A Single-Culture Bioprocess ofMethanothermobacter thermautotrophicusto Upgrade Digester Biogas by CO₂-to-CH₄Conversion with H₂

Abstract

We optimized and tested a postbioprocessing step with a single-culture archaeon to upgrade biogas (i.e., increase methane content) from anaerobic digesters via conversion of CO2 into CH4 by feeding H2 gas. We optimized a culture of the thermophilic methanogen Methanothermobacter thermautotrophicus using: (1) a synthetic H2/CO2 mixture; (2) the same mixture with pressurization; (3) a synthetic biogas with different CH4 contents and H2; and (4) an industrial, untreated biogas and H2. A laboratory culture with a robust growth (dry weight of 6.4–7.4 g/L; OD600 of 13.6–15.4), a volumetric methane production rate of 21 L/L culture-day, and a H2 conversion efficiency of 89% was moved to an industrial anaerobic digester facility, where it was restarted and fed untreated biogas with a methane content of ~70% at a rate such that CO2 was in excess of the stoichiometric requirements in relation to H2. Over an 8-day operating period, the dry weight of the culture initially decreased slightly before stabilizing at an elevated level of ~8 g/L to achieve a volumetric methane production rate of 21 L/L culture-day and a H2 conversion efficiency of 62%. While some microbial contamination of the culture was observed via microscopy, it did not affect the methane production rate of the culture.

A Critical Assessment of Microbiological Biogas to Biomethane Upgrading Systems

“…In this respect, one has to also keep in mind that enriching hydrogenotrophic methanogenic microbial cultures might require a pre-adaption period in order to make the culture fully acclimated to special fermentation temperatures and/or H 2 /CO 2 gassing rates, a modus operandi which is not required for pure culture of microorganisms. The results of above presented studies also show that a fully acclimated hydrogenotrophic methanogenic enrichment culture can sustain overpressure conditions and is furthermore capable of microbiological biogas upgrading as well as of achieving high MERs [28], a finding, in controversy to discussions found elsewhere [30]. Eventually, during experiments applying a monoculture of hydrogenotrophic methanogens it has been shown, that an increase of the pressure inside the bioreactor will elevate the MER, which is a consequence of increasing the maximum solubility of gasses inside the liquid phase [23,24,30].…”

“…The results of above presented studies also show that a fully acclimated hydrogenotrophic methanogenic enrichment culture can sustain overpressure conditions and is furthermore capable of microbiological biogas upgrading as well as of achieving high MERs [28], a finding, in controversy to discussions found elsewhere [30]. Eventually, during experiments applying a monoculture of hydrogenotrophic methanogens it has been shown, that an increase of the pressure inside the bioreactor will elevate the MER, which is a consequence of increasing the maximum solubility of gasses inside the liquid phase [23,24,30]. Hence, by applying overpressure an increase of the MER could be achieved, if a hydrogenotrophic and methanogenic enrichment culture would be utilized for ex situ microbiological biogas upgrading.…”

“…Yet, ex situ microbiological biogas upgrading by contacting biogas, H 2 and pure culture of hydrogenotrophic methanogens did also not receive much attention in the scientific community [22,29,30]. In all three available publications concerning pure culture microbiological biogas upgrading only different thermophilic hydrogenotrophic methanogenic strains were used for bioprocessing.…”

“…A postprocessing step for microbiological upgrading of synthetic and industrial biogas was performed by using a pure culture of a M. thermoautotrophicus DSM 3590 in a laboratory-bench top bioreactor at a temperature of 60°C and an agitation speed of 700 rpm in a working volume of 3 L in chemostat culture [30]. .…”

20 % Reduction of CO2-Emissions with Power-to-Gas in WWTP

“…The same research team observed that the H2 mass transfer is the main limitation in this process. The higher is the H2 retention time, the higher is the CH4 conversion efficiency [7]. Methane production of 20 m 3 CH4/m 3 reactor/day is achievable with low H2 remaining in the effluent gas (<10 %).…”