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.

Increase of antibiotic resistance genes via horizontal transfer in single- and two-chamber microbial electrolysis cells

Microbial electrolysis cells (MECs) have been applied for antibiotic degradation but simultaneously induced antibiotic resistance genes (ARGs), thus representing a risk to disseminate antibiotic resistance. However, few studies were on the potential and risk of ARGs transmission in the MECs. This work assessed conjugative transfer of ARGs under three tested conditions (voltages, cell concentration, and donor/recipient ratio) in both single- and two-chamber MECs. The results indicated that voltages (> 0.9 V) facilitated the horizontal frequency of ARGs in the single-chamber MECs and anode chamber of two-chamber MECs. The donor cell number (donor/recipient ratio was 2:1) increased the transfer frequency of ARGs. Furthermore, voltages ranged from 0.9 to 2.5 V increased reactive oxygen species (ROS) production and cell membrane permeability in MECs. These findings offer new insights into the roles of ARG transfer under different applied voltages in the MECs, which should not be ignored for horizontal transfer of antibiotic resistance.

Estimation of total energy requirement for sewage treatment by a microbial fuel cell with a one-meter air-cathode assuming Michaelis-Menten COD degradation

Calculations of chemical oxygen demand (COD) degradation in sewage by a microbial fuel cell (MFC) were used to estimate the total energy required for treatment of the sewage. Mono-exponential regression (MER) and the Michaelis-Menten equation (MME) were used to describe the MFC's COD removal rate (CRR). The tubular MFC used in this study (ϕ 5.0 × 100 cm) consisted of an air core surrounding a carbon-based cathode, an anion exchange membrane, and graphite non-woven fabric immersed in sewage. The MFC generated 0.26 A m-2 of the electrode area and 0.32 W m-3 of the sewage water, and 3.9 W h m-3 in a chemostat reactor supplemented continuously with sewage containing 180 mg L-1 of COD with a hydraulic retention time (HRT) of 12 h. The COD removal and coulombic efficiency (CE) were 46% and 19%, respectively, and the energy generation efficiency (EGE) was 0.054 kW h kg-1-COD. The CRR and current in the MFC were strongly dependent on the COD, which could be controlled by varying the HRT. The MER model predicted first-order rate constants of 0.054 and 0.034 for reactors with and without MFC, respectively. The difference in these values indicated that using MFC significantly increased the COD removal. The results of fitting the experimental data to the MME suggested that the total COD can be separated into nondegradable CODs (C n) and degradable CODs (C d) via MFC. The values of CRR for C d and CE suggest that MFC pretreatment for 12 hours prior to aeration results in a 75% decrease in net energy consumption while reducing sewage COD from 180 to 20 mg L-1.

Bio-Electrochemical Enhancement of Hydrogen and Methane Production in a Combined Anaerobic Digester (AD) and Microbial Electrolysis Cell (MEC) from Dairy Manure

Anaerobic digestion (AD) is a biological-based technology that generates methane-enriched biogas. A microbial electrolysis cell (MEC) uses electricity to initiate bacterial oxidization of organic matter to produce hydrogen. This study determined the effect of energy production and waste treatment when using dairy manure in a combined AD and MEC (AD-MEC) system compared to AD without MEC (AD-only). In the AD-MEC system, a single chamber MEC (150 mL) was placed inside a 10 L digester on day 20 of the digestion process and run for 272 h (11 days) to determine residual treatment and energy capacity with an MEC included. Cumulative H2 and CH4 production in the AD-MEC (2.43 L H2 and 23.6 L CH4) was higher than AD-only (0.00 L H2 and 10.9 L CH4). Hydrogen concentration during the first 24 h of MEC introduction constituted 20% of the produced biogas, after which time the H2 decreased as the CH4 concentration increased from 50% to 63%. The efficiency of electrical energy recovery (ηE) in the MEC was 73% (ηE min.) to 324% (ηE max.), with an average increase of 170% in total energy compared to AD-only. Chemical oxygen demand (COD) removal was higher in the AD-MEC (7.09 kJ/g COD removed) system compared to AD-only (6.19 kJ/g COD removed). This study showed that adding an MEC during the digestion process could increase overall energy production and organic removal from dairy manure.

