Electricity production and the analysis of the anode microbial community in a constructed wetland-microbial fuel cell

Improving performance of pilot-scale ecological bed coupled with microbial electrochemical system for urban tail water treatment

Recycling urban tail water for ecological base flow and landscape use offers a reliable solution for the problem of water resource shortage. But the long-term direct discharge of urban tail water can aggravate the eutrophication of surface water based on the present drainage standard of sewage plant. It is of great significance to develop low-cost and low-energy ecological technologies as transitional region between urban tail water and surface water. In this study, a pilot-scale ecological bed coupled with microbial electrochemical system (EB-MES) was established to treat urban tail water deeply. The system was operated for 96 days from June to September. Average TN removal efficiency in EB-MES under the condition of submerged plant coupled closed-circuit MES could reach 59.0 ± 16.6 %, which was 82.7 % and 38.1 % higher than that of open-circuit EB-MES and MES without plants, respectively. Microbial community structure testing indicated that multiple nitrogen metabolic mechanisms occurred in the system, including nitrification, electrode autotrophic denitrification, anammox, simultaneous nitrification and denitrification, and aerobic denitrification, which results in better denitrification efficiency under tail water. Our research provided a novel ecological technology with advantages of high-efficiency, low-energy and low-carbon and verified the feasibility in pilot scale for application in the advanced treatment of urban tail water.

The electron transport mechanism of downflow Leersia hexandra Swartz constructed wetland-microbial fuel cell when used to treat Cr(VI) and p-chlorophenol

Constructed wetland-microbial fuel cells are used to treat heavy metal and/or refractory organic wastewater. However, the electron transport mechanism of downflow Leersia hexandra constructed wetland-microbial fuel cells (DLCW-MFCs) is poorly understood when used to treat composite-polluted wastewater containing Cr(VI) and p-chlorophenol (4-CP) (C&P). In this study, metagenomics and in situ electrochemical techniques were used to investigate the electrochemical properties and the electricigens and their dominant gene functions. The DLCW-MFC was used to treat C&P and single-pollutant wastewater containing Cr(VI) (SC) and 4-CP (SP). The results showed that C&P had a higher current response and charge transfer capability and lower solution resistance plus charge transfer resistance. The anode bacteria solution of C&P contained more electron carriers (RF, FMN, FAD, CoQ10, and Cyt c). Metagenomic sequencing indicated that the total relative abundance of the microorganisms associated with electricity production (Desulfovibrio, Pseudomonas, Azospirillum, Nocardia, Microbacterium, Delftia, Geobacter, Acinetobacter, Bacillus, and Clostridium) was the highest in C&P (4.24%). However, Microbacterium was abundant in SP (0.12%), which exerted antagonistic effects on other electricigens. Among the 10 electricigens based on gene annotation, C&P had a higher overall relative abundance of the Unigene gene annotated to the KO pathway and CAZy level B compared with SC and SP, which were 1.31% and 0.582% respectively. Unigene153954 (ccmC), Unigene357497 (coxB), and Unigene1033667 (ubiG) were related to the electron carrier Cyt c, electron transfer, and CoQ biosynthesis, respectively. These were annotated to Desulfovibrio, Delftia, and Pseudomonas, respectively. Unigene161312 (AA1) used phenols and other substrates as electron donors and was annotated to Pseudomonas. Other functional carbohydrate enzyme genes (e.g., GT2, GT4, and GH31) used carbohydrates as donors and were annotated to other electricigens. This study provides a theoretical basis for electron transfer to promote the development of CW-MFCs.

Curbing per- and polyfluoroalkyl substances (PFASs): First investigation in a constructed wetland-microbial fuel cell system

