Performance of plug flow microbial fuel cell (PF-MFC) and complete mixing microbial fuel cell (CM-MFC) for wastewater treatment and power generation

Two flow patterns (plug flow (PF) and complete mixing (CM)) of microbial fuel cells (MFCs) with multiple anodes–cathodes were compared in continuous flow mode for wastewater treatment and power generation. The results indicated that PF-MFCs had higher power generation and columbic efficiency (CE) than CM-MFCs, and the power generation varied along with the flow pathway in the PF-MFCs. The gradient of substrate concentrations along the PF-MFCs was the driving force for the power generation. In contrast, the CM-MFCs had higher wastewater removal efficiency than PF-MFCs, but had lower power conversion efficiency and power generation. This work demonstrated that MFC configuration is a key factor for enhancing power generation and wastewater treatment.     

Enhanced sludge stabilization coupled with microbial fuel cells (MFCs)

This study presents research results on electricity production from waste activated sludge using MFCs during stabilization process. Different MFC configurations equipped with various electrodes were used. Voltage measurements were continuously done during 35 days of MFC operation. Experimental results showed that bioelectricity generation was linked to volatile solids (VS) and protein reductions as a fraction of extracellular polymeric substances (EPS). Double chamber MFC reactor equipped with graphite electrodes had better power and current densities as 312.98 mW/m² and 39.07 μA/cm² while single chamber MFC equipped with titanium electrodes revealed better power and current densities as 97.60 mW/m² and 17.63 μA/cm², respectively. Molecular results indicated that power outputs of MFCs effected by diverse microbial communities in anode biofilms. Although organic matter degradation is reported as 35%–55% VS reduction for digesters, this research provided a promising approach for sludge stabilization with enhanced degrading of organic matters up to 75% by using MFCs.     

Ceramic-based microbial fuel cells (MFCs): A review

Recently, porous ceramic and clayware membranes have been widely used in microbial fuel cells (MFCs) as separators. Chemical, thermal and mechanical stability, low-cost and many other advantages of ceramic membranes make them an appropriate substitute for expensive polymeric ion exchange membranes. Moreover, good power performances in short and long-term periods were observed using ceramic membranes. In this review, we attempted to gather and assort all the experiments which applied ceramic or other earthenware membranes as the separator of MFCs. The effects of physical and chemical properties of ceramic membranes on the power efficiency of MFCs as well as scale-up challenges and future aspects were also studied.     

Power generation using an activated carbon fiber felt cathode in an upflow microbial fuel cell

An activated carbon fiber felt (ACFF) cathode lacking metal catalysts is used in an upflow microbial fuel cell (UMFC). The maximum power density with the ACFF cathode is 315 mW m⁻², compared to lower values with cathodes made of plain carbon paper (67 mW m⁻²), carbon felt (77 mW m⁻²), or platinum-coated carbon paper (124 mW m⁻², 0.2 mg-Pt cm⁻²). The addition of platinum to the ACFF cathode (0.2 mg-Pt cm⁻²) increases the maximum power density to 391 mW m⁻². Power production is further increased to 784 mW m⁻² by increasing the cathode surface area and shaping it into a tubular form. With ACFF cutting into granules, the maximum power is 481 mW m⁻² (0.5 cm granules), and 667 mW m⁻² (1.0 cm granules). These results show that ACFF cathodes lacking metal catalysts can be used to substantially increase power production in UMFC compared to traditional materials lacking a precious metal catalyst.     

Power generation of microbial fuel cells (MFCs) with low cathodic platinum loading

This study aims at investigating the effects of platinum (Pt) loadings on the cathodic reactions in Single Chamber Microbial Fuel Cells (SCMFCs) and developing cost-effective MFC operational protocols. The power generation of SCMFCs was examined with different Pt loadings (0.005–1 mgPt/cm²) on cathodes. The results showed that the power generation of the SCMFCs with 0.5–1 mgPt/cm² were the highest in the tests, decreased 10–15% at 0.01–0.25 mgPt/cm², and decreased further 10–15% at 0.005 mgPt/cm². The SCMFCs with Pt-free cathode (graphite) had the lowest power generation. In addition, the power generation of SCMFCs with different Pt loadings were compared in raw wastewater (Chemical oxygen demand (COD): 0.36 g/L) and wastewater enriched with sodium acetate (COD: 2.95 g/L). The solution conductivity in SCMFCs decreased with the degradation of organic substrates. Daily polarization curves (V–I) showed a decrease in current generation and an increase in ohmic losses over the operational period (8 days). The SCMFCs (with 0.5–1 mgPt/cm² at cathode) fed with wastewater and sodium acetate (NaOAc) reached the highest power generation (786 mW/m²), while the SCMFCs (with 0.5–1 mgPt/cm² at cathode) fed only with wastewater obtained the lower power generation (81 mW/m²). The study demonstrated that lowering the Pt loadings in two magnitude orders (1 to 0.01, 0.5 to 0.005 mgPt/cm²) only reduced the power generation of 15–30%, and this reduction of the power generation become less substantial with the decrease in the solution conductivity of SCMFCs.     

