Carbon nanotube supported MnO₂ catalysts for oxygen reduction reaction and their applications in microbial fuel cells

Three types of manganese dioxide, α-MnO(2), β-MnO(2), γ-MnO(2) were tested as alternative cathode catalysts for oxygen reduction reaction (ORR) in air-cathode microbial fuel cells (MFCs). Prepared by solution-based methods, the MnO(2) nanomaterials were comprehensively characterized, and their electrocatalytic activities in neutral electrolyte were investigated with the supporting material of carbon nanotubes (CNTs) by cyclic voltammetry (CV). The CV results showed that all MnO(2) species could catalyze ORR in neutral NaCl solution with different catalytic activities. β-MnO(2) had the highest catalytic activity due to its intrinsic structure and better interaction with CNTs. Three MnO(2) species were further used as cathode catalysts under optimized conditions in air-cathode cubic MFCs, in which mixed culture was inoculated as biocatalysts and domestic wastewater was used as the substrate in the anode chamber. It was also found that β-MnO(2) based MFC yielded the best performance with a power density of 97.8 mWm(-2) which was 64.1% that of the Pt-based MFC, and a lower internal resistance of 165 Ω. Furthermore, the COD removal efficiency of β-MnO(2) based MFC was estimated as 84.8%, higher than that of the Pt-based MFC. This study demonstrated that using β-MnO(2) on CNT support instead of Pt could potentially improve the feasibility of scaling up air-cathode MFCs for practical applications by lowering the material cost.     

Use of inexpensive semicoke and activated carbon as biocathode in microbial fuel cells

In this study, two inexpensive semicoke and activated carbon packed bed biocathode were developed for oxygen reduction in microbial fuel cells (MFCs). These two materials were compared with two commonly used biocathode materials graphite and carbon felt in terms of material characteristic, power density, biomass density and price-performance ratio. MFCs with semicoke and activated carbon biocathode produced a maximum power density of 20.1 W/m3 (normalized liquid volume in cathodic compartment) and 24.3 W/m3, respectively, compared to 14.1 and 17.1 W/m3 obtained by MFCs with graphite and carbon felt biocathode, respectively. The bacteria attached on biocathode played a major role in oxygen reduction for all the materials investigated. The material cost per Watt produced for semicoke and activated carbon biocathode is only 2.8% and 22.7% of that for graphite biocathode, respectively. These two inexpensive carbon materials, especially semicoke, are very cost-effective biocathode materials for future large scale MFCs.     

Enzymatic hydrolysis of cellulose coupled with electricity generation in a microbial fuel cell

Electricity can be directly generated by bacteria in microbial fuel cells (MFCs) from a variety of biodegradable substrates, including cellulose. Particulate materials have not been extensively examined for power generation in MFCs, but in general power densities are lower than those produced with soluble substrates under similar conditions likely as a result of slow hydrolysis rates of the particles. Cellulases are used to achieve rapid conversion of cellulose to sugar for ethanol production, but these enzymes have not been previously tested for their effectiveness in MFCs. It was not known if cellulases would remain active in an MFC in the presence of exoelectrogenic bacteria or if enzymes might hinder power production by adversely affecting the bacteria. Electricity generation from cellulose was therefore examined in two-chamber MFCs in the presence and absence of cellulases. The maximum power density with enzymes and cellulose was 100 +/- 7 mW/m(2) (0.6 +/- 0.04 W/m(3)), compared to only 12 +/- 0.6 mW/m(2) (0.06 +/- 0.003 W/m(3)) in the absence of the enzymes. This power density was comparable to that achieved in the same system using glucose (102 +/- 7 mW/m(2), 0.56 +/- 0.038 W/m(3)) suggesting that the enzyme successfully hydrolyzed cellulose and did not otherwise inhibit electricity production by the bacteria. The addition of the enzyme doubled the Coulombic efficiency (CE) to CE = 51% and increased COD removal to 73%, likely as a result of rapid hydrolysis of cellulose in the reactor and biodegradation of the enzyme. These results demonstrate that cellulases do not adversely affect exoelectrogenic bacteria that produce power in an MFC, and that the use of these enzymes can increase power densities and reactor performance.     

