Performance Of Microbial Electrolysis Cell Research Articles

Microbial bioelectrosynthesis of hydrogen: Current challenges and scale-up.

Sustainable energy supplies are needed to supplement and eventually replace fossil fuels. Molecular hydrogen H2 is a clean burning, high-energy fuel that is also used as reducing gas in industrial processes. H2 is mainly synthesized by steam reforming of natural gas, a non-renewable fuel. There are biosynthetic strategies for H2 production; however, they are associated with poor yield and have high cost. The application of an electrochemical driving force in a microbial electrolysis cell (MEC) improves the yield of biological reactions. The performance of the MEC is influenced by experimental parameters such as the electrode material, reactor design, microbial consortia and the substrate. In this review, factors that affect the performance of MECs are discussed and critically analysed. The potential for scale-up of H2 bioelectrosynthesis is also discussed.

Domestic wastewater treatment in parallel with methane production in a microbial electrolysis cell

Microbial electrolysis cells (MECs) have great potential as a technology for wastewater treatment in parallel to energy production. In this study we explore the feasibility of using a low-cost, membraneless MEC for domestic wastewater treatment and methane production in both batch and continuous modes. Low-strength wastewater can be successfully treated by means of an MEC, obtaining significant amounts of methane. The results also suggest that hydrogenotrophic methanogenesis reduce the incidence of homoacetogenic activity, thus improving the overall MEC performance. However, gas production rates are low and important aspects such as methane solubility in water still remain a challenge. Overall, MECs can offer competitive advantages not only for low-strength wastewater treatment but also as an aid to anaerobic methane production by improving the chemical oxygen demand (COD) removal and methane production rates.

Microbial electrolysis cell performance using non-buffered and low conductivity wastewaters

Microbial electrolysis cells (MEC) are a novel technology aiming at producing hydrogen from wastewater. MEC performance gives successful results in lab-scale experiments with well buffered media and synthetically-increased conductivity, thus preventing operational problems and reducing the internal resistance of the cell. This is especially important in two-chamber configuration where membranes cause potential losses associated to the pH gradients across them. However, domestic and many industrial wastewaters have a limited buffer capacity and low conductivity. In this study, the performance of an MEC with a culture medium more like a real wastewater, in terms of buffer capacity and conductivity, was assessed in both single-chamber and two-chamber configurations and compared to that of a well-buffered cell. Single-chamber MEC tests demonstrated that the lack of buffer affected both the overpotentials of the anode and the cathode, although the overpotential of the latter was significantly higher. In two-chamber configuration, the non-buffered cell failed as a result of a high pH drop in the anode, which harmed the anodic biofilm. The conductivity increase from low (4mS/cm) to high values (13mS/cm) did not improve significantly any configuration tested.

Performance of microbial electrolysis cells with bioanodes grown at different external resistances.

Bioelectrochemical systems need an anode with a high abundance of exoelectrogenic bacteria for an optimal performance. Among all possible operational parameters for an efficient enrichment, the role of external resistance in microbial fuel cell (MFC) has gained a lot of interest since it indirectly poises an anode potential, a key parameter for biofilm distribution and morphology. Thus, this work aims at investigating and discussing whether bioanodes selected at different external resistances under MFC operation present different responses under both MFC and microbial electrolysis cell (MEC) operation. A better MEC performance (i.e. shorter start-up time, higher current intensity and higher H2 production rate) was obtained with an anode from an MFC developed under low external resistance. Quantitative real-time polymerase chain reaction (qPCR) confirmed that a low external resistance provides an MFC anodic biofilm with the highest content of Geobacter because it allows higher current intensity, which is correlated to exoelectrogenic activity. High external resistances such as 1,000 Ω led to a slower start-up time under MEC operation.

Modelling of biohydrogen generation in microbial electrolysis cells (MECs) using a committee of artificial neural networks (ANNs)

The enhancement of hydrogen yield in microbial electrolysis cells (MECs) requires a robust process model that accurately relates the effect of anodic physicochemical input variables to the process output. Artificial neural networks (ANNs) have been used for the modelling of complex and non-linear processes. This paper reports the modelling of biohydrogen yield in MECs by using a committee of five ANNs. A topology of 6–(6, 8, 11, 12, 14)–1 was adopted, corresponding to the number of neurons of inputs, hidden (varied) and output layers. The ANN inputs were substrate type, substrate concentration, pH, temperature, applied voltage and reactor configuration. Model development was carried out with 50 data points from 15 published studies. The coefficients of determination (R2) between the experimental and predicted hydrogen yields for the five models were as follows: 0.90, 0.81, 0.85, 0.70 and 0.80. Model validation on new MEC processes showed a strong correlation between the observed and predicted hydrogen yields. Sensitivity analysis revealed that the performance of MEC was highly affected by variations in the substrate type, followed by applied voltage, substrate concentration, pH, MEC configuration and temperature in decreasing order. This study showed that the committee model accurately modelled the non-linear relationship between the considered physicochemical parameters of MEC and hydrogen yield, and thus could be used to navigate the optimization window in MEC scale-up processes.

