## 4 Emerging Biogas Upgrading Technologies
### Biological Technologies
#### 4.4.2 Biological Methanation  
In biological hydrogen methanation (BHM), the catalysts are autotrophic hydrogenotrophic methanogens, and the process takes place with no additional energy requirement [61]. These bacteria already exist in every AD plant, so it is only necessary to provide them with the needed H2[102]. The BHM process uses the Sabatier reaction, catalyzed via specific archaea of the Methanothermobacter genus, which is capable of converting H2 and CO2 into CH4 with water as a by-product [144].  
Methanogens can operate under either mesophilic or thermophilic conditions within the pH range of 6.5-8. To render the process renewable, the hydrogen required for the enhancement of raw biogas must also be derived from renewable energy sources through water electrolysis to produce hydrogen [75].  
BHM can be achieved in three different ways: in situ, ex situ, and hybrid upgrade technology [102]. In situ technology consists of feeding hydrogen directly into a digester (Figure 6a), while ex situ upgrade technology involves CO\({}_{2}\) from external sources that is introduced together with hydrogen into a reactor containing hydrogenotrophic methanogens (Figure 6b). Finally, hybrid upgrade technology consists of a combination of both in situ and ex situ biogas upgrading in order to optimize the process [61]. It allows part of the CO\({}_{2}\) conversion to take place during the initial AD process, with the final step being a purification process [106]. Under optimal pH and temperature conditions, approximately 99% of potential methane can be recovered through an in situ process. Ex situ methanation is more similar to the biological PtM production pathway, but it can be used together with other technologies to convert the separated CO\({}_{2}\) from the biogas into more methane, thus increasing the carbon utilization of the facility. An example of a technology combination is the use of a dual-membrane aerated biofilm reactor (dMBfR), as H\({}_{2}\)-assisted ex situ biogas upgrading is hindered by a large solubility discrepancy between H\({}_{2}\) and CO\({}_{2}\). dMBfR enables a precise supply of CO\({}_{2}\) and H\({}_{2}\), making it an ideal combination for efficient biological biogas upgrading [145]. In situ biogas upgrading showed superior performance compared to ex situ biogas upgrading. BHM may require a significant amount of energy due to the processes necessary for the effective solubilization of H\({}_{2}\), allowing its absorption via archaea. An appropriate method for H\({}_{2}\) solubilization in liquid is agitation, but this option entails a high energy demand [144]. Opting for a low-energy-demand alternative would offer greater benefits. However, at a larger scale, employing intense agitation for enhanced H\({}_{2}\) solubilization could be reasonable. There are different reactor systems, such as continuously stirred tank reactors, diffusion-based reactors, fixed-film bioreactors, minimal-liquid bioreactors, soil-based fixed-film reactors, and hollow-fiber reactors [144].  
BHM is still at a pilot stage [102], but there is already one upgrading plant that injects biomethane into the gas grid that uses this method; it is located in Switzerland and has been in operation since 2019, providing biomethane with 96% CH\({}_{4}\)[80]. Pilot-scale experiments show that methane content of 95% in biogas has been achieved with a continuous injection of H\({}_{2}\) into the biogas reactor [66]. At a laboratory scale, in situ methanation under thermophilic operation (65 \({}^{\circ}\)C) not only obtained a methane concentration of 90%, but a content of 98% under mesophilic conditions (37 \({}^{\circ}\)C) has also been achieved [54]. Concentrations of 99% methane have also been reported [75]. For ex situ methanation, the final methane content can be between 79 and 98%, depending on the type of reactor used [75]. Furthermore, methanogens can even remove part of the H\({}_{2}\)S present in raw biogas [52].  
Figure 6: (a) In situ and (b) ex situ biological hydrogen methanation. Adapted from [102].