It had a COD of 175 ( 50 ppm (mg dm-3)and other characteristics shown in Table 1.No inoculumwasused. In some experiments the COD of the WW was in-creased by utilizing glucose as specified, or decreased bydilution with tap water. In this last case, an appropriatevolume of phosphate buffer (5 M, pH 7) was added in order

to keep the WW conductivity constant and equal to the nondiluted WW.

The wastewaters were prefiltered over a woven cloth toremove suspended solids before addition to the reactor.

Enrichment. The enrichment of the anode surface withelectrochemically active bacteria was performed in contin-uous mode by feeding the reactor at a fixed flow rate of0.353 cm3 min-1 with WW containing 1000 ppm of glucose.The COD loading rate, defined by considering the total fluidvolume (Vf), was 0.55 kg COD m-3 d-1 (0.38 mg COD L-1

min-1) in the case of graphite discs, and of 0.67 kg CODm-3

d-1 in the case of graphite granules.Analyses. The COD was determined using a standard

method with chromate as the oxidant as previously de-scribed.16 All samples were filtered through a 0.22 μm porediameter membrane filter (VWR International) prior toCOD measurements.

The COD consumption rate was calculated, when steadystate was reached, using the formula:

Q � ðCODin -CODoutÞ ð1Þ

where Q (dm3 h-1) was the fuel flow rate, and CODin andCODout (mg dm-3) were the COD of the influent and theeffluent, at the steady-state, respectively.

Total suspended solids of the wastewater was determinedas previously described.16 Sulfate and phosphate concentra-tion in the real wastewater were determined using a DionexICS-1000 ion chromatograph with an AS40 automatedsampler and with an IonPac AS14A, 4�125 mm analyticalcolumn; 8 mMNa2CO3 solution was used as eluent at a flowrate of 1 cm3 min-1. A sample loop of 25 μL was used; thedetector was an electrochemical conductivity detector.

The Coulombic efficiency (fractional), at the steady-statewas calculated with formula 2:

εc ¼ MI

FzQΔCOD� 100% ð2Þ

whereF is Faraday’s constant (96 485Cmol-1);M=32 is themolecular weight of oxygen, I (A) is the current at the steady-state; z=4 is the number of electron exchanged per mole ofoxygen,Q (dm3 s-1) is the flow rate through the system, andΔCOD (g dm3) the difference in the influent and effluentCOD.

Conductivity measurements were performed with a con-ductivity meter provided by Hanna instruments. The inter-nal cell resistance was measured by electrochemical

Figure 1. Single chamber microbial fuel cell utilized in this study.A: air cathode; B: inlet; C: outlet; D: polyacrylate plastic blockcontaining the electrodes; E: anode configuration with graphitediscs. The cross area of each cathode was of 16 cm2 for a totalsurface of 64 cm2.

Table 1. Characteristic of the Wastewater Utilized

parameter value

pH 7alkalinity, ppm 46 ( 3conductivitya, μS cm-1 1570 ( 10COD, ppm 175 ( 50BOD, ppm 88 ( 5TOC, ppm 98 ( 2NH3-N, ppm 21 ( 0.3phosphate, ppm 4.1 ( 0.9sulfate, ppm 49.4 ( 9total suspended solids, ppm 187 ( 18

aAt 20.7 �C.

(16) Greenberg, A.; Clesceri, L. S.; Eaton, A. D., Standard Methodsfor the Examination of Water and Wastewater, 18th ed.; American PublicHealth Association: Washington, 1992.

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impedance spectroscopy using a potentiostat (Gillac, ACMInstruments) with the cathode as the working electrode andthe anode as counter electrode and reference electrode.Impedance measurements were conducted at open circuitvoltage (OCV) over a frequency range of 104 down to 102 Hzwith a sinusoidal perturbation of 15 mV amplitude.

Polarization curves were recorded bymeans of a potentio-stat (Gillac, ACM Instruments) at a scan rate of 1 mV s-1

after establishing stable behavior at an open circuit potentialof over 4 h. The reference electrode was mercury/mecuroussulfate (MMS). The values of potentials reported are referredto the normal hydrogen electrode (NHE).

Community Analysis. The bacterial community cells werefixed by dipping the anode in a 1:1 solution of pure ethanoland sodium phosphate buffer (50 mM, pH 7.5), followed bystorage at -20 �C until analysis.

Total DNA was extracted from the biofilm, from thesuspension of sodium phosphate buffer, and from the influ-ent wastewater. In particular, the biofilmwas scratched fromgraphite discs placed at different height of the graphite rod:starting from the top, discs 2, 4, 6, 8, and 10 were considered.

The 16S rRNA gene fragments from the bacterial popula-tions were amplified using primers 1 and 3 as previouslydescribed,17 which were provided by Thermo ElectronGmbH (Germany). The 16S rRNA gene fragments weresubsequently analyzed by denaturing gradient gel electro-phoresis (DGGE) and bands were cut out of the gel with aclean scalpel and added to 50 μL of PCR grade water. After12 h of incubation at 4 �C, 1 μL of the solution was ream-plified with the same primers and sent for sequencing.