The presence of per- and polyfluoroalkyl substances (PFASs) in water environments has been linked to a slew of negative health effects in both animals and humans, but the green and eco-sustainable removal technologies remain largely unknown. Constructed wetland coupled microbial fuel cell (CW-MFC) is termed a "green process" to control pollutants and recover energy. However, so far, no study has investigated the removal of PFASs and their effects on the performance of the CW-MFC systems. Here, we investigated the removal performance of PFOA and PFOS in the CW-MFC systems both in the absence and presence of electricity circuit, and explored the distribution and fate of PFASs and their interactions with other elements in the systems. Our findings demonstrated excellent removal efficiency of >96% PFOA and PFOS in CW-MFC systems. PFOA and PFOS were distributed throughout the system via wastewater flow, while electrode material and plants are the main enrichment sites in which MFC enhanced up to 10% PFASs removal. However, a loss of 7.2-13.5% of nitrogen removal and a decrease of 7.3% in bioelectricity output were observed when PFASs were introduced in the system. The driven force led to the loss of nitrogen removal and bioelectricity generation lies in the accumulation of PFASs in system composition, which affected microbial activity and community composition, damaging the health of the plant, and in turn reducing CW-MFC's functioning. No doubt, CW-MFC systems provide an alternative technique for PFASs removal, alleviating some limitations to the physical and chemical techniques, but further investigation is highly needed.

Formation of biologically influenced palladium microstructures by Desulfovibrio desulfuricans and Desulfovibrio ferrophilus IS5

A range of Desulfovibrio spp. can reduce metal ions to form metallic nanoparticles that remain attached to their surfaces. The bioreduction of palladium (Pd) has been given considerable attention due to its extensive use in areas of catalysis and electronics and other technological domains. In this study we report, for the first time, evidence for Pd(II) reduction by the highly corrosive Desulfovibrio ferrophilus IS5 strain to form surface attached Pd nanoparticles, as well as rapid formation of Pd(0) coated microbial nanowires. These filaments reached up to 8 µm in length and led to the formation of a tightly bound group of interconnected cells with enhanced ability to attach to a low carbon steel surface. Moreover, when supplied with high concentrations of Pd (≥ 100 mmol Pd(II) g^-1 dry cells), both Desulfovibrio desulfuricans and D. ferrophilus IS5 formed bacteria/Pd hybrid porous microstructures comprising millions of cells. These three-dimensional structures reached up to 3 mm in diameter with a dose of 1200 mmol Pd(II) g^-1 dry cells. Under suitable hydrodynamic conditions during reduction, two-dimensional nanosheets of Pd metal were formed that were up to several cm in length. Lower dosing of Pd(II) for promoting rapid synthesis of metal coated nanowires and enhanced attachment of cells onto metal surfaces could improve the efficiency of various biotechnological applications such as microbial fuel cells. Formation of biologically stimulated Pd microstructures could lead to a novel way to produce metal scaffolds or nanosheets for a wide variety of applications.

Constructed Wetland Coupled Microbial Fuel Cell: A Clean Technology for Sustainable Treatment of Wastewater and Bioelectricity Generation

The availability of clean water and the depletion of non-renewable resources provide challenges to modern society. The widespread use of conventional wastewater treatment necessitates significant financial and energy expenditure. Constructed Wetland Microbial Fuel Cells (CW-MFCs), a more recent alternative technology that incorporates a Microbial Fuel Cell (MFC) inside a Constructed Wetland (CW), can alleviate these problems. By utilizing a CW’s inherent redox gradient, MFC can produce electricity while also improving a CW’s capacity for wastewater treatment. Electroactive bacteria in the anaerobic zone oxidize the organic contaminants in the wastewater, releasing electrons and protons in the process. Through an external circuit, these electrons travel to the cathode and produce electricity. Researchers have demonstrated the potential of CW-MFC technology in harnessing bio-electricity from wastewater while achieving pollutant removal at the lab and pilot scales, using both domestic and industrial wastewater. However, several limitations, such as inadequate removal of nitrogen, phosphates, and toxic organic/inorganic pollutants, limits its applicability on a large scale. In addition, the whole system must be well optimized to achieve effective wastewater treatment along with energy, as the ecosystem of the CW-MFC is large, and has diverse biotic and abiotic components which interact with each other in a dynamic manner. Therefore, by modifying important components and optimizing various influencing factors, the performance of this hybrid system in terms of wastewater treatment and power generation can be improved, making CW-MFCs a cost-effective, cleaner, and more sustainable approach for wastewater treatment that can be used in real-world applications in the future.

