Microbial electromethanogenesis for energy storage: Influence of acidic pH on process performance

[EN] Microbial electromethanogenesis (EM) has positioned itself as a promising technology for electrical energy storage using CO2 as a feedstock. However, the selectivity of the final product remains a challenge, being highly dependent of the operating conditions (temperature, pH, conductivity, etc.). This study tries to understand the role that pH plays on the start-up, performance and the structure of microbial communities of an EM system. To that end, two EM reactors were started at pH 7.0 and 5.5 respectively and were subsequently subjected to pH variations between 7.5 and 3.5. The reactor inoculated at pH 5.5 started to produce CH4 earlier than that inoculated at pH 7.0, and the acetogenic activity was gradually displaced by methanogenesis during the start-up period, regardless of the pH. In addition, as the pH of the catholyte became more acidic, the performance improved in terms of methane production, current density and columbic efficiency. Acidic environments – pH around 4.5 – promoted higher methane production due to the selection of Methanobacterium, an acid-tolerant hydrogenotrophic archaea. When pH was set at 3.5, the overall performance declined sharply, probably because it induced unfavourable physiological conditions.SIMinisterio de Ciencia e Innovació

Elucidating the impact of power interruptions on microbial electromethanogenesis

Preprint. Submitted version[EN] The need to accommodate power fluctuations intrinsic to high-renewable systems will demand in the future the implementation of large quantities of energy storage capacity. Electromethanogenesis (EM) can potentially absorb the excess of renewable energy and store it as CH4. However, it is still unknown how power fluctuations impact on the performance of EM systems. In this study, power gaps from 24 to 96 h were applied to two 0.5 L EM reactors to assess the effect of power interruptions on current density, methane production and current conversion efficiency. In addition, the cathodes where operated with and without external H2 supplementation during the power-off periods to analyse how power outages affect the two main metabolic stages of the EM (i.e.: the hydrogenic and methanogenic steps). Methane production rates kept around 1000 mL per m2 of electrode and per day regardless of the duration of the power interruptions and of the supplementation of hydrogen. Interestingly, current density increased in the absence of hydrogen (averaged current density during hydrogen supplementation was 0.36 A·m-2 , increasing up to 0.58 A·m-2 without hydrogen). However current was less efficiently used in the production of methane with no hydrogen supplementation. Nevertheless, when the electrical power was restored after the power interruption experiments, performance parameters were similar to those observed before. These results indicate that EM is resilient to power fluctuations, which reinforces the opportunity of using EM as a technology for renewable energy storage.N

Integrating microbial electrochemical technologies with anaerobic digestion to accelerate propionate degradation

[EN] The aim of this study is to evaluate the integration of microbial electrochemical technologies (MET) with anaerobic digestion (AD) to overcome AD limitations caused by propionate accumulation. The study focuses on understanding to what extent the inoculum impacts on the behaviour of the integrated systems (AD-MET) from the perspective of propionate degradation, methane production and microbial population dynamics. Three different inocula were used: two from environmental sources (anaerobic sludge and river sediment) and another one from a pre-enriched electroactive consortium adapted to propionate degradation. Contrary to expectations, the reactor inoculated with the pre-enriched consortium was not able to maintain its initial good performance in the long run, and the bioelectrochemical activity collapsed after three months of operation. On the other hand, the reactor inoculated with anaerobic sludge, although it required a relatively longer time to produce any observable current, was able to maintain an electrogenic activity operation (0.8 A m−2), whilst showcasing the positive contribution of AD-MET integration into tackling propionate accumulation and enhancing methane yield (338 mL gCOD−1). However, it must also be highlighted that from a purely energetic point of view the AD-MET was not favourable.SIMinisterio de Economía y CompetitividadJunta de Castilla y LeónEnte Regional de la Energía de Castilla y Leo

Reduced graphene oxide improves the performance of a methanogenic biocathode

.Microbial electrosynthesis (MES), a sub-branch of bioelectrochemical processes, takes advantage of a certain type of electroactive microorganism to produce added value products (such as methane) from carbon dioxide (CO2). The aim of this study is to quantify the benefits of using a carbon felt electrode modified with reduced graphene-oxide (rgoCF) as a methanogenic biocathode. The current density generated by the rgoCF was almost 30% higher than in the control carbon felt electrode (CF). In addition, charge transfer and ohmic resistances were, on average, 50% lower in the rgoCF electrode. These improvements were accompanied by a larger presence of bacteria (31% larger) and archaea (18% larger) in the rgoCF electrode. The microbial communities were dominated by hydrogenotrophic methanogenic archaea (Methanobacterium) and, to a lesser extent, by a low-diversity group of bacteria in both biocathodes. Finally, it was estimated that for a CO2 feeding rate in the range 15–30 g CO2 per m2 of electrode per day, it is possible to produce a high-quality biogas (>95% methane concentrationS