Integration of a microbial fuel cell with activated sludge process for energy-saving wastewater treatment: taking a sequencing batch reactor as an example

In the research and application of microbial fuel cell (MFC), how to incorporate MFCs into current wastewater infrastructure is an importance issue. Here, we report a novel strategy of integrating an MFC into a sequencing batch reactor (SBR) to test the energy production and the chemical oxygen demand (COD) removal. The membrane-less biocathode MFC is integrated with the SBR to recover energy from the aeration in the form of electricity and thus reduce the SBR operation costs. In a lab-scale integrated SBR-MFC system, the maximum power production of the MFC was 2.34 W/m(3) for one typical cycle and the current density reached up to 14 A/m(3) . As a result, the MFC contributed to the 18.7% COD consumption of the integrated system and also recovered energy from the aeration tank with a volume fraction of only 12% of the SBR. Our strategy provides a feasible and effective energy-saving and -recovering solution to upgrade the existing activated sludge processes.     

Enhanced wastewater treatment efficiency through microbially catalyzed oxidation and reduction: synergistic effect of biocathode microenvironment

Microbially catalyzed treatment of wastewater was evaluated in both the anode and cathode chambers in dual chambered microbial fuel cell (MFC) under varying biocathode microenvironment. MFC operation with aerobic biocathode showed significant increment in both TDS (cathode, 90.2±1%; anode, 39.7±0.5%) and substrate (cathode, 98.07±0.06%; anode, 96.2±0.3%) removal compared to anaerobic biocathode and abiotic cathode operations (COD, 80.25±0.3%; TDS, 30.5±1.2%). Microbially catalyzed reduction of protons and electrons at cathode will be higher during aerobic biocathode operation which leads to gradual substrate removal resulting in stable bio-potential for longer periods facilitating salts removal. Bio-electro catalytic behavior showed higher exchange current density during aerobic biocathode operation resulting in induced electrochemical oxidation which supports the enhanced treatment. Anaerobic biocathode operation depicted relatively less TDS removal (anode, 16.35%; cathode, 16.04%) in both the chambers in spite of good substrate degradation (anode, 84%; cathode, 87.39%). Both the chambers during anaerobic biocathode operation competed as electron donors resulting in negligible bio-potential development.     

Increased sustainable electricity generation in up-flow air-cathode microbial fuel cells

Sustainable electricity was generated from glucose in up-flow air-cathode microbial fuel cells (MFCs) with carbon cloth cathode and carbon granular anode. Plastic sieves rather than membrane were used to separate the anode and cathode. Based on 1g/l glucose as substrate, a maximum volumetric power density of 25+/-4 W/m(3) (89 A/m(3)) was obtained for the MFC with a sieve area of 30 cm(2) and 49+/-3 W/m(3) (215 A/m(3)) for the MFC with a sieve area of 60 cm(2). The increased power density with larger sieve area was mainly due to the decrease of internal resistance according to the electrochemistry impedance spectroscopy analysis. Increasing the sieve area from 30 cm(2) to 60 cm(2) resulted in a decrease of overall internal resistance from 41 ohm to 27.5 ohm and a decrease of ohmic resistance from 24.3 ohm to 14 ohm. While increasing operational recirculation ratio (RR) decreased internal resistance and increased power output at low substrate concentration, the effect of RR on cell performance was negligible at higher substrate concentration.     

Cattle wastes as substrates for bioelectricity production via microbial fuel cells

A new 2 l scale microbial fuel cell (MFC) configuration was developed to generate bioelectricity from particulate substrates. Voltage and power densities generated in this MFC fed with sucrose, particulate cattle manure, and manure wash-water as the substrates were evaluated in batch mode, with and without external mediators. Voltages averaged 0.5 V in open circuit, and 0.4 V under a resistive load of 470 Ω. Power densities (67 to 215 mW/m²) were comparable to previous work that had used liquid wastes as substrates. Based on the energy yield per unit mass of feedstock (~10 kJ/kg wet manure), cattle manure has limited potential to serve as a feedstock for electricity production via MFCs.     

