Two-chamber Microbial Electrolysis Cells Research Articles

The importance of OH− transport through anion exchange membrane in microbial electrolysis cells

In two-chamber microbial electrolysis cells (MECs) with anion exchange membranes (AEMs), a phosphate buffer solution (PBS) is typically used to avoid increases in catholyte pH as Nernst equation calculations indicate that high pHs adversely impact electrochemical performance. However, ion transport between the chambers will also impact performance, which is a factor not included in those calculations. To separate the impacts of pH and ion transport on MEC performance, a high molecular weight polymer buffer (PoB), which was retained in the catholyte due to its low AEM transport and cationic charge, was compared to PBS in MECs and abiotic electrochemical half cells (EHCs). In MECs, catholyte pH control was less important than ion transport. MEC tests using the PoB catholyte, which had a higher buffer capacity and thus maintained a lower catholye pH (<8), resulted in a 50% lower hydrogen production rate (HPR) than that obtained using PBS (HPR = 0.7 m3-H2 m−3 d−1) where the catholyte rapidly increased to pH = 12. The main reason for the decreased performance using PoB was a lack of hydroxide ion transfer into the anolyte to balance pH. The anolyte pH in MECs rapidly decreased to 5.8 due to a lack of hydroxide ion transport, which inhibited current generation by the anode, whereas the pH was maintained at 6.8 using PBS. In abiotic tests in ECHs, where the cathode potential was set at −1.2 V, the HPR was 133% higher using PoB than PBS due to catholyte pH control, as the anolyte pH was not a factor in the performance. These results show that maintaining charge transfer to control anolyte pH is more important than obtaining a more neutral pH catholyte.

Anaerobic digestion and electromethanogenic microbial electrolysis cell integrated system: Increased stability and recovery of ammonia and methane

Abstract The integration of anaerobic digestion (AD) and a microbial electrolysis cell (MEC) with an electromethanogenic biocathode is proposed to increase the stability and robustness of the AD process against organic and nitrogen overloads; to keep the effluent quality; to recover ammonium; and to upgrade the biogas. A thermophilic lab-scale AD, fed with pig slurry, was connected in series with the bioanode compartment of a two-chambered MEC. In turn, the biocathode of the MEC was poised at −800 mV vs Standard Hydrogen Electrode and fed with CO2 to increase the methane production of the system. After doubling its organic and nitrogen loading rate, the AD operation became stable thanks to the connection of a recirculation loop with the MEC effluent. Ammonium removal in the anode compartment of the MEC achieved 14.46 g N-NH4+ m−2 d−1, while obtaining on average 79 L CH4 m−3 d−1 through the conversion of CO2 in the cathode compartment. The microbial analysis showed that methylotrophic Methanossiliicoccaceae family (Methanomassiliicoccus genus) was the most abundant among the metabolically active archaea in the AD during the inhibited state; while, on the cathode, Methanobacteriaceae family (Methanobrevibacter and Methanobacterium genus) shared dominance with Methanomassiliicoccaceae and Methanotrichaceae families (Methanomassiliicoccus and Methanothrix genus, respectively).

Effects of electron acceptors on removal of antibiotic resistant Escherichia coli, resistance genes and class 1 integrons under anaerobic conditions

Anaerobic biotechnologies can effectively remove antibiotic resistant bacteria (ARB) and antibiotic resistance genes (ARGs), but there is a need to better understand the mechanisms. Here we employ bioelectrochemical systems (BES) as a platform to investigate the fate of a native tetracycline and sulfonamide-resistant Escherichia coli strain and its ARGs. The E. coli strain carrying intI1, sulI and tet(E) was isolated from domestic wastewater and dosed into a tubular BES. The BES was first operated as a microbial fuel cell (MFC), with aeration in the cathode, which resulted in enhanced removal of E. coli and ARGs by ~2 log (i.e., order of magnitude) when switched from high current to open circuit operation mode. The BES was then operated as a microbial electrolysis cell (MEC) to exclude the effects of oxygen diffusion, and the removal of E. coli and ARGs during the open circuit configuration was again 1–2 log higher than that at high current mode. Significant correlations of E. coli vs. current (R2=0.73) and ARGs vs. E. coli (R2 ranged from 0.54 to 0.87), and the fact that the BES substrate contained no electron acceptors, implied that the persistence of the E. coli and its ARGs was determined by the availability of indigenous electron acceptors in the BES, i.e., the anode electrode or the electron shuttles generated by the exoelectrogens. Subsequent experiments with pure-culture tetracycline and sulfonamide-resistant E. coli being incubated in a two-chamber MEC and serum bottles demonstrated that the E. coli could survive by respiring anode electrode and/or electron shuttles released by exoelectrogens, and ARGs persisted with their host E. coli.