Impact of the start-up process on the microbial communities in biocathodes for electrosynthesis

[EN]This study elucidates the impact of the start-up strategies on the microbial communities that evolve on the biofilm of a biocathode. Using reductive start-up potentials and a highly diverse inoculum, this start-up failed to produce any biofilm. When a less species richness inoculum from an anaerobic environment was used with the same reductive initial potential, a specialised biofilm was formed and a highly productive biocathode was developed in terms of acetic acid and also current production. However, using oxidative start-up potential led to rapid electroactive biofilm development, although the final composition of the biofilm was highly dependent on the inoculum used. So, using the diverse RM inoculum, a final specialised biofilm grew on the electrode, also giving high acetate and current generation. However, when using the less species richness AD inoculum, it was found that a nonspecialised biofilm was developed and lower acetic acid production was found. Importantly, a higher specialisation of the biofilm leads to an improvement in acetate generation, probably due to lowered influence of undesirable secondary methabolic pathways. Moreover, it has been shown that the coupling of H2 producing bacteria and acetic acid bacteria play an important role in acetate productionSIThis research was possible thanks to the financial support of the ‘Ministerio de Economía y Competitividad’ project ref: CTQ2015-68925-R, cofinanced by FEDER funds. Raúl Mateos thanks the Spanish ‘Ministerio de Educación, Cultura y Deporte’ for the FPU Grant (FPU14/01573). Ana Sotres thanks the regional ‘Junta de Castilla y León’ for the postdoctoral contract associated with project ref: LE060U16

Comparison of Activation Methods for 3D-Printed Electrodes for Microbial Electrochemical Technologies

Supplementary Materials: The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/app12010275/s1, Figure S1. Current density profiles of two successive cycles at the end of a 60 days period that allowed for the development of a stable biofilm; Table S1: Fitted parameters to EC 1 (DMF and acetone electrodes); Table S2: Fitted parameters to EC 2 (control and electrochemical-treated electrodes); Figure S2. Equivalent circuits for abiotic electrode essays’ modellization.1) DMF and acetone 2) Control and electrochemical treatment.[EN] Three-dimensional printing could provide flexibility in the design of a new generation of electrodes to be used in microbial electrochemical technologies (MET). In this work, we demonstrate the feasibility of using polylactic acid (PLA)/graphene—a common 3D-printing material—to build custom bioelectrodes. We also show that a suitable activation procedure is crucial to achieve an acceptable electrochemical performance (plain PLA/graphene bioanodes produce negligible amounts of current). Activation with acetone and dimethylformamide resulted in current densities similar to those typically observed in bioanodes built with more conventional materials (about 5 Am−2). In addition, the electrodes thus activated favored the proliferation of electroactive bacteria.SIMinisterio de Economía y CompetitividadEnte Regional de la Energía de Castilla y LeonThis research was funded by “Ministerio de Economía y Competitividad (Gobierno de España) grant number: PID2020-115948RB-I00 (MINECO/FEDER, EU) and “Ente Regional de la Energía de Castilla y Leon”, grant number: EREN_2019_L3_ULE

Microbial electrosynthesis for CO2 conversion and methane production: Influence of electrode geometry on biofilm development

[EN] Electromethanogenesis is a process of microbial electrosynthesis (MES) in whichelectroactive microorganisms reduce carbon dioxide (CO2) to produce methane (CH4), using a cathodeas an electron donor. The efficiency of this reaction is greatly determined by the establishment of arobust microbial community on the biocathodes, which eventually affects the global performance of thebioreactor. Moreover, the development of the biofilm depends on several characteristics of theelectrodes, more specifically their material and geometry. Since electrode geometry is a crucialparameter, this study aims at evaluating the sole influence of the electrode shape by installingcarbon-based electrodes with two different constructions (brush and carbon felt) of biocathodes in anelectromethanogenic reactor for CO2capture. The overall performance of the reactors showedcoulombic efficiencies around 100%, with high-quality biogas reaching methane concentrations above90%. The results reveal that the electrode geometry affects the individual biocathode performance, andthe carbon brush showed a bigger contribution to current generation and electrical capacitance,exhibiting higher peak hydrogen production compared to the carbon felt, which could be reflected inhigher CO2capture and methane generation. Both geometries showed a greater proliferation of archaeaover bacteria (between 53 and 85%), which was more significant on the brush than on the carbon felt.Archaea community was dominated byMethanobacteriumin carbon felt electrodes and codominatedwithMethanobrevibacterin brush electrodes, while bacteria analyses showed a very similar communityfor both geometriesS

