Damage of anodic biofilms by high salinity deteriorates PAHs degradation in single-chamber microbial electrolysis cell reactor

The anaerobic biodegradation of polycyclic aromatic hydrocarbons (PAHs) in high salinity wastewater is rather hard due to the inhibition of microorganisms by complex and high dosage of salts. Microbial electrolysis cell (MEC), with its excellent characteristic of anodic biofilms, can be an effective way to enhance the PAHs biodegradation. This work evaluated the impact of NaCl concentrations (0 g/L, 10 g/L, 30 g/L, and 60 g/L) on naphthalene biodegradation and analyzed the damage protection mechanism of anodic biofilms in batching MECs. Compared with the open circuit, the degradation efficiency of naphthalene under the closed circuit with 10 g/L NaCl concentration reached the maximum of 95.17% within 5 days. Even when NaCl concentration reached 60 g/L, the degradation efficiency only decreased by 10.02%, compared with the MEC without additional NaCl. Confocal scanning laser microscope (CSLM) proved the superiority of the biofilm states of MEC anode under high salinity in terms of thicker biofilms and higher proportion of live/dead bacteria cells. The highest dehydrogenase activity (DHA) was found in the MEC with 10 g/L NaCl concentration. Moreover, microbial diversity analysis demonstrated the classical electroactive microorganisms Geobacter and Pseudomonas were found on the anodic biofilms of MECs, which have both PAHs degradability and the electrochemical activity. Therefore, this study proved that high salinity had adverse effects on the anodic biofilms, but MEC alleviated the damage caused by high salinity.

Bioelectrochemical anoxic ammonium nitrogen removal by an MFC driven single chamber microbial electrolysis cell

Nitrogen removal from wastewater is an indispensable but highly energy-demanding process, and thus more energy-saving treatment processes are required. Here, we investigated the performance of bioelectrochemical ammonium nitrogen (NH₄⁺-N) removal from real domestic wastewater without energy-intensive aeration by a single chamber microbial electrolysis cell (MEC) that was electrically powered by a double chamber microbial fuel cell (MFC). Anoxic NH₄⁺-N oxidation and total nitrogen (TN) removal rates were determined at various applied voltages (0-1.2 V), provided by the MFC. The MEC achieved a NH₄⁺-N oxidation rate of 151 ± 42 g NH₄⁺-N m^-3 d^-1 and TN removal rate of 95 ± 42 g-TN m^-3 d^-1 without aeration at the applied voltage of 0.8 V (the anode potential E_anode = +0.633 ± 0.218 V vs. SHE). These removal rates were much higher than the previously reported values and conventional biological nitrogen removal processes. Open and closed-circuit MEC batch experiments confirmed that anoxic NH₄⁺-N oxidation was an electrochemically mediated biological process (that is, an anode acted as an electron acceptor) and denitrification occurred simultaneously without NO₂^- and NO₃^- accumulation. Moreover, ex-situ¹⁵N tracer experiment and microbial community analysis revealed that anammox and heterotrophic denitrification mainly contributed to the TN removal. Thus, the bioelectrochemical anodic NH₄⁺-N oxidation was coupled with anammox and denitrification in this MFC-assisted MEC system. Taken together, our MFC-driven single chamber MEC could be a high rate energy-saving nitrogen removal process without external carbon and energy input and high energy-demanding aeration.

Integrating microbial electrolysis cell based on electrochemical carbon dioxide reduction into anaerobic osmosis membrane reactor for biogas upgrading

It has been reported that anaerobic osmosis membrane bioreactors have the potential for energy recovery since dissolved methane was almost rejected by commercial forward osmosis membranes. Notwithstanding, upgraded biogas has to be achieved by removing as much carbon dioxide as possible. In this study, a novel anaerobic osmotic membrane bioreactor-microbial electrolysis cell (AnOMBR-MEC) system was developed for simultaneous biogas upgrading and wastewater treatment. The AnOMBR-MEC elicited an excellent and stable soluble chemical oxygen demand and phosphorus removal. As the experiment progressed, unwanted carbon dioxide produced from biogas was reduced to formate using a SnO₂ nanoparticles electrocatalytic cathode in an electrocatalytic-assisted MEC, with the highest faradic efficiency of formate being 85% at 1.2V. Compared to AnOMBR, the methane content increased from 55% to 90% at the end of operation and methane yield experienced a1.6-fold increment in the AnOMBR-MEC. Microbial community analysis revealed that hydrogenotrophic methanogens (e.g. Methanobacterium and Methanobrevibacter) converted the produced H₂ and formate to methane at saline conditions. These results have demonstrated an efficient strategy based on the integration of an electrocatalytic-assisted MEC into AnOMBR for upgrading biogas, enhancing methane yield and wastewater treatment.

