Identifying the microbial communities and operational conditions for optimized wastewater treatment in microbial fuel cells

A dual-function mixed-culture biofilm for sulfadiazine removal and electricity production using bio-electrochemical system

Sulfadiazine (SDZ) is frequently detected in environmental samples, arousing much concern due to its toxicity and hard degradation. This study investigated the electricity generation capabilities, SDZ removal and microbial communities of a highly efficient mixed-culture system using repeated transfer enrichments in a bio-electrochemical system. The mixed-culture biofilm (S160-T2) produced a remarkable current density of 954.12 ± 15.08 μA cm-2 with 160 mg/L SDZ, which was 32.9 and 1.8 times higher than that of Geobacter sulfurreducens PCA with 40 mg/L SDZ and without additional SDZ, respectively. Especially, the impressive SDZ removal rate of 98.76 ± 0.79% was achieved within 96 h using the further acclimatized mixed-culture. The removal efficiency of this mixed-culture for SDZ through the bio-electrochemical system was 1.1 times higher than that using simple anaerobic biodegradation. Furthermore, the current density and removal efficiency in this system gradually decreased with increasing SDZ concentrations from 0 to 800 mg/L. In addition, community diversity data demonstrated that the dominant genera, Geobacter and Escherichia-Shigella, were enriched in mixed-culture biofilm, which might be responsible for the current production and SDZ removal. This work confirmed the important roles of acclimatized microbial consortia and co-substrates in the simultaneous removal of SDZ and electricity generation in an electrochemical system.

Microbial fuel cell in long-term operation and providing electricity for intermittent aeration to remove contaminants from sewage

Microbial fuel cells (MFCs) show promise in sewage treatment because they can directly convert organic matter (OM) into electricity. This study aimed to demonstrate MFCs stability over 750 days of operation and efficient removal of OM and nitrogenous compounds from sewage. To enhance contaminant removal, oxygen was provided into the anode chamber via a mini air pump. This pump was powered by the MFCs' output voltage, which was boosted using a DC-DC converter. The experimental system consisted of 12 sets of cylindrical MFCs within a 246L-scale reactor. The boosted voltage reached 4.7 V. This voltage was first collected in capacitors every 5 min and then dispensed intermittently to the air pump for the MFCs reactor in 4 s. This corresponds to receiving average DO concentration reaching 0.34 ± 0.44 mg/L at 10 cm above the air-stone. Consequently, the degradation rate constants (k) for chemical oxygen demand (COD) and biological oxygen demand (BOD) in the presence of oxygen were 0.048 and 0.069, respectively, which surpassed those without oxygen by 0.039 and 0.044, respectively. Aeration also marginally improved the removal of ammonia because of its potential to create a favorable environment for the growth of anammox and ammonia-oxidizing bacteria such as Candidatus brocadia and Nitrospira. The findings of this study offer in-depth insight into the benefits of boosted voltage in MFCs, highlighting its potential to enhance contaminant degradation. This serves as a foundation for future research focused on improving MFCs performance, particularly for the removal of contaminants from wastewater.

Chemical-physical parameters and microbial community changes induced by electrodes polarization inhibit PCB dechlorination in a marine sediment

Microbial reductive dechlorination of organohalogenated pollutants is often limited by the scarcity of electron donors, that can be overcome with microbial electrochemical technologies (METs). In this study, polarized electrodes buried in marine sediment microcosms were investigated to stimulate PCB reductive dechlorination under potentiostatic (-0.7 V vs Ag/AgCl) and galvanostatic conditions (0.025 mA·cm-2-0.05 mA·cm-2), using graphite rod as cathode and iron plate as sacrificial anode. A single circuit and a novel two antiparallel circuits configuration (2AP) were investigated. Single circuit polarization impacted the sediment pH and redox potential (ORP) proportionally to the intensity of the electrical input and inhibited PCB reductive dechlorination. The effects on the sediment's pH and ORP, along with the inhibition of PCB reductive dechlorination, were mitigated in the 2AP system. Electrodes polarization stimulated sulfate-reduction and promoted the enrichment of bacterial clades potentially involved in sulfate-reduction as well as in sulfur oxidation. This suggested the electrons provided were consumed by competitors of organohalide respiring bacteria and specifically sequestered by sulfur cycling, which may represent the main factor limiting the applicability of METs for stimulating PCB reductive dechlorination in marine sediments.

Generation of bio-energy after optimization and controlling fluctuations using various sludge activated microbial fuel cell

An aerobic microbial fuel cell (MFC) was designed to produce bio-electricity using cow manure-pretreated slurry (CM) and sewage sludge (SS). A comparative study of parametric effects on power generation for various parameters like feed ratio of wastes, pH of anode media, and electrode depth was conducted. This experiment aimed to identify the most important system parameters and optimize them to develop a suitable controller for a stable output. Power production reached its maximum at an electrode depth of 7 cm, a pH of 6, and a feed ratio of 2:1 in the CM + SS system before applying the controller. Response surface methodology (RSM) was practiced to explore the relationships between various parameters and the response using MINITAB software. The regression equation of the most productive system deduced from the RSM result has an R2 value of 85.3%. The results show that an ON/OFF controller works satisfactorily in this study. The highest energy-generating setup has a chemical oxygen demand (COD) removal efficiency of 45%. The morphology and content of the used wastes indicate that they can be recycled in other applications.

