Factors affecting current production in microbial fuel cells using different industrial wastewaters

This study evaluated how different types of industrial wastewaters (bakery, brewery, paper and dairy) affect the performance of identical microbial fuel cells (MFCs); and the microbial composition and electrochemistry of MFC anodes. MFCs fed with paper wastewater produced the highest current density (125 ± 2 mA/m²) at least five times higher than dairy (25 ± 1 mA/m²), brewery and bakery wastewaters (10 ± 1 mA/m²). Such high current production was independent of substrate degradability. A comprehensive study was conducted to determine the factor driving current production when using the paper effluent. The microbial composition of anodic biofilms differed according to the type of wastewater used, and only MFC anodes fed with paper wastewater showed redox activity at −134 ± 5 mV vs NHE. Electrochemical analysis of this redox activity indicated that anodic bacteria produced a putative electron shuttling compound that increased the electron transfer rate through diffusion, and as a result the overall MFC performance.     

Dehydrogenase activity in association with poised potential during biohydrogen production in single chamber microbial electrolysis cell

Variation in the dehydrogenase (DH) activity and its simultaneous influence on hydrogen (H₂) production, substrate degradation rate (SDR) and volatile fatty acid (VFA) generation was investigated with respect to varying poised potential in single chambered membrane-less microbial electrolysis cell (MEC) using anaerobic consortia as biocatalyst. Poised potential showed significant influence on H₂ production and DH activity. Maximum H₂ production was observed at 1.0 V whereas the control system showed least H₂ production among the experimental variations studied. DH activity was observed maximum at 0.6 V followed by 0.8, 0.9 and 1.0 V, suggests the influence of poised potential on the microbial metabolism. Almost complete degradation of substrate was observed in all the experimental conditions studied irrespective of the applied potential. Experimental data was also analysed employing multiple regression analysis and 3D-surface plots to find out the best theoretical poised potential for maximum H₂ production and DH activity.     

Pre-acclimation of a wastewater inoculum to cellulose in an aqueous-cathode MEC improves power generation in air-cathode MFCs

Cellulose has been used in two-chamber microbial fuel cells (MFCs), but power densities were low. Higher power densities can be achieved in air-cathode MFCs using an inoculum from a two-chamber, aqueous-cathode microbial electrolysis cell (MEC). Air-cathode MFCs with this inoculum produced maximum power densities of 1070 mW m⁻² (cathode surface area) in single-chamber and 880 mW m⁻² in two-chamber MFCs. Coulombic efficiencies ranged from 25% to 50%, and COD removals were 50-70% based on total cellulose removals of 60-80%. Decreasing the reactor volume from 26 to 14 mL (while maintaining constant electrode spacing) decreased power output by 66% (from 526 to 180 mW m⁻²) due to a reduction in total mass of cellulose added. These results demonstrate that air-cathode MFCs can produce high power densities with cellulose following proper acclimation of the inoculum, and that organic loading rates are important for maximizing power densities from particulate substrates.     

Hydrogen consumption in microbial electrochemical systems (MXCs): The role of homo-acetogenic bacteria

Homo-acetogens in the anode of a microbial electrolysis cell (MEC) fed with H₂ as sole electron donor allowed current densities similar to acetate-fed biofilm anodes (∼10 A/m²). Evidence for homo-acetogens included accumulation of acetate at high concentrations (up to 18 mM) in the anode compartment; detection of formate, a known intermediate during reductive acetogenesis by the acetyl-CoA pathway; and detection of formyl tetrahydrofolate synthetase (FTHFS) genes by quantitative real-time PCR. Current production and acetate accumulation increased in parallel in batch and continuous mode, while both values decreased simultaneously at short hydraulic retention times (1 h) in the anode compartment, which limited suspended homo-acetogens. Acetate produced by homo-acetogens accounted for about 88% of the current density of 10 A/m², but the current density was sustained at 4 A/m² at short hydraulic retention time because of a robust partnership of homo-acetogens and anode respiring bacteria (ARB) in the biofilm anode.     

