Pau
Batlle-Vilanova
a,
Sebastià
Puig
*a,
Rafael
Gonzalez-Olmos
ab,
Anna
Vilajeliu-Pons
a,
M. Dolors
Balaguer
a and
Jesús
Colprim
a
aLEQUiA, Institute of the Environment, University of Girona, Campus Montilivi, E-17071 Girona, Catalonia, Spain. E-mail: sebastia@lequia.udg.cat; Fax: +34 972418150; Tel: +34 972418281
bChemical Engineering Department, IQS-School of Engineering, Ramon Llull University, Via Augusta 390, Barcelona 08017, Spain
First published on 5th June 2015
Biogas upgrading is an expanding field dealing with the increase in methane content of the biogas to produce biomethane. Biomethane has a high calorific content and can be used as a vehicle fuel or directly injected into the gas grid. Bioelectrochemical systems (BES) could become an alternative for biogas upgrading, by which the yield of the process in terms of carbon utilisation could be increased. The simulated effluent from a water scrubbing-like unit was used to feed a BES. The BES was operated with the biocathode poised at −800 mV vs. SHE to drive the reduction of the CO2 fraction of the biogas into methane. The BES was operated in batch mode to characterise methane production and under continuous flow to demonstrate its long-term viability. The maximum methane production rate obtained during batch tests was 5.12 ± 0.16 mmol m−2 per day with a coulombic efficiency (CE) of 75.3 ± 5.2%. The production rate increased to 15.35 mmol m−2 per day (CE of 68.9 ± 0.8%) during the continuous operation. Microbial community analyses and cyclic voltammograms showed that the main mechanism for methane production in the biocathode was hydrogenotrophic methanogenesis by Methanobacterium sp., and that electromethanogenesis occurred to a minor extent. The presence of other microorganisms in the biocathode, such as Methylocystis sp. revealed the presence of side reactions, such as oxygen diffusion from the anode compartment, which decreased the efficiency of the BES. The results of the present work offer the first experimental report on the application of BES in the field of biogas upgrading processes.
The term biomethane has been used to describe methane-rich biogas (95% v/v) from AD of organic wastes, which could be directly used as a vehicle fuel or injected into the gas grid.2 Biogas upgrading technologies imply (i) the removal of harmful trace components, and (ii) the upturn of the methane content to increase the calorific capacity of the biogas.3 According to the International Energy Agency (IEA), water scrubbing is the most used technique for biogas upgrading, accounting for about 40% of the total upgrading plants.4 This technology consists of an absorption tower where CO2 is absorbed in water and biomethane is obtained. The absorbed CO2 is released to the atmosphere in a subsequent stripping tower,5 what often implies the consumption of energy. The transformation of the CO2 contained in the effluent of the absorption tower into methane could be a useful strategy to increase the biomethane production and could contribute to reduce the CO2 emissions. In this light, bioelectrochemical systems (BES) can be a promising technology to drive the reduction of dissolved CO2 into methane. Some authors already proposed the coupling of BES with AD in single reactors to increase the yield of the whole process.6–8 This work proposes a new promising application for BES, which is the replacement of the stripping tower of a water scrubbing unit for biogas upgrading. The possibility of coupling water scrubbing and BES opens the door to an innovative hybrid process. The bioelectrochemical conversion of CO2 to methane is known as electromethanogenesis, and was first presented by Cheng and coworkers.9 Other researchers studied the methane production mechanism at different cathode potentials, and the performance at the long term operation.10,11 Accordingly, methane production can take place in a biocathode mainly by two mechanisms: (i) hydrogenotrophic methanogenesis (eqn (1)), using hydrogen as electrochemical mediator, which can be produced in situ either bioelectrochemically or electrochemically,12 and (ii) using directly electrons as reducing power source (eqn (2)).
