Valorization of gaseous streams via bioprocesses

Getting the most out of our gases

Almost any waste/wastewater biorefinery scheme proposed to date includes processes generating gaseous streams. Examples of these technologies are anaerobic digestion (AD) or dark fermentation (DF). These processes generate biogas (around 50% CH4 and 50% CO2) and biohydrogen (mostly H2 and CO2). At the LBE we are working on the upgrade and valorisation of these gases, aiming at generating valuable products other than energy, such as microbial proteins or chemicals. Effective (bio)gas upgrade and valorisation could be crucial for countries that depend on others for their natural gas supplies and to avoid future feeding crises.

Biomethanation

Biomethanation is based on a reaction carried out by methanogenic microorganisms, converting H2 and CO2  into CH4. This allows to increase the CH4 content in the biogas by valorising the CO2 produced.
Biomethanation can be carried out ex-situ, in which the biogas and H2 are injected into a dedicated reactor or in-situ, where H2 is directly injected into the digester (see Figure 1) [2]. H2 could be produced via Power-to-Gas (water electrolysis) or via DF, in which organic waste is valorised into H2 and organic acids. The resulting methane can be used as building block or injected directly into the natural gas grid.
At the LBE we are developing and optimising the in-situ biomethanation process because it has the advantage of avoiding the construction of additional infrastructure for post-gas treatments, hence lowering the cost of the process [3].

Figure 1: Schematic representation of ex-situ and in-situ biomethanation. Adapted from [1].

Purple bacteria for hydrogen valorization 

Purple phototrophic bacteria (PPB) are photosynthetic organisms able to use a range of gaseous electron donors, such as H2 or H2S to fix CO2. Under anaerobic-illuminated conditions, they can effectively grow with high biomass yields (up to 1 g CODbiomass·g CODconsumed-1) and with average protein contents of 60%. These features, together with their fast growth rates and easy enrichment, make them a promising candidate for gas valorization [4].
At the LBE we are working of the application of PPB for the simultaneous conversion of DF effluents, i.e. H2 and CO2 into single-cell protein [5]. In addition, we aim at using the liquid fraction of the digestates from DF and AD as source of nutrients for PPB growth.

 Figure 2: Application of purple phototrophic bacteria (PPB) for gas valorization into single-cell protein, coupled with dark fermentation, anaerobic digestion, and other H2 sources such as power-to-gas.

References

[1] Angelidaki et al. 2018. Biotech. Adv. 36:452-466.
[2] Braga Nan et al. 2020. Biotech. for Biofuels.13:1–17.
[3] Götz et al. 2016. Renew. Energy 85:1371-1390.
[4] Capson-Tojo et al. 2020. Biotech Adv. 43:107567
[5] Rodero et al. 2024. Sci. Tot. Env. 908:168471.