## 4 Conclusions  
The three case studies presented--LIFE Smart Agromobility, Smart Met Value, and UNDERGY--demonstrate that waste recovery and renewable energy integration are not only technically feasible but also scalable and high-impact strategies to advance the energy transition in Spain. Each project validates a different, albeit complementary, pathway: the local production of biomethane from agricultural and urban waste and the seasonal storage of surplus renewable energy in the form of hydrogen. Specifically, LIFE Smart Agromobility demonstrated that a single livestock facility processing 1300 tonnes of pig manure per year can generate 42.2 MWh of biomethane, equivalent to 89,000 km of vehicle travel per year. The Smart Met Value project demonstrated that a wastewater treatment plant serving 93,000 inhabitants can produce 285 kg/day of biomethane, enough for seven rubbish trucks travelling 150 km per day. In both cases, high levels of biomethane purity (>90%) were demonstrated that meet vehicle fuel standards, making these systems economically viable for fleet applications. Similarly, the UNDERGY project confirmed the technical feasibility of underground hydrogen storage from depleted natural gas fields. With a national storage potential of up to 2865 TWh, Spain has one of the largest capacities in Europe, enabling large-scale seasonal storage and helping to stabilise intermittent renewable generation. Advanced geomechanical and geochemical simulations validated the integrity and long-term security of these storage formations. From a future perspective, the combined potential of biomethane from pig manure and WWTPs in Spain exceeds 6.39 TWh/year, enough to power thousands of vehicles or replace a significant fraction of fossil heat demand in residential and industrial settings. At the same time, the enormous storage potential of hydrogen lies at the foundation of a hybrid energy system capable of balancing daily and seasonal fluctuations in renewable energy supply. Scaling up these models on a national scale would require targeted investments, political support for biomethane injection and hydrogen infrastructure, and incentives to integrate circular economy principles into local and regional planning. However, the basic technologies have already been validated. These projects offer applicable models that could be replicated to accelerate Europe's path towards climate neutrality, energy independence, and local economic revitalisation. Conceptualisation, J.P.d.I.F. ; methodology, E.G.-F.; software, M.-P.M.-H. and J.P.C. ; validation, M.F.O. and R.P. ; formal analysis, R.P. and J.P.C. ; investigation, E.G.-F.; data curation, M.-P.M.-H.; writing--original draft preparation, J.P.d.I.F, R.P., M.-P.M.-H., E.G.-F.; writing--review and editing, J.P.C., M.F.O. ; visualisation, J.P.C. and M.-P.M.-H.; supervision, M.F.O. ; project administration, M.F.O. and J.P.d.I.F; funding acquisition, M.F.O. and J.P.d.I.F. All authors have read and agreed to the published version of the manuscript. This research was funded by the European Commission, LIFE Smart Agromobility (LIFE19 CCM ES 515001206) founded by the LIFE program; by the Spanish Ministry of Science and Innovation AND the Next Generation EU Fund, Undergy (Ref. MIG-20211018) founded by the 'Program Misiones CDTI 2021'. Not applicable. The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author. The authors are grateful to SPANISH \"MINISTERIO DE UNIVERSIDADES\" for the grants FPU22/00900 and FPU19/06577. The authors declare no conflicts of interest. The following abbreviations are used in this manuscript: \begin{tabular}{l l} GHG & Greenhouse gas \\ UHS & Underground Hydrogen Storage \\ LHV & Low Heating Value \\ VS & Volatile Solids \\ WWTPs & Wastewater treatment plants \\ \end{tabular}