CO2 + 4H2 → CH4 + 2H2O | (1) |
CO2 + 8H+ + 8e− → CH4 + 2H2O; E′0 = −240 mV vs. SHE13 | (2) |
Methane production using hydrogen as intermediate was shown to be the predominant mechanism in methane producing biocathodes poised at low cathode potentials (i.e. <−750 mV vs. SHE).11 Nevertheless, Eerten-Jansen et al., also suggested the production of methane through other intermediates, such as acetate or formate.14 The conversion of CO2 to methane through direct electron transfer was suggested by Cheng et al., 2009 and Fu et al., 2015, who used a pure culture of Methanobacterium palustre and thermophilic methanogens, respectively.9,15 Finally, both direct and hydrogen mediated electron transfer took place in the biocathodes studied by Siegert and co-workers.16 Syntrophic relationships, such as interspecies hydrogen transfer and direct interspecies electron transfer (DIET) have been also proposed for the production of methane with mixed cultures, being the last one more energetically conservative due to the fact that electrons are directly transferred between microorganisms, avoiding the production of intermediates.17,18 So far, there is only one study that demonstrated the DIET between two pure cultures.19 Since mixed cultures have been usually utilised for the production of methane, the mechanism by which it is produced in a biocathode may differ among different studies. Thus, methane production mechanism in biocathodes is still unclear, so that more studies regarding the microorganisms involved and its electrochemical interaction with the electrode surface are needed to understand and optimise the whole process.
The present work proposes the coupling of water scrubbing with BES for biogas upgrading. The methane production and the electrochemical performance of the BES were studied. Electrochemical and microbiological analyses were carried out to understand how the microbial community dealt with the reducing power,13 which microorganisms were involved in the electron transfer, and which intermediates and final products affected the bioelectrochemical performance of the BES.
The BES was operated in a three-electrode configuration with a potentiostat (BioLogic, model VSP, France), which controlled the cathode potential and monitored the current demand. The biocathode was used as working electrode (WE) and the anode as counter electrode, the reference electrode (RE) was an Ag/AgCl (+197 mV vs. Standard Hydrogen Electrode [SHE], model RE-5B, BASI, United Kingdom) placed in the cathode chamber. All voltages within this study are reported with respect to SHE.
Gas samples were taken regularly to quantify the methane production, and liquid samples to check the concomitant production of other valuable compounds in the liquid phase (i.e. volatile fatty acids [VFA] and alcohols). A pH sensor (model 5303, Crison, Spain) was placed in the cathode recirculation loop to measure the pH with a transmitter (MultiMeter MM44, Crison, Spain) connected to a memograph (Graphic data manager RSG40, Memograph M, Endress+Hauser, Switzerland).
After the batch tests the BES was operated in continuous mode, under the same conditions than during the start-up period (Fig. S2†), to validate the operational stability of the BES in the long-term.
All of the production rates are given in mmol of carbon per m2 of electrode surface per day (mmol m−2 per day).
![]() | (3) |
The energetic efficiency of the process (ηE) was calculated according to eqn (4):10,24
![]() | (4) |
DNA was extracted using the Fast DNA® SPIN Kit for soil (MP Biomedicals, US) according to the manufacturer's instructions. DNA quantification was assessed with a nanodrop spectrophotometer (Thermo Scientific, US) characterized by 260/280 and 260/230 nm absorbance ratio in order to be comparable with quantitative real-time PCR (qPCR).
Bacterial diversity and microbial community structure of the cathode biofilm was analysed for 454 pyrosequencing (Research and Testing Laboratory, Lubbock, US). The 16S rRNA gene was analysed by primers 341F-907R for bacteria,27 and 341F-958R for archaea.28 Sequences obtained were analysed using MOTHUR software (v. 1.22.1) at 97% similarity.29 Sequences shorter than 250 bp and longer than 600 bp were excluded. Chimeric sequences were discarded by executing UCHIME.30 These sequences were then clustered into OTUs using the UPARSE algorithm.31 Taxonomic classification was done by RDP classifier.32 The most abundant sequences of bacteria and archaea were compared to BLAST supported by NCBI in order to obtain the closest similarity related to genbank database sequence. Alignments and identity matrix comparisons of deposited sequences were performed in BioEdit (v. 7.0).33 Post-data analysis was done by KRONA.34
Additionally, 16S rRNA gene was also quantified by qPCR to estimate the total amount of bacteria and archaea. All reactions were performed in a 7500 Real Time PCR system (Applied Biosystems, US) using the SYBRH Green PCR Mastermix. Primers and thermal cycling conditions for bacteria (341F-534R) and archaea (364af-A934b) 16S rRNA were used.35,36 Results were analysed using SDS software (Applied Biosystems, US). Standard curves were obtained using serial dilutions from 102 to 107 copies of linearised plasmids containing the respective functional genes. Controls without templates gave null or negligible values.