Enhanced performance and microbial interactions of shallow wetland bed coupling with functional biocathode microbial electrochemical system (MES)

It is essential to remediate the polluted urban river, which endangers the aquatic creatures and affected human body's senses. The treatment wetland combined with microbial electrochemical system (MES) used for the remediation is becoming a new research focus due to its ideal pollutants removal efficiency and small footprint. Here this paper provided a kind of novel shallow wetland bed coupling with close-circuit microbial electrochemical system (WB-CMES) to remove pollutants in surface water. In contrast to the shallow wetland bed coupling with open-circuit MES (WB-OMES) and the shallow wetland bed without MES (WB), the enhancing effects and pollutants removal pathway were evaluated. After 62-day operation, average TN removal efficiency in WB-CMES was 87.7%, which was 19.7% and 13.8% higher than that of WB-OMES and WB respectively. The rate coefficient k of NO₃^--N degradation in WB-CMES was 1.6 and 1.8 times higher than that in WB-OMES and WB. The results of chlorophyll, protein and superoxide dismutase (SOD) in WB-CMES were 27.3%, 44.3% and 12.9% higher than those in WB. The microbial community structure analysis indicated that electroactive bacteria on anode like Desulfobulbus could oxidize organics and generate electrons to compensate cathode, meanwhile, cathode could enrich more species of functional bacteria like Rhodobacter, Pirellula, Hyphomicrobium, Thauera, which had a synergistic effect on oxygen reduction, nitrogen removal and plant growth. The results indicated that oxygen produced by submerged plants could be utilized by the oxygen-reducing functional biocathode of MES and the proper aerobic and anoxic environment might enhance nitrate removal mainly through simultaneous nitrification and denitrification (SND), aerobic denitrification and anammox. This research provided a novel technology with advantages of simple operation, flexible configuration, easy scale-up and low cost for application in remediation of highly polluted surface water.

Performance optimization and microbial community evaluation for domestic wastewater treatment in a constructed wetland-microbial fuel cell

Constructed wetland-microbial fuel cell system (CW-MFC), an attractive technology still under study, has shown to improve domestic wastewater treatment efficiency and generate bioelectricity. This work investigated the effect of multiple factors on the performance optimization for the pollutants removal and bioelectricity production compared to a traditional CW, including influent chemical oxygen demand (COD) concentration, hydraulic retention time (HRT) and external resistance. The results showed that the optimal operating conditions of COD concentration, HRT and external resistance for CW-MFC were 200 mg/L, 24 h and 1000 Ω, respectively. The average COD, NH₄⁺-N, NO₃^--N and TP removal efficiencies were 6.06%, 3.85%, 3.68% and 3.68% higher than these in CW system, respectively. Meanwhile, the maximum output voltage and power density of CW-MFC were 388 ± 12 mV and 107.54 mW/m³. In addition, the microbial community analysis indicated that the pollution removal and bioelectricity generation might benefit from the gradual enrichment of electroactive bacteria (Tolumonas) and denitrifying bacteria (Denitratisoma, Methylotenera and Sulfuritales). The findings can provide the optimum operation parameters and mechanism insight for the performance of CW-MFC systems.

Removal of Cr(vi) and p-chlorophenol and generation of electricity using constructed wetland-microbial fuel cells based on Leersia hexandra Swartz: p-chlorophenol concentration and hydraulic retention time effects

Heavy metals and phenolic compounds existing in polluted wastewater are a threat to the environment and human safety. A downflow Leersia hexandra Swartz constructed wetland-microbial fuel cell (DLCW–MFC) was designed to treat polluted wastewater containing Cr(vi) and p-chlorophenol (4-CP). To determine the effect of 4-CP concentration and hydraulic retention time (HRT) on the performance of the DLCW–MFC system, the wastewater purification, electricity generation, electrochemical performance, and L. hexandra growth status were studied. Addition of 17.9 mg L⁻¹ 4-CP improved the power density (72.04 mW m⁻²) and the charge transfer capacity (exchange current, 4.72 × 10⁻³ A) of DLCW–MFC. The removal rates of Cr(vi) and 4-CP at a 4-CP concentration of 17.9 mg L⁻¹ were 98.8% and 38.1%, respectively. The Cr content in L. hexandra was 17.66 mg/10 plants. However, a 4-CP concentration of 35.7 mg L⁻¹ inhibited the removal of Cr(vi) and the growth of L. hexandra, and decreased the electricity generation (2.5 mW m⁻²) as well as exchange current (1.21 × 10⁻³ A) of DLCW–MFC. An increase in power density and removal of Cr(vi) and 4-CP, along with an enhanced transport coefficient of L. hexandra, was observed with HRT. At an optimal HRT of 6.5 d, the power density, coulomb efficiency, and exchange current of DLCW–MFC were 72.25 mW m⁻², 2.38%, and 4.99 × 10⁻³ A, respectively. The removal rates of Cr(vi) and 4-CP were 99.0% and 78.6%, respectively. The Cr content and transport coefficient of L. hexandra were 4.56 mg/10 plants and 0.451, respectively. Thus, DLCW–MFC is a promising technology that can be used to detoxify polluted wastewater containing composite mixtures and synchronously generate electricity.