Periodic polarity reversal for stabilizing the pH in two-chamber microbial electrolysis cells

A successful pH control is of critical importance for ensuring efficient H2 production in two-chamber MECs. In this study, a novel two-chamber MEC is developed with biotic/abiotic hybrid electrodes, which are composed of carbon felt (CF) and stainless steel (SS) mesh. The CF is used for the electrogenic microorganisms attachment catalyzing the oxidation of organic matter (OM); the SS serves as the current collector and the H2 evolution catalyst. The MEC is operated in the periodic polarity reversal (PPR) mode and two hybrid electrodes work alternately as an anode and a cathode. With the hybrid electrodes and the PPR mode, the pH fluctuation is limited within the range of 6.4–8.6, and the H2 production is 5.3 times that of control without the PPR mode. The optimal reversal interval is 2h for maximal H2 production. The H2 production increases with the amplitude of the applied voltage (0.7–1.0V). The using of hybrid electrodes, combining with the PPR mode, provides a simple but efficient pH control strategy and better H2 production performance in MECs. The findings of this study are expected to facilitate the design of pilot-scale MECs or similar bio-electrochemical reactors in future research.

Promoted electromethanosynthesis in a two-chamber microbial electrolysis cells (MECs) containing a hybrid biocathode covered with graphite felt (GF)

Microbial electromethanogenesis, relying on electrochemically active biofilm on biocathode to convert carbon dioxide to methane, provides a novel approach for renewable energy storage. One key factor that governs electron exchange and methane formation efficiencies is the electrode material. To promote methane production, a biocathode via modifying plain carbon stick with a layer of graphite felt (GF) (hereafter referred as “hybrid GF-biocathode”) was developed and evaluated in a two-chamber microbial electrolysis cells (MECs). Methane production with hybrid GF-biocathode reached 80.9mL/L at the potential of −1.4V after 24h of incubation with coulombic efficiency of 194.4%. The tests by flushing three substrates (CO2, N2 and H2–CO2 [80:20]) revealed that direct electron transfer rather than intermediate H2 contributed more to the electromethanogenesis. Cyclic voltammetry showed that GF enhanced the microbial electrocatalysis activity and reduced the cathode overpotential needed for methane production. Scanning electron microscope and fluorescence in situ hybridization analysis confirmed that the three-dimensional GF afforded the abundant space for the growth of electroactive microorganisms and promoted the electron exchange (e.g. cathode-to-cell etc.) via severing as “artificial pili”. This study reveals that GF with the open structure and high conductivity has the substantial potential to upgrade electromethanogenesis efficiency.

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.

Understanding methane bioelectrosynthesis from carbon dioxide in a two-chamber microbial electrolysis cells (MECs) containing a carbon biocathode

To better understand the underlying mechanisms for methane bioelectrosynthesis, a two-chamber MECs containing a carbon biocathode was developed and studied. Methane production substantially increased with increasing cathode potential. Considerable methane yield was achieved at a poised potential of −0.9V (vs. Ag/AgCl), reaching 2.30±0.34mL after 5h of operation with a faradaic efficiency of 24.2±4.7%. Confirmatory tests done at 0.9V by switching the type of flushed substrates (CO2/N2) or the electrical exposure modes (ON/OFF) demonstrated that cathode serving as an electron donor was the vital driving force for methanogenesis occurring at microbe–electrode surface. Fluorescence in situ hybridization reveled Methanobacteriaceae (particularly Methanobacterium) was the predominant methanogens, supporting the mechanisms of direct electron transfer between cell-electrode. Additionally, the analysis of scanning electron microscope confirmed that the multiple pathways of electron transfer, including direct cathode-to-cell, interspecies exchange and semi-conductive conduits all together ensured the successful electromethanogenesis process.