DNA sequencing of the fragments was carried out byGenevision T/A Geneius (INEX Business Centre, UK). TheDNA sequences were analyzed and the closest-matchingsequences in theNationalCentre forBiotechnology Information

nucleic acid sequence database, were identified using theBLAST algorithm.18

Results

Enrichment. As shown in Figure 2, during enrichment withelectro-active bacteria, the SCMFC with graphite disk anodesexhibited an initial lag phase with no substantial changes in thecurrent generated, for approximately four days. This phasewasdue to the build up of biomass and its adaptation to theenvironment in the cell.19 An exponential increase in currentfollowed and, after 9 days, a stable current output of 5 (0.6 mA was reached. The output power, measured with the10Ω resistor, was 0.25 mW and the Coulombic efficiency was7.40 ( 0.16%. The internal resistance was 15 Ω.

Figure 3 shows the polarization and power density curvesobtained with the SCMFC after the stable current wasachieved. The open circuit voltage was 0.46 V, and the cellvoltage fell graduallywith an increase of current density untila value of almost 0 V at a current density of approximately350 mA m-2. Over the full range of polarization the anodepotential contributed to approximately 250 mV of the po-tential loss, and the remainder was mainly due to Ohmicpotential losses and cathode polarization (Figure 3). Thereare indications of somemass transfer limitations in the anodeand cell polarizations curves. The maximum power outputwas approximately of 55 mWm-2 (1.4Wm-3) obtained at acurrent density of 202 mA m-2.

Effect of the External Resistance.Once the performance ofthe SCMFC with graphite discs stabilized, the effect of theexternal resistance (Rext) on the power output was investi-gated. For this purpose, the SCMFC was connected to aseries of external loads varying from 10 to 4000 Ω, with aconstant organic loading rate of 0.55 kg COD m-3 d-1.Table 2 reports the current generated at each value of Rext.The current decreased by increasing the external resistancefrom 10 to 4000 Ω: the highest fall in current was observedwhen the resistance was changed from 10 Ω to 50 Ω, with atotal current decrease of 40%. For higher values of Rext theeffects on the output current were minor, and in particular

Figure 2. Electrochemically active bacteria enrichment of theSCMFC. The SCMFC was fed in continuous mode until a stablecurrent output was obtained and the electrochemically activebacteria were considered to be enriched and stabilized at the anodesite. Anode material: graphite discs. External resistance: 10 Ω.Feeding rate: 0.35 cm3 min-1. Loading rate: 0.55 kg COD m-3

d-1. Fuel: sewage wastewater containing 1000 ppm of glucose. InletCOD: 1180 ( 70 ppm.

Figure 3. Polarization and current density curves from the SCMFC.Current density refers to the area of the anode, 84 cm2 (8.4 �10-3 m2). Power density refers to the anodic volume: 350 cm3. Anodematerial: graphite discs. The polarization curves were recorded usinga potentiostat (Gillac, ACM Instruments) at a scan rate of1 mV s-1 from an open circuit potential established after 4 h.

(17) Kowalchuck, G. A.; Stephen, J. R.; De Boer, W.; Prosser, J. I.;Embley, T. M.; Woldendorp, J. W. Appl. Environ. Microbiol. 1997, 63,1489–1497.(18) Altschul, S. F.;Madden, T. L.; Schaffer, A. A.; Zhang, J.; Zhang,

Z.; Miller, W.; Lipman, D. J. Nucleic Acid Res. 1997, 25, 3389–3402.(19) Rabaey, K.; Clauwaert, P.; Aelterman, P.; Verstraete, W.

Environ. Sci. Technol. 2005, 39 (20), 8077–8082.

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the higher the value of Rext the lower the current stepdecrease, with a reduction of less than 20% for a changefrom 3 kΩ to 4 kΩ.

Table 2 reports the reactor performance also in terms ofpower generated at the steady-state, COD removal, andmaximum Coulombic efficiency, that is, that measured forcurrent at the steady-state and based on the variation ofCODin - CODout at the steady-state. The Coulombic effi-ciency was a function of the steady-state current and there-fore increased by decreasing the external resistance as thecurrent increased. The external resistance therefore did nothave a major influence on the COD removal rate, which wasapproximately constant at 0.3 ( 0.06 mg COD min-1,corresponding to approximately 72% of COD removal.The maximum value of the Coulombic efficiency, obtainedat aRext of 10Ω, was 7.4( 0.16%, demonstrating that, evenin the case of the highest current production, a high percen-tage of the COD removed did not contribute to electricityproduction.

The highest power output was of 52.7 mW m-2 (definedwith regard to the anode surface area), as confirmed from thepolarization curve in Figure 3, and was reached at a Rext of100Ω (see Table 2 and Figure 4). For this reason, 100Ωwaschosen as Rext for all the following experiments.

