Scale-up and techno-economic analysis of microbial electrolysis cells for hydrogen production from wastewater.

Microbial electrolysis cells (MECs) have demonstrated high-rate H2 production while concurrently treating wastewater, but the transition in scale from laboratory research to systems that can be practically applied has encountered challenges. It has been more than a decade since the first pilot-scale MEC was reported, and in recent years, many attempts have been made to overcome the barriers and move the technology to the market. This study provided a detailed analysis of MEC scale-up efforts and summarized the key factors that should be considered to further develop the technology. We compared the major scale-up configurations and systematically evaluated their performance from both technical and economic perspectives. We characterized how system scale-up impacts the key performance metrics such as volumetric current density and H2 production rate, and we proposed methods to evaluate and optimize system design and fabrication. In addition, preliminary techno-economic analysis indicates that MECs can be profitable in many different market scenarios with or without subsidies. We also provide perspectives on future development needed to transition MEC technology to the marketplace.

Defining Genomic and Predicted Metabolic Features of the Acetobacterium Genus.

Acetogens are anaerobic bacteria capable of fixing CO2 or CO to produce acetyl coenzyme A (acetyl-CoA) and ultimately acetate using the Wood-Ljungdahl pathway (WLP). Acetobacterium woodii is the type strain of the Acetobacterium genus and has been critical for understanding the biochemistry and energy conservation in acetogens. Members of the Acetobacterium genus have been isolated from a variety of environments or have had genomes recovered from metagenome data, but no systematic investigation has been done on the unique and various metabolisms of the genus. To gain a better appreciation for the metabolic breadth of the genus, we sequenced the genomes of 4 isolates (A. fimetarium, A. malicum, A. paludosum, and A. tundrae) and conducted a comparative genome analysis (pan-genome) of 11 different Acetobacterium genomes. A unifying feature of the Acetobacterium genus is the carbon-fixing WLP. The methyl (cluster II) and carbonyl (cluster III) branches of the Wood-Ljungdahl pathway are highly conserved across all sequenced Acetobacterium genomes, but cluster I encoding the formate dehydrogenase is not. In contrast to A. woodii, all but four strains encode two distinct Rnf clusters, Rnf being the primary respiratory enzyme complex. Metabolism of fructose, lactate, and H2:CO2 was conserved across the genus, but metabolism of ethanol, methanol, caffeate, and 2,3-butanediol varied. Additionally, clade-specific metabolic potential was observed, such as amino acid transport and metabolism in the psychrophilic species, and biofilm formation in the A. wieringae clade, which may afford these groups an advantage in low-temperature growth or attachment to solid surfaces, respectively.IMPORTANCE Acetogens are anaerobic bacteria capable of fixing CO2 or CO to produce acetyl-CoA and ultimately acetate using the Wood-Ljungdahl pathway (WLP). This autotrophic metabolism plays a major role in the global carbon cycle and, if harnessed, can help reduce greenhouse gas emissions. Overall, the data presented here provide a framework for examining the ecology and evolution of the Acetobacterium genus and highlight the potential of these species as a source for production of fuels and chemicals from CO2 feedstocks.

Microbiome for the Electrosynthesis of Chemicals from Carbon Dioxide.