Rapid degradation of 2,4-dichloronitrobenzene in single-chamber microbial electrolysis cell with pre-acclimated bioanode: A comprehensive assessment

2,4-dichloronitrobenzene (DClNB) as a typical refractory pollutant, exists in multifarious industrial wastewater widely and poses a serious threat to the environment. An ion exchange membrane (IEM)-free microbial electrolysis cell (MEC) with pre-acclimated bioanode was built and evaluated systematically for treatment of DClNB containing wastewater. Results showed that compared with the non-acclimated or IEM-equipped MECs, the pre-acclimated IEM-free MECs had the best DClNB removal efficiency of 91.3% under COD and DClNB loading rates of nearly 1000 kg m^-3 d^-1 and 100 g m^-3 d^-1. Both of anode pre-acclimation and IEM removal reduced the electron transfer resistance by 71.1 and 194.5 Ω, respectively. Compared to the pre-acclimated IEM-equipped MEC, the cathode current efficiency of pre-acclimated IEM-free MEC increased by 13.7%. Analysis of live/dead cell staining indicated that a higher proportion of live cells was observed in the acclimated anode biofilm (66.1% vs. 47.3%), and the detoxification of DClNB in the pre-acclimated IEM-free MECs was significantly better (p < 0.05) than those of non-acclimated or IEM-equipped MECs. This study contributes to the performance improvement of the MEC process for treatment of toxic industrial wastewater.

Effect of low-intensity electric current field and iron anode on biological nitrate removal in wastewater with low COD to nitrogen ratio from coal pyrolysis

Mixotrophic nitrate removal in wastewater from coal pyrolysis was achieved in microbial electrolysis cell with iron anode (iron-MEC). The effect of voltage, iron anode and conductivity were investigated. The effluent TN concentration was 8.35 ± 1.94 mg/L in iron-MEC when the conductivity of the wastewater was adjusted to 3.97 ± 0.08 mS/cm, which was lower than that in no-treated reactor. The increase of current density, which was resulted from the elevation of conductivity, promoted the iron corrosion and Fe2+ ion generation. Therefore, more Fe2+ ion was utilized by nitrate reducing ferrous oxidation bacteria (NRFOB) used to reduce nitrate. The microbial community analysis demonstrated that NRFOB, including Acidovorax and Bradyrhizobium, possessed a higher abundance in iron-MEC. The enrichment of Geobacter in iron-MEC might imply that the part of Fe(III) produced by ferrous oxidation was reduced by Geobacter, which established an iron cycle. Moreover, the production of N2O was decreased by the formation of Fe2+ ion.

A comprehensive comparison of five different carbon-based cathode materials in CO2 electromethanogenesis: Long-term performance, cell-electrode contact behaviors and extracellular electron transfer pathways

Each carbon-based material, due to the discrepancy in critical properties, has distinct capability to enrich electroactive microbes able to electrosynthesize methane from CO₂. To optimize electromethanogenesis process, this study physically prepared and examined several carbon-based cathode materials: carbon stick (CS), CS twined by Ti wire (CS-Ti) or covered with carbon fiber (CS-CF), graphite felt (CS-GF) and carbon cloth (CS-CC). CS-GF electrode had constantly stable methane production (75.8 mL/L/d at -0.9 V vs. Ag/AgCl) while CS-CC showed a suppressed performance over time caused by the desposition of inorganic shell. Electrode material properties affected biofilms growth, cell-electrode contact behaviors and electron exchange. Methane formation with CS-CC biocathode was H₂-concnetration dependent; CS-GF cathode possessed high antifouling properties and extensive space, enriching the microorganisms capable of catalyzing electromethanogenesis through more efficient non-H₂ route. This study re-interpreted the application potentials of carbon-based materials in CO₂ electroreduction and electrofuel recovery, providing valuable guidance for materials' selection.