Effect of Organic Loading Rate. The effect of the loadingrate on the system performance was analyzed by consideringseveral input COD concentrations, at a constant flow rate,0.35 cm3 min-1, and, therefore, at a constant hydraulicretention time (HRT), which was approximately 17 h.

Starting with fuel made from wastewater plus 1000 ppmof glucose (total inlet COD: 1180 ( 70 ppm), the glucoseconcentration was reduced until the point at which purewastewater was utilized. Further COD reductions wereobtained by diluting the wastewater with tap water, withphosphate buffer added to maintain the conductivity(see Table 3). The fuel loading rate interval, obtained inthis way, varied from 0.04 kg COD m-3 d-1 to 0.55 kgCOD m-3 d-1, corresponding to a wastewater dilution of1:3.3, and to wastewater containing 1000 ppm of glucoserespectively.

As reported in Table 3, the power density increased withthe loading rate until the value of 0.08 kg COD d-1 m-3;further increase in the loading rate did not lead to improve-ments in the power output.

The Coulombic efficiency increased with the first increasein loading rate (i.e., from 0.037 to 0.055 kg COD d-1 m-3),reaching amaximumvalue of 63.4( 4.2%.Further increasesin loading rate caused a decrease of the Coulombic efficiencyuntil the value of 3.3( 0.38%, at themaximumCOD loadingrate (0.55 kg COD m-3 d-1).

On the other hand, at a highCOD loading rate, goodCODremoval was achieved (Table 3). For a loading rate of 0.55 kgCOD m-3 d-1, 80% removal was observed, while low inletCODs led to approximately half the values of percentageCOD removal. The explanation of this behavior can beattributed to the nature of the fuel. The addition of glucoseto the WW in fact, led to a high fraction of easily biodegrad-able COD in the influent, which would explain the very goodremoval efficiencies observed. Wastewaters, by contrast, arecharacterized by high amounts of complex substrates thatrequire longer time for digestion. This behavior was con-firmed when the effect of the flow rate on the systemperformance was investigated. The flow rate was in one casereduced to 0.18 cm3 min-1 and in another increased to0.52 cm3 min-1, and the results were compared with the rateof 0.35 cm3min-1 previously utilized (Table 4). At the lowestflow rate, the percentage COD removal increased by ap-proximately 60% with respect to that at the rate of 0.35 cm3

min-1, due to an increase in the HRT. When the flow ratewas increased a lower COD removal was reached.

Analysis of Anode Microbial Communities. The anodicperformance of an MFC is inextricably dependent on thenature and rate of electron transfer from themicrobial cell tothe anode. The composition of bacterial communitiesformed at the anode can be complex and may vary markedlyaccording to the nature of the electrode and of the substrateutilized. Understanding the nature of the microbial commu-nity that develops on the anode and themechanisms involved

Table 2. Effect of the External Resistance on the SCMFC Performancea

Rext (Ω) steady-state cell potential (mV) steady-state current (mA) COD removal rate (mg COD min-1) coulombic efficiency (%)

10 50 ( 8.4 5 ( 0. 6 0.25 0.33 ( 0.03 7.40 ( 0.1650 150 ( 17 3 ( 0.24 0.45 0.26 ( 0.05 6.27 ( 1.38100 220 ( 30 2.2 ( 0.21 0.484 0.30 ( 0.03 3.67 ( 0.04300 342 ( 38 1.14 ( 0.09 0.39 0.32 ( 0.05 1.76 ( 0.02500 400 ( 33 0.8 ( 0.047 0.32 0.30 ( 0.03 1.47 ( 0.03800 448 ( 57 0.56 ( 0.05 0.25 0.30 ( 0.09 0.92 ( 0.041000 355 ( 25 0.355 ( 0.018 0.126 0.28 ( 0.03 0.63 ( 0.052000 398 ( 71 0.199 ( 0.05 0.079 0.31 ( 0.02 0.32 ( 0.023000 456 ( 115 0.152 ( 0.027 0.069 0.30 ( 0.04 0.25 ( 0.024000 492 ( 85 0.123 ( 0.015 0.060 0.32 ( 0.09 0.19 ( 0.01

aLoading rate 0.55 kg CODm-3 d-1. Fuel: wastewater containing 1000 ppm of glucose. Inlet COD: 1180( 70 ppm. Anode material: graphite discs.Anode surface area: 89 cm2. The step changes of Rext were obtained with a resistor substitution box (RS 500, 1% accuracy, Elenco electronics).Measurements of COD removal rate and coulombic efficiency from three replicates.

Figure 4. Effect of the external resistance on current, voltage, andpower output. The reactor was continuously fed with wastewatercontaining 1000 ppm of glucose for a total inlet COD concentrationof 1180 ( 70 ppm and a loading rate of 0.55 kg COD m-3 d-1.Anode material: graphite discs.

