## 2 Materials and Methods
### Potential for Replication
#### 2.1.2 Biomethane Production from WWTP  
Wastewater treatment plants (WTTPs) have become essential infrastructure in cities around the world, enabling sewage treatment before reuse or discharge. Depending on the population size, a city may have one or several WWTPs to ensure efficient effluent treatment. The treatment process in WWTPs begins with passing the wastewater through a series of screens at the inlet of the plant, which remove large quantities of suspended or floating solids. It is then fed to primary treatment, where the suspended solids are separated by decantation, resulting in a cleaner liquid and sewage sludge that require specific treatment. In the next stage, secondary treatment and chemical and biological processes are applied to remove the remaining contaminants. Through flocculation reactions and the action of microorganisms, wastewater is purified to levels that allow it to be reused for tasks such as garden irrigation or street sweeping. Excess treated wastewater, now free of contaminants, is safely discharged into rivers or other natural waterways. As mentioned above, one of the by-products generated in this process is sewage sludge, with a high concentration of organic matter. The proper management of this by-product is the main objective of the project. Thus, to take advantage of its energy potential, the sludge is sent to anaerobic digesters, where, after a fermentation period, biogas is produced with a composition of approximately 66% methane, 30% carbon dioxide, and traces of other compounds, such as carbon monoxide, sulphur gases, and suspended microparticles. Traditionally, all biogas generated at WWTPs is burned in flares, resulting in a large loss/wastage of a potential valuable renewable energy source and unnecessary air pollution/release of air pollutants. To reverse this situation, the Smart Met Value project, implemented at the Guadalajara WWTP in Spain, developed a process to recover the sludge produced at the facility. The process developed started with the installation of cogeneration engines that used the biogas as fuel to generate electricity and reduce the WWTP's energy costs. However, the use of unpurified biogas caused serious maintenance problems in the engines, as the impurities present in the gas (such as sulphides, microparticles, and aromatics in the form of siloxanes) accelerate the wear of pistons, rings, and cylinder liners. To overcome this limitation, biogas refining and purification processes were incorporated. First, a backwash system was employed using treated water from the WWTP to separate methane from carbon dioxide and solid particles. Subsequently, the gas was passed through conventional absorption columns, where sulphur and aromatic compounds were removed and dried to avoid the presence of residual moisture. The overall valorisation project flowsheet is shown in Figure 2.  
The final result of the process was biomethane with a purity of more than 95%, which met the standards required by vehicle manufacturers for its use as a fuel. This high purity is essential to ensure efficient engine performance and avoid mechanical problems. To achieve this, biomethane must meet several technical conditions: a minimum concentration of 90% methane, absence of aromatic compounds, elimination of micro-particles, sulphurous gases, and moisture [28]. By meeting these conditions, the biomethane obtained could be used directly in trucks and light vehicles without the need to modify the engines. In addition, a refuelling station was set up at the WWTP itself, allowing vehicles to refuel autonomously, without relying on conventional filling stations or incurring their high costs. This case demonstrates not only the technical feasibility of the process but also its potential for replication on a larger scale. In this sense, the aim of this article is to analyse the general potential for biomethane production from WWTPs in the Spanish context.