Continuous micro-current stimulation to upgrade methanolic wastewater biodegradation and biomethane recovery in an upflow anaerobic sludge blanket (UASB) reactor

The dispersion of granules in upflow anaerobic sludge blanket (UASB) reactor represents a critical technical issue in methanolic wastewater treatment. In this study, the potentials of coupling a microbial electrolysis cell (MEC) into an UASB reactor for improving methanolic wastewater biodegradation, long-term process stability and biomethane recovery were evaluated. The results indicated that coupling a MEC system was capable of improving the overall performance of UASB reactor for methanolic wastewater treatment. The combined system maintained the comparatively higher methane yield and COD removal efficiency over the single UASB process through the entire process, with the methane production at the steady-state conditions approaching 1504.7 ± 92.2 mL-CH₄ L^-1-reactor d^-1, around 10.1% higher than the control UASB (i.e. 1366.4 ± 71.0 mL-CH₄ L^-1-reactor d^-1). The further characterizations verified that the input of external power source could stimulate the metabolic activity of microbes and reinforced the EPS secretion. The produced EPS interacted with Fe^2+/3+ liberated during anodic corrosion of iron electrode to create a gel-like three-dimensional [-Fe-EPS-]_n matrix, which promoted cell-cell cohesion and maintained the structural integrity of granules. Further observations via SEM and FISH analysis demonstrated that the use of bioelectrochemical stimulation promoted the growth and proliferation of microorganisms, which diversified the degradation routes of methanol, convert the wasted CO₂ into methane and accordingly increased the process stability and methane productivity.

The impact of anode acclimation strategy on microbial electrolysis cell treating hydrogen fermentation effluent

The impact of different anode acclimation methods for enhancing hydrogen production in microbial electrolysis cell (MEC) was investigated in this study. The anodes were first acclimated in microbial fuel cells using acetate, butyrate and corn stalk fermentation effluent (CSFE) as substrate before moving into MECs, respectively. Subsequently, CSFE was used as feedstock in all the three MECs. The maximum hydrogen yield with the anode pre-acclimated with butyrate (5.21±0.24L H₂/L CSFE) was higher than that pre-acclimated with acetate (4.22±0.19L H₂/L CSFE) and CSFE (4.55±0.14L H₂/L CSFE). The current density (480±11A/m³) and hydrogen production rate (4.52±0.13m³/m³/d) with the anode pre-acclimated with butyrate were also higher that another two reactors. These results demonstrated that the anode biofilm pre-acclimated with butyrate has significant advantages in CSFE treatment and could improve the performance of hydrogen production in MEC.

Unravelling the active microbial community in a thermophilic anaerobic digester-microbial electrolysis cell coupled system under different conditions

Thermophilic anaerobic digestion (AD) of pig slurry coupled to a microbial electrolysis cell (MEC) with a recirculation loop was studied at lab-scale as a strategy to increase AD stability when submitted to organic and nitrogen overloads. The system performance was studied, with the recirculation loop both connected and disconnected, in terms of AD methane production, chemical oxygen demand removal (COD) and volatile fatty acid (VFA) concentrations. Furthermore, the microbial population was quantitatively and qualitatively assessed through DNA and RNA-based qPCR and high throughput sequencing (MiSeq), respectively to identify the RNA-based active microbial populations from the total DNA-based microbial community composition both in the AD and MEC reactors under different operational conditions. Suppression of the recirculation loop reduced the AD COD removal efficiency (from 40% to 22%) and the methane production (from 0.32 to 0.03 m³ m^-3 d^-1). Restoring the recirculation loop led to a methane production of 0.55 m³ m^-3 d^-1 concomitant with maximum MEC COD and ammonium removal efficiencies of 29% and 34%, respectively. Regarding microbial analysis, the composition of the AD and MEC anode populations differed from really active microorganisms. Desulfuromonadaceae was revealed as the most active family in the MEC (18%-19% of the RNA relative abundance), while hydrogenotrophic methanogens (Methanobacteriaceae) dominated the AD biomass.