Use of Microbial Fuel Cells for the Treatment of Residue Effluents Discharged from an Anaerobic Digester Treating Food Wastes

One of practical challenges in anaerobic-digestion (AD) technology is the cost-effective treatment of residue effluents containing high concentrations of organics, nitrogen and phosphorus (CNP). In order to evaluate the utility of microbial fuel cells (MFCs) for treating anaerobic-digester effluents (ADEs) and generating power from them, laboratory-scale single-chamber MFCs were filled with ADE obtained from a commercial AD plant treating food wastes and thereafter operated by routinely supplying ADE at different hydraulic residence times (HRTs, 5 to 20 days). It is shown that MFCs were able to reduce not only organics in ADE but also nitrogen and phosphorus. For instance, data demonstrated that over 50% of CNP was removed in MFCs operated at an HRT of 10 days, at which the maximum power density reached over 200 mW m-2 (based on the projected area of anode). Metabarcoding of 16S rRNA genes showed that some bacteria were specifically enriched in anode biofilms, suggesting their involvement in power generation. Our study suggests that MFCs are applicable to reducing CNP in ADEs at reasonable rates, and provides subsequent work with fundamental data useful for setting targets for further developments.

Exploration of Bioinformatics on Microbial Fuel Cell Technology: Trends, Challenges, and Future Prospects

Microbial fuel cells (MFCs) are a cost-effective and environmentally friendly alternative energy method. MFC technology has gained much interest in recent decades owing to its effectiveness in remediating wastewater and generating bioelectricity. The microbial fuel cell generates energy mainlybecause of oxidation-reduction reactions. In this reaction, electrons were transferred between two reactants. Bioinformatics is expanding across a wide range of microbial fuel cell technology. Electroactive species in the microbial community were evaluated using bioinformatics methodologies in whole genome sequencing, RNA sequencing, transcriptomics, metagenomics, and phylogenetics. Technology advancements in microbial fuel cells primarily produce power from organic and inorganic waste from various sources. Reduced chemical oxygen demand and waste degradation are two added advantages for microbial fuel cells. From plants, bacteria, and algae, microbial fuel cells were developed. Due to the rapid advancement of sequencing techniques, bioinformatics approaches are currently widely used in the technology of microbial fuel cells. In addition, they play an important role in determining the composition of electroactive species in microorganisms. The metabolic pathway is also possible to determine with bioinformatics resources. A computational technique that reveals the nature of the mediators and the substrate was also used to predict the electrochemical properties. Computational strategies were used to tackle significant challenges in experimental procedures, such as optimization and understanding microbiological systems. The main focus of this review is on utilizing bioinformatics techniques to improve microbial fuel cell technology.

Multi-regional industrial wastewater metabolism analysis for the Yangtze River Economic Belt, China

Enormous wastewater discharges have significantly impeded the sustainable development. As several economic belt has been formed in China, systematic analysis of multi-regional wastewater metabolic system is required for advancing wastewater mitigation effectively and efficiently. In this study, a distributive environmental input-output model (DEIO) is developed for the Yangtze River Economic Belt (YREB) to provide bases for supporting sustainable development from inter-regional and inter-sectoral perspectives. The discharges and flows of wastewater and related pollutants (i.e., chemical oxygen demand (COD) and ammonia nitrogen (AN)) among sectors and regions are analyzed to providing solid bases for wastewater management within the YREB. The results show that the industrial wastewater mitigation in YREB is desired urgently. The industrial wastewater discharges in Jiangsu and Zhejiang provinces are numerous, while Hunan and Yunnan provinces are more inclined to suffer from serious COD and AN pollution. In addition, the manufacture of food, tobacco, chemical materials, and pharmaceutical are the typical sectors with a large amount of direct wastewater discharge, and the tertiary industry is ranked at the first in indirect wastewater discharge. According to the analysis, the implementation of the "Supply-side Structure Reform" and the "Replace Subsidies with Rewards" policy can benefit the wastewater mitigation in the YREB.

How Comparable are Microbial Electrochemical Systems around the Globe? An Electrochemical and Microbiological Cross-Laboratory Study

A cross-laboratory study on microbial fuel cells (MFC) which involved different institutions around the world is presented. The study aims to assess the development of autochthone microbial pools enriched from domestic wastewater, cultivated in identical single-chamber MFCs, operated in the same way, thereby approaching the idea of developing common standards for MFCs. The MFCs are inoculated with domestic wastewater in different geographic locations. The acclimation stage and, consequently, the startup time are longer or shorter depending on the inoculum, but all MFCs reach similar maximum power outputs (55±22 μW cm-2 ) and COD removal efficiencies (87±9 %), despite the diversity of the bacterial communities. It is inferred that the MFC performance starts when the syntrophic interaction of fermentative and electrogenic bacteria stabilizes under anaerobic conditions at the anode. The generated power is mostly limited by electrolytic conductivity, electrode overpotentials, and an unbalanced external resistance. The enriched microbial consortia, although composed of different bacterial groups, share similar functions both on the anode and the cathode of the different MFCs, resulting in similar electrochemical output.