Application of Co-naphthalocyanine (CoNPc) as alternative cathode catalyst and support structure for microbial fuel cells

Co-naphthalocyanine (CoNPc) was prepared by heat treatment for cathode catalysts to be used in microbial fuel cells (MFCs). Four different catalysts (Carbon black, NPc/C, CoNPc/C, Pt/C) were compared and characterized using XPS, EDAX and TEM. The electrochemical characteristics of oxygen reduction reaction (ORR) were compared by cyclic voltammetry (CV) and linear sweep voltammetry (LSV). The Co-macrocyclic complex improves the catalyst dispersion and oxygen reduction reaction of CoNPc/C. The maximum power of CoNPc/C was 64.7 mW/m² at 0.25 mA as compared with 81.3 mW/m² of Pt/C, 29.7 mW/m² of NPc/C and 9.3 mW/m² of carbon black when the cathodes were implemented in H-type MFCs. The steady state cell, cathode and anode potential of MFC with using CoNPc/C were comparable to those of Pt/C.     

Examination of microbial fuel cell start-up times with domestic wastewater and additional amendments

Rapid startup of microbial fuel cells (MFCs) and other bioreactors is desirable when treating wastewaters. The startup time with unamended wastewater (118 h) was similar to that obtained by adding acetate or fumarate (110-115 h), and less than that with glucose (181 h) or Fe(III) (353 h). Initial current production took longer when phosphate buffer was added, with startup times increasing with concentration from 149 h (25 mM) to 251 h (50 mM) and 526 h (100 mM). Microbial communities that developed in the reactors contained Betaproteobacteria, Acetoanaerobium noterae, and Chlorobium sp. Anode biomass densities ranged from 200 to 600 μg/cm² for all amendments except Fe(Ш) (1650 μg/cm²). Wastewater produced 91 mW/m², with the other MFCs producing 50 mW/m² (fumarate) to 103 mW/m² (Fe(III)) when amendments were removed. These experiments show that wastewater alone is sufficient to acclimate the reactor without the need for additional chemical amendments.     

Improving hydrogen production in microbial electrolysis cells through hydraulic connection with thermoelectric generators

Using alternative power sources to drive hydrogen production in microbial electrolysis cells (MECs) is important to implementation of MEC technology. Herein, thermoelectric generators (TEG) were to power MECs using simulated waste heat. With the MEC anolyte as a cold source for TEG, current generation of the MEC increased to 2.46 ± 0.06 mA and hydrogen production reached 0.14 m³ m⁻³ d^-1, higher than those of the TEG-MEC system without hydraulic connection (1.16 ± 0.07 mA and 0.07 ± 0.01 m³ m⁻³ d^-1). A high recirculation rate of 30 mL min^-1 doubled both current generation and hydrogen production with 10 mL min^-1, benefited from a stronger cooling effect that increased the TEG voltage output. However, the optimal recirculation rate was determined as 20 mL min^-1 because of comparable performance but potentially less energy requirement. Reducing anolyte hydraulic retention time to 4 h has increased hydrogen production to 0.25 ± 0.05 m³ m⁻³ d^-1 but decreased organic removal efficiency to 69 ± 2%. Adding three more TEG units that captured more heat energy further enhanced hydrogen production to 0.36 m³ m⁻³ d^-1. Those results have demonstrated a successful integration of TEG with MEC through both electrical and hydraulic connections for simultaneous wastewater treatment and energy recovery.     