Impact of applied voltage on methane generation and microbial activities in an anaerobic microbial electrolysis cell (MEC)

The microbial electrolysis cell (MEC) is a promising technology for anaerobic wastewater treatment and energy recovery. Applied voltage affects the activities of microorganisms in MEC and further impacts methane generation. Therefore, we explored the effects of applied voltages on the performance of MEC and the microbial activities under anaerobic conditions. With COD concentration of 2–3g/L in influent, COD removal efficiency and methane yield increased when the applied voltage was lower than 0.8V and decreased when the applied voltage was 1.0 and 2.0V. Lactic dehydrogenase (LDH), as an indicator for cell rupture, its activity increased 14% and 10% under 1.0 and 2.0V in anode and increased 35% and 41% in cathode, adenosine triphosphate (ATP), as an indicator for metabolism, its activity decreased sharply, 27% and 55% in anode while 46% and 66% in cathode, under the two voltages respectively, comparing to the control. Results indicated that the higher voltage led to the greater plasmatorrhexis, lower growth rate and lower metabolic activity. The destroyed cell membrance resulted in slower growth and metabolism rate, which meant the decrease of COD removal efficiency and methane yield, when the voltage higher than 0.8V conditions. In conclusion, the optimal applied voltage for wastewater treatment was 0.8V. Information provided will be useful to design reactor and maintain industry practice.

Grey relational analysis for comparative assessment of different cathode materials in microbial electrolysis cells

MEC (Microbial electrolysis cell) is an emerging and promising bio-electrochemical process for sustainable production of hydrogen gas. The performance and cost of cathode materials are the most important factors in MEC. In order to enhance MEC performance and reduce the fabrication and operational costs of electrodes, a wide range of cathode materials have been developed and tested in the recent years. However, no standard and systematic analytical method has been developed to make a comparative assessment of the cathode materials in MECs. In this study, a statistical method, namely GRA (grey relational analysis) was exploited to evaluate hydrogen evolution performance of eight different non-precious metal alloy cathodes, including stainless steel alloys 304, 316, 420, A286, and nickel alloys 201, 400, 625 with Ni HX in comparison to precious metal of Pt (platinum). Statistical results revealed that materials tested were ranked based on the grey relational grade values as ASS 286 > Pt > SS 304 > Ni 625 > SS 420 > Ni HX > Ni 400 > Ni 201 > SS 316. The results obtained showed stainless steel A286 had the best efficiency in MEC test based on grey relational grade. This study indicated that GRA could be used as a novel method to assess the performance of cathodes in MEC.

Impact of volatile fatty acids on microbial electrolysis cell performance

This study investigated the performance of microbial electrolysis cells (MECs) fed with three common fermentation products: acetate, butyrate, and propionate. Each substrate was fed to the reactor for three consecutive-batch cycles. The results showed high current densities for acetate, but low current densities for butyrate and propionate (maximum values were 6.0±0.28, 2.5±0.06, 1.6±0.14A/m2, respectively). Acetate also showed a higher coulombic efficiency of 87±5.7% compared to 72±2.0 and 51±6.4% for butyrate and propionate, respectively. This paper also revealed that acetate could be easily oxidized by anode respiring bacteria in MEC, while butyrate and propionate could not be oxidized to the same degree. The utilization rate of the substrates in MEC followed the order: acetate>butyrate>propionate. The ratio of suspended biomass to attached biomass was approximately 1:4 for all the three substrates.