A dual photoelectrode-based double-chambered microbial fuel cell applied for simultaneous COD and Cr (VI) reduction in wastewater

Cerium oxide (CeO₂) and cuprous oxide (Cu₂O) were used for the first time as photoanode and photocathode, respectively, in a microbial fuel cell (MFC) for simultaneous reduction of chemical oxygen demand (COD) and Cr(VI) in wastewater. The photoelectrodes, viz. Photoanode and photocathode were separately prepared by impregnating activated carbon fiber (ACF) with the respective metal oxide nanoparticles, followed by growing carbon nanofibers (CNFs) on the ACF substrate using catalytic chemical vapor deposition. The MFC, operated under visible light irradiation, showed reduction in COD and Cr(VI) by approximately 94 and 97%, respectively. The MFC also generated high bioelectricity with a current density of ~6918 mA/m² and a power density of ~1107 mW/m². The enhanced performance of the MFC developed in this study was attributed to the combined effects of the metal oxide photocatalysts, the graphitic CNFs, and the microporous ACF substrate. The MFC based on the inexpensive transition metal oxides-based photoelectrodes developed in this study has a potential to be used at a large scale for treating the industrial aqueous effluents co-contaminated with organics and toxic Cr(VI).     

Power generation using carbon mesh cathodes with different diffusion layers in microbial fuel cells

An inexpensive carbon material, carbon mesh, was examined to replace the more expensive carbon cloth usually used to make cathodes in air-cathode microbial fuel cells (MFCs). Three different diffusion layers were tested using carbon mesh: poly(dimethylsiloxane) (PDMS), polytetrafluoroethylene (PTFE), and Goretex cloth. Carbon mesh with a mixture of PDMS and carbon black as a diffusion layer produced a maximum power density of 1355 ± 62 mW m⁻² (normalized to the projected cathode area), which was similar to that obtained with a carbon cloth cathode (1390 ± 72 mW m⁻²). Carbon mesh with a PTFE diffusion layer produced only a slightly lower (6.6%) maximum power density (1303 ± 48 mW m⁻²). The Coulombic efficiencies were a function of current density, with the highest value for the carbon mesh and PDMS (79%) larger than that for carbon cloth (63%). The cost of the carbon mesh cathode with PDMS/Carbon or PTFE (excluding catalyst and binder costs) is only 2.5% of the cost of the carbon cloth cathode. These results show that low cost carbon materials such as carbon mesh can be used as the cathode in an MFC without reducing the performance compared to more expensive carbon cloth.     

Electricity generation and COD removal of microbial fuel cells (MFCs) operated with alkaline substrates

The characteristics of electricity generation and COD removal of dual-chamber microbial fuel cells (MFCs) operated with alkaline substrates were studied. Substrates with constant pH of either 7 or 9 as well as varying pH in a cycle of 7-8-9-8-7 were used. MFCs operated with these substrates were denoted as MFC-pH7, MFC-pH9 and MFC-pHV, respectively. The experimental results indicate that the MFC-pHV can generate the highest performance of 2554 ± 159 mW/m². Cyclic voltammetry (CV), active biomass and electrochemical impedance spectroscopy (EIS) measurements were conducted and these results suggested that the MFC-pHV had the highest electrochemical activity per unit biomass and the lowest internal resistance, which together contributed to the improved power output of the MFC-pHV. In addition, compared with the other two MFCs operated at fixed pH values, the COD removal efficiency of the MFC-pHV was improved due to the stronger adaptation to the varying pH-environment.     

