Addition of a carbon fiber brush improves anaerobic digestion compared to external voltage application
Graphical abstract
Introduction
Anaerobic digestion (AD) is an efficient method to degrade organic matter in wastewaters and produce methane but the process can require long hydraulic retention times for stable performance. The syntrophic relationship between fatty acid-degrading bacteria and methanogens is often considered as the rate-limiting step for soluble organic acid conversion to methane (Baek et al., 2018). Therefore, methane generation rates can be improved by reducing the background concentrations of volatile fatty acids (VFAs) produced from fermentation. Inserting electrodes like those used in a microbial electrolysis cell (MEC) into AD (i.e. AD-MEC), and applying a voltage to generate an electrical current and hydrogen gas, has been proposed to improve overall AD performance (Vu et al., 2020; Zhao et al., 2016). AD-MEC systems use exoelectrogenic bacteria on the anode to oxidize organic matter and reduce VFA concentrations, and the cathode can improve methane production either through electromethanogenesis (8H+ + 8e− + CO2 → CH4 + 2H2O) or by providing H2 for hydrogenotrophic methanogenesis (4H2 + CO2 → CH4 + 2H2O) (Cheng et al., 2009; Yu et al., 2018). AD-MEC systems have therefore been shown to increase methane production and organic removal efficiencies (Feng et al., 2015; Zhang et al., 2013; Zhao et al., 2016).
The main function of the electrodes in an AD-MEC has not been clearly distinguished between improvements due to additional surface area for biomass retention, and benefits of electrochemical processes that can remove additional organics and produce hydrogen gas. The operation of homogenized and continuously-fed AD reactors is dependent on the retention of biomass, especially methanogens (Choong et al., 2018). Adding supporting media with a high surface area into reactors has therefore been used to ensure a high biomass concentration (Show and Tay, 1999). Most of the materials used for bioelectrodes in AD-MECs provide a relatively high surface area compared to controls without electrodes. Carbon felt or carbon cloth materials have porous structures which could benefit AD performance by retaining biomass and preventing washout, in addition to stimulating electrochemical reactions. Despite of the importance of this surface area effect, the electrode packing density (i.e. electrode surface area to reactor volume ratio) has not been well documented or specifically examined in AD-MEC studies. Electrode packing densities have varied over a wide range in AD-MEC studies, from 0.7 to 15.1 m2/m3 of reactor volume, which could have impacted AD performance even without current production (Baek et al., 2020; Cai et al., 2016; De Vrieze et al., 2014; Feng et al., 2015; Liu et al., 2016; Vu et al., 2020). Low electrode packing densities (<1 m2/m3) would not be expected to impact methane production rates by more than a small amount (0.6–1.2%) due to a small percentage of the substrate being used for current generation relative to that for overall methane production (Baek et al., 2020; Feng et al., 2015). However, AD-MEC systems with a relatively small amount of current through the circuit have shown to achieve higher or more stable AD performance than conventional AD systems (Baek et al., 2020). In addition to increasing surface area available for microorganisms, the use of electrically conductive materials can also provide a beneficial platform for direct interspecies electron transfer (DIET) between electroactive bacteria and methanogens even in the absence of current generation (Baek et al., 2015; Cruz Viggi et al., 2014; Li et al., 2015). Through DIET, the electrons from organic oxidation can be transferred directly toward methanogens without redox intermediate (i.e. hydrogen), which could make the AD process more efficient and contribute to enhanced methane generation rates (Baek et al., 2018).
In this study, we investigated the impact of surface area relative to current generation using electrodes by adding carbon fiber brushes of different sizes into AD reactors. We hypothesized that the large surface area provided by the brushes was more critical to AD performance than current generation using smaller-size MEC electrodes. Carbon brushes were chosen for these tests because they provide a very high surface area-to-volume ratio, and the open brush structure has advantages of less potential for clogging compared to other carbon-based woven materials (e.g., pieces of carbon felt or carbon cloth) (Logan et al., 2007). In addition, the potential for DIET is enhanced by using carbon brushes due to their high electrical conductivity (650 S/cm for brushes used here). This strategy to add brushes into AD provide additional advantages compared to adding particles to stimulate DIET (e.g., magnetite, carbon nanotube, activated carbon) (Baek et al., 2015; Li et al., 2015; Liu et al., 2012) because there is no possibility of material washout using the brushes compared to particles. The impact of large carbon brushes on methane generation rates and COD removal rates was examined by adding brushes that filled, or half-filled the reactor volume compared to reactors with two smaller-size brush electrodes in the presence and absence of current production. The amount of attached biofilm to these different brushes was quantified in terms of protein, with the contribution of current generation evaluated in terms of coulombic efficiency and total internal resistances based on the electrode potential slope (EPS) method (Cario et al., 2019). To obtain a more comprehensive insight into process performance, the microbial communities on the brushes and in suspension were characterized using Illumina sequencing of both active (16S rRNA) and total (16S rRNA gene) microbial populations.