Hydrogen production in a microbial electrolysis cell fed with a dark fermentation effluent

The organic matter consumption and hydrogen production rate were evaluated in a two-chamber microbial electrolysis cell (MEC). Three chemical oxygen demand (COD) concentration levels (400, 600 and 1200 mg/L) were tested. The COD was composed of a mixture of volatile fatty acids (VFAs) present in the effluent of a dark fermentation process. The two levels of voltage studied were 350 and 550 mV. The performance of the MEC was evaluated using either an anionic (AEM) or cationic exchange membrane (CEM). The robustness of the MEC was tested using two dark fermentation effluents, one with VFAs and another containing 1100 mg/L glucose. The highest production rates (81 mL/L/day) were obtained with 550 mV, and 85 % COD consumption was attained. No considerable differences in the hydrogen production rate were observed when the COD was increased from 400 to 1200 mg/L using 550 mV. However, maximal hydrogen production rates were obtained with the lower COD concentration using 350 mV. Neither the employment of AEM and CEM nor the change from synthetic substrate to real substrate resulted in remarkable changes in MEC performance. The substrate containing glucose was more slowly degraded because glucose was first transformed into VFAs, and then the VFAs were consumed to produce hydrogen. In this case, methane and carbon dioxide were detected.

Anode acclimation methods and their impact on microbial electrolysis cells treating fermentation effluent

There is a lack of standardized acclimation procedures for evaluating treatability of different wastewaters, and such tests are often conducted using different types of microbial electrolysis cells (MECs). Two different types of MECs (mini or cube) were therefore acclimated using two different substrates (acetate or domestic wastewater) to see the impact of these procedures on the resulting treatment efficiency using the same cellulose fermentation effluent. COD removal was slightly larger using mini MECs (81–86%) than cube MECs (79–82%). Pre-acclimation of mini MECs to domestic wastewater increased COD removal slightly compared to non-acclimated tests with fermentation effluent, but acclimation differences for the cube MECs were not statistically significant. Gas production was not significantly different for cube pre-acclimated MECs compared to those acclimated only to the fermentation effluent. Current densities were higher for the cube reactors than the mini MECs, but they were unaffected by acclimation procedure (pre-acclimation or direct use of fermentation effluent). These results show that mini MECs acclimated to a readily available complex source of organic matter (domestic wastewater) can produce equivalent or slightly superior results for tests with a different complex wastewater (fermentation effluent), and that mini MEC performance is comparable to that of cube MECs. The similarity of reactor performance allows the use of simple and inexpensive mini MECs that can be acclimated to domestic wastewater and subsequently used to test different types of industrial effluents.

A Generalized Model for Complex Wastewater Treatment with Simultaneous Bioenergy Production Using the Microbial Electrochemical Cell

The objective of this study was to construct a novel model to be applied in a general manner to simulate microbial electrochemical cells (MXCs); for both microbial fuel cell (MFC) and microbial electrolysis cell (MEC). The liquid bulk was modeled based on the organic matters degradation to acetate via the anaerobic digestion process. Biofilm simulation was established based upon one-dimensional distribution and the dynamical electron transfer was completed by means of the conduction-based mechanism. We, for the first time, introduced biofilm local potential modeling for MEC simulation with general and simplified linear boundary conditions. The MFC-related part of the model was evaluated based on the experimental results from a gluconic acid-fed MFC with various simulated variables of liquid bulk and biofilm such as, bulk concentrations, distributions of microbial volume fractions and local potential in the biofilm, and biofilm thickness. The MEC-related part of the model with simplified linear boundary condition was assessed by the MEC experimental data fed with potato wastewater as the complex substrate. The MEC performance as an energy carrier generator was characterized based on the hydrogen production rate evolution, the variations of microbial distributions, and methane production at different applied potential differences. In addition, polarization characteristics were simulated and discussed based on the experimental results. These valuations showed that the presented model is successfully able to predict both MFC and MEC performance.

Optimization of high-solid waste activated sludge concentration for hydrogen production in microbial electrolysis cells and microbial community diversity analysis

To enhance hydrogen recovery from high-solid waste activated sludge (WAS), microbial electrolysis cells (MECs) were used as an efficient device. The effects of WAS concentrations were firstly investigated. Optimal concentration for hydrogen production was 7.6 g VSS/L. Maximum hydrogen yields reached to 4.66 ± 1.90 mg-H2/g VSS and 11.42 ± 2.43 mg-H2/g VSS for MECs fed with raw WAS (R-WAS) and alkaline-pretreated WAS (A-WAS) respectively, which was much higher than that obtained traditional anaerobic digestion. Moreover, no propionic acid accumulation was achieved at the optimal concentration. Effective sludge reduction was also achieved in MECs feeding with A-WAS. 52.9 ± 1.3% TCOD were removed in A-WAS MECs, meanwhile, protein degradation were 50.4 ± 0.8%. The 454 pyrosequencing analysis of 16S rRNA gene revealed the syntrophic interactions were existed between exoelectrogen Geobacter and fermentative bacteria Petrimonas, which apparently drove the efficient performance of MECs fed with WAS.