Power generation and contaminant removal in single chamber microbial fuel cells (SCMFCs) treating human urine

The potential of single chamber microbial fuel cells (SCMFC) to treat raw, fresh human urine was investigated. The power generation (55 μW) of the SCMFCs with platinum (Pt)-based cathode was higher than those with Pt-free cathodes (23 μW) at the beginning of the tests, but this difference decreased over time. Up to 75% of the chemical oxygen demand (COD) in urine was reduced after a 4-day treatment. During this time, the ammonium concentration increased significantly to 5 gNH₄⁺-N/L in SCMFCs due to urea hydrolysis, while sulfate concentration decreased and transformed into H₂S due to sulfate reduction reactions. Calcium and magnesium concentrations dropped due to precipitation at high pH, and phosphorous decreased 20–50% due to the formation of struvite that was found on the cathode surface and on the bottom of the anodic chamber. The advantages of power generation, COD removal, and nutrient recovery make SCMFCs treating human urine a cost-effective biotechnology.     

Power generation from furfural using the microbial fuel cell

Furfural is a typical inhibitor in the ethanol fermentation process using lignocellulosic hydrolysates as raw materials. In the literature, no report has shown that furfural can be utilized as the fuel to produce electricity in the microbial fuel cell (MFC), a device that uses microbes to convert organic compounds to generate electricity. In this study, we demonstrated that electricity was successfully generated using furfural as the sole fuel in both the ferricyanide-cathode MFC and the air-cathode MFC. In the ferricyanide-cathode MFC, the maximum power densities reached 45.4, 81.4, and 103 W m⁻³, respectively, when 1000 mg L⁻¹ glucose, a mixture of 200 mg L⁻¹ glucose and 5 mM furfural, and 6.68 mM furfural were used as the fuels in the anode solution. The corresponding Coulombic efficiencies (CE) were 4.0, 7.1, and 10.2% for the three treatments, respectively. For pure furfural as the fuel, the removal efficiency of furfural reached up to 95% within 12 h. In the air-cathode MFC using 6.68 mM furfural as the fuel, the maximum values of power density and CE were 361 mW m⁻² (18 W m⁻³) and 30.3%, respectively, and the COD removal was about 68% at the end of the experiment (about 30 h). Increase in furfural concentrations from 6.68 to 20 mM resulted in increase in the maximum power densities from 361 to 368 mW m⁻², and decrease in CEs from 30.3 to 20.6%. These results indicated that some toxic and biorefractory organics such as furfural might still be suitable resources for electricity generation using the MFC technology.     

A monetary comparison of energy recovered from microbial fuel cells and microbial electrolysis cells fed winery or domestic wastewaters

Microbial fuel (MFCs) and electrolysis cells (MECs) can be used to recover energy directly as electricity or hydrogen from organic matter. Organic removal efficiencies and values of the different energy products were compared for MFCs and MECs fed winery or domestic wastewater. TCOD removal (%) and energy recoveries (kWh/kg-COD) were higher for MFCs than MECs with both wastewaters. At a cost of $4.51/kg-H₂ for winery wastewater and $3.01/kg-H₂ for domestic wastewater, the hydrogen produced using MECs cost less than the estimated merchant value of hydrogen ($6/kg-H₂). 16S rRNA clone libraries indicated the predominance of Geobacter species in anodic microbial communities in MECs for both wastewaters, suggesting low current densities were the result of substrate limitations. The results of this study show that energy recovery and organic removal from wastewater are more effective with MFCs than MECs, but that hydrogen production from wastewater fed MECs can be cost effective.     

Sequential microbial activities mediated bioelectricity production from distillery wastewater using bio-electrochemical system with simultaneous waste remediation

A two-chambered microbial fuel cell (MFC), which can function on the self-driven bio-electrogenic activity operated on anaerobically digested distillery waste (ADDW) i.e. wastewater post anaerobic digestion was designed and fabricated in the laboratory. MFC was evaluated for production of bioelectricity with a simultaneous reduction in the carbon content. Using a surface response methodology with a Box-Behnken design (BBD), operating conditions such as the concentration of antifoam, pH, and resistance were optimized and it was found that the pH and resistance were optimum at 8.3 and 1000 Ω, respectively with no antifoam in the system. Under optimum conditions, 31.49 Wm^?3 was generated, and 60.78 ± 0.95% total organic carbon was degraded. We revealed that the fermentative bacteria generated organic acids mainly acetate from dextrose present in ADDW and electrogenic bacteria oxidized acetate in a successive manner to generate electrons, which was confirmed by gas chromatography. The development of biofilm analyzed by scanning electron microscope (SEM) was found to be crucial in the transfer of electrons directly to the anode and was confirmed by cyclic voltammetry experiments. Identification of bacteria from biofilm by both culture and denaturing gradient gel electrophoresis methods found bacteria belonging to phylum Firmicutes and γ-proteobacteria. The study of successive nature of bacterial metabolism to generate electricity could play an important role in the production of electricity in a continuous mode of operation using MFCs fed with ADDW for further reduction of carbon content post anaerobic digestion for the benefit for the environment. Thus MFC can be used as a complementary technology to anaerobic digestion.     

