Biomethanation is a promising solution to upgrade the CH4 content in biogas. This process consists in the injection of H2 into an anaerobic digester, using the capacity of indigenous hydrogenotrophic methanogens for converting the injected H2 and the CO2 generated from the anaerobic digestion process into CH4. However, the injection of H2 could cause process disturbances by impacting the microbial communities of the anaerobic digester. Better understanding on how the indigenous microbial community can adapt to high H2 partial pressures is therefore required. Seven microbial inocula issued from industrial bioprocesses treating different types of waste were exposed to a high H2 partial pressure in semi-continuous reactors. After 12 days of operation, even though both CH4 and volatile fatty acids (VFA) were produced as end products, one of them was the main product. Acetate was the most abundant VFA, representing up to 94% of the total VFA production. VFA accumulation strongly anti-correlated with CH4 production according to the source of inoculum. Three clusters of inocula were distinguished: (1) inocula leading to CH4 production, (2) inocula leading to the production of methane and VFA in a low proportion, and (3) inocula leading to the accumulation of mostly VFA, mainly acetate. Interestingly, VFA accumulation was highly correlated to a low proportion of archaea in the inocula, a higher amount of homoacetogens than hydrogenotrophic methanogens and, the absence or the very low abundance in members from the Methanosarcinales order. The best methanogenic performances were obtained when hydrogenotrophic methanogens and Methanosarcina sp. co-dominated all along the operation. New insights on the microbial community response to high H2 partial pressure are provided in this work. H2 injection in semi-continuous reactors showed a significant impact on microbial communities and their associated metabolic patterns. Hydrogenotrophic methanogens, Methanobacterium sp. or Methanoculleus sp. were highly selected in the reactors, but the presence of co-dominant Methanosarcinales related species were required to produce higher amounts of CH4 than VFA.

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Biomethanation processes: new insights on the effect of a high H2 partial pressure on microbial communities

Application of in-situ H2-assisted biogas upgrading in high-rate anaerobic wastewater treatment

Power-to-Gas technologies are considered essential for a green transition of the global energy system by producing fossil-free fuel from renewable electricity and CO2. Biological methanation can be used as a Power-to-Gas technology by harnessing hydrogenotrophic methanogenic archaea to produce CH4 from CO2 and electricity-derived H2. Advancing biological Power-to-Gas methanation technologies to full-scale requires a dedicated focus on H2 gas-liquid mass transfer, which generally limits the conversion of H2 to CH4. Process and reactor design and operation have accordingly received much research attention in recent years, but technology development is still characterized by a broad investigation of various system configurations and operational conditions, mainly in lab-scale. This review gives an extensive summary of the current biological methanation capacities of different reactor types in the context of Power-to-Gas and critically examines the influence of operational parameters on H2 gas-liquid mass transfer based on the two-film theory. The review provides a basis for critical comparison of reactor performances among biological methanation studies, which often vary in key process parameters that have a direct influence on H2 gas-liquid mass transfer and, therefore, product gas quality. The review hereby compiles knowledge essential to realize the microbial potential for biological Power-to-Gas methanation and support the technology's development to an industrially relevant scale.

H2 gas-liquid mass transfer: A key element in biological Power-to-Gas methanation

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Feasibility of hydrogen storage in depleted hydrocarbon chalk reservoirs: Assessment of biochemical and chemical effects

Biological life-support systems could greatly increase the sustainability of crewed missions to the Moon or Mars Understanding how bacteria react to hypobaria is critical to their optimization: if enclosed within crewed compartments, microbial modules may be exposed to the lower-than Earth atmospheric pressure considered for future space vehicles and habitats and, if deployed outside, they would best rely on a low pressure to minimize both engineering constraints and risks of leakage Bacterial behavior at low pressures is of relevance to other fields as well, both within astrobiology (eg, habitability and planetary protection) and outside of it (eg, aerobiology and food preservation) Unfortunately, while microbial survival under vacuum has been largely investigated, little work has focused on metabolism at low but growth-permissive pressures Nonetheless, recent studies brought some insights Limits were outlined: a few bacterial species can grow just above water’s triple point, more can multiply down to around 25 mbar, and shifting pressure within 100 mbar to 1 bar seems not to largely affect growth of most species when the partial pressures of metabolizable gases are not limiting Some mediating mechanisms have been proposed: hypobaria can affect bacteria by desiccation, via a reduced availability of specific gases, and through various other physico-chemical effects, interdependent and dependent on other environmental factors A limited number of studies also gave insights into how bacteria cope with low pressure, and how much they can adapt to it But, overall, much remains to be discovered on bacterial growth under hypobaric conditions

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Bacterial Growth at Low Pressure: A Short Review

Autotrophic hydrogenotrophic methanogens use H2/CO2 as sole carbon and energy source. In contrast to H2, CO2 is present in high concentrations in environments dominated by methanogens e.g. anaerobic digesters (AD), and is therefore rarely considered to be a limiting factor. Nonetheless, potential CO2 limitation can be relevant in the process of biomethanation, a power-to-gas technology, where biogas is upgraded by the addition of H2 and ideally reduce the CO2 concentration in the produced biogas to 0-6%. H2 is effectively utilized by methanogens even at very low concentrations, but little is known about the impact of unnaturally low CO2 concentrations on methanogenic activity. In this study, CO2 consumption and CH4 production kinetics under low CO2 concentrations were studied, using a hydrogenotrophic methanogen, Methanobacterium congolense, as model organism. We found that both cellular growth and methane production were limited at low CO2 concentrations (here expressed as Dissolved Inorganic Carbon, DIC). Maximum rates (Vmax) were reached at [DIC] of 100 mM (extrapolated), with a CO2 consumption rate of 69.2 fmol cell-1 d-1 and a CH4 production rate of 48.8 fmol cell-1 d-1. In our experimental setup, 80% of Vmax was achieved at [DIC] > 9 mM. DIC half-saturation concentrations (Km) was about 2.5 mM for CO2 consumption and 2.2 mM for CH4 production. No CH4 production could be detected below 44.4 µM [DIC]. These data revealed that the limiting concentration of DIC may be much higher than that of H2 for a hydrogenotrophic methanogen. However, DIC is not a limiting factor in ADs running under standard operating conditions. For biomethanation, the results are applicable for both in situ and ex situ biomethanation reactors and show that biogas can be upgraded to concentrations of 2% CO2 (98% CH4) while still retaining 80% Vmax at pH > 7.5 evaluated from M. congolense. Since DIC concentration can vary significantly with pH and pCO2 during biomethanation, monitoring DIC concentration through pH and pCO2 is therefore important for keeping optimal operational conditions for the biomethanation process.

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Xihan Chen

Papers

How Low Can You Go: Methane Production of Methanobacterium congolense at Low CO2 Concentrations.