Effects of hydraulic pressure on the performance of single chamber air-cathode microbial fuel cells

Microbial fuel cell in long-term operation and providing electricity for intermittent aeration to remove contaminants from sewage

Microbial fuel cells (MFCs) show promise in sewage treatment because they can directly convert organic matter (OM) into electricity. This study aimed to demonstrate MFCs stability over 750 days of operation and efficient removal of OM and nitrogenous compounds from sewage. To enhance contaminant removal, oxygen was provided into the anode chamber via a mini air pump. This pump was powered by the MFCs' output voltage, which was boosted using a DC-DC converter. The experimental system consisted of 12 sets of cylindrical MFCs within a 246L-scale reactor. The boosted voltage reached 4.7 V. This voltage was first collected in capacitors every 5 min and then dispensed intermittently to the air pump for the MFCs reactor in 4 s. This corresponds to receiving average DO concentration reaching 0.34 ± 0.44 mg/L at 10 cm above the air-stone. Consequently, the degradation rate constants (k) for chemical oxygen demand (COD) and biological oxygen demand (BOD) in the presence of oxygen were 0.048 and 0.069, respectively, which surpassed those without oxygen by 0.039 and 0.044, respectively. Aeration also marginally improved the removal of ammonia because of its potential to create a favorable environment for the growth of anammox and ammonia-oxidizing bacteria such as Candidatus brocadia and Nitrospira. The findings of this study offer in-depth insight into the benefits of boosted voltage in MFCs, highlighting its potential to enhance contaminant degradation. This serves as a foundation for future research focused on improving MFCs performance, particularly for the removal of contaminants from wastewater.

Impact of reactor configuration on pilot-scale microbial fuel cell performance

Different microbial fuel cell (MFC) configurations have been successfully operated at pilot-scale levels (>100 L) to demonstrate electricity generation while accomplishing domestic or industrial wastewater treatment. Two cathode configurations have been primarily used based on either oxygen transfer by aeration of a liquid catholyte or direct oxygen transfer using air-cathodes. Analysis of several pilot-scale MFCs showed that air-cathode MFCs outperformed liquid catholyte reactors based on power density, producing 233% larger area-normalized power densities and 181% higher volumetric power densities. Reactors with higher electrode packing densities improved performance by enabling larger power production while minimizing the reactor footprint. Despite producing more power than the liquid catholyte MFCs, and reducing energy consumption for catholyte aeration, pilot MFCs based on air-cathode configuration failed to produce effluents with chemical oxygen demand (COD) levels low enough to meet typical threshold for discharge. Therefore, additional treatment would be required to further reduce the organic matter in the effluent to levels suitable for discharge. Scaling up MFCs must incorporate designs that can minimize electrode and solution resistances to maximize power and enable efficient wastewater treatment.

Review on microbial fuel cells applications, developments and costs

The microbial fuel cell (MFC) technology has attracted significant attention in the last years due to its potential to recover energy in a wastewater treatment. The idea of using an MFC in industry is very attractive as the organic wastes can be converted into energy, reducing the waste disposal costs and the energy needs while increasing the company profit. However, taking aside these promising prospects, the attempts to apply MFCs in large-scale have not been succeeded so far since their lower performance and high costs remains challenging. This review intends to present the main applications of the MFC systems and its developments, particularly the advances on configuration and operating conditions. The diagnostic techniques used to evaluate the MFC performance as well as the different modeling approaches are described. Towards the introduction of the MFC in the market, a cost analysis is also included. The development of low-cost materials and more efficient systems, with high higher power outputs and durability, are crucial towards the application of MFCs in industrial/large scale. This work is a helpful tool for discovering new operation and design regimes.