Degradation of 2-mercaptobenzothizaole in microbial electrolysis cells: Intermediates, toxicity, and microbial communities

[EN] The compound 2-mercaptobenzothizaole (MBT) has been frequently detected in wastewater and surface water and is a potential threat to both aquatic organisms and human health (its mutagenic potential has been demonstrated). This study investigated the degradation routes of MBT in the anode of a microbial electrolysis cell (MEC) and the involved microbial communities. The results indicated that graphene-modified anodes promoted the presence of more enriched, developed, and specific communities compared to bare anodes. Moreover, consecutive additions of the OH substituent to the benzene ring of MBT were only detected in the reactor equipped with the graphene-treated electrode. Both phenomena, together with the application of an external voltage, may be related to the larger reduction of biotoxicity observed in the MEC equipped with graphene-modified anodes (46.2 eqtox∙m−3 to 27.9 eqtox∙m−3).SIThis research was possible thanks to the financial support by ‘Consejería de Educación de la Junta de Castilla y León’ (ref: LE320P18), a project co-financed by FEDER funds. R. M. Alonso thanks the University of León for the predoctoral contract. M. Canle acknowledges financial support from the Ministerio de Economía y Competitividad (Spain) through project CTQ2015-71238-R (MINECO/FEDER), and regional government Xunta de Galicia (project GPC ED431B 2017/59), respectively

Pilot-scale bioelectrochemical system for simultaneous nitrogen and carbon removal in urban wastewater treatment plants

[EN] This study aims to characterize the performance of a 150 L bioelectrochemical system-based plant, during the simultaneous carbon and nitrogen removal from several waste streams of wastewater treatment plants. The bioelectrochemical system (BES) contained five electrode pairs (operated hydraulically and electrically in parallel) and was fed with either wastewater, centrate (nutrient-rich liquid stream produced during the dewatering of digested biomass), or a mixture of both over 63 days, with a hydraulic retention time of one day. Total organic carbon and total nitrogen removal rates averaged 80% and 70%, respectively, with a specific energy consumption of 0.18 kWh·m−3 (BES + ancillary equipment). This work also underlines the challenges of using BES for nitrogen removal, highlighting the limitations of the current design, and suggesting some strategies for improvement.SIMinisterio de Educación, Cultura y Deport

Characterization of Anaerobic Biofilms Growing on Carbon Felt Bioanodes Exposed to Air

Supplementary Materials: The following are available online at https://www.mdpi.com/2073-4344/10/11/1341/s1, Figure S1: Dissolved oxygen profile. Rejected because the data acquisition system was accidentally interrupted at the end of the experiment; Figure S2: Another dissolved oxygen profile. Rejected because the data acquisition system was accidentally interrupted at the end of the experiment.[EN] The role of oxygen in anodic biofilms is still a matter of debate. In this study, we tried to elucidate the structure and performance of an electrogenic biofilm that develops on air-exposed, carbon felt electrodes, commonly used in bioelectrochemical systems. By simultaneously recording the current density produced by the bioanode and dissolved oxygen concentration, both inside and in the vicinity of the biofilm, it was possible to demonstrate the influence of a protective aerobic layer present in the biofilm (mainly formed by Pseudomonas genus bacteria) that prevents electrogenic bacteria (such as Geobacter sp.) from hazardous exposure to oxygen during its normal operation. Once this protective barrier was deactivated for a long period of time, the catalytic capacity of the biofilm was severely affected. In addition, our results highlighted the importance of the material’s porous structure for oxygen penetration in the electrode.SIJunta de Castilla y LeonThis research was possible thanks to the financial support by ‘Consejería de Educación de la Junta de Castilla y León’ (ref: LE320P18), a project co-financed by FEDER funds. R. M. Alonso thanks the University of León for his predoctoral contract