Section snippets
Inoculum and substrate
Sludge was collected from the anaerobic digester at the Pennsylvania State University wastewater treatment plant and used as the inoculum. The sludge was sieved using a screen (mesh size of 850 μm) to remove large particles that lead to clogging of the sampling port and obtain a homogenous inoculum, and then stored at 4°C. Before utilization as an inoculum, the sludge was held for 24 h at 35°C to activate microorganisms and decrease concentrations of easily degraded biodegradable organic
Methane production and COD removal
After successful biofilm acclimation cycles (Fig. S1) the reactors with the high-surface area brushes produced methane faster than the reactor containing only the MEC electrodes (NB-CC) (Fig. 2A). Methane production during the third batch cycle showed the lowest accumulated volume for the NB-CC reactor (208 ± 24 mL) and the highest gas volume for the FB reactor (249 ± 2 mL). The three configurations with the full or half brushes all showed similar overall trends in gas production, with similar
Conclusions
The addition of a large amount of conductive surface area using carbon brush was proved to be more efficient for AD performance than current generation through electrodes. FB, HB-CC, and HB-OC showed 57-82% higher methane generation rate parameters in the Gompertz model compared to the NB-CC, as well as the enhancement in VFA removals. Much higher amounts of protein in the large-size brushes and negligible impact of current production on the CH4 production were observed, suggesting a more
Conflict of interest statement
The authors certify that they have NO affiliations with or involvement in any organization or entity with any financial interest (such as honoraria; educational grants; participation in speakers’ bureaus; membership, employment, consultancies, stock ownership, or other equity interest; and expert testimony or patent-licensing arrangements), or non-financial interest (such as personal or professional relationships, affiliations, knowledge or beliefs) in the subject matter or materials discussed
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This research was funded by the Stan and Flora Kappe endowment and other funds through The Pennsylvania State University.
References (57)
- et al.
Minor revision to V4 region SSU rRNA 806R gene primer greatly increases detection of SAR11 bacterioplankton
Aquat. Microb. Ecol.
(2015) - et al.
Standard methods for the examination of water and wastewater
(1920) - et al.
The biostimulation of anaerobic digestion with (semi) conductive ferric oxides: their potential for enhanced biomethanation
Appl. Microbiol. Biotechnol.
(2015) - et al.
Role and potential of direct interspecies electron transfer in anaerobic digestion
Energies
(2018) - et al.
Individual and combined effects of magnetite addition and external voltage application on anaerobic digestion of dairy wastewater
Bioresour. Technol.
(2020) - et al.
Development of biocathode during repeated cycles of bioelectrochemical conversion of carbon dioxide to methane
Bioresour. Technol.
(2017) - et al.
Electricity production by Geobacter sulfurreducens attached to electrodes
Appl. Environ. Microbiol.
(2003) - et al.
Biocathodic methanogenic community in an integrated anaerobic digestion and microbial electrolysis system for enhancement of methane production from waste sludge
ACS Sustainable Chem. Eng.
(2016) - et al.
Applying the electrode potential slope method as a tool to quantitatively evaluate the performance of individual microbial electrolysis cell components
Bioresour. Technol.
(2019) - et al.
Turf soil enhances treatment efficiency and performance of phenolic wastewater in an up-flow anaerobic sludge blanket reactor
Chemosphere
(2018)
Carbon cloth stimulates direct interspecies electron transfer in syntrophic co-cultures
Bioresour. Technol.
Direct biological conversion of electrical current into methane by electromethanogenesis
Environ. Sci. Technol.
Strategies for improving biogas production of palm oil mill effluent (POME) anaerobic digestion: A critical review
Renewable Sustainable Energy Rev
Magnetite particles triggering a faster and more robust syntrophic pathway of methanogenic propionate degradation
Environ. Sci. Technol.
Enhancing anaerobic digestion of complex organic waste with carbon-based conductive materials
Bioresour. Technol.
Enrichment of Methanosaetaceae on carbon felt and biochar during anaerobic digestion of a potassium-rich molasses stream
Appl. Microbiol. Biotechnol.
Biomass retention on electrodes rather than electrical current enhances stability in anaerobic digestion
Water Res
Impact of applied voltage on methane generation and microbial activities in an anaerobic microbial electrolysis cell (MEC)
Chem. Eng. J.
Electroactive microorganisms in bulk solution contribute significantly to methane production in bioelectrochemical anaerobic reactor
Bioresour. Technol.
Enhanced production of methane from waste activated sludge by the combination of high-solid anaerobic digestion and microbial electrolysis cell with iron–graphite electrode
Chem. Eng. J.
Comparison of electrode reduction activities of Geobacter sulfurreducens and an enriched consortium in an air-cathode microbial fuel cell
Appl. Environ. Microbiol.
iTRAQ quantitative proteomic analysis reveals the pathways for methanation of propionate facilitated by magnetite
Water Res
Effects of brush-anode configurations on performance and electrochemistry of microbial fuel cells
Int. J. Hydrogen Energy
Methanogenesis facilitated by electric syntrophy via (semi) conductive iron‐oxide minerals
Environ. Microbiol.
Direct interspecies electron transfer accelerates syntrophic oxidation of butyrate in paddy soil enrichments
Environ. Microbiol.
Biological hydrogen production of the genus Clostridium: metabolic study and mathematical model simulation
Int. J. Hydrogen Energy
Enhancement of bioelectrochemical CO2 reduction with carbon brush electrode via direct electron transfer
ACS Sustainable Chem. Eng.
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