乌梁素海富营养化湖区浮游细菌多样性及系统发育分析

水生生态系统富营养化与细菌群落之间的关系尚不明确。本文通过构建和分析16S rRNA基因片段克隆文库, 以期揭示乌梁素海富营养化水体细菌的多样性及其系统发育关系, 并探讨富营养化与细菌多样性之间的关系。利用Hae III对文库中的87个克隆子进行单酶切, 产生了23种带型, 文库覆盖度达到了73.6%, 反映出文库有较好的代表性。选择每种OTU的一个代表克隆进行测序分析, 基因序列系统发育分析结果表明, 乌梁素海中多数细菌与淡水生态系统中常见的细菌门类相同, 即α-, β-, γ-Proteobacteria, Bacteroidetes, Actinobacteria, 它们分别占总菌数的10.3%、41.4%、4.6%和6.9%, 其中β-Proteobacteria和Bacteroidetes是优势细菌类群。与典型淡水生态系统细菌群落组成不同的是, 乌梁素海中存在约10.3%的轻度嗜盐碱细菌。水体中83.9%的细菌与已培养的细菌的同源性低于97%, 其中58.9%的细菌未能鉴定到属; 其余总菌数16.1%的克隆与具有降解污染物生物活性的已知菌相近。Bacteroidetes、Firmicutes和β-Proteobacteria中的某些类群成为优势菌群可能是对乌梁素海水体富营养化的响应。     

Remarkable impact of PAHs and TPHs on the richness and diversity of bacterial species in surface soils exposed to long-term hydrocarbon pollution

Nowadays, because of substantial use of petroleum-derived fuels the number and extension of hydrocarbon polluted terrestrial ecosystems is in growth worldwide. In remediation of aforementioned sites bioremediation still tends to be an innovative, environmentally attractive technology. Although huge amount of information is available concerning the hydrocarbon degradation potential of cultivable hydrocarbonoclastic bacteria little is known about the in situ long-term effects of petroleum derived compounds on the structure of soil microbiota. Therefore, in this study our aim was to determine the long-term impact of total petroleum hydrocarbons (TPHs), volatile petroleum hydrocarbons (VPHs), total alkyl benzenes (TABs) as well as of polycyclic aromatic hydrocarbons (PAHs) on the structure of bacterial communities of four different contaminated soil samples. Our results indicated that a very high amount of TPH affected positively the diversity of hydrocarbonoclastic bacteria. This finding was supported by the occurrence of representatives of the α-, β-, γ-Proteobacteria, Actinobacteria, Flavobacteriia and Bacilli classes. High concentration of VPHs and TABs contributed to the predominance of actinobacterial isolates. In PAH impacted samples the concentration of PAHs negatively correlated with the diversity of bacterial species. Heavily PAH polluted soil samples were mainly inhabited by the representatives of the β-, γ-Proteobacteria (overwhelming dominance of Pseudomonas sp.) and Actinobacteria.     

New clusters of arsenite oxidase and unusual bacterial groups in enrichments from arsenic-contaminated soil

In the present study cultivation-dependent and molecular methods were applied in combination to investigate the arsenite-oxidizing communities in enrichment cultures from arsenic and lead smelter-impacted soils with respect to both 16S rRNA and arsenite oxidase gene diversity. Enrichments with arsenite as the only electron donor resulted in completely different communities than enrichments with yeast extract and the simultaneous presence of arsenite. The lithoautotrophic community appeared to be dominated by Ferrimicrobium-related Actinobacteria, unusual Acidobacteria, Myxobacteria, and α-Proteobacteria but the heterotrophic community comprised many Dokdonella-related γ-Proteobacteria. Gene sequences of clones encoding arsenite oxidase from the enrichment for lithoautotrophs belonged to three major clusters with sequences from non-cultivated microorganisms. So, primers used to detect arsenite oxidase genes could amplify the genes from many α-, β- and γ-Proteobacteria, but not from various strains of the other phyla present in the enrichment for lithotrophs. This was also observed for the isolates where arsenite oxidase genes from new proteobacterial isolates of the genera Burkholderia, Bosea, Alcaligenes, Bradyrhizobium and Methylobacterium could be amplified but the genes of the new Rhodococcus isolate S43 could not. The results indicate that the ability to oxidize arsenite is widespread in various unusual taxa, and molecular methods for their detection require further improvement.     

Continuous electricity production from artificial wastewater using a mediator-less microbial fuel cell

A microbial fuel cell (MFC) was optimized in terms of MFC design factors and operational parameters for continuous electricity production using artificial wastewater (AW). The performance of MFC was analyzed through the polarization curve method under different conditions using a mediator-less MFC. The highest power density of 0.56 W/m2 was achieved with AW of 300 mg/l fed at the rate of 0.53 ml/min at 35 degrees C. The power per unit cell working volume was 102 mW/l, which was over 60 times higher than those reported in the previous mediator-less MFCs which did not use a cathode or an anode mediator. The power could be stably generated over 2 years.     