Enhanced Performance of Bioelectrochemical Hydrogen Production using a pH Control Strategy

The use of membranes in microbial electrolysis cells (MEC) is required to obtain high-purity hydrogen and to avoid the consumption of hydrogen by undesired microorganisms. However, its utilization results in pH gradients across the membrane that contribute to potential losses and reduce the efficiency of MEC. Several pH-controlled and noncontrolled scenarios were evaluated in this work, which evidenced that pH control is beneficial for the MEC performance. The best results were obtained if the anodic and cathodic pH were controlled at 7.5 and 2.0, respectively, to produce 0.58 m(3) m(-3) d(-1) of hydrogen at an applied voltage of only 0.2 V. The energy efficiency with respect to the electrical input was increased up to 883 %. Anodic pH control allowed us to maintain a stable exoelectrogenic activity with practically constant current intensity, whereas cathodic pH control at 2.0 allowed a fivefold decrease of the required electrical input, which opens new opportunities for the economy of its full-scale application.

Autotrophic biocathode for high efficient sulfate reduction in microbial electrolysis cells

The aim of this study was to utilize the biocathode microbial electrolysis cell (MEC) for sulfate removal from wastewater. Experiments were conducted using the two-chambered MEC under fed-batch and continuous flow modes, respectively, with different cathode potentials. With the fed-batch operation, the average reductive rate of sulfate was 0.49mgd−1 and the sulfide concentration increased to 3.1±0.7mgL−1 in the catholyte. Sulfate removal rate and electron production rate in the continuous flow mode were 49% and 11 times higher than in the fed-batch mode. With cathode potentials from −0.6 to −1.0V, electron recovery efficiencies ranged from 5.3% to 50% with the maximum obtained at −0.8V. The maximum sulfate removal efficiency of (39±9.2)% was achieved at −0.9V. This study suggests the MEC can be a valuable alternative to remove sulfate in wastewater treatment.

Electrochemical struvite precipitation from digestate with a fluidized bed cathode microbial electrolysis cell

Microbial electrolysis cells (MECs) can be used to simultaneously convert wastewater organics to hydrogen and precipitate struvite, but scale formation at the cathode surface can block catalytic active sites and limit extended operation. To promote bulk phase struvite precipitation and minimize cathode scaling, a two-chamber MEC was designed with a fluidized bed to produce suspended particles and inhibit scale formation on the cathode surface. MEC operation elevated the cathode pH to between 8.3 and 8.7 under continuous flow conditions. Soluble phosphorus removal using digester effluent ranged from 70 to 85% with current generation, compared to 10–20% for the control (open circuit conditions). At low current densities (≤2 mA/m2), scouring of the cathode by fluidized particles prevented scale accumulation over a period of 8 days. There was nearly identical removal of soluble phosphorus and magnesium from solution, and an equimolar composition in the collected solids, supporting phosphorus removal by struvite formation. At an applied voltage of 1.0 V, energy consumption from the power supply and pumping (0.2 Wh/L, 7.5 Wh/g-P) was significantly less than that needed by other struvite formation methods based on pH adjustment such as aeration and NaOH addition. In the anode chamber, current generation led to COD oxidation (1.1–2.1 g-COD/L-d) and ammonium removal (7–12 mM) from digestate amended with 1 g/L of sodium acetate. These results indicate that a fluidized bed cathode MEC is a promising method of sustainable electrochemical nutrient and energy recovery method for nutrient rich wastewaters.