Characterization of methane production and microbial community shifts during waste activated sludge degradation in microbial electrolysis cells

Microbial electrolysis cell (MECs) were investigated as a promising technology to manage waste activated sludge (WAS) reduction and bio-methane generation. The effect of WAS concentration on the MECs performance was discussed. At the optimal concentration of 15gCOD/L, maximum methane yield of MECs fed with alkaline pretreated WAS (A-WAS) were achieved with the value of 77.13±2.52LCH4/kg-COD on Day 3, which had been improved by 1.5-fold compared with MECs fed with raw WAS (R-WAS), while that was negligible in open circuit controls. Efficient sludge reduction was also obtained in terms of TCOD, total protein, TSS and VSS removal. Pyrosequencing revealed the dominance of exoelectrogen Geobacter and hydrogen-producing bacteria Petrimonas in MECs fed with WAS. Methanocorpusculum with the capacity of methane generation using CO2 and H2 also showed overwhelming dominance (96.01%). The large proportions of Petrimonas and Methanocorpusculum indicated the occurrence of hydrogenotrophic methanogenesis in our methane-producing MECs.

Improved hydrogen production in the microbial electrolysis cell by inhibiting methanogenesis using ultraviolet irradiation.

Methanogenesis inhibition is essential for the improvement of hydrogen (H2) yield and energy recovery in the microbial electrolysis cell (MEC). In this study, ultraviolet (UV) irradiation was proposed as an efficient method for methanogenesis control in a single chamber MEC. With 30 cycles of operation with UV irradiation in the MEC, high H2 concentrations (>91%) were maintained, while without UV irradiation, CH4 concentrations increased significantly and reached up to 94%. In the MEC, H2 yields ranged from 2.87 ± 0.03 to 3.70 ± 0.11 mol H2/mol acetate with UV irradiation and from 3.78 ± 0.12 to 0.03 ± 0.004 mol H2/mol acetate without UV irradiation. Average energy efficiencies from the UV-irradiated MEC were 1.5 times of those without UV irradiation. Energy production from the MEC without UV irradiation was a negative energy yield process because of large amount of CH4 produced over time, which was mainly attributable to cathodic hydrogenotrophic methanogenesis. Our results clearly showed that UV irradiation could effectively inhibit methanogenesis and improve MEC performance to produce H2.

Fermentation of glycerol into ethanol in a microbial electrolysis cell driven by a customized consortium.

The in situ generation of ethanol from glycerol-containing wastewater shows promise to improve the economics of the biodiesel industry. Consequently, we developed a microbial electrolysis cell (MEC) driven by the synergistic metabolisms of the exoelectrogen Geobacter sulfurreducens and the bacterium Clostridium cellobioparum, which fermented glycerol into ethanol in high yields (90%) and produced fermentative byproducts that served as electron donors for G. sulfurreducens. Syntrophic cooperation stimulated glycerol consumption, ethanol production, and the conversion of fermentation byproducts into cathodic H2 in the MEC. The platform was further improved by adaptively evolving glycerol-tolerant strains with robust growth at glycerol loadings typical of biodiesel wastewater and by increasing the buffering capacity of the anode medium. This resulted in additional increases in glycerol consumption (up to 50 g/L) and ethanol production (up to 10 g/L) at rates that greatly exceeded the capacity of the anode biofilms to concomitantly remove the fermentation byproducts. As a result, 1,3-propanediol was generated as a metabolic sink for electrons not converted into electricity syntrophically. The results highlight the potential of consortia to process glycerol in MECs and provide insights into genetic engineering and system design approaches that can be implemented to further improve MEC performance to satisfy industrial needs.

A quantitative method to evaluate microbial electrolysis cell effectiveness for energy recovery and wastewater treatment

Microbial electrolysis cells (MECs) are potential candidates for sustainable wastewater treatment as they allow for recovery of the energy input by producing valuable chemicals such as hydrogen gas. Evaluating the effectiveness of MEC treatment for different wastewaters requires new approaches to quantify performance, and the establishment of specific procedures and parameters to characterize the outcome of fed-batch treatability tests. It is shown here that Coulombic efficiency can be used to directly calculate energy consumption relative to wastewater treatment in terms of COD removal, and that the average current, not maximum current, is a better metric to evaluate the rate of the bioelectrochemical reactions. The utility of these methods was demonstrated using simulated current profiles and actual wastewater tests. Industrial and domestic wastewaters were evaluated using small volume MECs, and different inoculation strategies. The energy needed for treatment was 2.17kWhkgCOD−1 for industrial wastewater and 2.59kWhkgCOD−1 for domestic wastewater. When these wastewaters were combined in equal amounts, the energy required was reduced to 0.63kWhkgCOD−1. Acclimation of the MEC to domestic wastewater, prior to tests with industrial wastewaters, was the easiest and most direct method to optimize MEC performance for industrial wastewater treatment. A pre-acclimated MEC accomplished the same removal (1847 ± 53 mg L−1) as reactor acclimated to only the industrial wastewater (1839 ± 57 mg L−1), but treatment was achieved in significantly less time (70 h versus 238 h).