Microbial electrochemical systems for hydrogen peroxide synthesis: Critical review of process optimization, prospective environmental applications, and challenges

Hydrogen peroxide (H₂O₂) is an industrial chemical that has been widely adopted for various industrial applications, including water and wastewater treatment. Currently, the majority of H₂O₂ is being produced through the anthraquinone oxidation process, which is disadvantageous due to the requirement of toxic raw materials and high energy input. Recently, microbial electrochemical cells (MXCs), such as microbial fuel cells and microbial electrolysis cells, have demonstrated great potential for effective H₂O₂ production via cathodic oxygen-reduction reaction (ORR). Previous studies have specified key operational parameters for scaling-up of H₂O₂-producing MXCs, where improvements in production rate, conversion efficiency, product concentration and stability are attainable. Moreover, various systems have demonstrated their value proposition in the contaminant removal aspects through direct removal of various environmental pollutants, water disinfection, and many more. This review is intended to highlight promising ways of H₂O₂ production with MXCs and on-site environmental applications of bioelectrochemically-produced H₂O₂.

Studying the Impact of Wall Shear Stress on the Development and Performance of Electrochemically Active Biofilms

A laminar flow reactor was designed that provides constant and reproducible growth conditions for the bioelectrochemical observation of electroactive bacteria (EAB). Experiments were performed using four reactors in parallel to enable the comparison of EAB growth behavior and bioelectrochemical performance under different hydrodynamic conditions while simultaneously keeping biological conditions identical. With regard to the moderate flow conditions found in wastewater treatment applications, the wall shear stress was adjusted to a range between 0.4 mPa to 2.9 mPa. Chronoamperometric data indicate that early stage current densities are improved by a moderate increase of the wall shear stress. In the same way, current onset times were increasing slightly towards higher values of the applied wall shear stress. Long-term observations of EAB performance showed a decrease in current density and a leveling of the trend observed for the early stages of biofilm growth.

Quantification of the methane concentration using anaerobic oxidation of methane coupled to extracellular electron transfer

A biofilm anode acclimated with growth media containing acetate, then acetate+methane, and finally methane alone produced electrical current in a microbial electrochemical cell (MxC) fed with methane as the sole electron donor. Geobacter was the dominant genus for the bacterial domain (93%) in the biofilm anode, while methanogens (Methanocorpusculum labreanum and Methanosaeta concilii) accounted for 82% of the total archaeal clones in the biofilm. Fluorescence in situ hybridization (FISH) imaging clearly showed a biofilm of mixed bacteria and archaea, suggesting a syntrophic interaction between them for performing anaerobic oxidation of methane (AOM) in the biofilm anode. Measured cumulative coulombs were linearly correlated to the methane-gas concentration in the range of 10-99.97% (R²≥0.99) when the measurement was sustained for at least 50min Thus, cumulative coulombs over 50min could be used to quantify the methane concentration in gas samples.

Anaerobic oxidation of methane coupled with extracellular electron transfer to electrodes

Anaerobic oxidation of methane (AOM) is an important process for understanding the global flux of methane and its relation to the global carbon cycle. Although AOM is known to be coupled to reductions of sulfate, nitrite, and nitrate, evidence that AOM is coupled with extracellular electron transfer (EET) to conductive solids is relatively insufficient. Here, we demonstrate EET-dependent AOM in a biofilm anode dominated by Geobacter spp. and Methanobacterium spp. using carbon-fiber electrodes as the terminal electron sink. The steady-state current density was kept at 11.0 ± 1.3 mA/m² in a microbial electrochemical cell, and isotopic experiments supported AOM-EET to the anode. Fluorescence in situ hybridization images and metagenome results suggest that Methanobacterium spp. may work synergistically with Geobacter spp. to allow AOM, likely by employing intermediate (formate or H₂)-dependent inter-species electron transport. Since metal oxides are widely present in sedimentary and terrestrial environments, an AOM-EET niche would have implications for minimizing the net global emissions of methane.