A downflow Leersia hexandra Swartz constructed wetland-microbial fuel cell is used to treat polluted wastewater containing composite mixtures and synchronously generate electricity.

Effectiveness of constructed wetland integrated with microbial fuel cell for domestic wastewater treatment and to facilitate power generation

Constructed wetlands (CWs) have gained a lot of attention for wastewater treatment due to robustness and natural pollutant mitigation characteristics. This widely acknowledged technology possesses enough merits to derive direct electricity in collaboration with microbial fuel cell (MFC), thus taking advantage of microbial metabolic activities in the anoxic zone of CWs. In the present study, two identical lab-scale CWs were selected, each having 56 L capacity. One of the CW integrated with MFC (CW-MFC) contains two pairs of electrodes, i.e., carbon felt and graphite plate. The first pair of CW-MFC consists of a carbon felt cathode with a graphite plate anode, and the second pair contains a graphite plate cathode with a carbon felt anode. The other CW was not integrated with MFC and operated as a traditional CW for evaluating the performance. CW-MFC and CW were operated in continuous up-flow mode with a hydraulic retention time of 3 days and at different organic loading rates (OLRs) per unit surface area, such as 1.45 g m^-2 day^-1 (OLR-1), 2.43 g m^-2 day^-1 (OLR-2), and 7.25 g m^-2 day^-1 (OLR-3). The CW-MFC was able to reduce the organic matter, phosphate, and total nitrogen by 92%, 93%, and 70%, respectively, at OLR of 1.45 g m^-2 day^-1, which was found to be higher than that obtained in conventional CW. With increase in electrochemical redox activities, the second pair of electrodes made way for 3 times higher power density of 16.33 mW m^-2 as compared to the first pair of electrodes in CW-MFC (5.35 mW m^-2), asserting carbon felt as a good anode material to be used in CW-MFC. The CW-MFC with carbon felt as an anode material is proposed to improve the electro-kinetic activities for scalable applications to achieve efficient domestic wastewater treatment and electricity production.

A review of recent advances in electrode materials for emerging bioelectrochemical systems: From biofilm-bearing anodes to specialized cathodes

Bioelectrochemical systems (BES), mainly microbial fuel cells (MEC) and microbial electrolysis cells (MFC), are unique biosystems that use electroactive bacteria (EAB) to produce electrons in the form of electric energy for different applications. BES have attracted increasing attention as a sustainable, low-cost, and neutral-carbon option for energy production, wastewater treatment, and biosynthesis. Complex interactions between EAB and the electrode materials play a crucial role in system performance and scalability. The electron transfer processes from the EAB to the anode surface or from the cathode surface to the EAB have been the object of numerous investigations in BES, and the development of new materials to maximize energy production and overall performance has been a hot topic in the last years. The present review paper discusses the advances on innovative electrode materials for emerging BES, which include MEC coupled to anaerobic digestion (MEC-AD), Microbial Desalination Cells (MDC), plant-MFC (P-MFC), constructed wetlands-MFC (CW-MFC), and microbial electro-Fenton (BEF). Detailed insights on innovative electrode modification strategies to improve the electrode transfer kinetics on each emerging BES are provided. The effect of materials on microbial population is also discussed in this review. Furthermore, the challenges and opportunities for materials scientists and engineers working in BES are presented at the end of this work aiming at scaling up and industrialization of such versatile systems.

