Microbial Electrochemical Cells Research Articles

Performance evaluation of a microbial fuel cell for resource recovery as struvite and bioelectricity generation from slaughterhouse wastewater

AbstractBACKGROUNDThe microbial fuel cell (MFC) is a potential cost‐effective technology for the energy‐neutral treatment of wastewater. However, the successful implementation of this technology in resource recovery is still limited. In this study, a microbial electrochemical cell was designed and operated for 30 days. Critical factors for removal and recovery of nitrogen and phosphorus as struvite from wastewater were assessed.RESULTSOptimization studies on critical factors such as the chemical oxygen demand (COD) of wastewater (500–2000 mg L−1) and cathode aeration rate (45–135 mL min−1) were conducted using a pure culture of Escherichia coli. The system yielded an average power density of 465 mW m−2, average current density of 915 mA m−2 and phosphorus recovery at an extent of 40% as struvite. Additionally, a maximum reduction in the COD of 90% with an average coulombic efficiency of about 82% was obtained at a short interval of 30 days. Solubility studies of the recovered struvite for 12 h at different pH values from 4.5 to 9 showed a maximum solubility of 80% at pH 4.5 and a minimum of 3.5% at pH 9.CONCLUSIONThis study moves one step closer to applying MFC technology for nitrogen‐ and phosphorus‐rich wastewater treatment with concurrent struvite precipitation and electricity production. In this way, Sustainable Development Goals 2, 6 and 7 can be achieved through resource recovery, clean water and bioenergy. © 2024 Society of Chemical Industry (SCI).

Extracellular polymeric substances in electroactive biofilms play a crucial role in improving the efficiency of microbial fuel and electrolysis cells.

In microbial electrochemical cells (MECs), electroactive microbial biofilms can transmit electrons from organic molecules to anodes. To further understand the production of anodic biofilms, it is essential to investigate the composition and distribution of extracellular polymeric substance (EPS) in the MECs. In this study, the structure of EPS was examined in microbial electrolysis cells from mixed cultures forming biofilm using carbon fiber fabric anode. EPS was extracted from the anode biofilm of microbial electrolysis cells inoculated with mixed microbial culture. The anode biofilm yielded 0.4mg of EPS, of which 51.2% was humic substance, 16.2% was protein, 12.6% was carbohydrates, and 20% consisted of undetermined substances. Using epifluorescence microscopy, the composition of bacterial cells and their location inside EPS were studied, and the distribution of microbial communities was compared based on current density results in the presence of various carbohydrates. On the electrode surface, bacteria and EPS gathered or overlapped in various locations can affect microbial electrochemical performance. Our findings showed that EPS formation in electroactive biofilms would be important for enhanced efficiency of electricity- or hydrogen-producing microbial electrolysis cells.

Polyethylene glycol hydrogel coatings for protection of electroactive bacteria against chemical shocks

Loss of bioelectrochemical activity in low resource environments or from chemical toxin exposure is a significant limitation in microbial electrochemical cells (MxCs), necessitating the development of materials that can stabilize and protect electroactive biofilms. Here, polyethylene glycol (PEG) hydrogels were designed as protective coatings over anodic biofilms, and the effect of the hydrogel coatings on biofilm viability under oligotrophic conditions and ammonia-N (NH4+-N) shocks was investigated. Hydrogel deposition occurred through polymerization of PEG divinyl sulfone and PEG tetrathiol precursor molecules, generating crosslinked PEG coatings with long-term hydrolytic stability between pH values of 3 and 10. Simultaneous monitoring of coated and uncoated electrodes co-located within the same MxC anode chamber confirmed that the hydrogel did not compromise biofilm viability, while the coated anode sustained nearly a 4 × higher current density (0.44 A/m2) compared to the uncoated anode (0.12 A/m2) under oligotrophic conditions. Chemical interactions between NH4+-N and PEG hydrogels revealed that the hydrogels provided a diffusive barrier to NH4+-N transport. This enabled PEG-coated biofilms to generate higher current densities during NH4+-N shocks and faster recovery afterwards. These results indicate that PEG-based coatings can expand the non-ideal chemical environments that electroactive biofilms can reliably operate in.