Test | Operation mode | Current demand (mA m−2) | pH | r 2 | Production rate (mmol CH4 m−2 per day) | CE (%) | η E (%) |
---|---|---|---|---|---|---|---|
1 | Batch | 32.0 ± 2.4 | 6.1 ± 0.1 | 0.998 | 1.63 ± 0.01 | 45.3 ± 1.9 | 26.4 ± 2.0 |
2 | Batch | 40.8 ± 3.5 | 5.9 ± 0.3 | 0.979 | 1.66 ± 0.05 | 39.0 ± 1.6 | 21.2 ± 1.8 |
3 | Batch | 31.1 ± 6.8 | 6.3 ± 0.1 | 0.996 | 1.36 ± 0.02 | 47.9 ± 5.8 | 23.7 ± 5.8 |
4 | Batch | 32.8 ± 5.2 | 6.1 ± 0.2 | 0.981 | 1.89 ± 0.05 | 45.9 ± 3.6 | 30.3 ± 4.5 |
5 | Batch | 27.3 ± 2.4 | 6.0 ± 0.1 | 0.999 | 1.67 ± 0.02 | 51.7 ± 4.5 | 31.7 ± 2.1 |
6 | Batch-OCV | n/a | 4.6 ± 0.2 | 0.891 | 0.09 ± 0.02 | n/a | n/a |
7 | Batch | 11.6 ± 2.4 | 5.6 ± 0.1 | 0.976 | 0.14 ± 0.01 | 11.6 ± 0.9 | 6.4 ± 1.0 |
8 | Batch | 11.0 ± 1.9 | 5.5 ± 0.3 | 0.992 | 0.30 ± 0.01 | 25.1 ± 4.7 | 14.5 ± 2.7 |
9 | Batch | 6.9 ± 0.7 | 6.7 ± 0.1 | 0.952 | 0.19 ± 0.01 | 25.1 ± 2.3 | 14.5 ± 1.3 |
10 | Batch | 7.2 ± 0.4 | 6.7 ± 0.1 | n/a | n/a | n/a | n/a |
11 | Batch | 11.9 ± 3.3 | 6.6 ± 0.1 | n/a | n/a | n/a | n/a |
12 | Batch | 31.2 ± 10.2 | 6.8 ± 0.1 | 0.975 | 1.38 ± 0.05 | 54.2 ± 8.3 | 25.3 ± 7.8 |
13 | Batch | 70.7 ± 6.6 | 7.1 ± 0.1 | 0.989 | 5.12 ± 0.16 | 75.3 ± 5.2 | 39.7 ± 3.6 |
14 | Continuous | 201.7 ± 18.1 | 7.1 ± 0.2 | 0.999 | 15.35 ± 0.00 | 68.9 ± 0.8 | 39.7 ± 1.3 |
Analyses of the liquid phase revealed that compounds such as VFA and alcohols were not produced in the biocathode. Nevertheless, sulphate concentration in the influent was 3.6 mg L−1, and it was not detected in the effluent, so sulphates were completely consumed in the biocathode. Qualitative analyses demonstrated the presence of H2S in the off gas. Methane was detected in the gas phase with a volumetric concentration of 65–85%, being the rest CO2 (15–35%) and trace amount of oxygen (1–8%) and H2S.
The overall performance of the biocathode from tests 1 to 5 was very similar. The pH in those tests was around 6. The current demand ranged from 27.3 ± 2.4 to 40.8 ± 3.5 mA m−2, and the production rate from 1.36 to 1.89 mmol m−2 per day was obtained with associated CEs from 39.0 ± 1.6 to 51.7 ± 4.5%. Test 6 was performed in OCV mode. A 20-fold lower amount of methane was produced and the pH decreased 1.4 units compared to the previous test, which was caused by the lack of electrochemical activity in the biocathode. The BES was negatively affected after the OCV test.
Subsequent tests (i.e. 7 to 9) showed a lower performance in terms of current demand and methane production rate. Biogas was directly flushed to the biocathode once per day during test 10 and 11 to favour substrate availability, displace oxygen diffused to the cathode, and promote the growth of methanogenic microorganisms. The current demand was monitored as a parameter indicating the activity of the biocathode. By applying this strategy the system recovered, and after 2 batch tests (i.e. test 12) the performance was similar to the previous tests. Test 12 showed similar performance to tests 1 to 5 (Table 1). In the subsequent and last test, the activity increased, obtaining a higher current demand of 70.7 ± 6.6 mA m−2 and a production rate of 5.12 ± 0.16 mmol m−2 per day, with a CE of 75.3 ± 5.2%, which coincided with the higher pH (i.e. 7.1) of the biocathode. Comparative analysis (Fig. S5†) showed that methane production rate was linearly related (r2 = 0.99) to the current demand of the biocathode. The higher the current demand was, the higher the methane production rate. No evidence of dependence between the production rate and other parameters (i.e. pH and CE) was observed in this study (Fig. S5†). Although it was not investigated in the present study, previous studies suggested that other parameters, such as temperature, inoculum source or electrode materials could affect methane production.16,38,39 The ηE during the firsts tests (i.e. test 1 to test 5) were similar, ranging from 21.2 ± 1.8 to 31.7 ± 2.1%, and increased to 39.7 ± 3.6% in the last batch test (i.e. test 13).