How Comparable are Microbial Electrochemical Systems around the Globe? An Electrochemical and Microbiological Cross-Laboratory Study

A cross-laboratory study on microbial fuel cells (MFC) which involved different institutions around the world is presented. The study aims to assess the development of autochthone microbial pools enriched from domestic wastewater, cultivated in identical single-chamber MFCs, operated in the same way, thereby approaching the idea of developing common standards for MFCs. The MFCs are inoculated with domestic wastewater in different geographic locations. The acclimation stage and, consequently, the startup time are longer or shorter depending on the inoculum, but all MFCs reach similar maximum power outputs (55±22 μW cm-2 ) and COD removal efficiencies (87±9 %), despite the diversity of the bacterial communities. It is inferred that the MFC performance starts when the syntrophic interaction of fermentative and electrogenic bacteria stabilizes under anaerobic conditions at the anode. The generated power is mostly limited by electrolytic conductivity, electrode overpotentials, and an unbalanced external resistance. The enriched microbial consortia, although composed of different bacterial groups, share similar functions both on the anode and the cathode of the different MFCs, resulting in similar electrochemical output.

Nitric oxide reduction by microbial fuel cell with carbon based gas diffusion cathode for power generation and gas purification

Nitric oxide (NO) from anthropogenic emission is one of the main air contaminants and induces many environmental problems. Microbial fuel cells (MFCs) with gas diffusion cathode provide an alternative technology for NO reduction. In this work, pure NO as the sole electron acceptor of MFCs with gas diffusion cathode (NO-MFCs) was verified. The NO-MFCs obtained a maximum power density of 489 ± 50 mW/m². Compared with MFCs using O₂ in air as electron acceptor (Air-MFCs), the columbic efficiency increased from 23.2% ± 4.3% (Air-MFCs) to 55.7% ± 4.6% (NO-MFCs). The NO removal rate was 12.33 ± 0.14 mg/L/h and N₂ was the main reduction product. Cathode reduction was the dominant pathway of NO conversion in NO-MFCs, including abiotic electrochemical reduction and microbial denitrification process. The predominant genera in anodic microbial community changed from exoelectrogenic bacteria in Air-MFCs to denitrifying bacteria in NO-MFCs and effected the power generation.

Optimising the Hydraulic Retention Time in a Pilot-Scale Microbial Electrolysis Cell to Achieve High Volumetric Treatment Rates Using Concentrated Domestic Wastewater

Bioelectrochemical systems (BES) have the potential to deliver energy-neutral wastewater treatment. Pilot-scale tests have proven that they can operate at low temperatures with real wastewaters. However, volumetric treatment rates (VTRs) have been low, reducing the ability for this technology to compete with activated sludge (AS). This paper describes a pilot-scale microbial electrolysis cell (MEC) operated in continuous flow for 6 months. The reactor was fed return sludge liquor, the concentrated filtrate of anaerobic digestion sludge that has a high chemical oxygen demand (COD). The use of a wastewater with increased soluble organics, along with optimisation of the hydraulic retention time (HRT), resulted in the highest VTR achieved by a pilot-scale MEC treating real wastewater. Peak HRT was 0.5-days, resulting in an average VTR of 3.82 kgCOD/m³∙day and a 55% COD removal efficiency. Finally, using the data obtained, a direct analysis of the potential savings from the reduced loading on AS was then made. Theoretical calculation of the required tank size, with the estimated costs and savings, indicates that the use of an MEC as a return sludge liquor pre-treatment technique could result in an industrially viable system.

Enhanced removal of veterinary antibiotic from wastewater by photoelectroactive biofilm of purple anoxygenic phototroph through photosynthetic electron uptake

Purple anoxygenic phototrophs have been recently attracted substantial attention for their growing potential in wastewater treatment and their diverse metabolic patterns can be regulated for process control and optimization. In this study, the photoheterotrophic metabolism of Rhodopseudomonas palustris (R. palustris) was modified by photosynthetic electron uptake using a poised electrode which was explored to enhance removal of veterinary antibiotic from aqueous medium. The results showed that R. palustris grown as biofilm on electrode surface had excellent photoelectroactive activity and the photosynthetic electron uptake from the photoelectroactive biofilm significantly enhanced antibiotic florfenicol (FLO) degradation. The specific degradation rate of FLO at the set electrode potential of 0 V was 2.59-fold higher than that without applied potential. Enhanced co-metabolic reductive dehalogenation by use of the photosynthetic electrons extracted from co-substrate was mainly responsible for FLO degradation which eliminated the antibacterial activity of FLO. The electrode potential controlled the processes of photosynthetic electron uptake and its resultant FLO degradation. The fastest degradation of FLO was achieved at 0 V because the electrode poised at this potential stroke a proper balance between the enhancing photosynthetic electron uptake by serving as electron acceptor and minimizing competition with FLO for the photosynthetic electron from co-substrate. The activity of photoelectroactive biofilm was not negatively affected by FLO at environmental relevant concentration, suggesting its great potential for removal of antibiotic contaminants in wastewater. R. palustris could serve as a reservoir for floR resistance gene but its abundance can be diminished by choosing appropriate electrode potential.

