Abstract
Bioelectrochemical power-to-gas represents a novel solution for electrical energy storage, currently under development. It allows storing renewable energy surplus in the form of methane (CH4), while treating wastewater, therefore bridging the electricity and natural gas (and wastewater) grids. The technology can be coupled with membrane contactors for carbon dioxide (CO2) capture, dissolving the CO2 in wastewater before feeding it to the bioelectrochemical system. This way, the integrated system can achieve simultaneous carbon capture and energy storage objectives, in the scenario of a wastewater treatment plant application. In this study, such technology was developed in a medium-scale prototype (32 L volume), which was operated for 400 days in different conditions of temperature, voltage and CO2 capture rate. The prototype achieved the highest CH4 production rate (147 ± 33 L m-3 d-1) at the lowest specific energy consumption (1,0 ± 0,3 kWh m-3 CH4) when operated at 25 oC and applying a voltage of 0,7 V, while capturing and converting 22 L m-3 d-1 of CO2. The produced biogas was nearer to biomethane quality (CH4 > 90% v/v) when CO2 was not injected in the wastewater. Traces of hydrogen (H2) in the biogas, detectable during the periods of closed electrical circuit operation, indicated that hydrogenotrophic methanogenesis was taking place at the cathode. On the other hand, a relevant CH4 production during the periods of open electrical circuit operation confirmed the presence of acetoclastic methanogenic microorganisms in the microbial community, which was dominated by the archaeal genus Methanothrix (Euryarcheota). Different operational taxonomic units belonging to the bacterial Synergistes phylum were found at the anode and the cathode, having a potential role in organic matter degradation and H2 production, respectively. In the panorama of methanation technologies currently available for power-to-gas, the performances of this bioelectrochemical prototype are not yet competitive, especially in terms of volumetric CH4 production rate and power density demand. However, the possibility to obtain a high-quality biogas (almost reaching biomethane quality standards) at a minimal energy consumption represents a potentially favourable business scenario for this technology.
Highlights
The European Commission fixed a target of 80% CO2 emissions reduction by 2050 (European Commission, 2011)
This study reports for the first time the long-term operation of a stack of EMG-bioelectrochemical systems (BES) cells fed with municipal wastewater, which was integrated with a stack of membrane contactors for CO2 capture in wastewater
Strategies to reduce the competition of acetoclastic methanogenesis in the reactor, among other factors, must be developed to ensure an efficient conversion of electricity to CH4 by electro-active microorganisms (Flores-Rodriguez and Min, 2020)
Summary
The European Commission fixed a target of 80% CO2 emissions reduction by 2050 (European Commission, 2011). The methanation technologies, converting CO2 to CH4, were spotted by the energy industry, as the ones allowing an immediate integration into the existing infrastructure and the interconnection between electrical and natural gas distribution grids, with several advantages in terms of energy cost, system resilience and security (Zeng et al, 2016). Methanation can be performed through thermal, chemical, electrochemical, biological or bioelectrochemical processes (Geppert et al, 2016) It can be efficiently coupled with a biogas upgrading unit for cheap CO2 supply (Leonzio, 2019). This possibility opened the way to the idea of a distributed PtX application in wastewater treatment plants (WWTP), instead of centralized plants that would require huge piping of water and/or gaseous reactive streams (Inkeri et al, 2016)
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