Two-stage conversion of crude glycerol to energy using dark fermentation linked with microbial fuel cell or microbial electrolysis cell

Crude glycerol is a main byproduct of the biodiesel industry, and the beneficial use of waste glycerol has been a major challenge. This study characterises the conversion of crude glycerol into bioenergy such as H2 and electricity using a two-stage process linking dark fermentation with a microbial fuel cell (MFC) or microbial electrolysis cell (MEC). The results showed that fermentation achieved a maximum H2 rate of 332 mL/L and a yield of 0.55 mol H2/mol glycerol, accompanied by 20% of organic removal. Fed with the raw fermentation products with an initial COD of 7610 mg/L, a two-chamber MFC produced 92 mW/m(2) in power density and removed 50% of COD. The Columbic efficiency was 14%. When fed with 50% diluted fermentation product, a similar power output (90m W/m(2)) and COD removal (49%) were obtained, but the CE doubled to 27%. Similar substrates were used to produce H2 in two-chamber MECs, and the diluted influent had a higher performance, with the highest yield at 106 mL H2/g COD and a CE of 24%. These results demonstrate that dark fermentation linked with MFC/MEC can be a feasible option for conversion of waste glycerol into bioenergy.

Assessment of biotic and abiotic graphite cathodes for hydrogen production in microbial electrolysis cells

Hydrogen represents a promising clean fuel for future applications. The biocathode of a two-chambered microbial electrolysis cell (biotic MEC) was studied and compared with an abiotic cathode (abiotic MEC) in order to assess the influence of naturally selected microorganisms for hydrogen production in a wide range of cathode potentials (from −400 to −1800 mV vs SHE). Hydrogen production in both MECs increased when cathode potential was decreased. Microorganisms present in the biotic MEC were identified as Hoeflea sp. and Aquiflexum sp. Supplied energy was utilized more efficiently in the biotic MEC than in the abiotic, obtaining higher hydrogen production respect to energy consumption. At −1000 mV biotic MEC produced 0.89 ± 0.10 m3 H2 d−1 m−3NCC (Net Cathodic Compartment) at a minimum operational cost of 3.2 USD kg−1 H2. This cost is lower than the estimated market value for hydrogen (6 USD kg−1 H2).

Bioelectrochemical analyses of a thermophilic biocathode catalyzing sustainable hydrogen production

To achieve sustainable hydrogen production by microbial electrolysis cell (MEC) without precious metal catalysts, we examined the potential of thermophilic microorganisms as biocatalysts on the cathode of MEC. A biocathode was firstly developed in a single-chambered MEC operated at 55 °C and further analyzed in a two-chambered MEC. Linear sweep voltammetry showed that the biocathode had a reducing activity significantly higher than the control electrodes (bioanode or non-inoculated electrode). At the potential of −0.8 V vs. SHE, the thermophilic biocathode produced a current density of 1.28 ± 0.15 A m−2 and an H2 production rate of 376.5 ± 73.42 mmol day−1 m−2, which were around 10 times higher than those of the non-inoculated electrode, with the cathodic H2 recovery of ca. 70%. The molecular-phylogenetic analysis of the bacteria on the biocathode indicated that the community was comprised of six phyla, in which Firmicutes was the most populated phylum (77% of the clones in the 16S rRNA library).

Assessment of four different cathode materials at different initial pHs using unbuffered catholytes in microbial electrolysis cells