Competition of two highly specialized and efficient acetoclastic electroactive bacteria for acetate in biofilm anode of microbial electrolysis cell

Maintaining functional stability of microbial electrolysis cell (MEC) treating wastewater depends on maintaining functional redundancy of efficient electroactive bacteria (EAB) on the anode biofilm. Therefore, investigating whether efficient EAB competing for the same resources (electron donor and acceptor) co-exist at the anode biofilm is key for the successful application of MEC for wastewater treatment. Here, we compare the electrochemical and kinetic properties of two efficient acetoclastic EAB, Geobacter sulfurreducens (GS) and Desulfuromonas acetexigens (DA), grown as monoculture in MECs fed with acetate. Additionally, we monitor the evolution of DA and GS in co-culture MECs fed with acetate or domestic wastewater using fluorescent in situ hybridization. The apparent Monod kinetic parameters reveal that DA possesses higher jmax (10.7 ± 0.4 A/m2) and lower KS, app (2 ± 0.15 mM) compared to GS biofilms (jmax: 9.6 ± 0.2 A/m2 and KS, app: 2.9 ± 0.2 mM). Further, more donor electrons are diverted to the anode for respiration in DA compared to GS. In acetate-fed co-culture MECs, DA (98% abundance) outcompete GS for anode-dependent growth. In contrast, both EAB co-exist (DA: 55 ± 2%; GS: 24 ± 1.1%) in wastewater-fed co-culture MECs despite the advantage of DA over GS based on kinetic parameters alone. The co-existence of efficient acetoclastic EAB with high current density in MECs fed with wastewater is significant in the context of functional redundancy to maintain stable performance. Our findings also provide insight to future studies on bioaugmentation of wastewater-fed MECs with efficient EAB to enhance performance.

Socio-Economic and Environmental Impacts of Biomass Valorisation: A Strategic Drive for Sustainable Bioeconomy

In the late twentieth century, the only cost-effective opportunity for waste removal cost at least several thousand dollars, but nowadays, a lot of improvement has occurred. The biomass and waste generation problems attracted concerned authorities to identify and provide environmentally friendly sustainable solutions that possess environmental and economic benefits. The present study emphasises the valorisation of biomass and waste produced by domestic and industrial sectors. Therefore, substantial research is ongoing to replace the traditional treatment methods that potentially acquire less detrimental effects. Synthetic biology can be a unique platform that invites all the relevant characters for designing and assembling an efficient program that could be useful to handle the increasing threat for human beings. In the future, these engineered methods will not only revolutionise our lives but practically lead us to get cheaper biofuels, producing bioenergy, pharmaceutics, and various biochemicals. The bioaugmentation approach concomitant with microbial fuel cells (MFC) is an example that is used to produce electricity from municipal waste, which is directly associated with the loading of waste. Beyond the traditional opportunities, herein, we have spotlighted the new advances in pertinent technology closely related to production and reduction approaches. Various integrated modern techniques and aspects related to the industrial sector are also discussed with suitable examples, including green energy and other industrially relevant products. However, many problems persist in present-day technology that requires essential efforts to handle thoroughly because significant valorisation of biomass and waste involves integrated methods for timely detection, classification, and separation. We reviewed and proposed the anticipated dispensation methods to overcome the growing stream of biomass and waste at a distinct and organisational scale.

Advances in Circular Bioeconomy Technologies: From Agricultural Wastewater to Value-Added Resources

This review systematically outlines the recent advances in the application of circular bioeconomy technologies for converting agricultural wastewater to value-added resources. The properties and applications of the value-added products from agricultural wastewater are first summarized. Various types of agricultural wastewater, such as piggery wastewater and digestate from anaerobic digestion, are focused on. Next, different types of circular technologies for recovery of humic substances (e.g., humin, humic acids and fulvic acids) and nutrients (e.g., nitrogen and phosphorus) from agricultural wastewater are reviewed and discussed. Advanced technologies, such as chemical precipitation, membrane separation and electrokinetic separation, are evaluated. The environmental benefits of the circular technologies compared to conventional wastewater treatment processes are also addressed. Lastly, the perspectives and prospects of the circular technologies for agricultural wastewater are provided.

A novel microbial fuel cell electrode design: prototyping a self-standing one-step bacteria-encapsulating bioanode with electrospinning

In this study, the electrospinning technique is shown to be a viable method for the synthesis of a bacteria-encapsulating bioanode. A coaxial setup was designed to yield in one step a bioanode made of two fibers networks: one encapsulating the electroactive bacteria Shewanella oneidensis and the other one providing the necessary conductivity for electron transport throughout the bioelectrode. The electrical conductivity of this "integrated bioanode" (∼10-2 to 10-3 S cm-1) was deemed satisfactory and it was then included into a microbial fuel cells (MFC). The resulting MFC exhibited electricity generation. We further demonstrate that this electrode can be cryodesiccated and still exhibits an electrochemical activity once integrated into the MFC reactor. Its volume current and power densities were similar to those recorded for the fresh electrospun bioanode (up to 3260 A m-3 and 230 W m-3 for the thin cryodesiccated bioanode (∼410 μm)). Such impressive volume current densities for thin electrospun systems may be for instance envisioned to be applied to wearable or paper-based MFCs which require a certain flexibility.