Catalytic valorization of Kraft lignin into feedstock chemicals with methyltrioxorhenium (MTO) catalyst in microbial electrochemical cell

In this study, the authors investigate a novel approach to valorize Kraft lignin using the catalyst Methyltrioxorhenium (MTO) in tandem with in-situ produced H2O2 in a Microbial Electrochemical Cell (MEC). This study demonstrates the in-situ oxidation of Kraft lignin using different concentrations of MTO catalyst (2 mM to 8 mM) and H2O2 (5.24 ± 0.40 mM to 8.91 ± 0.70 mM) in three MECs. The depolymerized Kraft lignin samples were characterized using FTIR, CHNS/O, and 1H NMR analysis. The MTO/H2O2 combination showed high selectivity towards the oxidation of Kraft lignin, resulting in both aromatic ring and side chain cleavage reactions and the production of valuable feedstock chemicals. The oxidation also led to a reduction of 68.42 % to 78.18 % in Chemical Oxygen Demand (COD) of lignin. The selective oxidation favored the recovery of Guaiacyl (G) unit-derived feedstock chemicals, with Guaiacol being the most abundant compound (45.04 mg/mL) among the quantified products by HPLC. Additionally, the system demonstrated high efficiency in anodic wastewater treatment, achieving BOD and COD removal rates ranging from 67.68 % to 72.55 %. This method showcases the use of a sustainable system in combination with a selective catalyst to produce valuable products from usually discarded Kraft lignin while simultaneously treating wastewater.

Design and analysis of a low-cost potentiostat for application with microbial electrochemical sensors

Potentiostats are often limited to relatively costly laboratory-based analysis, which limits their access and application by researchers and practitioners, including for mobile in-field systems. This paper presents an alternative low-cost potentiostat design, sufficiently accurate for application with a microbial electrochemical sensor. The design of the device is described, evaluated using simulation and applied to a microbial electrochemical sensor to measure acetate. Incorporated into the design are a micro-controller, local data storage, and a standard industrial communication protocol to provide flexibility and data management. The device was able to produce a potential at ±2.5 V relative to an Ag-AgCl reference electrode and to measure current at ±2 mA, which was comparable to the performance of laboratory potentiostats on the market. Simulation analysis using fundamental electronic principles, such as Ohm's Law, confirmed that operation of the system was valid and as expected. Further testing using a pseudo-cell showed the device was accurate across a relevant potential range. Results from the initial calibration of the device show less than 1.6 % standard error of the mean. The device was further applied to a microbial electrochemical cell to measure acetate, which confirmed the device was functional for this application and recorded values were consistent with expectations based on the relevant literature.

Nitrogen Fixation and Ammonium Assimilation Pathway Expression of Geobacter sulfurreducens Changes in Response to the Anode Potential in Microbial Electrochemical Cells.

Nitrogen gas (N2) fixation in the anode-respiring bacterium Geobacter sulfurreducens occurs through complex, multistep processes. Optimizing ammonium (NH4+) production from this bacterium in microbial electrochemical technologies (METs) requires an understanding of how those processes are regulated in response to electrical driving forces. In this study, we quantified gene expression levels (via RNA sequencing) of G. sulfurreducens growing on anodes fixed at two different potentials (-0.15 V and +0.15 V versus standard hydrogen electrode). The anode potential had a significant impact on the expression levels of N2 fixation genes. At -0.15 V, the expression of nitrogenase genes, such as nifH, nifD, and nifK, significantly increased relative to that at +0.15 V, as well as genes associated with NH4+ uptake and transformation, such as glutamine and glutamate synthetases. Metabolite analysis confirmed that both of these organic compounds were present in significantly higher intracellular concentrations at -0.15 V. N2 fixation rates (estimated using the acetylene reduction assay and normalized to total protein) were significantly larger at -0.15 V. Genes expressing flavin-based electron bifurcation complexes, such as electron-transferring flavoproteins (EtfAB) and the NADH-dependent ferredoxin:NADP reductase (NfnAB), were also significantly upregulated at -0.15 V, suggesting that these mechanisms may be involved in N2 fixation at that potential. Our results show that in energy-constrained situations (i.e., low anode potential), the cells increase per-cell respiration and N2 fixation rates. We hypothesize that at -0.15 V, they increase N2 fixation activity to help maintain redox homeostasis, and they leverage electron bifurcation as a strategy to optimize energy generation and use. IMPORTANCE Biological nitrogen fixation coupled with ammonium recovery provides a sustainable alternative to the carbon-, water-, and energy-intensive Haber-Bosch process. Aerobic biological nitrogen fixation technologies are hindered by oxygen gas inhibition of the nitrogenase enzyme. Electrically driving biological nitrogen fixation in anaerobic microbial electrochemical technologies overcomes this challenge. Using Geobacter sulfurreducens as a model exoelectrogenic diazotroph, we show that the anode potential in microbial electrochemical technologies has a significant impact on nitrogen gas fixation rates, ammonium assimilation pathways, and expression of genes associated with nitrogen gas fixation. These findings have important implications for understanding regulatory pathways of nitrogen gas fixation and will help identify target genes and operational strategies to enhance ammonium production in microbial electrochemical technologies.