Mineralization of pentachlorophenol with enhanced degradation and power generation from air cathode microbial fuel cells

The combined anaerobic-aerobic conditions in air-cathode single-chamber MFCs were used to completely mineralize pentachlorophenol (PCP; 5 mg/L), in the presence of acetate or glucose. Degradation rates of 0.140 ± 0.011 mg/L-h (acetate) and 0.117 ± 0.009 mg/L-h (glucose) were obtained with maximum power densities of 7.7 ± 1.1 W/m(3) (264 ± 39 W/m(2), acetate) and 5.1 ± 0.1 W/m(3) (175 ± 5 W/m(2), glucose). At a higher PCP concentration of 15 mg/L, PCP degradation rates increased to 0.171 ± 0.01 mg/L-h (acetate) and 0.159 ± 0.011 mg/L-h (glucose). However, power was inversely proportional to initial PCP concentration, with decreases of 0.255 W/mg PCP (acetate) and 0.184 W/mg PCP (glucose). High pH (9.0, acetate; 8.0, glucose) was beneficial to exoelectrogenic activities and power generation, whereas an acidic pH = 5.0 decreased power but increased PCP degradation rates (0.195 ± 0.002 mg/L-h, acetate; 0.173 ± 0.005 mg/L-h, glucose). Increasing temperature from 22 to 35°C enhanced power production by 37% (glucose) to 70% (acetate), and PCP degradation rates (0.188 ± 0.01 mg/L-h, acetate; 0.172 ± 0.009 mg/L-h, glucose). Dominant exoelectrogens of Pseudomonas (acetate) and Klebsiella (glucose) were identified in the biofilms. These results demonstrate that PCP degradation using air-cathode single-chamber MFCs may be a promising process for remediation of water contaminated with PCP as well as for power generation.     

Simultaneous carbon and nitrogen removal using an oxic/anoxic-biocathode microbial fuel cells coupled system

A coupled microbial fuel cell (MFC) system comprising of an oxic-biocathode MFC (O-MFC) and an anoxic-biocathode MFC (A-MFC) was implemented for simultaneous removal of carbon and nitrogen from a synthetic wastewater. The chemical oxygen demand (COD) of the influent was mainly reduced at the anodes of the two MFCs; ammonium was oxidized to nitrate in the O-MFC’s cathode, and nitrate was electrochemically denitrified in the A-MFC’s cathode. The coupled MFC system reached power densities of 14 W/m³ net cathodic compartment (NCC) and 7.2 W/m³ NCC for the O-MFC and the A-MFC, respectively. In addition, the MFC system obtained a maximum COD, NH₄⁺-N and TN removal rate of 98.8%, 97.4% and 97.3%, respectively, at an A-MFC external resistance of 5 Ω, a recirculation ratio (recirculated flow to total influent flow) of 2:1, and an influent flow ratio (O-MFC anode flow to A-MFC anode flow) of 1:1.     

Bioelectricity production from wastewater treatment in dual chambered microbial fuel cell (MFC) using selectively enriched mixed microflora: Effect of catholyte

The performance of aerated and ferricyanide catholytes on the bioelectricity production was evaluated in dual chambered microbial fuel cell (MFC) (mediatroless anode; graphite electrodes) employing selectively enriched H(2) producing mixed consortia as anodic inoculum. Two MFCs with aerated catholyte (MFC(AC)) and ferricyanide catholyte (MFC(FC)) were operated separately to elucidate the difference in power generation potential and carbon removal efficiency under similar operating conditions [ambient pressure; room temperature (28+/-2 degrees C); acidophilic microenvironment (pH 6)]. The experimental data demonstrated the feasibility of in situ bioelectricity generation along with wastewater treatment. Effective power generation and substrate removal efficiency was documented in the fuel cell operated with ferricyanide catholyte (586 mV; 2.37 mA; 0.559 kg COD/m(3) day) than aerated catholyte (572 mV; 1.68 mA; 0.464 kg COD/m(3) day). Maximum power yield (0.635 W/kg COD(R) and 0.440 W/kg COD(R)) and current density (222.59 mA/m(2) and 190.28 mA/m(2)) was observed at 100 Omega resistor with ferricyanide and aerated catholytes, respectively. The study documented both wastewater treatment and electricity production through direct conversion of H(2) in a single system.     