On-line monitoring of repeated copper pollutions using sediment microbial fuel cell based sensors in the field environment

Most microbial fuel cells (MFCs) based sensors rely on exoelectrogenic bacteria to sense contaminants. However, these sensors cannot monitor repeated pollutions unless the exoelectrogenic bacteria are recovered or re-inoculated. To overcome this drawback, a novel sediment microbial fuel cell (SMFC) based sensor was developed for online and in situ monitoring of repeated Cu²⁺ shocks to the overlaying water of paddy soil. The SMFC sensor was operated for a period of eight months in the field environment and a group of CuCl₂ solutions ranging from 12.5 to 400 mg L^-1 Cu²⁺ were repeatedly applied on sunny and rainy days in different seasons. Results show that the SMFC sensor generates one voltage peak in less than 20 s after each Cu²⁺ shock, regardless of the seasons and weather conditions, and the voltage increments from baseline to peak exhibit linear correlation (R² > 0.92) with the logarithm of Cu²⁺ concentrations. Repeated Cu²⁺ pollutions do not decrease the baseline voltage, indicating that the activity of exoelectrogenic bacteria was not significantly inhibited. Soil adsorbed and inactivated approximately 99% of total Cu²⁺. Only 1% of total Cu²⁺ was the toxic exchangeable fraction, of which the concentrations were 0.73, 0.23, and 0.22 mg kg^-1 in the surface (0-3 cm), middle (3-6 cm), and bottom (6-11 cm) layers, respectively. The abundance of 16S rRNA gene transcripts of exoelectrogenic bacteria-associated genera is the lowest in the surface layer (2.86 × 10¹¹ copies g^-1) and the highest in the bottom layer (7.99 × 10¹¹ copies g^-1). Geobacter, Clostridium, Anaeromyxobacter, and Bacillus are the most active exoelectrogenic bacteria-associated genera in the soil. This study suggests that the SMFC sensor could be applied in wetlands to monitor the repeated discharge of Cu²⁺ and other heavy metals.

Sediment microbial fuel cells as a barrier to sulfide accumulation and their potential for sediment remediation beneath aquaculture pens

Sediment microbial fuel cells (SMFCs) generate electricity through the oxidation of reduced compounds, such as sulfide or organic carbon compounds, buried in anoxic sediments. The ability to remove sulfide suggests their use in the remediation of sediments impacted by point source organic matter loading, such as occurs beneath open pen aquaculture farms. However, for SMFCs to be a viable technology they must remove sulfide at a scale relevant to the environmental contamination and their impact on the sediment geochemistry as a whole must be evaluated. Here we address these issues through a laboratory microcosm experiment. Two SMFCs placed in high organic matter sediments were operated for 96 days and compared to open circuit and sediment only controls. The impact on sediment geochemistry was evaluated with microsensor profiling for oxygen, sulfide, and pH. The SMFCs had no discernable effect on oxygen profiles, however porewater sulfide was significantly lower in the sediment microcosms with functioning SMFCs than those without. Depth integrated sulfide inventories in the SMFCs were only 20% that of the controls. However, the SMFCs also lowered pH in the sediments and the consequences of this acidification on sediment geochemistry should be considered if developing SMFCs for remediation. The data presented here indicate that SMFCs have potential for the remediation of sulfidic sediments around aquaculture operations.

Individual and combined effects of magnetite addition and external voltage application on anaerobic digestion of dairy wastewater

Direct interspecies electron transfer (DIET) between exoelectrogenic fatty acid oxidizers and electrotrophic methanogens plays an important role in keeping the overall anaerobic digestion (AD) process well-balanced. This study examined the individual and combined effects of two different DIET-promoting strategies, i.e., magnetite addition (20 mM Fe) and external voltage application (0.6 V), in continuous digesters treating dairy wastewater. Although the strategies were both effective in enhancing the process performance and stability, adding magnetite had a much greater stimulatory effect. External voltage contributed little to the methane yield, and the digester with magnetite addition alone achieved stable performance, comparable to that of the digester where both strategies were combined, at short hydraulic retention times (down to 7.5 days). Diverse (putative) electroactive microorganisms were significantly enriched under DIET-promoting conditions, particularly with magnetite addition. The overall results suggest that magnetite addition could effectively enhance AD performance and stability by promoting DIET-based electro-syntrophic microbial interactions.

Continuous flow, large-scale, microbial fuel cell system for the sustained treatment of swine waste

Microbial fuel cells (MFCs) have long held the promise of being a cost-effective technology for the energy-neutral treatment of wastewater. However, successful pilot-scale demonstrations for this technology are still limited to very few. Here, we present a large-scale MFC system, composed of 12 MFCs with a total volume of 110 L, successfully treating swine wastewater at a small educational farm. The system was operated for over 200 days in continuous mode with hydraulic residence time of 4 hr. Very stable electrochemical and waste treatment performance was observed with up to 65% of chemical oxygen demand (COD) removed and a maximum treatment rate of 5.0 kg COD/m³ .day. Robust microbial enrichment was performed and adapted to metabolize and transform a diversity of compounds present. The Net Energy Recovery (NER = 0.11 kWhr/kg COD) is not only competitive with conventional cogeneration processes, but is in fact sufficient to sustain the operational energy requirements of the system. PRACTITIONER POINTS: This study demonstrates the design and operation of a large-scale microbial fuel cells (MFC) system for continuous treatment of swine wastewater. The system achieved a high chemical oxygen demand removal rate within a short hydraulic residence time. This study moves one-step closer to applying MFC technology for real wastewater treatment.