The price for renewable electricity is rapidly decreasing, and the availability of such energy is expected to increase in the coming years. This is a welcomed outcome considering that mitigation of climate disruption due to the use of fossil carbon is reaching a critical stage. However, the economy will remain dependent on carbon-based chemicals and the problem of electricity storage persists. Therefore, the development of electrosynthetic processes that convert electricity and CO2 into chemicals and energy dense fuels, perhaps even food, would be desirable. Electrochemistry has been applied to the manufacture of many valuable products and at a large industrial scale, but it is difficult to produce multicarbon chemicals from CO2 by chemistry alone. Being that the biological world possesses expertise at the construction of C-C bonds, it is being examined in conjunction with electrochemistry to discover new ways of synthesizing chemicals from electricity and CO2. One approach is microbial electrosynthesis. This Account describes the development of a microbial electrosynthesis system by the authors. A biocathode consisting of a carbon-based electrode and a microbial community produced short chain fatty acids, primarily acetate. The device works by electrolysis of water, but microbes facilitate electron transfer from the cathode while reducing CO2 by the Wood-Ljungdahl pathway possessed by an Acetobacterium sp. While this acetogenic microorganism dominates the microbiome growing on the cathode surface, 13 total species of microbes overall were ecologically selected on the cathode and genomes for each have been assembled. The combined species may contribute to the stability of the microbiome, a common feature of naturally selected microbial communities. The microbial electrosynthesis system was demonstrated to operate continuously at a cathode for more than 2 years and could also be used with intermittent power, thus demonstrating the stability of the microbiome living at the cathode. In addition to the description of reactor design and startup procedures, the possible mechanisms of electron transfer are described in this Account. While mysteries remain to be solved, much evidence indicates that the microbiome may facilitate electron transfer by supplying catalyst(s) external to the bacterial cells and onto the cathode surface. This may be in the form of a hydrogen-producing catalyst that enhances hydrogen generation by an inert carbon-based electrode. Through the enrichment of the electrosynthetic microbiome along with several modifications in reactor design and operation, the productivity and efficiency were improved. In addition to the intrinsic value of the current products, coupling the process with a secondary stage might be used to produce more valuable products from the acetic acid stream such as lipids, biocrude oil, or higher value food supplements. Alternatively, additional work on the mechanism of electron transfer, reactor design/operation, and modification of the microbes through synthetic biology, particularly to enhance carbon efficiency into higher value chemicals, are the needed next steps to advance microbial electrosynthesis so that it may be used to transform renewable electrons and CO2 directly into products and help solve the problem of climate disruption.

A Pilot-Scale Field Study: In Situ Treatment of PCB-Impacted Sediments with Bioamended Activated Carbon.

A combined approach involving microbial bioaugmentation and enhanced sorption was demonstrated to be effective for in situ treatment of polychlorinated biphenyls (PCBs). A pilot study was conducted for 409 days on PCB impacted sediments in four 400 m2 plots located in a watershed drainage pond in Quantico, VA. Treatments with activated carbon (AC) agglomerate bioamended with PCB dechlorinating and oxidizing bacteria decreased the PCB concentration in the top 7.5 cm by up to 52% and the aqueous concentrations of tri- to nonachlorobiphenyl PCB congeners by as much as 95%. Coplanar congeners decreased by up to 80% in sediment and were undetectable in the porewater. There was no significant decrease in PCB concentrations in non-bioamended plots with or without AC. All homologue groups decreased in bioamended sediment and porewater, indicating that both anaerobic dechlorination and aerobic degradation occurred concurrently. The titer of the bioamendments based on quantitative PCR of functional marker genes decreased but were still detectable after 409 days, whereas indigenous microbial diversity was not significantly different between sites, time points, or depths, indicating that bioaugmentation and the addition of activated carbon did not significantly alter total microbial diversity. In situ treatment of PCBs using an AC agglomerate as a delivery system for bioamendments is particularly well-suited for environmentally sensitive sites where there is a need to reduce exposure of the aquatic food web to sediment-bound PCBs with minimal disruption to the environment.

Carbon dioxide and organic waste valorization by microbial electrosynthesis and electro-fermentation.

Carbon-rich waste materials (solid, liquid, or gaseous) are largely considered to be a burden on society due to the large capital and energy costs for their treatment and disposal. However, solid and liquid organic wastes have inherent energy and value, and similar as waste CO2 gas they can be reused to produce value-added chemicals and materials. There has been a paradigm shift towards developing a closed loop, biorefinery approach for the valorization of these wastes into value-added products, and such an approach enables a more carbon-efficient and circular economy. This review quantitatively analyzes the state-of-the-art of the emerging microbial electrochemical technology (MET) platform and provides critical perspectives on research advancement and technology development. The review offers side-by-side comparison between microbial electrosynthesis (MES) and electro-fermentation (EF) processes in terms of principles, key performance metrics, data analysis, and microorganisms. The study also summarizes all the processes and products that have been developed using MES and EF to date for organic waste and CO2 valorization. It finally identifies the technological and economic potentials and challenges on future system development.