Acidogenic fermentation of vegetable based market waste to harness biohydrogen with simultaneous stabilization

Vegetable based market waste was evaluated as a fermentable substrate for hydrogen (H₂) production with simultaneous stabilization by dark-fermentation process using selectively enriched acidogenic mixed consortia under acidophilic microenvironment. Experiments were performed at different substrate/organic loading conditions in concurrence with two types of feed compositions (with and without pulp). Study depicted the feasibility of H₂ production from vegetable waste stabilization process. H₂ production was found to be dependent on the concentration of the substrate and composition. Higher H₂ production and substrate degradation were observed in experiments performed without pulp (23.96 mmol/day (30.0 kg COD/m³); 13.96 mol/kg COD_R (4.8 kg COD/m³)) than with pulp (22.46 mmol/day (32.0 kg COD/m³); 12.24 mol/kg COD_R (4.4 kg COD/m³)). Generation of higher concentrations of acetic acid and butyric acid was observed in experiments performed without pulp. Data enveloping analysis (DEA) was employed to study the combined process efficiency of system by integrating H₂ production and substrate degradation.     

Electrocatalytic activity of anodic biofilm responses to pH changes in microbial fuel cells

This study investigates the effects of anodic pH on electricity generation in microbial fuel cells (MFCs) and the intrinsic reasons behind them. In a two-chamber MFC, the maximum power density is 1170 ± 58 mW m⁻² at pH 9.0, which is 29% and 89% higher than those working at pH 7.0 and 5.0, respectively. Electrochemical measurements reveal that pH affects the electron transfer kinetics of anodic biofilms. The apparent electron transfer rate constant (k_app) and exchange current density (i₀) are greater whereas the charge transfer resistance (R_ct) is smaller at pH 9.0 than at other conditions. Scanning electron microscopy verifies that alkaline conditions benefit biofilm formation in MFCs. These results demonstrate that electrochemical interactions between bacteria and electrodes in MFCs are greatly enhanced under alkaline conditions, which can be one of the important reasons for the improved MFC output.     

Treatment of biodiesel production wastes with simultaneous electricity generation using a single-chamber microbial fuel cell

Biodiesel production through transesterification of lipids generates large quantity of biodiesel waste (BW) containing mainly glycerin. BW can be treated in various ways including distillation to produce glycerin, use as substrate for fermentative propanediol production and discharge as wastes. This study examined microbial fuel cells (MFCs) to treat BW with simultaneous electricity generation. The maximum power density using BW was 487 ± 28 mW/m² cathode (1.5 A/m² cathode) with 50 mM phosphate buffer solution (PBS) as the electrolyte, which was comparable with 533 ± 14 mW/m² cathode obtained from MFCs fed with glycerin medium (COD 1400 mg/L). The power density increased from 778 ± 67 mW/m² cathode using carbon cloth to 1310 ± 15 mW/m² cathode using carbon brush as anode in 200 mM PBS electrolyte. The power density was further increased to 2110 ± 68 mW/m² cathode using the heat-treated carbon brush anode. Coulombic efficiencies (CEs) increased from 8.8 ± 0.6% with carbon cloth anode to 10.4 ± 0.9% and 18.7 ± 0.9% with carbon brush anode and heat-treated carbon brush anode, respectively.     

Anode microbial communities produced by changing from microbial fuel cell to microbial electrolysis cell operation using two different wastewaters

Conditions in microbial fuel cells (MFCs) differ from those in microbial electrolysis cells (MECs) due to the intrusion of oxygen through the cathode and the release of H₂ gas into solution. Based on 16S rRNA gene clone libraries, anode communities in reactors fed acetic acid decreased in species richness and diversity, and increased in numbers of Geobacter sulfurreducens, when reactors were shifted from MFCs to MECs. With a complex source of organic matter (potato wastewater), the proportion of Geobacteraceae remained constant when MFCs were converted into MECs, but the percentage of clones belonging to G. sulfurreducens decreased and the percentage of G. metallireducens clones increased. A dairy manure wastewater-fed MFC produced little power, and had more diverse microbial communities, but did not generate current in an MEC. These results show changes in Geobacter species in response to the MEC environment and that higher species diversity is not correlated with current.     