Anode modification by biogenic gold nanoparticles for the improved performance of microbial fuel cells and microbial community shift

In this study, carbon cloth anodes were modified using biogenic gold nanoparticles (BioAu) and nanohybrids of multi-walled carbon nanotubes blended with BioAu (BioAu/MWCNT) to improve the performance of microbial fuel cells (MFCs). The results demonstrated that BioAu modification significantly enhanced the electricity generation of MFCs. In particular, BioAu/MWCNT nanohybrids as the modifier displayed a better performance. The MFC with the BioAu/MWCNT electrode had the shortest start-up time (6.74 d) and highest power density (178.34 ± 4.79 mW/m²), which were 141.69% shorter and 56.11% higher compared with those of the unmodified control, respectively. These improvements were attributed to the excellent electrocatalytic activity and strong affinity towards exoelectrogens of the BioAu/MWCNT nanohybrids on the electrode. High throughput sequencing analysis indicated that the relative abundance of electroactive bacteria in the biofilm community, mostly from the classes of Gammaproteobacteria and Negativicutes, increased after anode modification.

Selection and community analysis of halophilic mixed exoelectrogens from salt lake soils

In this study, two slightly different halophilic mixed exoelectrogens were enriched and selected from salt lake soils. The results showed that the selected mixed exoelectrogens ESA from the sample OSA (Xiaochaidan Lake soil) and ESB from the sample OSB (Dachaidan Lake soil), without additional NaCl, produced current densities of 1231.1 and 1050.2 μA cm-2, which were 89.6% and 61.7% higher than the typical exoelectrogen G. sulfurreducens PCA, respectively. ESA and ESB could produce 2.7 and 1.9 times higher currents than that obtained using G. sulfurreducens PCA with an additional 1.5% NaCl, respectively. The community diversity data demonstrated that Proteobacteria was the most abundant phylum, in which Enterobacteriaceae and Rhodocyclaceae were the dominant families for both ESA and ESB. Furthermore, at the genus level, the dominant genera Propionivibrio and Escherichia-Shigella were also shared by both. ESA had higher species diversity compared to ESB.

Metagenomic insights into the ecology and physiology of microbes in bioelectrochemical systems

In bioelectrochemical systems (BESs), electrons are transferred between electrochemically active microbes (EAMs) and conductive materials, such as electrodes, via extracellular electron transfer (EET) pathways, and electrons thus transferred stimulate intracellular catabolic reactions. Catabolic and EET pathways have extensively been studied for several model EAMs, such as Shewanella oneidensis MR-1 and Geobacter sulfurreducens PCA, whereas it is also important to understand the ecophysiology of EAMs in naturally occurring microbiomes, such as those in anode biofilms in microbial fuel cells treating wastewater. Recent studies have exploited metagenomics and metatranscriptomics (meta-omics) approaches to characterize EAMs in BES-associated microbiomes. Here we review recent BES studies that used meta-omics approaches and show that these studies have discovered unexpected features of EAMs and deepened our understanding of functions and behaviors of microbes in BESs. It is desired that more studies will employ meta-omics approaches for advancing our knowledge on microbes in BESs.

Food waste biorefinery: Sustainable strategy for circular bioeconomy

Enormous quantity of food waste (FW) is becoming a global concern. To address this persistent problem, sustainable interventions with green technologies are essential. FW can be used as potential feedstock in biological processes for the generation of various biobased products along with its remediation. Enabling bioprocesses like acidogenesis, fermentation, methanogenesis, solventogenesis, photosynthesis, oleaginous process, bio-electrogenesis, etc., that yields various products like biofuels, platform chemicals, bioelectricity, biomaterial, biofertilizers, animal feed, etc can be utilized for FW valorisation. Integrating these bioprocesses further enhances the process efficiency and resource recovery sustainably. Adapting biorefinery strategy with integrated approach can lead to the development of circular bioeconomy. The present review highlights the various enabling bioprocesses that can be employed for the generation of energy and various commodity chemicals in an integrated approach addressing sustainability. The waste biorefinery approach for FW needs optimization of the cascade of the individual bioprocesses for the transformation of linear economy to circular bioeconomy.

The Identification of Cable Bacteria Attached to the Anode of a Benthic Microbial Fuel Cell: Evidence of Long Distance Extracellular Electron Transport to Electrodes

Multicellular, filamentous, sulfur-oxidizing bacteria, known as cable bacteria, were discovered attached to fibers of a carbon brush electrode serving as an anode of a benthic microbial fuel cell (BMFC). The BMFC had been operated in a temperate estuarine environment for over a year before collecting anode samples for scanning electron microscopy and phylogenetic analyses. Individual filaments were attached by single terminus cells with networks of pilus-like nano-filaments radiating out from these cells, across the anode fiber surface, and between adjacent attachment locations. Current harvesting by the BMFC poised the anode at potentials of ~170-250 mV vs. SHE, and these surface potentials appear to have allowed the cable bacteria to use the anode as an electron acceptor in a completely anaerobic environment. A combination of catalyzed reporter deposition fluorescent in situ hybridization (CARD-FISH) and 16S rRNA gene sequence analysis confirmed the phylogeny of the cable bacteria and showed that filaments often occurred in bundles and in close association with members of the genera Desulfuromonas. However, the Desulfobulbaceae Operational Taxonomic Units (OTUs) from the 16S sequencing did not cluster closely with other putative cable bacteria sequences suggesting that the taxonomic delineation of cable bacteria is far from complete.

