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ARTICLE

Energy From Algae Using Microbial Fuel Cells

Sharon B. Velasquez-Orta,1 Tom P. Curtis,1 Bruce E. Logan2

1

School of Civil Engineering and Geosciences, Newcastle University,Newcastle upon Tyne NE17RU, United Kingdom; telephone: 44-191-222-6415;

fax: 44-191-222-6502; e-mail: [email protected] of Civil and Environmental Engineering, The Pennsylvania State University,

University Park, Pennsylvania 16802

Received 3 December 2008; revision received 19 February 2009; accepted 27 March 2009

Published online 3 April 2009 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/bit.22346

ABSTRACT: Bioelectricity production from a phytoplankton,Chlorella vulgaris, and a macrophyte, Ulva lactuca wasexamined in single chamber microbial fuel cells (MFCs).MFCs were fed with the two algae (as powders), obtainingdifferences in energy recovery, degradation efficiency, andpower densities.C. vulgarisproduced more energy generationper substrate mass (2.5 kWh/kg), butU. lactucawas degradedmore completely over a batch cycle (73 1% COD).Maximum power densities obtained using either single cycleor multiple cycle methods were 0.98 W/m2 (277 W/m3) usingC. vulgaris, and 0.76 W/m2 (215 W/m3) using U. lactuca.Polarization curves obtained using a common method oflinear sweep voltammetry (LSV) overestimated maximumpower densities at a scan rate of 1 mV/s. At 0.1 mV/s,however, the LSV polarization data was in better agreementwith single- and multiple-cycle polarization curves. Thefingerprints of microbial communities developed in reactors

had only 11% similarity to inocula and clustered according tothe type of bioprocess used. These results demonstrate thatalgae can in principle, be used as a renewable source ofelectricity production in MFCs.

Biotechnol. Bioeng. 2009;103: 10681076.

2009 Wiley Periodicals, Inc.

KEYWORDS: microbial fuel cell; algae; bioenergy; substratecomposition; polarization

Introduction

The concern about global warming effects and fossil fuelcosts has encouraged the search for alternative sources ofenergy (Brecha, 2008). Biomass products tested for energygeneration include a wide range of growth plants, crops and

wastes (Deublein and Steinhauser, 2008). In this context,algae are seen as an alternative to land based alternatives.The cultivation of algae has several advantages overterrestrial plants; they require less space (1/7th less surfacearea), have higher growth rates, and do not compete withfood production (Hartman, 2008). Algae are divided intotwo groups: phytoplankton (microalgae), or macrophytes(macroalgae). Microalgae are unicellular green plants richin chlorophyll that lack lignin or cellulose, and containproteins, carbohydrates and lipids in strain-specific propor-tions (Schenk et al., 2008). They are abundant in oceans,suitable for cultivation in rivers, and serve as the primarysource of carbohydrates and proteins for aquatic organisms.Macroalgae are more resistant to predators and environ-mental conditions than microalgae. They are abundantin costal zones (Riegman et al., 1993), lack lignin, and

largely consist of polysaccharides (alginate, laminaran, andmannitol) and unsaturated fatty acids which are easilyhydrolyzed, and low concentrations of cellulose (Vergara-Fernandez et al., 2008). Both macrophytes and phyto-plankton are suitable for cultivation using river water, seawater, and some wastewaters (de-Bashan et al., 2004; Walkeret al., 2005).

The generation of energy products from microalgae andmacroalgae has been examined by a number of workers.Microalgae have been tested as raw material for theproduction of bio-oil (Li et al., 2007), methane (Goluekeand Oswald, 1959; Minowa and Sawayama, 1999), methanol

(Hirano et al., 1998) and hydrogen (Kim et al., 2006; Turneret al., 2008). Macroalgae have primarily been used toproduce methane (Hansson, 1983; Morand and Briand,1999; Troiano et al., 1976). A disadvantage of all thesetechnologies is that the fuel produced must be stored,transported and further processed to produce electricity.