Removal of volatile fatty acids and ammonia recovery from unstable anaerobic digesters with a microbial electrolysis cell

Continuous assays with a microbial electrolysis cell (MEC) fed with digested pig slurry were performed to evaluate its stability and robustness to malfunction periods of an anaerobic digestion (AD) reactor and its feasibility as a strategy to recover ammonia. When performing punctual pulses of volatile fatty acids (VFA) in the anode compartment of the MEC, simulating a malfunction of the AD process, an increase in the current density was produced (up to 14 times, reaching values of 3500mAm(-2)) as a result of the added chemical oxygen demand (COD), especially when acetate was used. Furthermore, ammonium diffusion from the anode to the cathode compartment was enhanced and the removal efficiency achieved up to 60% during daily basis VFA pulses. An AD-MEC combined system has proven to be a robust and stable configuration to obtain a high quality effluent, with a lower organic and ammonium content.

Reductive Transformation of p-chloronitrobenzene in the upflow anaerobic sludge blanket reactor coupled with microbial electrolysis cell: performance and microbial community

A microbial electrolysis cell (MEC) combined with an upflow anaerobic sludge blanket (UASB) reactor was operated to degrade p-chloronitrobenzenes (p-ClNB) effectively. The results indicated that p-ClNB was transformed to p-chloroaniline (p-ClAn) and then reduced via dechlorination pathways. In the MEC-UASB coupled system, p-ClNB, p-ClAn removal efficiency and dechlorination efficiency reached 99.63±0.37%, 40.39±9.26% and 32.16±8.12%, respectively, which was significantly improved in comparison with the control UASB system. In addition, the coupled system could maintain appropriate pH and promote anaerobic sludge granulation to exert a positive effect on reductive transformation of p-ClNB. PCR-DGGE experiment and 454 pyrophosphate sequencing analysis indicated that applied voltage would significantly influence the succession of microbial community and promote oriented enrichment of the functional bacteria, which could be the underlying reasons for the improved performance. This study demonstrated that MEC-UASB coupled system had a promising application prospect to remove the recalcitrant pollutants effectively.

Overcoming organic and nitrogen overload in thermophilic anaerobic digestion of pig slurry by coupling a microbial electrolysis cell

The combination of the anaerobic digestion (AD) process with a microbial electrolysis cell (MEC) coupled to an ammonia stripping unit as a post-treatment was assessed both in series operation, to improve the quality of the effluent, and in loop configuration recirculating the effluent, to increase the AD robustness. The MEC allowed maintaining the chemical oxygen demand removal of the whole system of 46±5% despite the AD destabilization after doubling the organic and nitrogen loads, while recovering 40±3% of ammonia. The AD-MEC system, in loop configuration, helped to recover the AD (55% increase in methane productivity) and attained a more stable and robust operation. The microbial population assessment revealed an enhancement of AD methanogenic archaea numbers and a shift in eubacterial population. The AD-MEC combined system is a promising strategy for stabilizing AD against organic and nitrogen overloads, while improving the quality of the effluent and recovering nutrients for their reutilization.

Biomethane recovery from Egeria densa in a microbial electrolysis cell-assisted anaerobic system: Performance and stability assessment

Renewable energy recovery from submerged aquatic plants such as Egeria densa (E. densa) via continuous anaerobic digestion (AD) represents a bottleneck because of process instability. Here, a single-chamber membrane-free microbial electrolysis cell (MEC) equipped with a pair of Ti/RuO2 mesh electrodes (i.e. the combined MEC-AD system) was implemented at different applied voltages (0-1.0 V) to evaluate the potential effects of bioelectrochemical stimulation on methane production and process stability of E. densa fermentation. The application of MEC effectively stabilized E. densa fermentation and upgraded overall process performance, especially solid matters removal. E. densa AD process was operated steadily throughout bioelectrochemical process without any signs of imbalance. The solubilization-removal of solid matters and methane conversion efficiency gradually increased with increasing applied voltage, with an average methane yield of approximately 248.2 ± 21.0 mL L(-1) d(-1) at 1.0 V. Whereas, the stability of the process became worse immediately once the external power was removed, with weaken solid matters removal along with methane output, evidencing the favorable and indispensable role in maintaining process stability. The stabilizing effect was further quantitatively demonstrated by statistical analysis using standard deviation (SD), coefficient of variance (CV) and box-plots. The syntrophic and win-win interactions between fermenting bacteria and electroactive bacteria might have contributed to the improved process stability and bioenergy recovery.