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is fundamental to optimising the overall system and has beenthe subject of several reviews.20,21

In this study, the bioelectrocatalysis was achieved by theinteraction of a mixed community of bacteria, usually re-ferred to as the “electrochemically active bacterial consor-tium”. The bacterial community composition of the biofilmformed on the graphite discs was investigated (Figure 5). Inorder to estimate the influence of the position of the graphitediscs in the reactor on the consortium composition, weanalyzed the biofilm formed on discs placed at differentheight on the titanium rod (Figure 5).

DGGEanalysis of 16S rRNAgene fragments showed onlysmall differences in biofilm community composition betweengraphite discs at different locations in the reactor. Themicrobial community on the anodes was distinct from thecommunity in the influent wastewater and, in particular, twobands were predominant.

Comparative analysis of partial 16S rRNAgene sequencesof from themost intense bands in theDGGEprofiles showedhigh similarity with the 16S rRNA from Chlorobium spp.(95% identity over 154 bp) andMesorhizobium species (99%identity over 136 bp). Chlorobium spp. are anoxygenicphotosyntheticbacteriaand itspredominancecouldbeexplainedby the fact that the reactor was open to the light.

Chlorobium spp. normally grow in environments rich inhydrogen sulphide, which they use as electron donor togenerate reducing power for biosynthesis and produceelemental sulfur as a product of sulfide oxidation. Inaddition, they can photo-oxidize hydrogen as well as othersulfur compounds such as sulphide, polysulfide, and thio-sulfate.22,23 Fermentation of organic matter and sulfatereduction are potential sources of methane in the MFC,however the sulfate concentrations were approximately0.5 mM and were unlikely to sustain high levels of sulfide

generation. Fermentation of organic matter may thereforebe the most likely source of sulfide in the MFC system.Hydrogen produced as an intermediate in organic matterfermentation may also have supported the growth of Chlor-obium spp.

Mesorhizobium spp. are Alphaproteobacteria and includeorganisms that enter into symbiotic relationships with plantswhere the bacteria perform nitrogen fixation. The genus alsocontains nonsymbiotic organisms.

Many studies have regardedMFCs operated with specificaxenic cultures, suchasShewanella sp.,Pseudomonasaeruginosa,

Table 3. Effect of the Loading Rate on the SCMFC Performancea

influentCOD loading rate(kg COD m-3 d-1)

current(mA)

COD removal(%)

Coulombic efficiency(%)

power density(mW m-2)

WW diluted 1:3.3 0.04 1.48 ( 0.08 45 ( 1.2 58 ( 2.2 24.7 ( 3.7WW diluted 1:2 0.05 2.01 ( 0.02 41 ( 9.2 63.4 ( 4.2 38.0 ( 1.2WW diluted 1:1.3 0.07 1.90 ( 0.04 46 ( 1.1 40 ( 1.2 39.3 ( 2.3WW 0.08 2.27 ( 0.04 48 ( 1.5 37.1 ( 1.4 57.9 ( 3.2WW þ 100 ppm of glucose 0.13 2.23 ( 0.06 49 ( 5 22.1 ( 3.2 55.4 ( 4.6WW þ 300 ppm of glucose 0.22 2.21 ( 0.06 78 ( 1.7 8.24 ( 0.2 54.9 ( 4.1WW þ 600 ppm of glucose 0.35 2.18 ( 0.09 81 ( 1.2 4.9 ( 0.0736 53.4 ( 6.4WW þ 1000 ppm of glucose 0.55 2.21 ( 0.08 80 ( 2 3.3 ( 0.38 54.9 ( 5.8

a Rext 100 Ω. Fuel: wastewater characterized by several input CODs. Power density referred to the anode surface area: 89 cm2. Anode material:graphite discs.

Table 4. Effect of the HRTa

flow rate(cm3 min-1)

COD loadingrate

(kg CODm-3 d-1)

currentoutput(mA)

CODremoval(%)

Coulombicefficiency

(%)

0.18 0.028 1.47 ( 0.05 65 ( 5.0 57 ( 5.80.35 0.055 2.01 ( 0.02 41 ( 9.2 63.4 ( 4.20.52 0.081 2.4 ( 0.1 31.2 ( 1.7 64.1 ( 3.3

aAnodematerial: graphite discs. Fuel: sewage wastewater diluted 1:2.Inlet COD: 120 ppm. Loading rate: 0.055 kg COD m-3 d-1.

Figure 5. DGGE analysis of 16S rRNA gene fragments from themicrobial communities in the SCMFC. Anode material: graphitediscs. 1: influent wastewater; 2: biofilm from disk 2; 3: biofilm fromdisk 4; 4: biofilm from disk 6; 5: biomass from the 1:1 ethanol/sodiumphosphate buffer solution used to store the anode; 6: biofilmfrom disk 8; 7: biofilm from disk 10. The bands which were cut outand sequenced are indicated with the arrows. A: similarity withChlorobium sp, B: similarity with Mesorhizobium sp.