Microbial fuel cells: From fundamentals to applications. A review

In the past 10-15 years, the microbial fuel cell (MFC) technology has captured the attention of the scientific community for the possibility of transforming organic waste directly into electricity through microbially catalyzed anodic, and microbial/enzymatic/abiotic cathodic electrochemical reactions. In this review, several aspects of the technology are considered. Firstly, a brief history of abiotic to biological fuel cells and subsequently, microbial fuel cells is presented. Secondly, the development of the concept of microbial fuel cell into a wider range of derivative technologies, called bioelectrochemical systems, is described introducing briefly microbial electrolysis cells, microbial desalination cells and microbial electrosynthesis cells. The focus is then shifted to electroactive biofilms and electron transfer mechanisms involved with solid electrodes. Carbonaceous and metallic anode materials are then introduced, followed by an explanation of the electro catalysis of the oxygen reduction reaction and its behavior in neutral media, from recent studies. Cathode catalysts based on carbonaceous, platinum-group metal and platinum-group-metal-free materials are presented, along with membrane materials with a view to future directions. Finally, microbial fuel cell practical implementation, through the utilization of energy output for practical applications, is described.

An Overview of Electron Acceptors in Microbial Fuel Cells

Microbial fuel cells (MFC) have recently received increasing attention due to their promising potential in sustainable wastewater treatment and contaminant removal. In general, contaminants can be removed either as an electron donor via microbial catalyzed oxidization at the anode or removed at the cathode as electron acceptors through reduction. Some contaminants can also function as electron mediators at the anode or cathode. While previous studies have done a thorough assessment of electron donors, cathodic electron acceptors and mediators have not been as well described. Oxygen is widely used as an electron acceptor due to its high oxidation potential and ready availability. Recent studies, however, have begun to assess the use of different electron acceptors because of the (1) diversity of redox potential, (2) needs of alternative and more efficient cathode reaction, and (3) expanding of MFC based technologies in different areas. The aim of this review was to evaluate the performance and applicability of various electron acceptors and mediators used in MFCs. This review also evaluated the corresponding performance, advantages and disadvantages, and future potential applications of select electron acceptors (e.g., nitrate, iron, copper, perchlorate) and mediators.

Resilience, Dynamics, and Interactions within a Model Multispecies Exoelectrogenic-Biofilm Community

Anode-associated multispecies exoelectrogenic biofilms are essential for the function of bioelectrochemical systems (BESs). The individual activities of anode-associated organisms and physiological responses resulting from coculturing are often hard to assess due to the high microbial diversity in these systems. Therefore, we developed a model multispecies biofilm comprising three exoelectrogenic proteobacteria, Shewanella oneidensis, Geobacter sulfurreducens, and Geobacter metallireducens, with the aim to study in detail the biofilm formation dynamics, the interactions between the organisms, and the overall activity of an exoelectrogenic biofilm as a consequence of the applied anode potential. The experiments revealed that the organisms build a stable biofilm on an electrode surface that is rather resilient to changes in the redox potential of the anode. The community operated at maximum electron transfer rates at electrode potentials that were higher than 0.04 V versus a normal hydrogen electrode. Current densities decreased gradually with lower potentials and reached half-maximal values at -0.08 V. Transcriptomic results point toward a positive interaction among the individual strains. S. oneidensis and G. sulfurreducens upregulated their central metabolisms as a response to cultivation under mixed-species conditions. G. sulfurreducens was detected in the planktonic phase of the bioelectrochemical reactors in mixed-culture experiments but not when it was grown in the absence of the other two organisms.IMPORTANCE In many cases, multispecies communities can convert organic substrates into electric power more efficiently than axenic cultures, a phenomenon that remains unresolved. In this study, we aimed to elucidate the potential mutual effects of multispecies communities in bioelectrochemical systems to understand how microbes interact in the coculture anodic network and to improve the community's conversion efficiency for organic substrates into electrical energy. The results reveal positive interactions that might lead to accelerated electron transfer in mixed-species anode communities. The observations made within this model biofilm might be applicable to a variety of nonaxenic systems in the field.

The performance of microbial anodes in municipal wastewater: Pre-grown multispecies biofilm vs. natural inocula

In this study, different inoculation strategies for continuously operated microbial anodes are analyzed and compared. After 20daysof operation with municipal wastewater anodes pre-incubated with a biofilm of the exoelectrogenic species Geobacter and Shewanella showed current densities of (65±8) μA/cm². This is comparable to the current densities of non-inoculated anodes and anodes inoculated with sewage sludge. Analysis of the barcoded pre-grown multispecies biofilms reveal that 99% of the original biofilm was detached after 20daysof operation with municipal wastewater. This is in contrast to previous experiments where a pre-grown biofilm of exoelectrogens was operated in batch mode. To implement pre-grown biofilms in continuous systems it will thus be necessary to reveal a window of process parameters in which typical exoelectrogenic microorganisms including model organisms can be kept and/or enriched on anodes.