After batch tests, the BES was operated in continuous flow under the same conditions than the start-up period (Fig. S2†). Fig. 1 shows the evolution of the current demand and the production rate during this period. The average methane production rate during the first days was 3.99 ± 0.06 mmol m−2 per day, which increased and stabilised at 15.35 mmol m−2 per day after 43 days of continuous operation. The results regarding the continuous operation are included in Table 1 (test 14) and correspond to the results obtained at the end of the period, when current demand and production rate were stable. The CE and the ηE obtained during the continuous operation were 68.9 ± 0.8% and 39.7 ± 1.3%, respectively, which were close to the last batch test, whilst the current demand and the methane production rate were triplicated.
![]() | ||
Fig. 1 Current demand and methane production rate of the biocathode during the continuous operation after the batch tests. |
![]() | ||
Fig. 2 Results of the microbial community analysis of archaea (A) and bacteria (B) present the biocathode. |
Bacteria community presented higher diversity of phylums than archaea (Fig. 2B). Proteobacteria composed 51% of the sample, more concretely Alphaproteobacteria (32%). The dominant genus of the Alphaproteobacteria was identified as Methylocystis sp. (20%). The rest of the community was divided in several phylums, highlighting the presence of Firmicutes (Clostridium sp. 12.0%) and Bacteroidetes (Anaerophaga sp. 5.6%).
The lower current demand and methane production after the OCV test were likely caused by the lack of available reducing power for a period of about 70 hours. Thus, non-electrochemical microorganisms present in the biocathode, such as Methylocystis sp. were favoured over methanogens during the OCV test. Another hypothesis was that the lack of activity caused the pH inside the biocathode being similar to the influent pH, which was close to 5. At such low pH values, the methanogenic metabolism could be inhibited,40 so that, after the OCV test it took time to recover the previous performance. Otherwise, the increased activity of the last batch test could have been caused by a higher pH of the biocathode. While previous tests had not significant differences of pH, being its value around 6, in test 13 it increased to 7.1, likely due to the higher current demand and the increase of the bioelectrochemical activity. Although substrate availability is lower at high pH values,20 some microorganisms from Methanobacterium sp. have been reported to grow better at pH higher than 6.5, with optimum growth values even higher than 7.41 Thus, the pH conditions were more favourable for the methanogens. The effect of the pH in the electromethanogenic biocathode cannot be contrasted to other studies because it has not been reported yet.
The results regarding the continuous operation (i.e. test 14) were similar to the last batch tests in terms of pH, CE and ηE. However, the current demand and the methane production rate, were triplicated, likely because the higher substrate availability caused by the continuous CO2 supply in the form of a saturated solution.
In contrast, the bacterial community was highly-diversified. The most abundant species which could play a role in the biocathode were examined. Methylocystis sp. is especially common near environments where methane is produced. The presence of methane in the biocathode favoured this microorganism. It has been described in previous articles that Methylocystis sp. is able to transform methane to CO2 under aerobic conditions.43 Its activity could advantage the archaea activity, removing the oxygen diffused from the anode to the biocathode, and creating an anaerobic environment.20 However, this reaction decreased the efficiency of the process due to the methane consumption, as suggested by other authors.42
Firmicutes members, such as Clostridium sp. have several environmental roles. Hydrogen production from organic compounds was demonstrated by six mesophilic Clostridia.44 Recently, Firmicutes were identified in an autotrophic hydrogen producing biocathode.45
Anaerophaga sp. was identified in consortium of sulphate-reducing bacteria in biofilms used to analyse the corrosion behavior.46 It was detected also in electricity producing bioanodes and its activity decreased in the long term because of the lack of dissolved electron acceptors,47 such as sulphate.