Electrochemical techniques reveal that total ammonium stress increases electron flow to anode respiration in mixed-species bacterial anode biofilms

When anode-respiring bacteria (ARB) respire electrons to an anode in microbial electrochemical cells (MXCs), they harvest only a small amount of free energy. This means that ARB must have a high substrate-oxidation rate coupled with a high ratio of electrons used for respiration compared to total electrons removed by substrate utilization. It also means that they are especially susceptible to inhibition that slows anode respiration or lowers their biomass yield. Using several electrochemical techniques, we show that a relatively high total ammonium-nitrogen (TAN) concentration (2.2 g TAN/L) induced significant stress on the ARB biofilms, lowering their true yield and forcing the ARB to boost the ratio of electrons respired per electrons consumed from the substrate. In particular, a higher respiration rate, measured as current density (j), was associated with slower growth and a lower net yield, compared to an ARB biofilm grown with a lower ammonium concentration (0.2 g TAN/L). Further increases in influent TAN (to 3 and then to 4.4 g TAN/L) caused nearly complete inhibition of anode respiration. However, the ARB could recover from high-TAN inhibition after a shift of the MXC's feed to 0.2 g TAN/L. In summary, ARB biofilms were inhibited by a high TAN concentration, but could divert more electron flow toward anode respiration with modest inhibition and recover when severe inhibition was relieved. Biotechnol. Bioeng. 2017;114: 1151-1159. © 2017 Wiley Periodicals, Inc.

Hydrogen production from sugar beet juice using an integrated biohydrogen process of dark fermentation and microbial electrolysis cell

An integrated dark fermentation and microbial electrochemical cell (MEC) process was evaluated for hydrogen production from sugar beet juice. Different substrate to inoculum (S/X) ratios were tested for dark fermentation, and the maximum hydrogen yield was 13% of initial COD at the S/X ratio of 2 and 4 for dark fermentation. Hydrogen yield was 12% of initial COD in the MEC using fermentation liquid end products as substrate, and butyrate only accumulated in the MEC. The overall hydrogen production from the integrated biohydrogen process was 25% of initial COD (equivalent to 6 mol H2/mol hexoseadded), and the energy recovery from sugar beet juice was 57% using the combined biohydrogen.

Characterization and optimization of cathodic conditions for H2O2 synthesis in microbial electrochemical cells

Cathode potential and O2 supply methods were investigated to improve H2O2 synthesis in an electrochemical cell, and optimal cathode conditions were applied for microbial electrochemical cells (MECs). Using aqueous O2 for the cathode significantly improved current density, but H2O2 conversion efficiency was negligible at 0.3-12%. Current density decreased for passive O2 diffusion to the cathode, but H2O2 conversion efficiency increased by 65%. An MEC equipped with a gas diffusion cathode was operated with acetate medium and domestic wastewater, which presented relatively high H2O2 conversion efficiency from 36% to 47%, although cathode overpotential was fluctuated. Due to different current densities, the maximum H2O2 production rate was 141 mg H2O2/L-h in the MEC fed with acetate medium, but it became low at 6 mg H2O2/L-h in the MEC fed with the wastewater. Our study clearly indicates that improving anodic current density and mitigating membrane fouling would be key parameters for large-scale H2O2-MECs.

