## 2 Materials and Methods  
### Potential for Replication  
#### 2.1.1 Biomethane Production from Pig Manure  
Potential for replication was estimated using primary data under the Review and Records methodology from LIFE Smart Agromobility, a LIFE European project developed by a consortium formed by Universidad Politecnica de Madrid, Universidad de Valladolid, NTTDATA, COPISO, GASNAM, and Ente Publico Regional de la Energia de Castilla y Leon [19]. This project, which is located in Sauquillo de Boices (Soria, Spain), produced biogas using pig manure through an anaerobic digestion process, while an absorption column containing microalgae was applied to transform the biogas into biomethane.  
The process begins with storing manure in a pond for 25 days; thereafter, the manure is subjected to anaerobic digestion inside a low-cost biodgester under psychrophilic conditions, resulting in the production of biogas (with approximately 70% CH\({}_{4}\)) and digestate. Only 3% of digestate is used for microalgae cultivation in the raceway section, and the rest is used as fertiliser. The grown microalgae are combined with biogas in an absorption column to upgrade the biogas to 90% CH\({}_{4}\). This is facilitated through an absorption process whereby microalgae remove CO\({}_{2}\) from biogas. The biogas is further refined to 95% CH\({}_{4}\) using silica gel and activated carbon filters to meet vehicular biomethane standards. Finally, the biomethane is compressed to 250 bar and stored in a gas station to fuel light vehicles and agricultural machinery [22]. Figure 1 shows a diagram of the project's process flow chart.  
The potential of biomethane production depends on the amount of VS, which represents the portion of organic material that can be degraded. A typical value for biomethane production from pig manure (\(\eta\)) is 0.5 m\({}^{3}\) CH\({}_{4}\) per kg of VS [23], and for WWTP, it is also 0.5 kg CH\({}_{4}\) per kg of VS [24]. The potential production for one swine head was estimated using primary data from LIFE Smart Agromobility, considering mesophilic conditions (i.e., 35 \({}^{\circ}\)C). This range has been considered because production in the psychrophilic range is strongly dependent on outdoor temperature, and it is not possible to extrapolate results between two places with different temperatures. To operate under  
Figure 1: Process Flow Chart of the Life Smart Agromobility project. Photos have been taken by the authors or other members of the consortium of the project. The authors obtained permission from the project consortium to use these photographs.  
is necessary to consume part of the biogas generated in the boiler to maintain the required temperature (35 \({}^{\circ}\)C). This amount is called %biogas_boiler. The amount of biogas consumed also depends inversely on the outdoor temperature.  
Data from pig manure parameters is obtained in [25]. The energy content of biogas could be calculated with the Wobbe index of biomethane, correcting the value with the percentage of methane (%CH4) in biogas. Total production of biomethane per head could be calculated using Equation (1):  
\[\begin{array}{ll}\text{Biomethane}&\left(\text{GWh}/\text{pig}\right)\\ &=\eta\left(\text{m}^{3}/\text{kg}\,\text{VS}\right)\times\text{VS}\left(\text {kg}\,\text{VS}/\text{kg}\,\text{mature}\right)\times\text{pig}\,\text{mature} \left(\text{kg}/\text{pig}\right)\\ &\times\%\text{biogas\_boiler}\times\%\text{CH}_{4}\times\text{Wobbe Index}\left(\text{GWh}/\text{m}^{3}\right)\end{array} \tag{1}\]  
The potential production of pig manure was estimated using primary data from LIFE Smart Agromobility and data from the Agricultural Census of the Spanish National Institute of Statistics [26]. In Appendix B.1, there is a table with the swine heads per province in Spain.  
All maps presented in this study were created using Education Site ArcGIS Pro 3.2, using a template obtained from Instituto Geografico Nacional (IGN) [27] and the ETRS 1989 as the coordinate system.