(20) Schroder, U. Phys. Chem. Chem. Phys. 2007, 9, 2619–2629.(21) Logan, B. E.; Regan, J.M.Trends Biotechnol. 2006, 14 (12), 512–

518.(22) Cork, D.; Mathers, J.; Maka, A.; Srnak, A. Appl. Environ.

Microbiol. 1985, 49 (2), 269–272.(23) Eisen, J. A.; Nelson, K. E.; Paulsen, I. T.; Heldelberg, J. F.; Wu,

M.; Dodson, R. J.; Fraser, C. M. Proc. Natl. Acad. Sci. U.S.A. 2002, 99(14), 9509–9514.

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Geobacter sp., Rhodoferax ferrireducens, Bacillus lichenifor-mis and Bacillus thermoglucosidasius.24-28 However, MFCsoperated with mixed culture have previously demonstratedhigh stability in the face of perturbations, larger substrateversatility, and also higher power output when compared toMFCs using pure cultures.4,29

Chlorobium andMesorhizobium species have not previouslybeen identified as electro-active bacteria. However, on thebasis of the DGGE profiles they constitute a large proportionof the anodic biomass. Given the high Coulombic efficienciesobserved (64%, obtained shortly before the anodes weresampled) it would seem likely that one or both of theseorganisms have some role to play in anodic electron transfer.

Stability.Figure 6 reports the current output generated bythe SCMFC fed with a COD loading rate of 0.083 kg CODd-1 m-3 over a period of 5 months, during which the presentstudy was conducted. As shown, during the first threemonths of operation the current output was very stable, withminimum variance (less than 0.04%). From the fourthmonth a decrease in current occurred, with a maximumreduction in the output current, after the fifth month, ofapproximately 22% with respect to the initial value.

A gradual decrease in the performance of anMFCoperatingwithwastewater in continuousmodewas reportedpreviously.30

This may result from clogging of the electrode and/or the PEMmembrane. The reduced performance of the SCMFC couldalso be due to bacterial growth observed at the cathode side ofthe MFC from the fourth month of operation reducing masstransfer of oxygen to the cathode surface.

Effect of Anode Surface Area. In order to test the effect ofanode surface area on the performance of the SCMFC, analternative anode configuration was considered, whichwas obtained by using graphite granules. The gran-ules increased the theoretical anode surface, from 89 to1496 cm2

As shown in Figure 7, enrichment of the anode withelectro-active bacteria required a much longer period thanfor the graphite discs. The lag characterized by minimalcurrent lasted 18 days. An exponential increase of the outputcurrent followed and finally, after 20 days, the current outputstabilized at 1.91 ( 0.029 mA, with a power density of0.243 mW m-2 (130 mW m-3) and a Coulombic efficiencyof 2.7 ( 1.12. The internal resistance was 10 Ω.

The performances of the SCMFC with graphite granuleswas lower than that with the disk anode. After enrichment,the output current was 62% lower with respect to graphitediscs, and the power was 86% lower.

Figure 6. Long-term stability of the SCMFC. Steady state currentobtained during each month of the SCMFC operation. TheSCMFC was fed in continuous mode with sewage wastewater afterprimary clarify. Inlet COD: 180 ppm. COD loading rate: 0.083 kgm-3 d-1. External load: 100 Ω. Anode material: graphite discs.

Figure 7. Electrochemically active bacteria enrichment of theSCMFC with graphite granules as anode. The SCMFC was fed incontinuousmode until a stable current outputwas obtained, and theelectrochemically active bacteria were considered to be enriched andstabilized at the anode. Anodematerial: graphite granules. Externalresistance: 10Ω. Feeding rate: 0.35 cm3min-1. Loading rate: 0.67 kgCOD m-3d-1. Fuel: sewage wastewater containing 1000 ppm ofglucose. Inlet COD: 1180 ( 70 ppm.

Figure 8. Polarization and current density curves from the SCMFCwith a graphite granule anode. Anode material: graphite granules.Current density refers to the anode surface area: 1496 cm2 (0.15m2).Power density refers to the anodic volume: 280 cm3. The polariza-tion curves were recorded at a scan rate of 1mV s-1 and a prior opencircuit potential of over 4 h.