Microbial fuel cells (MFCs) offer an alternative way toobtain electricity from the hydrolysis and fermentation ofalgae in only one process unit. MFCs consist of an anode andcathode connected by a load (usually a resistor in laboratorystudies). The anode contains mixed or pure cultures of

Correspondence to: S.B. Velasquez-Orta

Contract grant sponsor: Consejo Nacional de Ciencia y Technologia (CONACyT)

Contract grant number: 196298

Contract grant sponsor: National Science Foundation

Contract grant number: CBET-0730359

Contract grant sponsor: King Abdullah University of Science and Technology (KAUST)

Global Research Partnership

Contract grant number: KUS-I1-003013

1068 Biotechnology and Bioengineering, Vol. 103, No. 6, August 15, 2009 2009 Wiley Periodicals, Inc.


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microorganisms that are used to catalyze the decompositionof the organic matter into electrons and protons. Power isproduced through the reduction of oxygen or anotherchemical at the cathode. A metal is usually used to catalyzeoxygen reduction, although it has recently been shown thatmicroorganisms can be used for this purpose as well (He andAngenent, 2006).

In this study, we evaluated the performance of MFCsusing two different types of algae as substrates. Chlorella

vulgaris (a microalgae) andUlva lactuca(a macroalgae) wereselected because they have been widely tested in othertechnologies for energy generation. Moreover, they havevery different organic matter composition, with C. vulgariscontaining more than 50% protein (Becker, 2007) andU. lactucahaving around 60% carbohydrates (Ventura andCastanon, 1998). The substrate degradation and microbialcomposition in MFCs and in anaerobic reactors werecompared. To better understand the degradation process,we monitored the production of by-products (biogas andvolatile fatty acids) using U. lactuca. To estimate themaximum power obtainable, we used different methods toobtain polarization and power density curves. We foundthat potentiostat methods usually employed for hydrogenfuel cells can overestimate maximum power production byMFCs with algae at scan rates often used in MFC tests.

Methods

Reactors Construction

Twelve reactors were used, each having a liquid volume of25 mL. Four MFCs were operated at closed circuit (CC)using a circuit of titanium wire containing a resistor of

1,000 V (except as noted), four were operated in opencircuit (OC) (no connection), and four were constructed tobe completely anaerobic reactors (ARs) by sealing thereactors with an end plate. MFCs and ARs had graphite fiberbrush anodes 3.0 cm in outer diameter and 3.0 cm long(PANEX33 160K, ZOLTEK) (Logan et al., 2007) that weretreated using a high-temperature ammonia gas process(Cheng and Logan, 2007). MFCs had air-cathodes that wereprepared according to the procedures of Cheng et al.(2006), with a platinum (Pt) catalyst (0.5 mg/m2 Pt) andfour diffusion layers. All materials were initially sterilized(using UV light for 2 h or autoclaved at 1218C for 15 min);although all tests were run using mixed cultures under non-sterile conditions.

Algae Characteristics and Medium Preparation

U. lactuca was purchased as a powder with homogenousparticles of 90200 mm (SigmaAldrich, St. Louis, MO). Itconsisted of 317 45 mg/g-substrate of total carbohydrates,65 1 mg/g-substrate of total protein and had a COD of833 mg/g-substrate. C. vulgaris was also obtained in a

powder form (Federal Laboratory Corporation, New York,NY) with particle sizes in the same range as those forU. lactuca. It consisted of 157 46 mg/g-substrate of totalcarbohydrates, 428 15 mg/g-substrate of total protein andhad a COD of 1587 mg/g-substrate. Both materials werecertified to contain the same composition as the naturalsources. Pre-treatment of theU. Lactucaconsisted of dryingin a hot-drum (losing 93% of total water) followed bygrinding in stainless steel and tungsten carbide mills.

C. Vulgariswas pre-treated using spray drying. Samples werestored in a cool (48C) dry environment. No chemicaltreatment processes were used. The use of algae in a powderform ensured MFC efficiencies could be directly comparedbased on substrate composition, and not for example algaecell size.

Algae powders were added to in a medium containing(g/L): NH4Cl, 0.31; KCl, 0.13; NaH2PO4H2O, 2.45 andNa2HPO47H2O, 4.57. The conductivity of the medium was4.9 mS/cm, and it had a pH of 6.8. No additional minerals orvitamins were added to the medium.

Reactors Inoculation and Operation

Reactors were inoculated using primary clarifier overflowfrom the Pennsylvania State University wastewater treat-ment plant in State College. All reactors were operated infed-batch mode at a fixed temperature of 308C. Voltage wasmonitored across the resistor every 20 min. Data wereobtained when MFCs produced a repeatable cycle of currentgeneration. The solution was replaced when the voltage wasreduced to


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was fitted on the top of selected reactors to create aheadspace for gas accumulation. Gases were sampled using agastight syringe (250 mL, Hamilton Samplelock Syringe).Bulk solution (0.5 mL) was obtained using a sterile syringe,filtered using glass fiber filters (Whatman, GF/C, 4.7 cm ofdiameter, 1.2mm of pore size), and stored at 208C. Onceexperiments were completed, samples were obtained tocharacterize the microbial community as described below.