Enhancement of hydrogen production in a single chamber microbial electrolysis cell through anode arrangement optimization

Reducing the inner resistances is crucial for the enhancement of hydrogen generation in microbial electrolysis cells (MECs). This study demonstrates that the optimization of the anode arrangement is an effective strategy to reduce the system resistances. By changing the normal MEC configuration into a stacking mode, namely separately placing the contacted anodes from one side to both sides of cathode in parallel, the solution, biofilm and polarization resistances of MECs were greatly reduced, which was also confirmed with electrochemical impedance spectroscopy analysis. After the anode arrangement optimization, the current and hydrogen production rate (HPR) of MEC could be enhanced by 72% and 118%, reaching 621.3 ± 20.6 A/m³ and 5.56 m³/m³ d respectively, under 0.8 V applied voltage. A maximum current density of 1355 A/m³ with a HPR of 10.88 m³/m³ d can be achieved with 1.5 V applied voltage.     

Scalable air cathode microbial fuel cells using glass fiber separators, plastic mesh supporters, and graphite fiber brush anodes

The combined use of brush anodes and glass fiber (GF1) separators, and plastic mesh supporters were used here for the first time to create a scalable microbial fuel cell architecture. Separators prevented short circuiting of closely-spaced electrodes, and cathode supporters were used to avoid water gaps between the separator and cathode that can reduce power production. The maximum power density with a separator and supporter and a single cathode was 75 ± 1 W/m³. Removing the separator decreased power by 8%. Adding a second cathode increased power to 154 ± 1 W/m³. Current was increased by connecting two MFCs connected in parallel. These results show that brush anodes, combined with a glass fiber separator and a plastic mesh supporter, produce a useful MFC architecture that is inherently scalable due to good insulation between the electrodes and a compact architecture.     

Evaluation of bioenergy recovery processes treating organic residues from ethanol fermentation process

This study evaluates a two-stage bioprocess for recovering bioenergy in the forms of hydrogen and methane while treating organic residues of ethanol fermentation from tapioca starch. A maximum hydrogen production rate of 0.77 mmol H₂/g VSS/h can be achieved at volumetric loading rate (VLR) of 56 kg COD/m³/day. Batch results indicate that controlling conditions at S₀/X₀ = 12 with X₀ = 4000 mg VSS/L and pH 5.5-6 are important for efficient hydrogen production from fermentation residues. Hydrogen-producing bacteria enriched in the hydrogen bioreactor are likely utilizing lactate and acetate for biohydrogen production from ethanol-fermentation residues. Organic residues remained in the effluent of hydrogen bioreactor can be effectively converted to methane with a rate of 0.37 mmol CH₄/g VSS/h at VLR of 8 kg COD/m³/day. Approximately 90% of COD in ethanol-fermentation residues can be removed and among that 2% and 85.1% of COD can be recovered in the forms of hydrogen and methane, respectively.     

Long-term cathode performance and the microbial communities that develop in microbial fuel cells fed different fermentation endproducts

To better understand how cathode performance and substrates affected communities that evolved in these reactors over long periods of time, microbial fuel cells were operated for more than 1 year with individual endproducts of lignocellulose fermentation (acetic acid, formic acid, lactic acid, succinic acid, or ethanol). Large variations in reactor performance were primarily due to the specific substrates, with power densities ranging from 835 ± 21 to 62 ± 1 mW/m³. Cathodes performance degraded over time, as shown by an increase in power of up to 26% when the cathode biofilm was removed, and 118% using new cathodes. Communities that developed on the anodes included exoelectrogenic families, such as Rhodobacteraceae, Geobacteraceae, and Peptococcaceae, with the Deltaproteobacteria dominating most reactors. Pelobacter propionicus was the predominant member in reactors fed acetic acid, and it was abundant in several other MFCs. These results provide valuable insights into the effects of long-term MFC operation on reactor performance.     