Enhancement of the denitrification activity by exoelectrogens in single-chamber air cathode microbial fuel cells

Single-chamber microbial fuel cells (MFCs) can efficiently treat wastewater containing nitrate, probably because the interaction between exoelectrogens and denitrifying bacteria may enhance the denitrification activity of MFCs. In this study, the denitrification of nitrate with a wide range of concentrations was investigated by using single-chamber air cathode MFCs. The maximum average denitrification rate of the MFCs inoculated and operated under closed-circuit conditions (Group N-CC) was up to 12.2 ± 0.6 kg NO₃^--N m^-3 d^-1 at a high nitrate concentration of 2000 mg NO₃-N L^-1, which was 74.3% higher than that of the MFCs inoculated and operated under open-circuit conditions and which was significantly higher than those of other MFC systems and many traditional bioreactors. The high denitrification activity of the MFCs of Group N-CC was attributed to the significant reduction of nitrite accumulation through the possible bioelectrochemical nitrite reduction by exoelectrogens that were only enriched at the anodes of the MFCs of Group N-CC. In addition, the MFCs of Group N-CC showed good stability (over 3.5 years) and low apparent activation energy (34.0 kJ mol^-1) of the denitrification, indicating the good coexistence of exoelectrogens (Geobacter) and denitrifying bacteria (Thauera) with high performance on denitrification during the long-term operation.

Evaluating a multi-panel air cathode through electrochemical and biotic tests

To scale up microbial fuel cells (MFCs), larger cathodes need to be developed that can use air directly, rather than dissolved oxygen, and have good electrochemical performance. A new type of cathode design was examined here that uses a "window-pane" approach with fifteen smaller cathodes welded to a single conductive metal sheet to maintain good electrical conductivity across the cathode with an increase in total area. Abiotic electrochemical tests were conducted to evaluate the impact of the cathode size (exposed areas of 7 cm², 33 cm², and 6200 cm²) on performance for all cathodes having the same active catalyst material. Increasing the size of the exposed area of the electrodes to the electrolyte from 7 cm² to 33 cm² (a single cathode panel) decreased the cathode potential by 5%, and a further increase in size to 6200 cm² using the multi-panel cathode reduced the electrode potential by 55% (at 0.6 A m^-2), in a 50 mM phosphate buffer solution (PBS). In 85 L MFC tests with the largest cathode using wastewater as a fuel, the maximum power density based on polarization data was 0.083 ± 0.006 W m^-2 using 22 brush anodes to fully cover the cathode, and 0.061 ± 0.003 W m^-2 with 8 brush anodes (40% of cathode projected area) compared to 0.304 ± 0.009 W m^-2 obtained in the 28 mL MFC. Recovering power from large MFCs will therefore be challenging, but several approaches identified in this study can be pursued to maintain performance when increasing the size of the electrodes.

In situ biofilm removal from air cathodes in microbial fuel cells treating domestic wastewater

One challenge in using microbial fuel cells (MFCs) for wastewater treatment is the reduction in performance over time due to cathode fouling. An in-situ technique was developed to clean air cathodes using magnets on either side of the electrode, with the air-side magnet moved to clean the water-side magnet by scraping off the biofilm. The power output of the magnet-cleaned cathodes after one month of operation was 132 ± 7 mW m^-2, which was 42% higher than the controls with no magnet (93 ± 4 mW m^-2) (no separator, NS), and 110% higher (116 ± 4 mW m^-2) than controls with separators (Sp, 55 ± 7 mW m^-2). Cleaning cathodes using magnets reduced the biofilm by 75% (NS) and 28% (Sp). The in-situ cleaning technique thus improved the performance of the MFC over time by reducing biofouling due to biofilm formation on the air cathodes.