Performance of a semi-pilot tubular microbial electrolysis cell (MEC) under several hydraulic retention times and applied voltages

The influence of applied voltage and hydraulic retention time on the performance of a semi-pilot modular tubular wastewater-fed microbial electrolysis cell (MEC) with high scalability was investigated. A chemical oxygen demand (COD) removal efficiency of 80%, as well as an energy consumption of 0.3-1.1 Wh g-COD(-1) removed, were achieved. Hydrogen production was limited by the reduced amounts of organic matter fed into the reactor, the poor performance of the cathode, and COD consuming by non electrogenic microorganisms. The presence of COD consuming microorganism that do not contribute to electrogenic metabolism severely affected the MEC performance.

Optimization of catholyte concentration and anolyte pHs in two chamber microbial electrolysis cells

The hydrogen production rate in a microbial electrolysis cell (MEC) using a non-buffered saline catholyte (NaCl) can be optimized through proper control of the initial anolyte pH and catholyte NaCl concentration. The highest hydrogen yield of 3.3 ± 0.4 mol H2/mole acetate and gas production rate of 2.2 ± 0.2 m3 H2/m3/d were achieved here with an initial anolyte pH = 9 and catholyte NaCl concentration of 98 mM. Further increases in the salt concentration substantially reduced the anolyte pH to as low as 4.6, resulting in reduced MEC performance due to pH inhibition of exoelectrogens. Cathodic hydrogen recovery was high (rcat > 90%) as hydrogen consumption by hydrogenotrophic methanogens was prevented by separating the anode and cathode chambers using a membrane. These results show that the MEC can be optimized for hydrogen production through proper choices in the concentration of a non-buffered saline catholyte and initial anolyte pH in two chamber MECs.

Current generation in microbial electrolysis cells with addition of amorphous ferric hydroxide, Tween 80, or DNA

Abstract Iron-oxide nanoparticles and the Tween 80 have previously been shown to improve power generation in microbial fuel cells (MFCs), presumably by improving electron transfer from the bacteria to the anode. We examined whether several chemicals would affect current production in single-chamber microbial electrolysis cells (MECs), where hydrogen gas is produced at the cathode, using mixed cultures and Geobacter sulfurreducens. Tween 80 did not increase the current. Fe(OH)3 addition increased the maximum current density of both the mixed cultures (from 6.1 ± 0.9 A/m2 to 8.8 ± 0.3 A/m2) and pure cultures (from 4.8 ± 0.5 A/m2 to 7.4 ± 1.1 A/m2). Improved current production was sustained even after iron was no longer added to the medium. It was demonstrated that increased current resulted from improved cathode performance. Analysis using electrochemical impedance spectroscopy (EIS) showed that the iron primarily reduced the diffusion resistances of the cathodes, and scanning electron microscopy (SEM) images showed the formation of highly porous structures on the cathode. The addition of DNA also did not improve MEC or MFC performance. These results demonstrated that among these treatments only Fe(OH)3 addition was a viable method for enhancing current densities in MECs, primarily by improving cathode performance.

Nickel ion removal from wastewater using the microbial electrolysis cell

In this study, the microbial electrolysis cell (MEC) was examined for treatment of wastewater containing nickel ion (Ni2+). Effects of initial Ni2+ concentrations in catholyte, pH values of catholyte, and applied voltages on MEC performance were determined. Results showed that the Ni2+ removal efficiency with the MEC was three times higher than those with an electrolysis cell and a microbial fuel cell. Ni2+ removal efficiencies decreased from 99±0.6% to 33±4.2% with the initial Ni2+ concentrations from 50 to 1000mg/L, while the mass removal of Ni2+ increased consistently with the initial concentrations. Ni2+ removal efficiencies in the MEC increased from 51±4.6% to 67±5.3% with the applied voltages from 0.5 to 1.1V. These results demonstrate that the MEC can be an alternative process to effectively remove Ni2+ from wastewater.