Effects of pre-fermentation and pulsed-electric-field treatment of primary sludge in microbial electrochemical cells

The aim of this study was to investigate the combination of two technologies - pulsed electric field (PEF) pre-treatment and semi-continuous pre-fermentation of primary sludge (PS) - to produce volatile fatty acids (VFAs) as the electron donor for microbial electrolysis cells (MECs). Pre-fermentation with a 3-day solids retention time (SRT) led to the maximum generation of VFAs, with or without pretreatment of the PS through pulsed-electric-fields (PEF). PEF treatment before fermentation enhanced the accumulation of the preferred VFA, acetate, by 2.6-fold. Correspondingly, MEC anodes fed with centrate from 3-day pre-fermentation of PEF-treated PS had a maximum current density ∼3.1 A/m(2), which was 2.4-fold greater than the control pre-fermented centrate. Over the full duration of batch MEC experiments, using pre-fermented centrate led to successful performance in terms of Coulombic efficiency (95%), Coulombic recovery (80%), and COD-removal efficiency (85%).

Evaluation of limiting factors for current density in microbial electrochemical cells (MXCs) treating domestic wastewater

This study quantitatively assessed three limiting factors for current density in a microbial electrochemical cell (MXC) treating domestic wastewater: (1) buffer concentration, (2) biodegradability, and (3) particulates. Buffer concentration was not significant for current density in the MXC fed with filtered domestic wastewater (180 mg COD/L). Current density reduced by 67% in the MXC fed with filtered sewage having similar COD concentration to acetate medium, which indicates poor biodegradability of soluble organics in the wastewater. Particulate matters seriously decreased current density down to 76%, probably due to the accumulation of particulates on biofilm anode. Our study quantitatively showed that buffer concentration does not limit current density much, but biodegradability of soluble organics and fermentation rate of particulate matters in domestic wastewater mainly control current density in MXCs.

Occurrence and implications of voltage reversal in stacked microbial fuel cells

Voltage reversal in stacked microbial fuel cells (MFCs) is a significant challenge that must be addressed, and the information on its definite cause and occurrence process is still obscure. In this work, we first demonstrated that different anodic reaction rates caused voltage reversal in a stacked MFC. Sluggish reaction rates on the anode in unit 1 of the stacked MFC resulted in a significantly increased anode overpotential of up to 0.132 V, as compared to negligible anode overpotential (0.0247 V) in unit 2. This work clearly verified the process of voltage reversal in the stacked MFC. As the current was gradually increased in the stacked MFC, the voltage in the stacked unit 1 decreased to 0 V prior to that of the stacked unit 2. Then, when the voltage in unit 1 became 0 V, it was converted from a galvanic cell to an electrochemical cell powered by unit 2. We found that the stacked unit 2 provided electrical energy for the stacked unit 1 as a power supply. Finally, the anode potential of the stacked unit 1 significantly increased over cathode potential as current increased further, which caused voltage reversal in unit 1. Voltage reversal occurs in stacked MFCs as a result of non-spontaneous anode overpotential in a unit MFC that has sluggish anode kinetics compared to the other unit MFCs.

Separation of competitive microorganisms using anaerobic membrane bioreactors as pretreatment to microbial electrochemical cells

Anaerobic membrane bioreactors (AnMBRs) as pretreatment to microbial electrochemical cells (MECs) were first assessed for improving energy recovery. A dual-chamber MEC was operated at hydraulic retention time (HRT) ranging from 1 to 8d, while operating conditions for an AnMBR were fixed. Current density was increased from 7.5 ± 0 to 14 ± 1A/m(2) membrane with increasing HRT. MEC tests with AnMBR permeate (mainly propionate and acetate) and propionate medium confirmed that propionate was fermented to acetate and hydrogen gas, and anode-respiring bacteria (ARB) utilized these fermentation products as substrate. Membrane separation in the AnMBR excluded fermenters and methanogens from the MEC, and thus no methane production was found in the MEC. The lack of fermenters, however, slowed down propionate fermentation rate, which limited current density in the MEC. To symphonize fermenters, H2-consumers, and ARB in biofilm anode is essential for improving current density, and COD removal.