Impact of Anodic Respiration on Biopolymer Production and Consequent Membrane Fouling

Microbial fuel cells (MFCs) have recently been integrated with membrane bioreactors (MBRs) for wastewater treatment and energy recovery. However, the impact of integration of the two reactors on membrane fouling of MBR has not been reported yet. In this study, MFCs equipped with different external resistances (1-10 000 ohm) were operated, and membrane-fouling potentials of the MFC anode effluents were directly measured to study the impact of anodic respiration by exoelectrogens on membrane fouling. It was found that although the COD removal efficiency was comparable, the fouling potential was significantly reduced due to less production of biopolymer (a major foulant) in MFCs equipped with lower external resistance (i.e., with higher current generation) as compared with aerobic respiration. Furthermore, it was confirmed that Geobacter sulfurreducens strain PCA, a dominant exoelectrogen in anode biofilms of MFCs in this study, produced less biopolymer under anodic respiration condition than fumarate (anaerobic) respiration condition, resulting in lower membrane-fouling potential. Taken together, anodic respiration can mitigate membrane fouling of MBR due to lower biopolymer production, suggesting that development of an electrode-assisted MBR (e-MBR) without aeration is feasible.

Community structure dynamics during startup in microbial fuel cells - The effect of phosphate concentrations

For microbial fuel cells (MFCs) to become a cost-effective wastewater treatment technology, they must produce a stable electro-active microbial community quickly and operate under realistic wastewater nutrient conditions. The composition of the anodic-biofilm and planktonic-cells communities was followed temporally for MFCs operated under typical laboratory phosphate concentrations (134mgL(-1)P) versus wastewater phosphate concentrations (16mgL(-1)P). A stable peak voltage was attained two-fold faster in MFCs operating under lower phosphate concentration. All anodic-biofilms were composed of well-known exoelectrogenic bacterial families; however, MFCs showing faster startup and a stable voltage had a Desulfuromonadaceae-dominated-biofilm, while biofilms co-dominated by Desulfuromonadaceae and Geobacteraceae characterized slower or less stable MFCs. Interestingly,planktonic-cell concentrations of these bacteria followed a similar trend as the anodic-biofilm and could therefore serve as a biomarker for its formation. These results demonstrate that wastewater-phosphate concentrations do not compromise MFCs efficiency, and considerably speed up startup times.

Microbial community dynamics in continuous microbial fuel cells fed with synthetic wastewater and pig slurry

Two-chambered microbial fuel cells (MFCs) operating with synthetic wastewater and pig slurry were assessed. Additionally, the use of 2-bromoethanesulfonate (BES-Inh) was studied. The synthetic wastewater-fed MFC (MFCSW) showed a maximum power density (PDmax) of 2138mWm(-3), and the addition of BES-Inh (10mM) did not show any improvement in its performance (PDmax=2078mWm(-3)). When pig slurry was used as feed (MFCPS), PDmax increased up to 5623mWm(-3). The microbial community composition was affected by the type of substrate used. While, Pseudomonadaceae and Clostridiaceae were the most representative families within the acetate-based medium, Flavobacteriaceae, Chitinophagaceae, Comamonadaceae and Nitrosomonadaceae were predominant when pig slurry was used as feed. Otherwise, only the Eubacterial microbial community composition was strongly modified when adding BES-Inh, thus leading to an enrichment of the Bacteroidetes phylum. Oppositely, the Archaeal community was less affected by the addition of BES-Inh, and Methanosarcina sp., arose as the predominant family in both situations. Despite all the differences in microbial communities, 6 operational taxonomic units (OTUs) belonging to Bacteroidetes (Porphyromonadaceae and Marinilabiaceae) and Firmicutes (Clostridiales) were found to be common to both MFCs, also for different contents of COD and N-NH4(+), and therefore could be considered as the bioanode core microbiome.

Effects of storage on mixed-culture biological electrodes

Storage methods are important to preserve the viability and biochemical characteristics of microbial cultures between experiments or during periods when bioreactors are inactive. Most of the research on storage has focused on isolates; however, there is an increasing interest in methods for mixed cultures, which are of relevance in environmental biotechnology. The purpose of this study was to investigate the effect of different storage methods on electrochemically active enrichment cultures. Acetate-oxidizing bioanodes generating a current density of about 5 A m⁻² were enriched in a microbial electrolysis cell. The effect of five weeks of storage was evaluated using electrochemical techniques and microbial community analysis. Storage by refrigeration resulted in quicker re-activation than freezing in 10% glycerol, while the bioelectrochemical activity was entirely lost after storage using dehydration. The results showed that the bioelectrochemical activity of bioanodes stored at low temperature could be retained. However, during the re-activation period the bioanodes only recovered 75% of the current density generated before storage and the bacterial communities were different in composition and more diverse after storage than before.

Methods for enrichment of novel electrochemically-active microorganisms

Electrochemically-active microorganisms (EAM) are relevant to metal biogeochemistry and have applications in microbial fuel cells (MFCs), bioremediation, and bioelectrocatalysis. Most research conducted to date focuses on EAM hailing from two distinct genera, namely Shewanella and Geobacter, with a relatively limited number of EAM discovered in recent years. This review article summarises current approaches to novel EAM enrichment, in terms of inoculum choice, growth medium, reactor configuration, electrochemical characterisation and community profiling through metagenomics and metatranscriptomics. A novel roadmap for EAM enrichment and subsequent characterisation using environmental samples as a starting material is provided in order to increase throughput and hence the likelihood of discovering novel EAM.