(24) Biffinger, J. C.; Pietron, J.; Ray, R.; Little, B.; Ringeisen, B. R.Biosens. Bioelectron. 2007, 22, 1672–1679.(25) Kim,H. J.; Park,H. S.; Hyun,M. S.; Chang, I. S.; Kim,M.;Kim,

B. H. Enzyme Microb. Technol. 2002, 30, 145–152.(26) Kim, B. H.; Kim, H. J.; Hyun, M. S.; Park, H. S. J. Mol.

Microbiol. Biotechnol. 1999, 9, 127–131.(27) Rabaey,K.; Boon,N.; Siciliano, S. D.; Verhaege,M.; Verstraete,

W. Appl. Environ. Microbiol. 2004, 70, 5373–5382.(28) Choi, Y.; Jung, E.; Park, H.; Paik, S. R.; Jung, S.; Kim, S. Bull.

Kor. Chem. Soc. 2004, 25, 813–818.(29) Rabaey, K.; Lissens, G.; Siciliano, S. D.; Verstraete, W. Biotech-

nol. Lett. 2003, 25, 1531–1535.(30) Kim,M.; Youn, S.M.; Shin, S. H.; Jang, J. G.; Han, S. H.; Hyun,

M. S.; Gadd, G. M.; Kim, H. J. J. Environ. Monitor. 2003, 5, 640–643.

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The reduced performance of the SCMFC with graphitegranules was confirmed by the polarization and powerdensity curves reported in Figure 8. The peak power densitywas, in fact, of 1.36 mWm-2 (730 mWm-3), approximately40 times lower than the case of graphite discs.

The OCV was of around 0.46 V, very similar to that withthe graphite discs. The cell voltage variation (Vcell, Figure 8)in the SCMFC with the graphite pellet anode exhibited atypical fuel cell polarization curve with a relatively rapidvoltage loss at low current density, followed by a secondphase of linear voltage drop. Consequently, the anodevoltage was characterized by a rapid increase at low currentdensity as opposed to a slow initial increase ofVan in the caseof graphite discs. The change in the cathode potential overthe current density range was similar for both MFCs usingeither graphite discs or granules. The polarization behaviorof the cathode in the cells using both anodes was effectivelythe same. The graphite granule packed bed gave currentsapproximately half those obtained with the disk stack cell.The major difference in performance was caused by thedifferent anode configurations; noting that the internalresistance with the graphite granule packed bed anode waslower than that with the disk anode.

The effect of external resistance on the performance of theSCMFC with graphite granules was investigated (Table 5).The external load was changed from 10 to 1000 Ω, and thecurrent output evaluated. With the graphite granules thehighest power of 0.17 mWwas reached at an external load of100Ω, which compares with 0.484 mW for the graphite diskcell.

The COD removal rate did not change with the externalload and therefore the Coulombic efficiency decreased withthe current, that is, higher external resistance correspondedto lower Coulombic efficiency, as observed for the graphitediscs anode.

The values of Coulombic efficiencies were in general verylow, meaning that a large amount of COD removed did notcontribute to energy production. These values were lowerthan values obtained with graphite discs at the correspond-ing external load.

The effect of the organic loading rate was also investigated(Table 6). LowerCOD loading rates led to higher Coulombicefficiency, and the highest value was 28 ( 3%. On the otherhand, COD removal was improved: at 100Ω and for a WW

dilution of 1:2, the COD removal increased by approxi-mately 42% for the anode made with graphite discs. How-ever the best removal was still reached when the system wasfedwith easily biodegradable organicmatter, that is, glucose.

Discussion

The water treatment business is fairly mature in the devel-oped world. Europe represents around one-third of theworld’s environmental market and is driven by the EuropeanUnion (EU) environmental regulations that are among thetoughest in the world. The total European municipal waterand wastewater treatment market has a compound annualgrowth rate of 4.1%per year and is estimated to reach a valueof $ 3.09 billion in 2010. It is estimated that wastewatertreatment is growing at about 7%globally per year, and everyday it requires large amountsof energy.Mostof the plants relyon biological methods for secondary treatment, and these aregenerally based on aerobic processes. Considering a standardorganic loading rate in the range of 0.07-0.22 kg BOD m-3

d-1, aerobic treatments require approximately (2-4)�103kWhm-3 every day with BOD removal efficiencies in the range of80-90%.31

As supplies of fossil fuels dwindle and concerns aboutanthropogenic contributions of carbon dioxide to the atmo-sphere intensify, there is an increasing trend toward the use ofnew sources of energy from renewable carbon-neutral sourceswith minimal negative environmental impact.

In addition to conventional anaerobic treatments, micro-bial fuel cells represent a potential, alternative, low energywastewater treatment system. Yet, to enhance their practicalapplicability in wastewater treatment, it is important to focuson implementing systems that can be easily scaled up, work incontinuous mode, and are fed with real wastewater instead ofpure compounds.