Metagenome-Assembled Genome Sequences of Acetobacterium sp. Strain MES1 and Desulfovibrio sp. Strain MES5 from a Cathode-Associated Acetogenic Microbial Community.

Draft genome sequences of Acetobacterium sp. strain MES1 and Desulfovibrio sp. strain MES5 were obtained from the metagenome of a cathode-associated community enriched within a microbial electrosynthesis system (MES). The draft genome sequences provide insight into the functional potential of these microorganisms within an MES and a foundation for future comparative analyses.

Metabolic Reconstruction and Modeling Microbial Electrosynthesis.

Microbial electrosynthesis is a renewable energy and chemical production platform that relies on microbial cells to capture electrons from a cathode and fix carbon. Yet despite the promise of this technology, the metabolic capacity of the microbes that inhabit the electrode surface and catalyze electron transfer in these systems remains largely unknown. We assembled thirteen draft genomes from a microbial electrosynthesis system producing primarily acetate from carbon dioxide, and their transcriptional activity was mapped to genomes from cells on the electrode surface and in the supernatant. This allowed us to create a metabolic model of the predominant community members belonging to Acetobacterium, Sulfurospirillum, and Desulfovibrio. According to the model, the Acetobacterium was the primary carbon fixer, and a keystone member of the community. Transcripts of soluble hydrogenases and ferredoxins from Acetobacterium and hydrogenases, formate dehydrogenase, and cytochromes of Desulfovibrio were found in high abundance near the electrode surface. Cytochrome c oxidases of facultative members of the community were highly expressed in the supernatant despite completely sealed reactors and constant flushing with anaerobic gases. These molecular discoveries and metabolic modeling now serve as a foundation for future examination and development of electrosynthetic microbial communities.

Mesocosm Studies on the Efficacy of Bioamended Activated Carbon for Treating PCB-Impacted Sediment.

This report describes results of a bench-scale treatability study to evaluate the efficacy of bioaugmentation with bioamended activated carbon (AC) for in situ treatment of polychlorinated biphenyl (PCB) impacted sediments. To this end, the ability of PCB transforming microorganisms to degrade and reduce the overall concentration of PCBs in sediment was determined in 2 L recirculating mesocosms designed to simulate conditions in Abraham's Creek in Quantico, Virginia. Ten sediment mesocosms were tested for the effects of AC alone, AC with slow release electron donor (cellulose) and different concentrations and combinations of PCB dehalogenating and degrading microorganisms added as bioamendments. A 78% reduction of total PCBs was observed using a cell titer of 5 × 105 Dehalobium chlorocoercia and Paraburkholderia xenovorans cells g-1 sediment with 1.5% AC as a delivery system. Levels of both higher and lower chlorinated congeners were reduced throughout the sediment column indicating that both anaerobic reductive dechlorination and aerobic degradation occurred concurrently. Porewater concentrations of all PCB homologues were reduced 94-97% for bioaugmented treatments. Toxicity associated with coplanar PCBs was reduced by 90% after treatment based on toxic equivalency of dioxin-like congeners. These results suggest that an in situ treatment employing the simultaneous application of anaerobic and aerobic microorganisms on AC could be an effective, environmentally sustainable strategy to reduce PCB levels in contaminated sediment.

Energy Efficiency and Productivity Enhancement of Microbial Electrosynthesis of Acetate.

It was hypothesized that a lack of acetogenic biomass (biocatalyst) at the cathode of a microbial electrosynthesis system, due to electron and nutrient limitations, has prevented further improvement in acetate productivity and efficiency. In order to increase the biomass at the cathode and thereby performance, a bioelectrochemical system with this acetogenic community was operated under galvanostatic control and continuous media flow through a reticulated vitreous carbon (RVC) foam cathode. The combination of galvanostatic control and the high surface area cathode reduced the electron limitation and the continuous flow overcame the nutrient limitation while avoiding the accumulation of products and potential inhibitors. These conditions were set with the intention of operating the biocathode through the production of H2. Biofilm growth occurred on and within the unmodified RVC foam regardless of vigorous H2 generation on the cathode surface. A maximum volumetric rate or space time yield for acetate production of 0.78 g/Lcatholyte/h was achieved with 8 A/Lcatholyte (83.3 A/m2projected surface area of cathode) supplied to the continuous flow/culture bioelectrochemical reactors. The total Coulombic efficiency in H2 and acetate ranged from approximately 80-100%, with a maximum of 35% in acetate. The overall energy efficiency ranged from approximately 35-42% with a maximum to acetate of 12%.