Chemical Analyses

Total and soluble protein, carbohydrate and chemicaloxygen demand (COD) concentrations were monitored forsoluble and total fractions of the bulk liquid according toStandard Methods (American Public Health Association(APHA), 1995). For the soluble fraction, samples were pre-filtered using glass fiber filters (Whatman, GF/C, 4.7 cm ofdiameter, 1.2 mm of pore size). Total and soluble CODanalyses were carried out using method 5220 (Hach CODsystem, Hach Company, Loveland, Colorado). Soluble andtotal protein concentrations were quantified using the

bicinchoninic acid protein assay kit (SigmaAldrich). Acalibration curve was prepared using standard solutionsranging from 0.1 to 1.0 g/L of bovine serum albumin.Soluble and total carbohydrate concentrations weremeasured using a colorimetric method based on ananthrone reagent (Gerhardt et al., 1994). For this measure-ment, 1 mL of sample was transfer to a COD tube and 2 mLof a chilled H2SO4 solution (75%, v/v) were added. Aftervortexing (30 s), 4 mL of a chilled fresh anthrone solution (2g/L, 75% (v/v) H2SO4) was added and the sample vortexedagain. COD tubes were placed in a heating-block at 1008Cfor 15 min, cooled to room temperature, and analyzed

using disposable cuvettes at 578 nm (UV-1601, ShimadzuCorporation). Every measurement included a calibrationcurve using glucose at concentrations of 503,000 mg/L(R2 0.99).

Volatile fatty acid (VFA) concentrations (acetate, butyrateand propionate) were measured in triplicate using agas chromatograph (Agilent 6890N) and a 30 m0.32 mm 0.5 mm fused-silica capillary column. BeforeGC analysis, 50 mL 50% formic acid (v/v in water) wereadded to 1 mL samples. Gases (carbon dioxide, methane,and nitrogen) were analyzed using a gas chromatograph(helium carrier gas; model 310, SRI Instruments). Sincenitrogen served as a dilution gas, it was removed from the

calculations in order to determine the CO2 and CH4headspace fractions.

Microbial Analyses

Biofilms from the anodes were removed from MFCs bycutting off anode fibers in a sterile laminar flow cabinet.Fibers were immediately placed in 15 mL centrifugetubes containing sterile PBS solution (130 mM NaCl,10 mM Na2HPO4, pH 7.2). Centrifuge tubes were vortexed

for 30 s and fibers removed. Then, tubes were centrifuged at5,000g for 7 min. Total DNA was extracted using thePowersoil DNA isolation kit (Mobio Laboratories, Carlsbad,CA) and stored at 208C. 16S rRNA gene fragments wereamplified from DNA samples using 16S rRNA genefragments amplified using primer 3 50-CCT ACG GGAGGC AGC AG-30 with a GC-clamp, and primer 2 50-ATTACC GCG GCT GCT GG-30 (Muyzer et al., 1993), andanalyzed by denaturing gradient gel electrophoresis (DGGE)

with a denaturing gradient ranging from 30% to 55% (100%denaturant is 7 M urea plus 40% (v/v) formamide in 1TAE; 40 mM Tris-acetate, 1 mM EDTA, pH 8). DGGE gelswere processed using the Bionumerics software package(version 3.5, Applied Maths, Austin, TX) to determine theposition and intensity of all bands in all DGGE profiles inrelation to markers run alongside samples in the gel(Verseveld and Roling, 2008) and to analyze bands presentin DGGE profiles using Dice cluster analysis.

Calculations

MFC potentials were obtained using a data acquisitionsystem (2700, Keithly, Cleveland, OH). Potentials wereconverted to current using Ohms law,E IR, whereEis thevoltage, I is the current, and R is the resistance. Powerdensities ( Pd) were obtained using the equation: Pd IV/AAn, where AAn is the cathode surface area (7.07 cm

2).Columbic efficiencies (Ec) were calculated using (Loganet al., 2006):Ec M

Rtf

0 Idt=FzvAnDCOD, where:M 32, isthe molecular weight of oxygen, z 4 the number ofelectrons transferred per mole of oxygen, F 96485.4Amol1 Faradays constant, vAn the volume of liquid inthe anode compartment, and DCOD the change in CODover the batch period of time. Statistical analysis of data wasperformed using Minitab1 15.1.0.0 for Windows usinganalysis of variance (ANOVA). The regression analysisrelating maximum power densities algae concentrationswas assessed using Sigmaplot1 9.0 for Windows. Thefirst order exponential rise-to-maximum model gavethe best fit (R2 0.95 0.05) according to the equation:Pd Pmax1 e

kCS, where Pmaxis the maximum powerdensity,kis the growth rate constant, andCsis the substrateconcentration.