Power generation from cellulose using mixed and pure cultures of cellulose-degrading bacteria in a microbial fuel cell

Microbial fuel cells (MFCs) have been used to generate electricity from various organic compounds such as acetate, glucose, and lactate. We demonstrate here that electricity can be produced in an MFC using cellulose as the electron donor source. Tests were conducted using two-chambered MFCs, the anode medium was inoculated with mixed or pure culture of cellulose-degrading bacteria Nocardiopsis sp. KNU (S strain) or Streptomyces enissocaesilis KNU (K strain), and the catholyte in the cathode compartment was 50mM ferricyanide as catholyte. The power density for the mixed culture was 0.188mW (188mW/m(2)) at a current of 0.5mA when 1g/L cellulose was used. However, the power density decreased as the cellulose concentration in the anode compartment decreased. The columbic efficiencies (CEs) ranged from 41.5 to 33.4%, corresponding to an initial cellulose concentration of 0.1-1.0g/L. For the pure culture, cellobioase enzyme was added to increase the conversion of cellulose to simple sugars, since electricity production is very low. The power densities for S and K strain pure cultures with cellobioase were 162mW/m(2) and 145mW/m(2), respectively. Cyclic voltammetry (CV) experiments showed the presence of peaks at 380, 500, and 720mV vs. Ag/AgCl for the mixed bacterial culture, indicating its electrochemical activity without an external mediator. Furthermore, this MFC system employs a unique microbial ecology in which both the electron donor (cellulose) and the electron acceptor (carbon paper) are insoluble.     

Yellowstone Lake: high-energy geochemistry and rich bacterial diversity

Yellowstone Lake is central to the balanced functioning of the Yellowstone ecosystem, yet little is known about the microbial component of its food chain. A remotely operated vehicle provided video documentation (http://www.tbi.montana.edu/media/videos/) and allowed sampling of dilute surface zone waters and enriched lake floor hydrothermal vent fluids. Vent emissions contained substantial H(2)S, CH(4), CO(2) and H(2), although CH(4) and H(2) levels were also significant throughout the lake. Pyrosequencing and near full-length sequencing of Bacteria 16S rRNA gene diversity associated with two vents and two surface water environments demonstrated that this lake contains significant bacterial diversity. Biomass was size-fractionated by sequentially filtering through 20-μm-, 3.0-μm-, 0.8-μm- and 0.1-μm-pore-size filters, with the >0.1 to <0.8 μm size class being the focus of this study. Major phyla included Acidobacteria, Actinobacteria, Bacteroidetes, α- and β-Proteobacteria and Cyanobacteria, with 21 other phyla represented at varying levels. Surface waters were dominated by two phylotypes: the Actinobacteria freshwater acI group and an α-Proteobacteria clade tightly linked with freshwater SAR11-like organisms. We also obtained evidence of novel thermophiles and recovered Prochlorococcus phylotypes (97-100% identity) in one near surface photic zone region of the lake. The combined geochemical and microbial analyses suggest that the foundation of this lake's food chain is not simple. Phototrophy presumably is an important driver of primary productivity in photic zone waters; however, chemosynthetic hydrogenotrophy and methanotrophy are likely important components of the lake's food chain.     

Power production enhancement with a polyaniline modified anode in microbial fuel cells

In this paper, an approach of improving power generation of microbial fuel cells (MFCs) by using a HSO(4)(-) doped polyaniline modified carbon cloth anode was reported. The modification of carbon cloth anode was accomplished by electrochemical polymerization of aniline in 5% H(2)SO(4) solution. A dual-chamber MFC reactor with the modified anode achieved a maximum power density of 5.16 Wm(-3), an internal resistance of 90 Ω, and a start-up time of 4 days, which was respectively 2.66 times higher, 65.5% lower, and 33.3% shorter than the corresponding values of the MFC with unmodified anode. Evidence from X-ray photoelectron spectroscopy and scanning electron microscopy results proved that the formation of biofilm on the anode surface could prevent the HSO(4)(-) doped polyaniline to be de-doped, and the results from electrochemical tests confirmed that the electrochemical activity of the modified anode was enhanced significantly after inoculation. Charge transfer was facilitated by polyaniline modification. All the results indicated that the polyaniline modification on the anode was an efficient approach of improving the performance of MFCs.     