Improving the outcomes from electroactive constructed wetlands by mixing wastewaters from different beverage-processing industries

Denitrification in electroactive constructed wetland (EW) systems is constrained by the carbon source and the carbon/nitrogen (C/N) ratio (the COD/TN ratio). In this study, wastewater with a high C/N from a brewery was added to wastewater with a low C/N (dairy wastewater) in an EW system, and the pollutant removal, bioelectricity generation, transformations of dissolved organic matter, and microbial community structures were evaluated. The results showed that the average removal rates of ammonium nitrogen, total nitrogen, and chemical oxygen demand from the wastewater mixture were 6.40%, 46.44%, and 23.85% higher than those from the wastewater with a low C/N, respectively. Dissimilatory nitrate reduction to ammonium was effectively inhibited, and the NH₄⁺-N removal was 25.52% higher, when the wastewater mixture was used instead of the high C/N wastewater. Similarly, the output voltage was significantly increased, and the internal resistance of the device was reduced, for the wastewater mixture. The structure of the microbial community improved, the relative abundance of electrochemically active bacteria was higher, and the protein-like and humic-like components were lower, in the mixture treatment than in the individual treatment. The results show that the nitrogen removal and biopower generation improved in an EW system when high C/N wastewater was used as the carbon source.

Advances in technological control of greenhouse gas emissions from wastewater in the context of circular economy

This review paper aims to identify the main sources of carbon dioxide (CO₂) emissions from wastewater treatment plants (WWTPs) and highlights the technologies developed for CO₂ capture in this milieu. CO₂ is emitted in all the operational units of conventional WWTPs and even after the disposal of treated effluents and sludges. CO₂ emissions from wastewater can be captured or mitigated by several technologies such as the production of biochar from sludge, the application of constructed wetlands (CWs), the treatment of wastewater in microbial electrochemical processes (microbial electrosynthesis, MES; microbial electrolytic carbon capture, MECC; in microbial carbon capture, MCC), and via microalgal cultivation. Sludge-to-biochar and CW systems showed a high cost-effectiveness in the capture of CO₂, while MES, MECC, MCC technologies, and microalgal cultivation offered efficient capture of CO₂ with associate production of value-added by-products. At the state-of-the-art, these technologies, utilized for carbon capture and utilization from wastewater, require more research for further configuration, development and cost-effectiveness. Moreover, the integration of these technologies has a potential internal rate of return (IRR) that could equate the operation or provide additional revenue to wastewater management. In the context of circular economy, these carbon capture technologies will pave the way for new sustainable concepts of WWTPs, as an essential element for the mitigation of climate change fostering the transition to a decarbonised economy.

A taxonomy of design factors in constructed wetland-microbial fuel cell performance: A review

The past decade has seen the rapid development of constructed wetland-microbial fuel cell (CW-MFC) technology in many aspects. The first publication on the combination of constructed wetland (CW) and microbial fuel cell (MFC) appeared in 2012, subsequently, research on the subject has grown exponentially to improve the performance of CW-MFCs in their dual roles of wastewater treatment and power generation. Although significant research has been conducted on this technology worldwide, a comprehensive and critical review of effective controlling parameters is lacking. More broadly, research is needed to draw up-to-date conclusions on recent developments and to identify knowledge gaps for further studies. This review paper systematically enumerates and reviews research studies published in this area to determine the key design factors and their role in CW-MFC performance. Moreover, a taxonomy of all CW-MFC design parameters has been synthesised from the literature. Importantly, this original work provides a comprehensive conceptual framework for future researchers, designers, builders, and users to understand CW-MFC technology. Within the taxonomy, parameters are placed in three main categories (physical/environmental, chemical, and biological/electrochemical) and comprehensive details are given for each parameter. Finally, a comprehensive summary of the parameters has been tabulated showing their impact on CW-MFC operation, design recommendations from literature, and the significant research gaps that this review has identified within the existing literature. It is hoped that this paper will provide a clear and rich picture of this technology at its current stage of development and furthermore, will facilitate a deeper understanding of CW-MFC performance for long-term and large-scale development.