Results

MFCs Performance Using Different AlgaeConcentrations

A repeatable cycle of current generation was obtained byC. vulgaris and U. lactuca(510 mg COD/L) after five batchcycles (6 days). The maximum current for each batch cyclewas obtained within 0.76.7 h depending on the substrateconcentration. Batch cycles for MFCs using differentconcentrations ofC. vulgarisrequired 25 days, while those

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forU. lactucalasted 27 days (data not shown). When MFCswere fed algae at a concentration of 2,500 mg COD/L, asteady state current generation was rapidly achieved (Fig. 1).MFCs produced a steady state current between 0.52 and0.55 mA for a period of 3 days in MFCs using U. lactuca, and1.5 days using C. vulgaris. Following this steady output,current generation gradually decreased from 0.5 to 0.05 mAover a period of 4 days.

Maximum power densities increased with substrate

concentration until reaching a plateau (Fig. 2). At highorganic matter concentrations the maximum powerdensities ( Pmax) obtained using the different algae were inthe same range, with 440 mW 60 for MFCs usingC. vulgaris and 450 mW 50 for MFCs using U. lactuca.MFCs fed C. vulgaris had a higher power growth rateconstant (k 2:6 103 0:0012) and hence achievedPmax at lower organic matter concentrations than MFCsusingU. lactuca(k 0:9 103 0:0003).

Coulombic efficiencies peaked at relatively low algaeconcentrations (from 100 mg COD/L and 500 mg COD/L)with a maximum of 28% for C. vulgaris and 23% forU. lactuca(Fig. 3). At substrate concentrations higher than600 mg/L, coulombic efficiencies decreased and reached aplateau at 10%. COD removal increased to a maximum of85 5% at a loading of 1,000 mg COD/L.

Effect of Polarization Method on the CalculatedMaximum Power

There were no significant differences in the maximumpower produced between the single cycle and multiple cyclemethods ( P 0.45, ANOVA; Fig. 4). The average maximum

power density produced using these two methods was of0.98 0.39 W/m2 (277 W/m3) for MFCs usingC. vulgarisin

a current density between 0.20 and 0.25 mA/ cm2, and0.76 0.15 W/m2 (215 W/m3) for MFCs usingU. lactucaat

a current density of 0.20 mA/cm2 (Fig. 4). The maximumpower density obtained using LSV was dependant on thescan rate selected (Fig. 5). A scan rate of 1 mV/s produceda very high maximum power density of 1.6 0.3 W/m2

(453 W/m3). At a scan rate of 0.1 mV/s, the power densitywas much lower (0.9 0.2 W/m2, 255 W/m3; P 0.007,ANOVA). Based on a comparison of these LSV scan resultswith other methods (single and multiple cycles), it appearsthat a scan rate of 0.1 mV/s is a better choice than 1 mV/s forobtaining polarization curves (Fig. 4). The slower scanrates of 0.1 mV/s required 34 h to complete a polarizationcurve. It is likely that a scan rate of 1 mV/s is too fast (30 min

for a cycle) for the bacteria to respond to the changes involtage.

Figure 1. MFCs current generation with 2,500 mg COD/L ofChlorella vulgarisorUlva lactuca. Reactors were operated using an external resistance of 1,000 V. [Color

figure can be seen in the online version of this article, available at www.interscience.

wiley.com.]

Figure 2. Maximum currents with Chlorella vulgaris and Ulva lactuca as afunction of COD concentration.

Figure 3. Effect of substrate concentration in CE and COD removal efficiency.Filled symbols are for CE and empty symbols are for COD removal efficiency

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Organic Matter Consumption

Removal efficiencies were compared among three types ofreactors: operating MFCs or those with a closed circuit(CC); MFCs kept in an open circuit (OC); and anaerobicreactors (ARs) for one operational batch (Fig. 6). COD inC. vulgaris was better removed in MFCs at CC (1,000 V)than in other reactors ( P 0.00, ANOVA). Differences inCOD removals among reactors fed U. lactuca were notsignificant ( P 0.60, ANOVA). COD removals in reactorsusing C. vulgaris were two times higher in the operatingMFCs (60 6%) than in the AR (28%), while CODremovals in reactors usingU. lactucawere in the same rangefor all reactors (approximately 73 1%). Carbohydrateremoval using both types of algae was highest in operatingMFCs.

