## 2 Methodology
### Presumptions for the Successful Integration of Green Hydrogen and Biomethane Production Processes
#### 2.3.1 Utilization Possibility of All Products from Electrolysis in WWTP
By using green hydrogen and methanation of CO\({}_{2}\), separated from biogas, it is possible to produce an additional 50-70% biomethane in the WWTP [13; 66]. The methanation of CO\({}_{2}\) from anaerobic digestion within the power to gas concept has recently emerged as a promising technology to upgrade biogas, to decarbonize the domestic and industrial heat sector, provide long-term energy storage and deliver grid balancing services [14]. Two approaches exist to produce methane via the Sabatier reaction (Equation (2)), i.e., chemical (or catalytic) and biological methanation [54; 67]. \[4\mathrm{H}_{2}+\mathrm{CO}_{2}\rightarrow\mathrm{CH}_{4}+2\mathrm{H}_{2} \mathrm{O},\,\Delta\mathrm{G}_{0}=-165\,\mathrm{kJ}/\mathrm{mol} \tag{2}\]  
During the chemical methanation, various catalysts, such as nickel (Ni) or ruthenium (Ru), can be used under increased temperature (e.g., 300-600 \({}^{\circ}\)C) and pressure (e.g., 50-200 bars) [28; 54; 68]. Chemical methanation requires much smaller reactor sizes for the same feed gas flow as biological methanation. Use of chemical methanation also leads to higher efficiencies because no stirrer in the reactor is required, and the waste heat can easily be utilized [69]. High rates of CO\({}_{2}\) conversion can be achieved with chemical methanation. The results of the study [28] showed that, while the cooled reactor with no previous CO\({}_{2}\) separation from the biogas had a CO\({}_{2}\) conversion of 91.8%, the adiabatic reactors showed an overall conversion of 93.0%. Another study [68] revealed that, through catalytic methanation, the outlet gas mixture (CH\({}_{4}\geq\) 95%, H\({}_{2}\leq\) 5% and CO\({}_{2}\leq\) 2.5%) fulfils the requirements for gas grid injection. On the other hand, chemical methanation requires that input CO\({}_{2}\) is free from impurities (such as siloxanes), however, biological methanation requires less stringent quality and may use the CO\({}_{2}\) in raw biogas derived from anaerobic digestion to produce methane [67]. This means that the biological methanation process does not require biogas upgrading prior to CO\({}_{2}\) methanation--this can save capital investment and operating costs, especially for electricity. The most popular biotechnology strategies for increasing the methane content in the biogas include: (i) H\({}_{2}\)-assisted CO\({}_{2}\) bioconversion, (ii) fermentative CO\({}_{2}\) reduction, (iii) microbial electrochemical CO\({}_{2}\) utilization, and (iv) CO\({}_{2}\) capture by microalgae [54]. During the last decade, H\({}_{2}\)-assisted CO\({}_{2}\) bioconversion, or biological hydrogen methanation (BHM), has been widely investigated and well-proven with sufficient experimental results as a means of upgrading biogas [54; 66; 70; 71]. BHM is a promising technology for the conversion of green electricity to biomethane via electrolysis and an application of the Sabatier-reaction (Equation (2)). During the BHM process, catalyzed by specific Methanothermobacter microorganisms, it is possible to convert H\({}_{2}\) and CO\({}_{2}\) to CH\({}_{4}\) with a small amount of water as a byproduct. The BHM process can be applied both within an anaerobic digester system, known as in situ, or in a separate reactor known as ex situ [14; 66; 70; 72]. In situ biomethanation takes place within the anaerobic digester. H\({}_{2}\) gas is introduced typically through mixing or diffusion to maximize the contact area with hydrogenotrophic methanogenic archaea, which produce CH\({}_{4}\) from CO\({}_{2}\) and H\({}_{2}\). Ex situ methanation takes place in a separate external reactor, typically tailored to suit the hydrogenotrophic methanogens. Gases (biogas, CO\({}_{2}\), H\({}_{2}\)) and specific nutrient media are supplied to the ex situ reactor under controlled conditions [66]. Hybrid systems can also be used, in which in situ and ex situ biogas upgrading are taking place together, aiming to process optimization [54; 73; 74; 75; 76]. While some studies have attempted in situ upgrading, although at lower efficiencies, ex situ is more developed [66].