The bioelectrosynthesis of acetate.

Risks associated with climate change are driving the search for new technologies to produce fuels and chemicals. The microbial electrosynthesis of chemical compounds, using electricity and CO2 as feedstock and microbes to deliver the catalysts, has the potential to be one of those technologies. Central to the production of multicarbon compounds by this process is the bioelectrosynthesis of acetate (electroacetogenesis), and significant improvements in productivity and insightful discoveries concerning the extracellular transfer of electrons to the acetogenic microorganisms have been made recently. This review examines these advances and how they are influencing the development of microbial electrosynthesis into a new biotechnology for the sustainable production of fuels and chemicals.

Comparative Genomic Analysis of Sulfurospirillum cavolei MES Reconstructed from the Metagenome of an Electrosynthetic Microbiome.

Sulfurospirillum spp. play an important role in sulfur and nitrogen cycling, and contain metabolic versatility that enables reduction of a wide range of electron acceptors, including thiosulfate, tetrathionate, polysulfide, nitrate, and nitrite. Here we describe the assembly of a Sulfurospirillum genome obtained from the metagenome of an electrosynthetic microbiome. The ubiquity and persistence of this organism in microbial electrosynthesis systems suggest it plays an important role in reactor stability and performance. Understanding why this organism is present and elucidating its genetic repertoire provide a genomic and ecological foundation for future studies where Sulfurospirillum are found, especially in electrode-associated communities. Metabolic comparisons and in-depth analysis of unique genes revealed potential ecological niche-specific capabilities within the Sulfurospirillum genus. The functional similarities common to all genomes, i.e., core genome, and unique gene clusters found only in a single genome were identified. Based upon 16S rRNA gene phylogenetic analysis and average nucleotide identity, the Sulfurospirillum draft genome was found to be most closely related to Sulfurospirillum cavolei. Characterization of the draft genome described herein provides pathway-specific details of the metabolic significance of the newly described Sulfurospirillum cavolei MES and, importantly, yields insight to the ecology of the genus as a whole. Comparison of eleven sequenced Sulfurospirillum genomes revealed a total of 6246 gene clusters in the pan-genome. Of the total gene clusters, 18.5% were shared among all eleven genomes and 50% were unique to a single genome. While most Sulfurospirillum spp. reduce nitrate to ammonium, five of the eleven Sulfurospirillum strains encode for a nitrous oxide reductase (nos) cluster with an atypical nitrous-oxide reductase, suggesting a utility for this genus in reduction of the nitrous oxide, and as a potential sink for this potent greenhouse gas.

Draft Genome Sequence of Sulfurospirillum sp. Strain MES, Reconstructed from the Metagenome of a Microbial Electrosynthesis System.

A draft genome of Sulfurospirillum sp. strain MES was isolated through taxonomic binning of a metagenome sequenced from a microbial electrosynthesis system (MES) actively producing acetate and hydrogen. The genome contains the nosZDFLY genes, which are involved in nitrous oxide reduction, suggesting the potential role of this strain in denitrification.

Influence of acidic pH on hydrogen and acetate production by an electrosynthetic microbiome.

Production of hydrogen and organic compounds by an electrosynthetic microbiome using electrodes and carbon dioxide as sole electron donor and carbon source, respectively, was examined after exposure to acidic pH (∼ 5). Hydrogen production by biocathodes poised at -600 mV vs. SHE increased >100-fold and acetate production ceased at acidic pH, but ∼ 5-15 mM (catholyte volume)/day acetate and >1,000 mM/day hydrogen were attained at pH ∼ 6.5 following repeated exposure to acidic pH. Cyclic voltammetry revealed a 250 mV decrease in hydrogen overpotential and a maximum current density of 12.2 mA/cm2 at -765 mV (0.065 mA/cm2 sterile control at -800 mV) by the Acetobacterium-dominated community. Supplying -800 mV to the microbiome after repeated exposure to acidic pH resulted in up to 2.6 kg/m3/day hydrogen (≈ 2.6 gallons gasoline equivalent), 0.7 kg/m3/day formate, and 3.1 kg/m3/day acetate ( = 4.7 kg CO2 captured).