Nickel foam (NF), stainless steel wool (SSW), platinum coated stainless steel mesh (Pt), and molybdenum disulfide coated stainless steel mesh (MoS2) electrodes have been proposed as catalysts for hydrogen gas production, but previous tests have primarily examined their performance in well buffered solutions. These materials were compared using two-chamber microbial electrolysis cells (MECs), and linear sweep voltammetry (LSV) in unbuffered saline solutions at two different initial pHs (7 and 12). There was generally no appreciable effect of initial pH on production rates or total gas production. NF produced hydrogen gas at a rate of 1.1 m3 H2/m3·d, which was only slightly less than that using Pt (1.4 m3 H2/m3·d), but larger than that obtained with SSW (0.52 m3 H2/m3·d) or MoS2 (0.67 m3 H2/m3·d). Overall hydrogen gas recoveries with SSW (29.7 ± 0.5 mL), MoS2 (28.6 ± 1.3 mL) and NF (32.4 ± 2 mL) were only slightly less than that of Pt (37.9 ± 0.5 mL). Total energy recoveries, based on the gas produced versus electrical energy input, ranged from 0.75 ± 0.02 for Pt, to 0.55 ± 0.02 for SSW. An LSV analysis showed no effect of pH for NF and Pt, but overpotentials were reduced for MoS2 and SSW by using an initial lower pH. At cathode potentials more negative than −0.85 V (vs Ag/AgCl), NF had lower overpotentials than the MoS2. These results provide the first assessment of these materials under practical conditions of high pH in unbuffered saline catholytes, and position NF as the most promising inexpensive alternative to Pt.

Enhanced hydrogen production from waste activated sludge by cascade utilization of organic matter in microbial electrolysis cells

Fermentative hydrogen production from waste activated sludge (WAS) has low H 2 yield because WAS contains limited amounts of carbohydrate suitable for use by hydrogen-producing bacteria. Here, augmentation of hydrogen production from WAS by microbial electrolysis cells (MECs) was implemented. H 2 yields of 3.89 ± 0.39 mg-H 2/g-DS (5.67 ± 0.61 mg-H 2/g-VSS) from raw WAS and 6.78 ± 0.94 mg-H 2/g-DS (15.08 ± 1.41 mg-H 2/g-VSS) from alkaline-pretreated WAS were obtained in the two-chamber MECs (TMECs). This was several times higher than yields obtained previously by fermentation. Single-chamber MECs (SMECs) with low internal resistance showed a H 2 production rate that 13 times that of TMECs with similar H 2 yield when alkaline-pretreated WAS was used. However, methanogenesis was detected after several batch cycles. A yield balance calculation revealed that carbohydrates were not the only substrates for electrohydrogenesis. Protein and its acidification products, such as volatile fatty acids are also responsible for a portion of H 2 generation in MEC. Characterization of WAS in TMECs by three-dimensional excitation-emission matrix (EEM) fluorescence spectroscopy with parallel factor analysis indicated that electrohydrogenesis reacted on the extracellular polymeric substances and intracellular substances of WAS. Cascade utilization of organic matter in MECs increased hydrogen production from WAS. MECs showed high hydrogen yield from WAS, fewer H 2 sinks, and insensitivity to temperature. Optimizing MEC configurations and operation conditions and improving the pretreatment processes of WAS are necessary before practical application can take place on a large scale.

Characterization of microbial communities during anode biofilm reformation in a two-chambered microbial electrolysis cell (MEC)

GeoChip (II) and single strand conformation polymorphism (SSCP) were used to characterize anode microbial communities of a microbial electrolysis cell (MEC). Biofilm communities, enriched in a two-chamber MEC (R1, 0.6 V applied) having a coulombic efficiency (CE) of 35±4% and a hydrogen yield (Y(H₂))of 31±3%, were used as the inoculum for a new reactor (R2). After three months R2 achieved stable performance with CE=38±4% and (Y(H₂)). Few changes in the predominant populations were observed from R1 to R2. Unlike sludge inoculation process in R1 in the beginning, little further elimination was aroused by community competitions in anode biofilm reformation in R2. Functional genes detection of biofilm indicated that cytochrome genes enriched soon in new reactor R2, and four genera (Desulfovibrio, Rhodopseudomonas, Shewanella and Geobacter) were likely to contribute to exoelectrogenic activity. This work also implied that symbiosis of microbial communities (exoelectrogens and others) contribute to system performance and stability.

Bio-community analysis during the anode biofilm reformation in a two-chamber microbial electrolysis cell (MEC) for H 2 production

Bio-community analysis during the anode biofilm reformation in a two-chamber microbial electrolysis cell (MEC) for H 2 production