Molecular transformation and metabolic insights of microbial electrolysis treatment and valorization of post-hydrothermal liquefaction wastewater

Microbial electrochemical cells can effectively treat post-hydrothermal liquefaction wastewater with the synergistic metabolism of fermentative bacteria and electroactive bacteria while producing H2 for biocrude upgrading.

A Robust Controller of a Reactor Electromicrobial System Based on a Structured Fractional Transformation for Renewable Energy

The focus on renewable energy is increasing globally to lessen reliance on conventional sources and fossil fuels. For renewable energy systems to work at their best and produce the desired results, precise feedback control is required. Microbial electrochemical cells (MEC) are a relatively new technology for renewable energy. In this study, we design and implement a model-based robust controller for a continuous MEC reactor. We compare its performance with those of traditional methods involving a proportional integral derivative (PID), H-infinity (H∞) controller and PID controller tuned by intelligent genetic algorithms. Recently, a dynamic model of a MEC continuous reactor was proposed, which describes the complex dynamics of MEC through a set of nonlinear differential equations. Until now, no model-based control approaches for MEC have been proposed. For optimal and robust output control of a continuous-reactor MEC system, we linearize the model to state a linear time-invariant (LTI) state-space representation at the nominal operating point. The LTI model is used to design four different types of controllers. The designed controllers and systems are simulated, and their performances are evaluated and compared for various operating conditions. Our findings show that a structured linear fractional transformation (LFT)-based H∞ control approach is much better than the other approaches against various performance parameters. The study provides numerous possibilities for control applications of continuous MEC reactor processes.

Microalgal pandora for potent bioenergy production: A way forward?

Microalgae have shown their extravagant potency as a prominent source for the production of biomass, which has opened the gates for biofuel, bioenergy generation, and down-streaming in today’s third-world developing countries. They are competent in producing smaller land footprints while engendering high yields of biomass and fuel overall. By their nature, they have the potential to avail the land that is incapacitated for food production and flourishes there without bringing forth disruption to the economy in any form. Since microalgae are perennial along with excellent tolerance towards pH changes, their current demand is skyrocketing. The review paper, sails on the generation of biohydrogen, biomethane, and bioelectricity using microalgae as the primordial constituents, with the mechanisms and pitfalls confronted throughout the process. Alongside, this paper also covers the involvement of microalgae in the treatment of wastewater and biohydrogen production using dark fermentation. Biomethane is one of the prime biogas generated upon methanogenesis and fermentation when carried out employing microalgae. The paper’s greater aims are also to highlight the production of bioelectricity using microbial electrochemical cells and microbial fuel cells. The outcome of this review can provide a research update on the efficiency of algal biomass as a promising raw material for various biofuels paving a way for finding research gaps towards future exploration.

Optimization of biofilm conductance measurement with two-electrode microbial electrochemical cells (MECs)

This study was conducted to develop a standardized and consistent method for biofilm conductance measurement for an improved comprehension of extracellular electron transfer. Biofilm conductance (2.12 ± 0.25 × 10−4 S) with and without a fixed anode potential did not show significant difference. The conductance showed a sigmoidal relationship with anode potential. The current-voltage profile of the tested biofilm at applied voltage larger than 100 mV showed deviation from Ohm's law. Up to 69% decrease in biofilm conductance and deviation from Ohm's law were observed in the current-voltage profile when the measurement time increased. By choosing the voltage range (0– 100 mV) and step (25 mV), measurement time (100-s at each voltage step), and anode control mode, these operation settings were found more suitable for consistent and accurate biofilm conductance measurement in the 2-Au MEC system. This represents the first study that comprehensively evaluated the environmental and instrumental parameters for biofilm conductance measurement.