Long-term operation of microbial electrosynthesis systems improves acetate production by autotrophic microbiomes.

Microbial electrosynthesis is the biocathode-driven production of chemicals from CO2 and has the promise to be a sustainable, carbon-consuming technology. To date, microbial electrosynthesis of acetate, the first step in order to generate liquid fuels from CO2, has been characterized by low rates and yields. To improve performance, a previously established acetogenic biocathode was operated in semi-batch mode at a poised potential of -590 mV vs SHE for over 150 days beyond its initial development. Rates of acetate production reached a maximum of 17.25 mM day(-1) (1.04 g L(-1) d(-1)) with accumulation to 175 mM (10.5 g L(-1)) over 20 days. Hydrogen was also produced at high rates by the biocathode, reaching 100 mM d(-1) (0.2 g L(-1) d(-1)) and a total accumulation of 1164 mM (2.4 g L(-1)) over 20 days. Phylogenetic analysis of the active electrosynthetic microbiome revealed a similar community structure to what was observed during an earlier stage of development of the electroacetogenic microbiome. Acetobacterium spp. dominated the active microbial population on the cathodes. Also prevalent were Sulfurospirillum spp. and an unclassified Rhodobacteraceae. Taken together, these results demonstrate the stability, resilience, and improved performance of electrosynthetic biocathodes following long-term operation. Furthermore, sustained product formation at faster rates by a carbon-capturing microbiome is a key milestone addressed in this study that advances microbial electrosynthesis systems toward commercialization.

Production of fuels and chemicals from waste by microbiomes.

The demand for chemicals and fuels will continue to grow simultaneously with the costly requirement to treat solid waste, wastewater, and regarding climate change, carbon dioxide. A dual benefit is at hand if waste could be converted to valuable chemicals. The application of stable chemical producing microbiomes adapted to these waste streams may turn this challenge into an opportunity.

Remediation of polychlorinated biphenyl impacted sediment by concurrent bioaugmentation with anaerobic halorespiring and aerobic degrading bacteria.

Bioremediation of sediments contaminated with commercial polychlorinated biphenyls (PCBs) is potentially achievable by the sequential activity of anaerobic halorespiration to convert higher chlorinated congeners to less chlorinated congeners that are susceptible to aerobic respiratory degradation. The efficacy of bioaugmentation with anaerobic halorespiring Dehalobium chlorocoercia DF1 and aerobic Burkholderia xenovorans LB400 added concurrently with granulated activated carbon (GAC) as a delivery system was determined in 2 L laboratory mesocosms containing weathered Aroclor-contaminated sediment from Baltimore Harbor, MD, USA. The greatest effect was seen in the mesocosm bioaugmented with both DF1 and LB400 together, which resulted in an 80% decrease by mass of PCBs, from 8 to <2 mg/kg after 120 days. There was no significant increase in lesser-chlorinated congeners, indicating that both anaerobic dechlorination by DF1 and aerobic degradation by LB400 occurred. In contrast, nonbioaugmented controls containing filtered culture supernatant showed only a 25% decrease in total levels of PCBs after 365 days, which was likely due to biostimulation of the indigenous population by the medium. Direct colony counts and molecular analysis targeting a putative reductive dehalogenase gene of D. chlorocoercia or the bphA gene of LB400 showed the presence of viable DF1 and LB400 in bioaugmented mesocosms after 365 days, indicating that both nonindigenous strains were sustainable within the indigenous microbial community. These results suggest that an in situ treatment employing the simultaneous application of anaerobic and aerobic microorganisms could be an effective and environmentally sustainable strategy to reduce PCBs levels in contaminated sediment.

Electrical stimulation of microbial PCB degradation in sediment.