Effect of dissolved oxygen concentration on nitrogen removal and electricity generation in self pH-buffer microbial fuel cell

A double-chamber self pH-buffer microbial fuel cell (MFC) was used to investigate the effect of dissolved oxygen (DO) concentration on cathodic nitrification coupled with anodic denitrification MFC. It was found that nitrogen and COD removal, electricity generation were positively correlated with DO concentration in the cathode chamber. When total inorganic nitrogen of influent was 202.51 ± 7.82 mg/L at DO 6.8 mg/L, the maximum voltage output was 282 mV and the maximum power density was 149.76 mW/m². After 82 h operation, the highest removal rate of total inorganic nitrogen was 91.71 ± 0.38%. Electrochemical impedance spectroscopy (EIS) test showed that the internal resistance of the reactor with different DO concentration was related to the diffusion internal resistance. The data of bacterial analysis in the cathode chamber revealed that there were not only ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB), but also a large number of exoelectrogens. Compared with the traditional biological denitrification and related MFC denitrification research, this method does not need pH-buffer solution and external circulation device through the anion exchange membrane (AEM). It can generate electricity and remove nitrogen simultaneously, and the oxygen utilization rate in the cathode can also be enhanced.     

Double-chamber microbial fuel cells started up under room and low temperatures

This study examined the performances of two double-chamber microbial fuel cells (MFCs) at 25 °C and 15 °C. After successful startup, the cell temperature of MFC A was decreased from 25 to 15 °C, yielding a sudden breakdown of the entire system. Conversely, the MFC B, started up at 15 °C, delivering higher power density at 25 °C than MFC A at the same temperature. The electrochemical analysis revealed that the MFC B had lower anodic resistance than MFC A. Additionally, a negative temperature dependence of the polarization resistances of the anodic biofilm was noted, a novel phenomenon only reported in this double-chambered study. Microbial analysis showed that the psychrophilic bacteria were enriched in anodic biofilms of MFC B, which likely contributed to the robust cell performance of the present double-chambered MFCs.     

Power recovery with multi-anode/cathode microbial fuel cells suitable for future large-scale applications

Multi-anode/cathode microbial fuel cells (MFCs) incorporate multiple MFCs into a single unit, which maintain high power generation at a low cost and small space occupation for the scale-up MFC systems. The power production of multi-anode/cathode MFCs was similar to the total power production of multiple single-anode/cathode MFCs. The power density of a 4-anode/cathode MFC was 1184 mW/m³, which was 3.2 times as that of a single-anode/cathode MFC (350 mW/m³). The effect of chemical oxygen demand (COD) was studied as the preliminary factor affecting the MFC performance. The power density of MFCs increased with COD concentrations. Multi-anode/cathode MFCs exhibited higher power generation efficiencies than single-anode/cathode MFCs at high CODs. The power output of the 4-anode/cathode MFCs kept increasing from 200 mW/m³ to 1200 mW/m³ as COD increased from 500 mg/L to 3000 mg/L, while the single-anode/cathode MFC showed no increase in the power output at CODs above 1000 mg/L. In addition, the internal resistance (R_in) exhibited strong dependence on COD and electrode distance. The R_in decreased at high CODs and short electrode distances. The tests indicated that the multi-anode/cathode configuration efficiently enhanced the power generation.     

