Abstract
The biological conversion of hydrogen (H2) and carbon dioxide (CO2) to methane (CH4), is accomplished by the hydrogenotrophic methanogens (HM). HMs are difficult to cultivate in pure culture, but they are readily available in the mixed culture of effluents from the anaerobic degradation of organic matter, i.e. the fermentation effluent of biogas plants. The rate-limiting step in the work of CH4-forming microbial communities is the low solubility of H2 in the aqueous environment. In our approach, the simple fed-batch fermentation technique was selected to supply the gaseous substrates for the microbial community at laboratory scale and mesophilic temperature. Periodically withdrawn samples were analyzed for process parameters and the microbial communities were studied using Terminal Restriction Fragment Length Polymorphism (T-RFLP) of the mcrA gene and Ion Torrent whole metagenome DNA sequencing. The metagenome data were evaluated by both read-based and genome-centric bioinformatics tools. The rearrangements in the mixed microbial communities, triggered by switching the operating conditions to biological power-to-biomethane (bio-P2M), have been established. The production rates were 6.30 mL CH4 L-1 h-1 during the acclimation phase and 9.21 mL CH4 L-1 h-1 by the fully adapted community, respectively. The diversity of the anaerobic microbiota decreased as the bio-P2M process progressed. Feeding the community with H2 apparently promoted the abundance of several genera, in particular Candidatus Cloacimonas and Herbinix. The diversity of the Archaea community decreased considerably upon daily feeding with H2 and CO2. The predominant Archea genus was Methanobacterium in every reactor, Methanothrix persisted for the first 4 weeks, while the initially less abundant genus Methanoculleus gained advantage during the adaptation to the sustained bio-P2M process. The accumulation of acetate indicated a strong involvement of homoacetogenic bacteria.
Highlights
There is a common understanding worldwide about the importance of renewable energy carriers (RECs) and the need for increasing their proportion in the portfolio of the global energy consumption
After the gas chromatography (GC) measurement, the gas phase was replaced with N2, using a manifold, and the pressure was adjusted to atmospheric level
The control reactors (CN) produced 46.1 ± 1.4 mL (1.81 ± 0.05 mmol) CH4, this could be accounted for the residual biogas potential present in the inoculum
Summary
There is a common understanding worldwide about the importance of renewable energy carriers (RECs) and the need for increasing their proportion in the portfolio of the global energy consumption. Due to the inherently fluctuating nature of the most widely used wind and photovoltaic technologies, the stable operation of the grid is jeopardized as the fluctuations cannot be synchronized with electricity consumption. This leads to the reduction of the overall efficiency and economic feasibility. The H2 from water electrolysis is used for biological or chemical conversion of CO2 to CH4, the process employing microbes for the CH4 conversion is called bio-power-to-methane (Angelidaki et al, 2018; Treu et al, 2018a; Nap et al, 2019) The CO2 to be reduced with H2 may originate from biogas, which contains 30–50% CO2 or exhaust gas of internal combustion engines. Through coupling the excess REC electricity utilization with CO2 mitigation the economic and environmental value of bio-P2M improves (Lewandowska-Bernat, 2017)
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