Decolorization mechanisms of reactive yellow 145 and ponceau S in microbial fuel cells during simultaneous electricity production

Environmental pollution is increasing in parallel with the increase in the world population. Azo dyes are one of the most important causes of environmental pollution. Microbial electrochemical cells are biotechnological systems that generate energy from renewable sources such as electricity. This study investigated simultaneous electricity generation with the decolorization of two different azo dyes in microbial fuel cells. And also, changes in pH values, chemical oxygen demand analysis, hourly color removal rate, dye spectral scanning were investigated. Reactive Yellow 145 dye with a concentration of 10 mg/L, 20 mg/L, and 40 mg/L, and Ponceau S dye with 20 mg/L and 40 mg/L concentration were tested in microbial fuel cells, respectively. Results indicate that the maximum voltage obtained was 0.11 V at the same time as the 100% decolorization rate in Reactive Yellow 145 and was achieved at a concentration of 10 mg/L also, the maximum voltage obtained was 0.24 V at the same time as the 100% decolorization rate in Ponceau S. It was achieved at a concentration of 20 mg/L. In conclusion, microbial fuel cells appear to be promising tools in treating textile azo dye wastewaters, and computational methods can be applied to estimate the degradation mechanisms of complex organic molecules found in wastewaters.

Biohydrogen production and purification: Focusing on bioelectrochemical systems

Innovative technologies on green hydrogen production become significant as the hydrogen economy has grown globally. Biohydrogen is one of green hydrogen production methods, and microbial electrochemical cells (MECs) can be key to biohydrogen provision. However, MECs are immature for biohydrogen technology due to several limitations including extracellular electron transfer (EET) engineering. Fundamental understanding of EET also needs more works to accelerate MEC commercialization. Interestingly, studies on biohydrogen gas purification are limited although biohydrogen gas mixture requires complex purification for use. To facilitate an MEC-based biohydrogen technology as the green hydrogen supply this review discussed EET kinetics, engineering of EET and direct interspecies electron transfer associated with hydrogen yield and the application of advanced molecular biology for improving EET kinetics. Finally, this article reviewed biohydrogen purification technologies to better understand purification and use appropriate for biohydrogen, focusing on membrane separation.

Maize processing wastewater for electricity production in a microbial electrochemical cell

Electrochemical cells have been fed organic matter in wastewater for depollution and energy production via microbial catalysis. Maize processing wastewater (MPWW) is rich in organics but is extremely alkaline, hindering the survival of microorganisms. The aim of the present work was to determine the feasibility of using MPWW to produce electricity in a bioelectrochemical cell. Raw MPWW was tested for electrochemically active microorganisms, thereafter a dilution strategy was implemented (1:2, 1:4, 1:10). A potential of +0.1 V was applied to the bioelectrochemical cell for 12 days and the current production was monitored as well as the decrease in chemical oxygen demand (COD). The raw MPWW contained electrochemically active microorganisms and the 1:2 dilution was the most favorable for current production reaching 100 mA m-2 in 9 days. The highest COD removal was 14%. This work demonstrates that MPWW can be exploited by industries for a cleaner production.

Primary sludge-based blackwater favors electrical current over methane production in microbial electrochemical cells

Microbial electrochemical cells (MXCs) represent a technology that could potentially be used for decentralized blackwater treatment and resource recovery in remote locations, such as space. One challenge for blackwater treatment in MXCs is achieving sufficient conversion of complex organics to electrical current, especially by preventing electron diversion to dead-end products, such as methane. In this study, we evaluated the effect of source separation (treating feces and urine separately versus combined) on electron diversion to methane in MXCs. We operated five microbial electrolysis cells (MECs) in batch mode, two treating a synthetic primary sludge-based blackwater (PS-BW) with total ammonia nitrogen (TAN) concentration of 475 mg N/L representing combined blackwater (urine and feces), two treating PS-BW with a TAN concentration of 40 mg N/L representing a source separated blackwater (feces only), and one control fed only inoculum, which was anaerobic digester sludge. Chemical oxygen demand (COD) conversion to electrical current in the low and high TAN PS-BW fed MXC was 22% and 21%, respectively, while 1% or less was to methane. Biochemical methane potential assays demonstrated possible methane production from the PS-BW fed to the MXCs. Dysgonomonadaceae and Rikenellaceae dominated the suspension microbial communities of PS-BW fed MECs compared to the inoculum. Bacteria belonging to family Geobacteraceae, known anode-respiring bacteria, dominated the biofilms. Overall, PS-BW, whether combined or source-separated, appeared to limit electron diversion to undesirable methane likely due to the complexity of the PS-BW and the pre-establishment of a robust anode biofilm.