Impact of salinity on cathode catalyst performance in microbial fuel cells (MFCs)

Several alternative cathode catalysts have been proposed for microbial fuel cells (MFCs), but effects of salinity (sodium chloride) on catalyst performance, separate from those of conductivity on internal resistance, have not been previously examined. Three different types of cathode materials were tested here with increasingly saline solutions using single-chamber, air-cathode MFCs. The best MFC performance was obtained using a Co catalyst (cobalt tetramethoxyphenyl porphyrin; CoTMPP), with power increasing by 24 ± 1% to 1062 ± 9 mW/m² (normalized to the projected cathode surface area) when 250 mM NaCl (final conductivity of 31.3 mS/cm) was added (initial conductivity of 7.5 mS/cm). This power density was 25 ± 1% higher than that achieved with Pt on carbon cloth, and 27 ± 1% more than that produced using an activated carbon/nickel mesh (AC) cathode in the highest salinity solution. Linear sweep voltammetry (LSV) was used to separate changes in performance due to solution conductivity from those produced by reductions in ohmic resistance with the higher conductivity solutions. The potential of the cathode with CoTMPP increased by 17–20 mV in LSVs when the NaCl addition was increased from 0 to 250 mM independent of solution conductivity changes. Increases in current were observed with salinity increases in LSVs for AC, but not for Pt cathodes. Cathodes with CoTMPP had increased catalytic activity at higher salt concentrations in cyclic voltammograms compared to Pt and AC. These results suggest that special consideration should be given to the type of catalyst used with more saline wastewaters. While Pt oxygen reduction activity is reduced, CoTMPP cathode performance will be improved at higher salt concentrations expected for wastewaters containing seawater.     

Anodic biofilm in single-chamber microbial fuel cells cultivated under different temperatures

The microorganisms in anodic biofilms of a microbial fuel cell (MFC) oxidize substrates to generate electrons, protons, and metabolic products. This study started up two single-chamber MFCs at different temperatures (25 °C for MFC A and 15 °C for MFC B); after successful startup, the cell temperatures were swapped. The MFC A had peak voltage at 540 mV at 25 °C, which was decreased rapidly as fed substrate was consumed. At 15 °C, the MFC A yielded a nearly constant voltage of 500 mV over complete feed cycle. Conversely, the MFC B produced higher maximum power than MFC A, and can deliver nearly constant voltage over the entire feed cycle at either 15 or 25 °C. Electrochemical analysis revealed that the MFC B had lower internal resistance than MFC A, with the former having much lower anodic resistance than the latter. Microbial analysis showed that the MFC started up at low temperatures had anodic biofilm enriched with psychrophilic bacteria Simplicispira psychrophila LMG 5408(T)[AF078755] and Geobacter psychrophilus P35(T)[AY653549]. This study suggests the strategy to promote the development of anodic biofilms at low temperatures that are capable of yielding electricity at constant voltage.     

Processing of electroplating industry wastewater through dual chambered microbial fuel cells (MFC) for simultaneous treatment of wastewater and green fuel production

Microbial fuel cells (MFC) provide a breakthrough development for wastewater treatment combined with electricity production. Though, MFC applications are restricted in laboratory scale level. Present study an effort has been made to employ the electroplating industrial wastewater as feedstock in dual chambered anaerobic microbial fuel cell for organic content removal as well as energy production. The ultimate goal of this research is to analyze the effect of organic load (OL) on removal of organic matter and power production. The maximum removal efficiency of total, soluble oxygen demands (TCOD, SCOD) and total suspended solids (TSS) of about 87%, 79% and 72% respectively was obtained at the OL of 1.5 gCOD/L. The maximum power and current density of about 260 mW/m² (6.2 W/m³) and 364 mA/m² was also recorded at a same OL of 1.5 gCOD/L. From the above findings proposed that utilization of high strength organic wastewater in MFC could pave the way to handle the problem of electroplating industries as well as minimize a small portion of energy demand.     

Multi-electrode microbial fuel cell (MEMFC): A close analysis towards large scale system architecture

Microbial fuel cells are capable of producing electricity through the treatment of wastewater, however, the low power density poses main hurdles towards their wide application. In present work, microbial fuel cell based on multiple anodes, acting as baffle is constructed for achieving higher performance which can be scaled up for real life application. It is investigated for continuous sixty two days using distillery wastewater (WW) in three batches under ambient condition. During first batch, the WW is maintained under stagnant condition inside the anode chamber where as in the rest of the two batches WW is recirculated in the chamber. A maximum power density 427 mW m⁻², is achieved under stagnant condition which is further enhanced to 597 mW m⁻² under recirculation mode. Recirculation of WW reduces the internal resistance arising from the mass transfer by 50%. Maximum COD removal and Coulombic efficiency obtained is 43% and 23%. Biofouling on the surface of the membrane facing anode chamber is observed.