Microbial metabolic networks in a complex electrogenic biofilm recovered from a stimulus-induced metatranscriptomics approach

Microorganisms almost always exist as mixed communities in nature. While the significance of microbial community activities is well appreciated, a thorough understanding about how microbial communities respond to environmental perturbations has not yet been achieved. Here we have used a combination of metagenomic, genome binning, and stimulus-induced metatranscriptomic approaches to estimate the metabolic network and stimuli-induced metabolic switches existing in a complex microbial biofilm that was producing electrical current via extracellular electron transfer (EET) to a solid electrode surface. Two stimuli were employed: to increase EET and to stop EET. An analysis of cell activity marker genes after stimuli exposure revealed that only two strains within eleven binned genomes had strong transcriptional responses to increased EET rates, with one responding positively and the other responding negatively. Potential metabolic switches between eleven dominant members were mainly observed for acetate, hydrogen, and ethanol metabolisms. These results have enabled the estimation of a multi-species metabolic network and the associated short-term responses to EET stimuli that induce changes to metabolic flow and cooperative or competitive microbial interactions. This systematic meta-omics approach represents a next step towards understanding complex microbial roles within a community and how community members respond to specific environmental stimuli.

All ecosystems potentially host electrogenic bacteria

Instead of requiring metal catalysts, MFCs utilize bacteria that oxidize organic matter and either transfer electrons to the anode or take electrons from the cathode. These devices are thus based on a wide microbial diversity that can convert a large array of organic matter components into sustainable and renewable energy. A wide variety of explored environments were found to host electrogenic bacteria, including extreme environments. In the present review, we describe how different ecosystems host electrogenic bacteria, as well as the physicochemical, electrochemical and biological parameters that control the currents from MFCs. We also report how using new molecular techniques allowed characterization of electrochemical biofilms and identification of potentially new electrogenic species. Finally we discuss these findings in the context of future research directions.

Influence of anode surface chemistry on microbial fuel cell operation

Self-assembled monolayers (SAMs) modified gold anodes are used in single chamber microbial fuel cells for organic removal and electricity generation. Hydrophilic (N(CH3)3(+), OH, COOH) and hydrophobic (CH3) SAMs are examined for their effect on bacterial attachment, current and power output. The different substratum chemistry affects the community composition of the electrochemically active biofilm formed and thus the current and power output. Of the four SAM-modified anodes tested, N(CH3)3(+) results in the shortest start up time (15 days), highest current achieved (225 μA cm(-2)) and highest MFC power density (40 μW cm(-2)), followed by COOH (150 μA cm(-2) and 37 μW cm(-2)) and OH (83 μA cm(-2) and 27 μW cm(-2)) SAMs. Hydrophobic SAM decreases electrochemically active bacteria attachment and anode performance in comparison to hydrophilic SAMs (CH3 modified anodes 7 μA cm(-2) anodic current and 1.2 μW cm(-2) MFC's power density). A consortium of Clostridia and δ-Proteobacteria is found on all the anode surfaces, suggesting a synergistic cooperation under anodic conditions.

Innovative statistical interpretation of Shewanella oneidensis microbial fuel cells data

The last decade of research has made significant strides toward practical applications of Microbial Fuel Cells (MFCs); however, design improvements and operational optimization cannot be realized without equally considering engineering designs and biological interfacial reactions. In this study, the main factors contributing to MFCs' overall performance and their influence on MFC reproducibility are discussed. Two statistical approaches were used to create a map of MFC components and their expanded uncertainties, principal component analysis (PCA) and uncertainty of measurement results (UMR). PCA was used to identify the major factors influencing MFCs and to determine their ascendency over MFC operational characteristics statistically. UMR was applied to evaluate the factors' uncertainties and estimate their level of contribution to the final irreproducibility. In order to simplify the presentation and concentrate on the MFC components, only results from Shewanella spp. were included; however, a similar analysis could be applied for any DMRB or microbial community. The performed PCA/UMR analyses suggest that better reproducibility of MFC performance can be achieved through improved design parameters. This approach is exactly opposite to the MFC optimization and scale up approach, which should start with improving the bacteria-electrode interactions and applying these findings to well-designed systems.

Bioelectrogenic role of anoxic microbial anode in the treatment of chemical wastewater: microbial dynamics with bioelectro-characterization

A membrane-less anoxic bioelectrochemical treatment (AxBET) system was evaluated to study the influence of bioelectrogenic activity during the treatment of chemical wastewater (CW). Increment in power generation was observed with increase in substrate loading (61-204 mW/m(2)) indicating the ability of anodic bacteria in BET system to utilize the complex chemicals as the sole carbon source. Derivative analysis of voltammograms depicted by positive and negative peak potentials which relate to the extracellular electron transport sites (EETs) that presumably play a significant role in electron transfer. These self-driven redox mediators varied with respect to the substrate load. The microbial population was dominated by anaerobic microorganisms which are commonly involved in effluent treatment plants during the initial phase of operation. A gradual shift in the microbial community was observed towards enrichment of electrogenically active bacteria belonging to phyla viz., Firmicutes and Proteobacteria after prolonged operation. Shannon Index and principal component analysis correlated with the microbial profile studies. The feasibility of self-driven bioremediation of chemical wastewater in an AxBET system demonstrated bioelectricity production along with multipollutant removal simultaneously.