Accelerating the extracellular electron transfer of Shewanella oneidensis MR-1 by carbon dots: The role of carbon dots concentration

The high performance of microbial extracellular electron transfer (EET) is essential for bioremediation and bioenergy generation. Carbon dots (CDs) modified electroactive microorganisms showed an increased electron transfer efficiency. However, the influence of CDs’ concentration the EET has not been illustrated before. Herein, we reported that the addition of CDs increased the current generation by accelerating the EET process. The maximum current output of microbial electrochemical cells with the addition of CDs was increased by more than 5 times compared to pure Shewanella oneidensis MR-1. With CDs addition, the increase of biofilm immobilization and flavins secretion improved the direct and mediated EET process. Electrochemical analyses clarified that the combination of cytochrome c with CDs resulted in a faster electron transfer rate. The overload of CDs would restrict their promotion effect on EET, which may be due to their biotoxicity to bacteria. This work offered the insight of CDs’ concentration on the efficiency of EET, not only promoting the related applications of EET but also inspiring the regulation of microorganism function via nanomaterial.

Enhancing quorum sensing in biofilm anode to improve biosensing of naphthenic acids

The feasibility of enhancing quorum sensing (QS) in anode biofilm to improve the quantifications of commercial naphthenic acid concentrations (9.4–94 mg/L) in a microbial electrochemical cell (MXC) based biosensor was demonstrated in this study. First, three calibration methods were systematically compared, and the charging-discharging operation was selected for further experiments due to its 71–227 folds higher electrical signal outputs than the continuous closed-circuit operation and cyclic voltammetry modes. Then, the addition of acylase (5 μg/L) as an exogenous QS autoinducer (acylase) was investigated, which further improved the biosensor's electrical signal output by ∼70%, as compared to the control (without acylase). The addition of acylase increased the relative expression of QS-associated genes (lasR, lasI, rhlR, rhlI, lasA, and luxR) by 7–100%, along with increased abundances of known electroactive bacterial genera, such as Geobacter (from 42% to 47%) and Desulfovibrio (from 6% to 11%). Furthermore, toxicities of different NAs concentrations measured with the Microtox bioassay test were correlated with corresponding electrical signals, indicating that MXC-biosensor can provide a dual platform for rapid assessment of both NA concentrations and NA-associated toxicity.

Green electricity-driven simultaneous ammonia recovery and in-situ upcycling for microbial protein production

Currently, hydrogen-oxidizing bacteria (HOB) based power-to-protein is a promising approach to produce alternative microbial protein (MP), however, the nitrogen source used was either derived from commercial products or was firstly recovered from waste streams and then diluted for HOB growth. In the present study, simultaneous ammonium recovery from wastewater and in-situ utilization for the green MP (derived from Cupriavidus necator 335) production was successfully demonstrated using a microbial electrochemical recovery conversion cell (MERC). 0.41 ∼ 0.82 g/L of dried biomass (protein content 49 ∼ 63%) was yielded in 36 h with a power supply of 3, 4, and 5 V. C. necator 335 could grow in the MERC system receiving wastewater with a broad range of ammonium (0.05 ∼ 8 g N/L) and the highest biomass production of 0.9 g/L (protein content 54%) was achieved at 2 g N/L. 2.69 g/L of dried biomass containing 57% protein was obtained in 120 h with an initial supply of 1 L CO2 and 2 g N-NH4+. Applied voltages and ammonium concentrations showed a minor impact on the amino acid profile. Furthermore, the MERC system was tested with real waste streams (e. g., municipal wastewater, and digestate) and 0.45 ∼ 1.22 g biomass/L (protein content 52 ∼ 62%) were harvested. The characteristics of the wastewater streams (e. g., ammonium concentration and conductivity) could significantly affect the system performances. The harvested MP from real wastewater showed a high quality of amino acid profile and implied a potential in substituting traditional plant/animal-based protein.