A comprehensive review on emerging constructed wetland coupled microbial fuel cell technology: Potential applications and challenges

Constructed wetlands (CWs) integrated with bioelectrochemical systems (BESs) are being intensively researched with the names like constructed wetland-microbial fuel cell (CW-MFC), electro-wetlands, electroactive wetlands, and microbial electrochemical technologies-based constructed wetland since the last decade. The implantation of BES in CW facilitates the tuning of redox activities and electron flow balance in aerobic and anaerobic zones in the CW bed matrix, thereby alleviating the limitation associated with electron acceptor availability and increasing its operational controllability. The benefits of CW-MFC include high treatment efficiency, electricity generation, and recalcitrant pollutant abatement. This article presents CW-MFC technology's journey since its emergence to date, encompassing the research done so far, including the basic principle and functioning, bio-electrocatalysts as its machinery, influential factors for microbial interactions, and operational parameters controlling different processes. A few key challenges and potential applications are also discussed for the CW-MFC systems.

Effect of heterotrophic anodic denitrification on anolyte pH control and bioelectricity generation enhancement of bufferless microbial fuel cells

Heterotrophic anodic denitrification (HAD) in the single-chamber microbial fuel cell (MFC) is a promising nitrogen removal technology. In this paper, the benefit (anolyte pH increase) and challenge (substrate consumption) brought by the heterotrophic anodic denitrification process for the electricity generation of bufferless MFCs were studied for the first time. Substrate anaerobic hydrolysis dramatically decreased the anolyte pH to 5.1, which seriously restricted the electric power output of the Control. The anolyte pH of the heterotrophic anodic denitrification MFCs (HADMFCs) with 60 mg/L (HADMFC-60), 90 mg/L (HADMFC-90), and 120 mg/L (HADMFC-120) nitrate nitrogen (NO₃^--N), retained above 6.0, 6.5, and 6.8 in every running cycles, due to the protons (H⁺) consumption by nitrate reduction. In the HADMFC-60 and HADMFC-90, 17.6% and 26.1% of the total organic carbons (TOC) were used for the nitrate reduction, but their electric power output significantly increased. The maximum power densities of the HADMFC-60 and HADMFC-90 were 3.3 and 5.4 times higher than that of the Control. However, when the proportion of TOC consumption for nitrate reduction increased to 35.8%, substrate insufficiency became a serious limitation for the electricity generation. The P_max of the HADMFC-120 dramatically decreased to 17.3 mW/m². Dysgonomonas was the dominant electro-active genus, and Petrimonas, Acidovorax and Devosia appeared as the denitrifying bacteria genera.

Robust nitrate removal and bioenergy generation with elucidating functional microorganisms under carbon constraint in a novel multianode tidal constructed wetland coupled with microbial fuel cell

This study investigated synthetic wastewater treatment under low inflow C/N ratio and characterized NO₃^--N-transforming and electricity-producing bacteria in a multi-anode tidal constructed wetland-microbial fuel cell (TFCW-MFC). The optimal concurrent average removal rates of NH₄⁺-N and NO₃^--N were 73% and 78%, respectively, under a flood/rest/flood time of 4 h/2h/4h in "tide" mode accompanied by one recirculation. The lowest NO₃^--N concentration among all anodes was observed when the electrode gap was 45 cm. Similarly, the 45 cm anode exhibited selective enrichment of Variovorax and Azoarcus. Correction analysis showed that the high relative abundance of Azoarcus was crucial in enhancing NO₃^--N removal, and the internal resistance significantly decreased as the relative abundance of Acidovorax increased. These results suggest that NO₃^--N removal and bioelectricity generation can be promoted in a TFCW-MFC with limited carbon by improving the culture conditions for specific genera.

Effects of biofilm transfer and electron mediators transfer on Klebsiella quasipneumoniae sp. 203 electricity generation performance in MFCs

BACKGROUND

Extracellular electron transfer (EET) is essential in improving the power generation performance of electrochemically active bacteria (EAB) in microbial fuel cells (MFCs). Currently, the EET mechanisms of dissimilatory metal-reducing (DMR) model bacteria Shewanella oneidensis and Geobacter sulfurreducens have been thoroughly studied. Klebsiella has also been proved to be an EAB capable of EET, but the EET mechanism has not been perfected. This study investigated the effects of biofilm transfer and electron mediators transfer on Klebsiella quasipneumoniae sp. 203 electricity generation performance in MFCs.