Upgrading of straw hydrolysate for production of hydrogen and phenols in a microbial electrolysis cell (MEC)

In a microbial electrolysis cell (MEC), hydrolysate produced by hydrothermal treatment of wheat straw was used for hydrogen production during selective recovery of phenols. The average H₂ production rate was 0.61 m³ H₂/m³ MEC·day and equivalent to a rate of 0.40 kg COD/m³ MEC·day. The microbial community in the anode biofilm was adapted by establishment of xylose-degrading bacteria of the Bacteriodetes phylum (16%) and Geobacter sulfurreducens (49%). During the process, 61% of the chemical oxygen demand was removed as hydrogen at 64% yield. The total energy production yield was 78% considering the energy content in the consumed compounds and the cell voltage of 0.7 V. The highest hydrogen production was equivalent to 0.8 kg COD/m³ MEC·day and was obtained at pH 7–8 and 25°C. Accumulation of 53% w/v phenolic compounds in the liquor was obtained by stepwise addition of the hydrolysate during simultaneous production of hydrogen from consumption of 95% for the hemicellulose and 100% of the fatty acids. Final calculations showed that hydrolysate produced from 1 kg wheat straw was upgraded by means of the MEC to 22 g hydrogen (266 L), 8 g xylan, and 9 g polyphenolics for potential utilization in biobased materials.     

Performance of microbial fuel cell in response to change in sludge loading rate at different anodic feed pH

Performance of two dual chambered mediator-less microbial fuel cells (MFCs) was evaluated at different sludge loading rate (SLR) and feed pH. Optimum performance in terms of organic matter removal and power production was obtained at the SLR of 0.75 kg COD kg VSS⁻¹ d⁻¹. Maximum power density of 158 mW/m² and 600 mW/m² was obtained in MFC-1 (feed pH 6.0) and MFC-2 (feed pH 8.0), respectively. Internal resistance of the cell decreased with increase in SLR. When operated only with biofilm on anode, the maximum power density was 109.5 mW/m² in MFC-1 and 459 mW/m² in MFC-2, which was, respectively, 30% and 23.5% less than the value obtained in MFC-1 and MFC-2 at SLR of 0.75 kg COD kg VSS⁻¹ d⁻¹. Maximum volumetric power of 15.51 W/m³ and 36.72 W/m³ was obtained in MFC-1 and MFC-2, respectively, when permanganate was added as catholyte. Higher feed pH (8.0) favoured higher power production.     

Effects of ammonium concentration and charge exchange on ammonium recovery from high strength wastewater using a microbial fuel cell

Ammonium recovery using a two chamber microbial fuel cell (MFC) was investigated at high ammonium concentration. Increasing the ammonium concentration (from 0.07 to 4 g ammonium-nitrogen/L) by addition of ammonium chloride did not affect the performance of the MFC. The obtained current densities by DC-voltammetry were higher than 6 A/m² for both operated MFCs. Also continuous operation at lower external resistance (250 Ω) showed an increased current density (0.9 A/m²). Effective ammonium recovery can be achieved by migrational ion flux through the cation exchange membrane to the cathode chamber, driven by the electron production from degradation of organic substrate. The charge transport was proportional to the concentration of ions. Nonetheless, a concentration gradient will influence the charge transport. Furthermore, a charge exchange process can influence the charge transport and therefore the recovery of specific ions.     

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.     

Effect of external resistance on bacterial diversity and metabolism in cellulose-fed microbial fuel cells

External resistance affects the performance of microbial fuel cells (MFCs) by controlling the flow of electrons from the anode to the cathode. The purpose of this study was to determine the effect of external resistance on bacterial diversity and metabolism in MFCs. Four external resistances (20, 249, 480, and 1000 Ω) were tested by operating parallel MFCs independently at constant circuit loads for 10 weeks. A maximum power density of 66 mW m⁻² was achieved by the 20 Ω MFCs, while the MFCs with 249, 480, and 1000 Ω external resistances produced 57.5, 27, and 47 mW m⁻², respectively. Denaturing gradient gel electrophoresis analysis of partial 16S rRNA genes showed clear differences between the planktonic and anode-attached populations at various external resistances. Concentrations of short chain fatty acids were higher in MFCs with larger circuit loads, suggesting that fermentative metabolism dominated over anaerobic respiration using the anode as the final electron acceptor.