In this study we analyzed the performance of an up-flowSCMFC fed with real wastewater after primary clarification.Although platinum was still used as a catalyst at the cathode,we tested a very cost-effective polypropylene/silicate mem-brane, which is usually used as a separator in lead acidbatteries. The performance of theMFCwas analyzed in terms

Table 5. Effect of the External Resistance on the SCMFC Performance with Graphite Granules As Anodea

Rext

(Ω)steady-state voltage

(mV)steady-state current

(mA)steady-state power

(mW)COD removal rate(mg COD min-1)

Coulombic efficiency(%)

10 19.1 ( 0.3 1.91 ( 0. 029 0.036 0.35 ( 0.02 2.7 ( 1.1250 70 ( 0.99 1.5 ( 0.014 0.105 0.3 ( 0.008 1.87 ( 0.13100 132 ( 0.42 1.32 ( 0.004 0.174 0.33 ( 0.012 1.61 ( 1.4500 195 ( 1.88 0.39 ( 0.0038 0.076 0.31 ( 0.02 0.63 ( 0.081000 270 ( 1.54 0.27 ( 0.0016 0.073 0.33 ( 0.04 0.35 ( 0.009

aLoading rate: 0.55 kg CODm-3 d-1. Fuel: wastewater containing 1000 ppm of glucose. Inlet COD: 1180( 70 ppm. Anode surface area: 1496 cm2.Measurements of COD removal rate and coulombic efficiency from three replicates.

Table 6. Effect of the Loading Rate on the SCMFC Performance with Graphite Granules As Anodea

influentCOD loading rate(kg COD m-3 d-1)

current(mA)

COD removal(%)

Coulombic efficiency(%)

Power density(mW m-2)

WW diluted 1:2 0.07 1.4 ( 0.009 58 ( 2.2 28 ( 3 1.31WW 0.099 1.4 ( 0.05 61 ( 3.0 17.7 ( 0.8 1.31WW þ 600 ppm of glucose 0.43 1.34 ( 0.006 80 ( 0.9 3.06 ( 1.1 1.20WW þ 1000 ppm of glucose 0.67 1.32 ( 0.004 83 ( 3.3 1.61 ( 1.4 1.16

a Rext = 100 Ω. Fuel: wastewater (WW) characterized by several input CODs. Power density referred to the anode surface area (0.15 m2).

(31) Metcalf, L.; Eddy,H. P.,Wastewater Engineering: Treatment andReuse, fourth ed.; MacGraw-Hill Companies: New York, 2003.

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of COD removal, current and power output, and energyrecovery.

The maximal power density generated was 59 mW m-2, ifdefinedwith respect to the anode surface area, or 1.5Wm-3, ifdefined with respect to the anode chamber volume. Higherpower outputs have been previously reported for an MFC.8

However, a direct comparison of the performance with pre-vious studies is difficult because of differences in the anodeconfiguration and reactor volume.

The system used in this work has the advantage of beingrelatively easily scaled up, although the use of platinum ascatalyst still represents a big obstacle. Recently, in ourresearch group we have successfully tested manganese oxideas cathode catalyst in the MFC.9 A next step could be there-fore to investigate the performance of the SCMFC of thisstudy with manganese oxide as catalyst. Another attractivealternative to platinum catalysis of oxygen reduction at thecathode is the use of biocathodes.13,14

In the treatment of wastewater, the COD loading ratewas akey factor in the performance of the SCMFC: lower organicloadings gave higher Coulombic efficiency with a maximumof 63.4 ( 4.2% when the system was fed with 0.055 kg CODm-3 d-1, meaning that the electro-active biofilm was respon-sible for the majority of the COD removal.

Working at low external resistances has been shown toreduce methanogenesis,32 presumably by allowing more effi-cient oxidation of the organic carbon by the electrogenicbacteria, allowing them to out compete methanogens. How-ever, it was also demonstrated that, even at low externalresistances, at higher organic loading rates, the Coulombicefficiency was reduced; with organic carbon oxidation beingchannelled through methanogenesis possibly being responsi-ble, aside from increased potential losses.19

High COD loadings, corresponding to highly saturatedconditions with respect to anode surface area may lead tocompetition among bacteria involved in the electricity pro-duction and other types of bacteria, thus leading to greateramount of COD removal not related to current generationand, consequently, to low Coulombic efficiencies.

When the fuel concentration decreased, the conditionsbecamemore restrictive for the other bacteria and the electro-chemically active bacteria were at an advantage. The hypoth-esis of saturation conditions for high loading rates wassupported by the fact that the current output did not decreasewith the loading rate until a value of 0.08 kgCODm-3 d-1, orless, was used. Switching from pure wastewater to wastewaterdiluted 1.3 times caused a current decease of only 20%.

Coulombic efficiencies as high as 89% have been reachedwithpure compounds suchasglucose;29 but 63%is thehighestefficiency reported so far for MFCs fed real wastewater.8

On the other hand, the percent COD removal was very lowif compared, for example, with treatment performed withUASBreactors.Bydecreasing the flow rate to 0.18 cm3min-1,the percent COD removal increased by 60%. This raterepresents the best compromise between treatment efficiencyand energy recovery (57%Coulombic efficiency), but impliesa HRT of approximately 102 h. In contrast, with UASBreactors operating at loading rates one thousand-fold higher(12-20 kg COD m-3 d-1 compared to 0.028 kg COD m-3

d-1),HRTsas lowas 4-8 h can be used.31TheseHRTs canbefurther decreased when less than 90% COD removal and

higher-effluent TSS concentration are acceptable. This higherloading rate in the UASB reactor can only be applicable forhigh strength wastewater. For wastewater having COD lessthan 1000 mg/L, the loading generally used for this design isless than 3 kg COD/(m3 d) due to hydraulic loading limita-tions.