Protein removal from C. vulgaris was highest in theoperating MFCs (51 16%) and lowest in ARs (24 3%).Protein removal fromU. lactucawas highest in open circuitMFCs (62 7%) followed by the closed circuit MFCs(41 1%) and ARs (15 6%).

Concentrations of the substrate on the basis of soluble and

particulate matter were analyzed for the operating MFCs atthe start and at the end of the experiments (Fig. 7). Initially,organic matter from both algae contained more particulateorganic matter than soluble organic matter. C. vulgaris

contained two times more protein than carbohydrates whileU. lactuca contained five times more carbohydrates thanprotein. Soluble protein was the highest fraction consumed(83%) in C. vulgaris. In contrast, particulate carbohydratewas the highest fraction consumed (90%) in U. lactuca.Total COD consumption was higher for MFCs usingU. lactuca than for MFCs using C. vulgaris. All MFCsremoved more than 50% of the soluble and particulate

COD.

Figure 4. MFCs power density outputs from algae using differentpolarization methods. Reactors were fed a concentration of 2,500 mgCOD/L of

C. vulgaris (a) and of U. lactuca (b). Measurements were done using different

resistances in duplicate reactors. Dotted lines are cell voltages and plain lines are

for power densities. [Color figure can be seen in the online version of this article,

available at www.interscience.wiley.com.]

Figure 5. Comparison of short single cycle polarizations in MFCs using2,500 mg COD/L Chlorella vulgarisat different scan rates. [Color figure can be seen

in the online version of this article, available at www.interscience.wiley.com.]

Figure 6. Comparison of organic matter removal in MFCs at closed circuit(CC), MFCs at open circuit (OCV) and anaerobic reactors (AR) on the basis of protein,

carbohydrates and COD using 2,500 mg/L of algae substrate. Measurements were

done in duplicates, if the difference between replicates was low errors bar do not

appear.



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By-Products in MFCs and Anaerobic ReactorsUsing U. lactuca

Differences in headspace composition and volatile fattyacids production were assessed over one batch cycle forMFCs and ARs fed 2,500 mg/L U. lactuca. The headspacecomposition changed over a period of 7 days (96 h; Fig. 8a).Methane concentrations were higher for ARs than for MFCs.ARs reached a maximum methane concentration of 30%between the 3rd and 5th day while methane concentrationsfor MFCs remained below 5%. Carbon dioxide concentra-tions in MFCs reactors peaked on the first day (14%) andgradually decreased until the end of the experiment. For ARscarbon dioxide concentrations remained between 5% and10%. Acetate concentrations rose rapidly during the first day

for all reactors, following by a gradual decrease in MFCs(Fig. 8b). ARs had sharps increases and decreases in acetateconcentrations. Propionate was detected in low quantities(


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(Aelterman et al., 2008; Chen et al., 2008; Clauwaert et al.,2008). This indicates that polarization data developed usingLSV should be confirmed by data obtained using fixedresistances over longer period of times. For example, in thesingle-cycle method an interval of 20 min is used for eachresistor, allowing time for the bacteria to adjust to this newload. At high scanning rates of 1 mV/s, the LSV approach

applies a potential change that is too rapid for some MFCs toreach sufficiently steady conditions. Further work should bedone to determine if these changes can also be observed inMFCs using simple substrates.

Better energy recovery was obtained with microalgae,C. vulgaris, than with macroalgae,U. lactuca. MFCs fed withmicroalgae produced greater power densities at a similarCOD concentration than the macroalgae. In addition,the COD per gram of C. vulgaris was higher than thatof U. lactuca on a dry weight basis. This difference ispresumably attributable to the nature of the carbon sourcein each alga; presumably C. vulgaris contained a higher

quantity of organic compounds than U. lactuca per unitmass. Apart from the proteins and carbohydrates measuredinC. vulgaris other molecules known to be present such aslipids and vitamins may have contributed to energyproduction. While Coulombic Efficiencies (CEs) obtainedwere low (1025%), they were typical for MFCs usingcomplex substrates, for example, wastewater (16%, Huangand Logan, 2008) and cellulose (23%, Rezaei et al., 2008).Presumably as we begin to better understand the factors thataffect CE, these values can be improved. CEs achieved inMFCs fedC. vulgariswere also slightly higher that those fed

