## 2 Methodology
### Presumptions for the Successful Integration of Green Hydrogen and Biomethane Production Processes
#### 2.3.3 Availability of External Infrastructure  
The next important factor for the successful integration of green hydrogen and biomethane technologies is the availability of various infrastructure. The most important thing is access to the following types of infrastructure: natural gas, district heating, power networks, as well as transport refueling infrastructure. In the future, access to the developing European hydrogen network will also need to be assessed [91]. Direct access to energy networks can reduce energy losses associated with the transportation of energy produced by WWTPs [47]. The importance and consumption of natural gas in WWTPs should be continuously reduced, aiming for complete energy neutrality of WWTPs [5]. However, direct access to the natural gas network is relevant to ensure efficient sale, distribution and use of one of the main energy products produced by WWTPs--biomethane. By integrating green hydrogen production into WWTPs, biomethane production can increase by 50-70% (e.g., by implementing biogenic CO\({}_{2}\) methanation) [13; 66]. The value of biomethane consistsof two components--the energy value, which is equal to the price of natural gas, and the additional green gas value, which is justified by the green gas guarantee of origin and the green certificate (Proof of Sustainability (PoS)). According to Lithuanian law, the guarantee of origin is granted only to biomethane injected into the natural gas network. Although at the end of 2023, the legal possibility of transporting produced biomethane from the production site to the injection point became available, direct injection of biomethane is the most efficient option from an economic and environmental point of view, as there is no need to use excess energy for compression or liquifaction of biomethane and to burn transport fuel for biomethane transportation by roads [47]. Another advantage of direct connection to the natural gas network is the ability to inject hydrogen produced by WWTP and transport it through the natural gas network. In order to transport hydrogen by road, hydrogen must be compressed to high pressure or liquefied. The low volumetric energy density of hydrogen requires energy-intensive methods for its storage and transportation. Compressing to 700 bars incurs energy losses of 10-15%, liquefaction at \(-253\)\({}^{\circ}\)C expends up to 30% of its energy [47]. In countries like Germany, Italy, the UK or Lithuania, where natural gas infrastructure is well developed, biomethane and hydrogen can be supplied together with natural gas through the existing pipeline network, a concept known as the \"green gas concept\" [47]. Additionally, natural gas transmission lines operate at higher pressures of up to 80 bar, while local distribution networks operate at much lower pressures of 4-30 bar [92]. However, the injection of hydrogen into the natural gas network is limited by existing technical and regulatory conditions. The technical conditions include the 3.3 times lower volumetric energy density of hydrogen compared to methane, the associated need for additional transmission and storage capacities of the gas network, hydrogen embrittlement in natural gas pipelines, accuracy requirements for gas flow monitoring, metering and regulation stations, the sensitivity of end-user appliances to gas composition and others [93; 94; 95]. Therefore, depending on the age and technology of the natural gas system, various countries limit the hydrogen blending level in mixtures with natural gas. This blending level ranges from 0.1% (Sweden, UK, Ireland, Latvia) to 6% (France) or 10% (Germany, limited to 2% for \"sensitive\" consumers) [95]. In Lithuania, the permitted hydrogen blending level is 2% in pipelines with a pressure of \(p\) < 1.6 MPa and 0.1% in pipelines with a pressure of \(p\) \(\geq\) 1.6 MPa (with a special permit, it can be increased to 2%) [96]. This means that the amount of hydrogen that can be injected into the natural gas network directly depends on the natural gas flow at the injection point.