Impacts of process parameters optimization on the performance of the annular single chamber microbial fuel cell in wastewater treatment

Energy harvest from optimized annular single chamber microbial fuel cell (ASCMFC) with novel configuration, which treats chocolate industry wastewater, was investigated. In this study, optimization of operational parameters of the ASCMFC in terms of efficiency water-soluble organic matter reduction and capability of electricity generation was evaluated. During the experiment, effluent from the anode compartment was examined through current and power density curves for variation in temperature and pH, chemical oxygen demand (COD), and turbidity removal, and substrate concentration. The performance analyzed at different temperature ranges such as 25, 30, 35, and 40°C, which showed 88% increase by uprising temperature from 25 to 35°C. The ASCMFC was used to produce electricity by adjusting pH between 5 and 9 at resistance of 100 Ω. Under the condition of pH 7 power density (16.75 W/m³) was highest, which means natural pH is preferred to maximize microbial activities. Wastewater concentration with COD of 700 and 1400 mg/L were investigated to determine its affection on current production. Reduction of current density was observed due to decrease in wastewater concentration. Significant reduction in COD and turbidity of effluent were 91 and 78%, respectively. The coulombic efficiency of 45.1% was achieved.

CeO2 doped Pt/C as an efficient cathode catalyst for an air-cathode single-chamber microbial fuel cell

Incorporation of nanophase ceria into the cathode catalyst Pt/C was used as alternative cathode catalysts for the oxygen reduction reaction in an air-cathode single-chamber microbial fuel cell (SCMFC) for the first time.

Temporal-spatial changes in viabilities and electrochemical properties of anode biofilms

Sustained current generation by anodic biofilms is a key element for the longevity and success of bioelectrochemical systems. Over time, however, inactive or dead cells can accumulate within the anode biofilm, which can be particularly detrimental to current generation. Mixed and pure culture (Geobacter anodireducens) biofilms were examined here relative to changes in electrochemical properties over time. An analysis of the three-dimensional metabolic structure of the biofilms over time showed that both types of biofilms developed a live outer-layer that covered a dead inner-core. This two-layer structure appeared to be mostly a result of relatively low anodic current densities compared to other studies. During biofilm development, the live layer reached a constant thickness, whereas dead cells continued to accumulate near the electrode surface. This result indicated that only the live outer-layer of biofilm was responsible for current generation and suggested that the dead inner-layer continued to function as an electrically conductive matrix. Analysis of the electrochemical properties and biofilm thickness revealed that the diffusion resistance measured using electrochemical impedance spectroscopy might not be due to acetate or proton diffusion limitations to the live layer, but rather electron-mediator diffusion.

N-type Cu2O doped activated carbon as catalyst for improving power generation of air cathode microbial fuel cells

A novel n-type Cu2O doped activated carbon (AC) air cathode (Cu/AC) was developed as an alternative to Pt electrode for oxygen reduction in microbial fuel cells (MFCs). The maximum power density of MFCs using this novel air cathode was as high as 1390±76mWm(-2), almost 59% higher than the bare AC air cathode. Specifically, the resistance including total resistance and charge transfer resistance significantly decreased comparing to the control. Tafel curve also showed the faster electro-transfer kinetics of Cu/AC with exchange current density of 1.03×10(-3)Acm(-2), which was 69% higher than the control. Ribbon-like Cu2O was deposited on the surface of AC with the mesopore surface area increasing. Cubic Cu2O crystals exclusively expose (111) planes with the interplanar crystal spacing of 2.48Å, which was the dominate active sites for oxygen reduction reaction (ORR). N-type Cu2O with oxygen vacancies played crucial roles in electrochemical catalytic activity.