U. lactuca. CEs decreased at COD concentrations higherthan 600 mg/L. This trend towards a reduction in CE withorganic loading has previously been observed, and isthought to be due to increased substrate losses over a longerbatch cycle time either through methanogenesis or due toaerobic degradation of organic matter sustained by oxygenleakage through the cathode (Feng et al., 2008). Since low

methane compositions and VFA concentrations weremeasured for MFCs using U. lactuca it appears that, inthis case, methanogenesis was not the main contributor tothe decrease in CE.

At a high organic matter load, the difference in algaecomposition produced different COD removals amongMFCs. The best COD removals were obtained in reactorsusing U. lactuca. This was correlated with a higherdegradation of carbohydrate in MFCs using U. lactucathanof protein in MFCs usingC. vulgaris. Moreover, particulatecarbohydrates in MFCs using U. lactucaor C. vulgarisweremore readily consumed than particulate proteins. Analysis

of microbial communities showed that bacterial selectionoccurred in all reactors (MFCs and ARs) as a result of thetreatments. Microbial communities clustered according tothe type of bioprocess (MFC at OC, MFC at CC and AR)used. This implies that the reactor conditions had an effectin the final microbial selection suggesting that there werepossible differences in the metabolisms occurring in eachreactor.

The utility of power generation from algae using MFCscan be evaluated in comparison to other methods of energyproduction exploiting this source of organic matter. The

Figure 9. Dice cluster analysis of bands present in biofilms. Indicates similarities between the DGGE bands obtained from three different reactor configurations and twodifferent substrates (Chlorella vulgarisand Ulva lactuca). The cluster analysis was obtained using Bionumerics software.



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energy obtained per gram of algae with an MFC is currentlylow relative to other technologies, based on assuming a 30%efficiency from heat to power with other technologies(Table I). For C. vulgaris the highest power output wasderived from oil extraction and methane production fromanaerobic digestion. MFCs and hydrogen production had alow energy recovery indicating the need for technologyimprovement. For U. lactuca the best option seems to beincineration, but the sustainability of this technology isdebatable (Minowa and Sawayama, 1999). Incinerationcould result in the production of hazardous ash andemission of partially combusted products.

Conclusions

Using microalgae and macroalgae for energy generationpresents several benefits over other biomass sources: they

can grow using different substrates such as CO2 orwastewater, have high growth rates, and require less spacefor cultivation. MFCs using either microalgae or macroalgaeproduced relatively high power densities compared tothe use of other substrates in this MFC configuration.Maximum power densities were best obtained either usingfixed resistances in the circuit for a sufficiently long time forthe system to reach steady conditions, or by LSV using aslow scan rate (0.1 mV/s). C. vulgaris gave the highestenergy generation per gram of substrate while, themacroalgae U. lactuca was degraded more efficiently inMFCs. The substrate composition and bioprocess had aneffect on the final COD removal obtained and the type ofmicrobial communities present. Carbohydrates contained inU. lactucawere more completely degraded than proteins inC. vulgaris. Bioelectricity production using algae in MFCs isuseful as a low temperature method of power generation,but it needs to be further improved in order to make itcompetitive with alternative energy technologies.

The authors greatly appreciate the help of David W. Jones during

laboratory measurements.

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Table I. Comparison of total energy production using different technologies with Chlorella vulgaris and Ulva lactuca.

Technology Chlorella vulgaris(kW-h/kg-DW) References Ulva lactuca (kWh/kg DW) References

Incineration 9.3a Minowa and Sawayama (1999) 13.5f Grahame (1973)

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Microbial fuel cells 2.5e This study 2.0h This study

n.a, not available.a,b,c,e,f,g,h2.77 kWh/MJ, assuming 30% (for a, b, d, f, g) and 70% (for c) conversion efficiencies to convert energy from heat to power.bUsing mixed cultures of microalgae containing Chlorella vulgaris, 1 mol CH4 981 kJ.cConverted to energy assuming 1 mol of H2 237 10

3 J (Turner et al., 2008).dConverted to grams of algae assuming 0.48 gbiodiesel/g-algae.e,hCalculated from the linear regression equation obtained in the plot of joules produced over a range of algae concentrations.

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A1 2009 Velasquez Orta Etal B&B