Power production in MFCs inoculated with Shewanella oneidensis MR‐1 or mixed cultures

Power densities and oxidation–reduction potentials (ORPs) of MFCs containing a pure culture of Shewanella oneidensis MR‐1 were compared to mixed cultures (wastewater inoculum) in cube shaped, 1‐, 2‐, and 3‐bottle batch‐fed MFC reactor configurations. The reactor architecture influenced the relative power produced by the different inocula, with the mixed culture generating 68–480% more power than MR‐1 in each MFC configuration. The mixed culture produced the maximum power density of 858 ± 9 mW m^?2 in the cubic MFC, while MR‐1 produced 148 ± 20 mW m^?2. The higher power by the mixed culture was primarily a result of lower internal resistances than those produced by the pure culture. Power was a direct function of ohmic resistance for the mixed culture, but not for strain MR‐1. ORP of the anode compartment varied with reactor configuration and inoculum, and it was always negative during maximum power production but it did not vary in proportion to power output. The ORP varied primarily at the end of the cycle when substrate was depleted, with a change from a reductive environment during maximum power production (approximately ?175 mV for mixed and approximately ?210 mV for MR‐1 in cubic MFCs), to an oxidative environment at the end of the batch cycle (～250 mV for mixed and ～300 mV for MR‐1). Mixed cultures produced more power than MR‐1 MFCs even though their redox potential was less negative. These results demonstrate that differences between power densities produced by pure and mixed cultures depend on the MFC architecture. Biotechnol. Bioeng. 2010; 105: 489–498. © 2009 Wiley Periodicals, Inc.     

Convergent development of anodic bacterial communities in microbial fuel cells

Microbial fuel cells (MFCs) are often inoculated from a single wastewater source. The extent that the inoculum affects community development or power production is unknown. The stable anodic microbial communities in MFCs were examined using three inocula: a wastewater treatment plant sample known to produce consistent power densities, a second wastewater treatment plant sample, and an anaerobic bog sediment. The bog-inoculated MFCs initially produced higher power densities than the wastewater-inoculated MFCs, but after 20 cycles all MFCs on average converged to similar voltages (470±20 mV) and maximum power densities (590±170 mW m⁻²). The power output from replicate bog-inoculated MFCs was not significantly different, but one wastewater-inoculated MFC (UAJA3 (UAJA, University Area Joint Authority Wastewater Treatment Plant)) produced substantially less power. Denaturing gradient gel electrophoresis profiling showed a stable exoelectrogenic biofilm community in all samples after 11 cycles. After 16 cycles the predominance of Geobacter spp. in anode communities was identified using 16S rRNA gene clone libraries (58±10%), fluorescent in-situ hybridization (FISH) (63±6%) and pyrosequencing (81±4%). While the clone library analysis for the underperforming UAJA3 had a significantly lower percentage of Geobacter spp. sequences (36%), suggesting that a predominance of this microbe was needed for convergent power densities, the lower percentage of this species was not verified by FISH or pyrosequencing analyses. These results show that the predominance of Geobacter spp. in acetate-fed systems was consistent with good MFC performance and independent of the inoculum source.     

Investigating microbial fuel cell bioanode performance under different cathode conditions

A compact, three‐in‐one, flow‐through, porous, electrode design with minimal electrode spacing and minimal dead volume was implemented to develop a microbial fuel cell (MFC) with improved anode performance. A biofilm‐dominated anode consortium enriched under a multimode, continuous‐flow regime was used. The increase in the power density of the MFC was investigated by changing the cathode (type, as well as catholyte strength) to determine whether anode was limiting. The power density obtained with an air‐breathing cathode was 56 W/m³ of net anode volume (590 mW/m²) and 203 W/m³ (2160 mW/m²) with a 50‐mM ferricyanide‐based cathode. Increasing the ferricyanide concentration and ionic strength further increased the power density, reaching 304 W/m³ (3220 mW/m², with 200 mM ferricyanide and 200 mM buffer concentration). The increasing trend in the power density indicated that the anode was not limiting and that higher power densities could be obtained using cathodes capable of higher rates of oxidation. The internal solution resistance for the MFC was 5–6 Ω, which supported the improved performance of the anode design. A new parameter defined as the ratio of projected surface area to total anode volume is suggested as a design parameter to relate volumetric and area‐based power densities and to enable comparison of various MFC configurations. Published 2009 American Institute of Chemical Engineers Biotechnol. Prog., 2009