Bioremediation of polychlorinated biphenyls (PCBs) has been precluded in part by the lack of a cost-effective method to stimulate microbial degradation in situ. A common limitation is the lack of an effective method of providing electron donors and acceptors to promote in situ PCB biodegradation. Application of an electric potential to soil/sediment could be an effective means of providing electron-donors/-acceptors to PCB dechlorinating and degrading microorganisms. In this study, electrical stimulation of microbial PCB dechlorination/degradation was examined in sediment maintained under simulated in situ conditions. Voltage was applied to open microcosms filled with PCB-impacted (Aroclor 1242) freshwater sediment from a Superfund site (Fox River, WI). The effect of applied low voltages (1.5-3.0 V) on the microbial transformation of PCBs was determined with: 1) spiked PCBs, and 2) indigenous weathered PCBs. The results indicate that both oxidative and reductive microbial transformation of the spiked PCBs was stimulated but oxidation was dominant and most effective with higher voltage. Chlorobenzoates were produced as oxidation metabolites of the spiked PCBs, but increasing voltage enhanced chlorobenzoate consumption, indicating that overall degradation was enhanced. In the case of weathered PCBs, the total concentration decreased 40-60% in microcosms exposed to electric current while no significant decrease of PCB concentration was observed in control reactors (0 V or sterilized). Single congener analysis of the weathered PCBs showed significant loss of di- to penta-chlorinated congeners, indicating that microbial activity was not limited to anaerobic dechlorination of only higher chlorinated congeners. Degradation was most apparent with the application of only 1.5 V where anodic O(2) was not generated, indicating a mechanism of degradation independent of electrolytic O(2). Low voltage stimulation of the microbial degradation of weathered PCBs observed in this study suggests that this approach could be a cost-effective, environmentally sustainable strategy to remediate PCBs in situ.

In situ treatment of PCBs by anaerobic microbial dechlorination in aquatic sediment: are we there yet?

The remediation of polychlorinated biphenyls (PCBs) in soils and sediments remains a particularly difficult problem to solve. The possibility of in situ degradation by microorganisms has been pursued for many years since this approach has the potential to provide a cost-effective and environmentally sustainable alternative to dredging for treatment of PCB impacted sites. Being hydrophobic, PCBs partition into organic material and accumulate in anoxic environments well poised to support anaerobic dechlorination of highly chlorinated congeners; products of which are susceptible to complete aerobic degradation. Laboratory research over the past 25 years is now leading to new microbial technologies that could soon be tested for treatment of PCB impacted sediments in the field.

Electrosynthesis of commodity chemicals by an autotrophic microbial community.

A microbial community originating from brewery waste produced methane, acetate, and hydrogen when selected on a granular graphite cathode poised at -590 mV versus the standard hydrogen electrode (SHE) with CO(2) as the only carbon source. This is the first report on the simultaneous electrosynthesis of these commodity chemicals and the first description of electroacetogenesis by a microbial community. Deep sequencing of the active community 16S rRNA revealed a dynamic microbial community composed of an invariant Archaea population of Methanobacterium spp. and a shifting Bacteria population. Acetobacterium spp. were the most abundant Bacteria on the cathode when acetogenesis dominated. Methane was generally the dominant product with rates increasing from <1 to 7 mM day(-1) (per cathode liquid volume) and was concomitantly produced with acetate and hydrogen. Acetogenesis increased to >4 mM day(-1) (accumulated to 28.5 mM over 12 days), and methanogenesis ceased following the addition of 2-bromoethanesulfonic acid. Traces of hydrogen accumulated during initial selection and subsequently accelerated to >11 mM day(-1) (versus 0.045 mM day(-1) abiotic production). The hypothesis of electrosynthetic biocatalysis occurring at the microbe-electrode interface was supported by a catalytic wave (midpoint potential of -460 mV versus SHE) in cyclic voltammetry scans of the biocathode, the lack of redox active components in the medium, and the generation of comparatively high amounts of products (even after medium exchange). In addition, the volumetric production rates of these three commodity chemicals are marked improvements for electrosynthesis, advancing the process toward economic feasibility.