![]() | ||
Fig. 4 Schematic representation of the methane producing (solid lines), and cross-over (dashed lines) reactions that took place in the biocathode. |
The relatively low CE observed during the operation of the BES was caused by the presence of cross-over reactions (Fig. 4). Some of these cross-over reactions were suggested and supported by the microbial community identification. The relatively high abundance of Methylocystis sp. suggested the presence of oxygen in the cathode. Since water oxidation occurred in the anode chamber, the oxygen generated in this reaction can diffuse to the cathode through the membrane.10 In the cathode it was partly electrochemically reduced to water, due to the reducing conditions, and partly used by Methylocystis sp. to consume methane, which decreased the CE (see ESI, Table S1†). The presence of sulphate in the influent promoted the growth of sulphate reducing bacteria. This was suggested by the presence of Anaerophaga sp. which reduced SO42− in the liquid phase to H2S. Since sulphate reducers and methanogens are direct competitors for hydrogen, the activity of the methanogens, and therefore the CE, decreased due to the presence of this microorganisms in the biocathode (Table S1†).51 Although oxygen was the main electron sink, sulphate reduction was another cross-over reaction that decreased the CE of the process. However, the effect of sulphate reduction on the CE compared to oxygen was 100 and 10 times lower during batch and continuous operation, respectively (Table S1†). Another potential electron sink is the production of hydrogen as intermediate. At such low cathode potential, hydrogen was used as mediator to produce methane through hydrogenotrophic methanogenesis.10,11 Gas quantification, especially when hydrogen is present, is not accurate as it may diffuse out of the reactor through the connectors, the tubes, or the membrane, therefore part of the CE losses could have been caused by these reasons.52–54
Nowadays, the methane content of the biogas can be increased through water scrubbing, releasing CO2 to the atmosphere in a stripping unit according to Fig. 5A. However, this study has shown the potential of BES in the field of biogas upgrading. The replacement of the stripping unit by a BES according to Fig. 5B could increase the yield of the process in terms of carbon utilisation, and therefore reduce the CO2 emissions, especially when renewable energy is used to drive the process. Typically, the effluent from an absorption tower for biogas upgrading has a CO2 and methane content in the solution of around 96% and 4%, respectively, when this effluent was treated with the BES, an additional amount of biogas was produced, which was composed by 65–85% CH4, 15–35% CO2 and 1–8% O2. The present study demonstrated the robustness of the operation at long term (i.e. more than 420 days) of a BES using CO2 absorbed from biogas and producing methane. The methane production rate obtained during continuous operation was 15.35 mmol m−2 per day with a CE of 68.9 ± 0.8%, which was three times higher compared to the best results obtained during batch tests. A recent study by Siegert and co-workers suggested that the production rate could be even more increased by choosing a good combination of anode and cathode materials.16
![]() | ||
Fig. 5 Schematic representation of a typical water scrubbing-like biogas upgrading process (A), and the proposed modification of the process by the use of a BES instead of a stripping tower (B). |
The energy efficiency of the process was found in this study as the key bottleneck for scalability and applicability of BES for biogas upgrading. The kWh recovered in the form of methane represented 39.7 ± 1.3% of the kW h supplied to the BES. Accordingly, the actual energy efficiency could restrict the potential application of this technology to the energy storage during off-peak energy production periods,55 or renewable energy harvesting and storage in the form of methane, which is easier handled than electricity and can be distributed through the existing gas grid.2 Regarding the energy efficiency, the oxidative conditions of the anode could be used to carry out profitable reactions to increasing the overall energy efficiency and the value of the process. In a study carried out by Luo and colleagues, the authors demonstrated the production of methane obtaining the electrons from a salinity gradient in the anode by reverse electrodialysis.56 This process avoided part of the energy consumption and the presence of oxygen, which allowed for the production of nearly pure methane in the biocathode, with relatively high CE. Thus, avoiding oxygen in the anode could be a useful strategy to increase the efficiency in future applications.
Finally, an exhaustive cost-benefit analysis needs to be performed to discern whether the application of the technology proposed in the present study would be economically feasible to replace a stripping tower.
The results presented in this study highlight the application of BES in the field of biogas upgrading technologies, to increase the overall yield of the process and to reduce CO2 emissions. These results open the door to further applications such as energy storage or production of biomethane to inject into the gas grid or use as a vehicle fuel.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra09039c |
This journal is © The Royal Society of Chemistry 2015 |