RESULTS

Herein, we covered the anode of MFC with a layer of microfiltration membrane to block the effect of the biofilm mechanism, and then explore the EET of the electron mediator mechanism of K. quasipneumoniae sp. 203 and electricity generation performance. In the absence of short-range electron transfer, we found that K. quasipneumoniae sp. 203 can still produce a certain power generation performance, and coated-MFC reached 40.26 mW/m2 at a current density of 770.9 mA/m2, whereas the uncoated-MFC reached 90.69 mW/m2 at a current density of 1224.49 mA/m2. The difference in the electricity generation performance between coated-MFC and uncoated-MFC was probably due to the microfiltration membrane covered in anode, which inhibited the growth of EAB on the anode. Therefore, we speculated that K. quasipneumoniae sp. 203 can also perform EET through the biofilm mechanism. The protein content, the integrity of biofilm and the biofilm activity all proved that the difference in the electricity generation performance between coated-MFC and uncoated-MFC was due to the extremely little biomass of the anode biofilm. To further verify the effect of electron mediators on electricity generation performance of MFCs, 10 µM 2,6-DTBBQ, 2,6-DTBHQ and DHNA were added to coated-MFC and uncoated-MFC. Combining the time-voltage curve and CV curve, we found that 2,6-DTBBQ and 2,6-DTBHQ had high electrocatalytic activity toward the redox reaction of K. quasipneumoniae sp. 203-inoculated MFCs. It was also speculated that K. quasipneumoniae sp. 203 produced 2,6-DTBHQ and 2,6-DTBBQ.

CONCLUSIONS

To the best of our knowledge, the three modes of EET did not exist separately. K. quasipneumoniae sp.203 will adopt the corresponding electron transfer mode or multiple ways to realize EET according to the living environment to improve electricity generation performance.

Effect of Cathode Material and Its Size on the Abundance of Nitrogen Removal Functional Genes in Microcosms of Integrated Bioelectrochemical-Wetland Systems

Constructed wetland-microbial electrochemical snorkel (CW-MES) systems, which are short-circuited microbial fuel cells (MFC), have emerged as a novel tool for wastewater management, although the system mechanisms are insufficiently studied in process-based or environmental contexts. Based on quantitative polymerase chain reaction assays, we assessed the prevalence of different nitrogen removal processes for treating nitrate-rich waters with varying cathode materials (stainless steel, graphite felt, and copper) and sizes in the CW-MES systems and correlated them to the changes of N2O emissions. The nitrate and nitrite removal efficiencies were in range of 40% to 75% and over 98%, respectively. In response to the electrochemical manipulation, the abundances of most of the nitrogen-transforming microbial groups decreased in general. Graphite felt cathodes supported nitrifiers, but nirK-type denitrifiers were inhibited. Anaerobic ammonium oxidation (ANAMMOX) bacteria were less abundant in the electrochemically manipulated treatments compared to the controls. ANAMMOX and denitrification are the main nitrogen reducers in CW-MES systems. The treatments with 1:1 graphite felt, copper, plastic, and stainless-steel cathodes showed higher N2O emissions. nirS- and nosZI-type denitrifiers are mainly responsible for producing and reducing N2O emissions, respectively. Hence, electrochemical manipulation supported dissimilatory nitrate reduction to ammonium (DNRA) microbes may play a crucial role in producing N2O in CW-MES systems.

Progress and Prospects of Bioelectrochemical Systems: Electron Transfer and Its Applications in the Microbial Metabolism

Bioelectrochemical systems are revolutionary new bioengineering technologies which integrate microorganisms or enzymes with the electrochemical method to improve the reducing or oxidizing metabolism. Generally, the bioelectrochemical systems show the processes referring to electrical power generation or achieving the reducing reaction with a certain potential poised by means of electron transfer between the electron acceptor and electron donor. Researchers have focused on the selection and optimization of the electrode materials, design of electrochemical device, and screening of electrochemically active or inactive model microorganisms. Notably, all these means and studies are related to electron transfer: efflux and consumption. Thus, here we introduce the basic concepts of bioelectrochemical systems, and elaborate on the extracellular and intracellular electron transfer, and the hypothetical electron transfer mechanism. Also, intracellular energy generation and coenzyme metabolism along with electron transfer are analyzed. Finally, the applications of bioelectrochemical systems and the prospect of microbial electrochemical technologies are discussed.