*6.2.1 Chemoautotrophic techniques*

The chemoautotrophic biogas upgrading techniques rely on the action of hydrogenotrophic methanogens that can use H2 to transform CO2 to CH4 depending on the following equation:

$$\text{4H}\_2 + \text{CO}\_2 \rightarrow \text{CH}\_4 + \text{H}\_2\text{O} \,\text{4G}^\circ = -\text{130.7 kJ/mol} \tag{4}$$

But, in order to make the biological upgrading technology renewable, the necessary H2 in the reaction has to be extracted from renewable source. Thus, the using renewable electricity concept for generation of H2 by hydrolyzing water has attracted great attention, particularly in cases that residual electricity from solar panels or wind mills is exploited. Whereas, in the concept of in-situ biological biogas upgrading, H2 is injected into a biogas digester in order to be connected with the endogenous CO2, which is generated in the anaerobic reactor and be transformed into CH4 by the action of autochthonous methanogenic archaea [11, 166].

## *6.2.2 Photoautotrophic methods*

The photosynthetic biogas upgrading is an alternative technology to isolate the CO2 in order to produce a CH4-rich gas. By performing these techniques, H2S elimination is further achieved; whereas, >54% of CO2 is devoured. The methane recovery of photoautotrophic methods can reach up to roughly 97% relying on the reactor kinds and the selection of algal species.

Physicochemical methods are in general at high technology readiness levels, while biological methods are still new and not commercial yet. However, they offer huge potential in respect to feasibility, technological easiness, and potential. Biological upgrading opens new horizons for integrating different forms of renewable energy and besides upgrading can offer electricity storage advances and decoupling bioenergy production from biomass availability [11].