Energy efficient reconcentration of diluted human urine using ion exchange membranes in bioelectrochemical systems

Nutrients can be recovered from source separated human urine; however, nutrient reconcentration (i.e., volume reduction of collected urine) requires energy-intensive treatment processes, making it practically difficult to utilize human urine. In this study, energy-efficient nutrient reconcentration was demonstrated using ion exchange membranes (IEMs) in a microbial electrolysis cell (MEC) where substrate oxidation at the MEC anode provides energy for the separation of nutrient ions (e.g., NH4(+), HPO4(2-)). The rate of nutrient separation was magnified with increasing number of IEM pairs and electric voltage application (Eap). Ammonia and phosphate were reconcentrated from diluted human urine by a factor of up to 4.5 and 3.0, respectively (Eap = 1.2 V; 3-IEM pairs). The concentrating factor increased with increasing degrees of volume reduction, but it remained stationary when the volume ratio between the diluate (urine solution that is diluted in the IEM stack) and concentrate (urine solution that is reconcentrated) was 6 or greater. The energy requirement normalized by the mass of nutrient reconcentrated was 6.48 MJ/kg-N (1.80 kWh/kg-N) and 117.6 MJ/kg-P (32.7 kWh/kg-P). In addition to nutrient separation, the examined MEC reactor with three IEM pairs showed 54% removal of COD (chemical oxygen demand) in 47-hr batch operation. The high sulfate concentration in human urine resulted in substantial growth of both of acetate-oxidizing and H2-oxidizing sulfate reducing bacteria, greatly diminishing the energy recovery and Coulombic efficiency. However, the high microbial activity of sulfate reducing bacteria hardly affected the rate of nutrient reconcentration. With the capability to reconcentrate nutrients at a minimal energy consumption and simultaneous COD removal, the examined bioelectrochemical treatment method with an IEM application has a potential for practical nutrient recovery and sustainable treatment of source-separated human urine.

Performance of a semi-pilot tubular microbial electrolysis cell (MEC) under several hydraulic retention times and applied voltages

The influence of applied voltage and hydraulic retention time on the performance of a semi-pilot modular tubular wastewater-fed microbial electrolysis cell (MEC) with high scalability was investigated. A chemical oxygen demand (COD) removal efficiency of 80%, as well as an energy consumption of 0.3-1.1 Wh g-COD(-1) removed, were achieved. Hydrogen production was limited by the reduced amounts of organic matter fed into the reactor, the poor performance of the cathode, and COD consuming by non electrogenic microorganisms. The presence of COD consuming microorganism that do not contribute to electrogenic metabolism severely affected the MEC performance.

Relating MEC population dynamics to anode performance from DGGE and electrical data

The microbial electrolysis cell (MEC) is a promising system for H2 production, but little is known about the active microbial population in MEC systems. Therefore, the microbial community of five different MEC graphite felt anodes was analyzed using denaturing gradient gel electrophoresis (DGGE) profiling. The results showed that the bacterial population was very diverse and there were substantial differences between microorganisms in anolyte and anode samples. The archaeal population in the anolyte and at the anodes, and between the different MEC anodes, was very similar. SEM and FISH imaging showed that Archaea were mainly present in the spaces between the electrode fibers and Bacteria were present at the fiber surface, which suggested that Bacteria were the main microorganisms involved in MEC electrochemical activity. Redundancy analysis (RDA) and QR factorization-based estimation (QRE) were used to link the composition of the bacterial community to electrochemical performance of the MEC. The operational mode of the MECs and their consequent effects on current density and anode resistance on the populations were significant. The results showed that the community composition was most strongly correlated with current density. The DGGE band mostly correlated with current represented a Clostridium sticklandii strain, suggesting that this species had a major role in current from acetate generation at the MEC anodes. The combination of RDA and QRE seemed especially promising for obtaining an insight into the part of the microbial population actively involved in electrode interaction in the MEC.