The utilization of an alternative anode configuration basedon graphite granules led to a better treatment of the waste-water but did not improve the performance of the SCMFCwith regard to the current and power output. These para-meters decreased, even though the surface area of the anodewas markedly increased. The optimal external resistance was100 Ω for both anode configurations, and the highest Cou-lombic efficiency was again found at low COD loading rates.Nevertheless, the power outputwas approximately three timeslower with the graphite granule anode compared to thegraphite disk anode (0.17 mW vs 0.484 mW), and the max-imum Coulombic efficiency was only around 28%.

One of the reasons for the difference in performance of theMFC with graphite granules compared with that for graphitediscswas due to the variation in current distribution in the cell.It is known that in porous electrode structures, higher currentsare experienced at the surface nearer to the cathode and thecurrent densities fall as the distance increases from the cath-ode. Eventually, with relatively thick beds the current near tothe current collector canbe lowandnear zero.Thus, the actualeffective area of the anode is not the actual surface areabecause of this nonuniform current distribution. The effectivearea that is used will tend to decrease as higher currents areimposedandwill fall with adecrease in fluid ionic conductivityand with thicker granular beds. In comparison, the variationin the current density distribution on the disk anode surfaceswould be smaller and thus most of the available surface areawould be used.33 Thus, for granular porous electrodes orsimilar geometries there is in fact an “optimum” thicknessbeyond which little improvement in power performance canbe achieved. Modeling current distribution in anodes withdifferent configurations will be vital for the design of effectiveMFCs. Overall, the results obtained here may be due to acombination of the anode characteristics, bacterial activityand growth, and the effective surface area of the anodeexposed to the feed.On one hand, the granules lead to a bettercontact between the biofilm and the bulk solution resulting inhigher COD removal. On the other hand, the shape ofgranules and bed porosity allow only a small fraction of theirtotal surface to be connected, resulting in lower current out-put.

Our results demonstrated that even if graphite granulesincrease the surface area for anodic biofilm formation, theymay not be suitable for implementation of the SCMFC in realwastewater treatment.

Onaccount of the highCODremoval reachedwhen glucosewas added to the influent, pretreatment of wastewater prior toMFC treatment could be considered, in order to increase theconcentration of easily biodegradable organic compounds.However, the total energy recovery and treatment efficiencyshould be able to justify the extra expenses related to thepretreatment.

An interesting option would be to consider MFC andanaerobic digestion as two complementary technologies andintegrate them in a treatment plant, as previously suggested.11

(32) Rabaey,K.; Boon,N.; Siciliano, S. D.; Verhaege,M.; Verstraete,W. Appl. Environ. Microbiol. 2004, 70, 5373–5382.

(33) Scott, K. Electrochemical Reaction Engineering; ElsevierAcademic Press: London, 1991.

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The plant would include a prior anaerobic digestion to treathigh COD effluent and a secondary treatment based onMFCto remove the residual organic carbon.

Stacked microbial fuel cells could represent a good optionto increase the anodic volume. It was recently shown thatstacked configuration can lead to high current and poweroutput.34

Conclusions

This study investigate the performance of a SCMFC, withair cathodes surrounding a series of graphite discs anodes. Theair cathode was supported on a low-cost polypropylene/silicacompositemembrane. The systemwas operated in continuousmode and fed with wastewater from the primary clarifier of awastewater treatment plant. In the treatment of the waste-water, theCOD loading ratewas akey factor in theperformance

of the SCMFC: lower organic loadings gave higher Coulom-bic efficiency with a maximum of 63.4 ( 4.2% when thesystemwas fed with 0.055 kgCODm-3 d-1, meaning that theelectro-active biofilm was responsible for the majority of theCOD removal.

When an alternative anode configuration made of graphitegranules was analyzed, a better treatment of the wastewaterwas reached, but the electro-chemical performance of theSCMFC decreased with a power output approximately threetimes lower compared to the graphite disk anode (0.17 mWvs0.484mW), and themaximumCoulombic efficiency was onlyof 28% .

Acknowledgment. This work was supported by the EuropeanUnion for Transfer of Knowledge award on biological fuel cells(contract MTKD-CT-2004-517215) and Northumbria Water.We thank Northumbria Water for providing anaerobic sludgeand wastewater from the primary clarifier from CramlingtonWWTP. We thank Entek for supply of the RhinoHide mem-brane.

(34) Aelterman, P.; Rabaey, K.; Pham, T. H.; Boon, N.; Verstraete,W. Environ. Sci. Technol. 2006, 40 (10), 3388–3394.