Theory of transport and recovery in microbial electrosynthesis of acetate from [formula omitted

Microbial electrosynthesis (MES) provides a sustainable route for the conversion of CO2 and electricity into acetate and other organics. The conversion of CO2 takes place at a biologically active cathode (‘biocathode’), which is typically separated from the anode by an ion exchange membrane. Since both charged and uncharged species participate in the reaction, understanding the transport of these species through the membrane, and how this depends on the type of membrane, is of key importance. We develop a theory for ion mass transport and conversion in these types of microbial electrochemical cells. The theory includes ion transport, acid-base reactions, as well as electrochemical reactions at the electrodes. We first analyze a cell configuration including three compartments, in which the acetate recovery compartment in the middle is separated from the outer compartments by one cation exchange membrane and one anion exchange membrane, and we compare with experimental data from literature. Analysis of ion transport across the ion exchange membranes revealed that acetic acid/acetate and carbonic acid/bicarbonate species were used as proton shuttles between the catholyte compartment and the recovery compartment. We also analyzed a system including a bipolar membrane (BPM). Our results showed that a commonly made assumption that in BPMs the charge is solely carried by protons and hydroxyl ions, produced inside the BPM, is not generally correct. In our calculation charge is mainly carried by protons in the cation exchange layer of the BPM, while bisulphate and sulphate ions carry the charge in the anion exchange layer. In conclusion, we show that the ions which participate in acid-base reactions have to be considered in detail to describe and explain ion transport in MES cells and in the elements thereof such as BPMs.

Coupling Microbial Electrolysis Cell and Activated Carbon Biofilter for Source-Separated Greywater Treatment

Reclamation and reuse of wastewater are increasingly viewed as a pragmatic tool for water conservation. Greywater, which includes water from baths, washing machines, dishwashers, and kitchen sinks, is a dilute wastewater stream, making it an attractive stream for extraction of non-potable water. However, most previous studies primarily focused on passively aerated biological and physicochemical treatment processes for greywater treatment. Here, we investigated an integrated process of a microbial electrochemical cell (MEC) followed by granular activated carbon (GAC) biofilter for greywater treatment. The integrated system could achieve 99.3% removal of total chemical oxygen demand (TCOD) and 98.7% removal of the anionic surfactants (linear alkylbenzene sulphonates) from synthetic greywater at a total hydraulic residence time (HRT) of 25 h (1 day for MEC and 1 h for GAC biofilter). For one-day HRT, the maximum peak volumetric current density from MEC was 0.65 A/m3, which was comparable to that achieved at four-day HRT (0.66 A/m3). The adsorption by GAC was identified as a key mechanism for the removal of organics and surfactants. In addition, recirculation of liquid within the GAC biofilter was identified as a critical factor in achieving high-rate treatment. Although results indicated that GAC biofilter could be a standalone process for greywater, MEC can provide an opportunity for potential energy recovery from greywater. However, further studies should focus on developing high-rate MECs with higher energy recovery potential for practical operation.

Hydrogen production automatic control in continuous microbial electrolysis cells reactors used in wastewater treatment

In this paper, two control laws are proposed and applied in a model for a continuous Microbial Electrochemical Cells system. The used model is based on mass balances describing the behavior of substrate consumption, microbial growth, competition between anodophilic and methanogenic microorganisms for the carbon source in the anode, hydrogen generation, and electrical current production. The main control objective is to improve the electrical current generated and thus the production of bio-hydrogen gas in the reactor, using the dilution rate and the applied potential as individual control input variables. The control laws implemented are nonlinear adaptive type. In order to demonstrate its usefulness, numerical simulation runs involving multiple set-point changes and input perturbations were conducted for each control variable. The results of these simulations show that both control laws were able to respond adequately and efficiently to the disturbances and reach the reference value to which they were subjected. Moreover, it is possible to control both the electrical current produced and the hydrogen produced. Finally, these simulations also show that the highest rate of hydrogen production can be obtained using the applied potential as a control input, but such productivity is only attainable for a short period of time.