## 1 Introduction  
The energy transition in Spain faces urgent challenges, marked by the need to reduce greenhouse gas emissions and to guarantee a reliable and sustainable energy supply. On this path, Spain has experienced a significant increase in electricity generation from renewable sources between 1999 and 2025 [1]. In line with its National Integrated Energy and Climate Plan (PNIEC) 2021-2030, the government has set an ambitious target of 74% of electricity generation from renewable sources by 2030 [2]. In this context, biomethane represents a key alternative to non-renewable sources of energy generation. It is estimated that the production of biomethane could replace more than 50% of the coal-based primary energy used in Spain in 2023 [3]. This potential replacement capacity offers the opportunity to provide an efficient and strategic energy solution that takes advantage of locally available resources to advance waste-to-energy research, innovation, and technologies, thereby promoting environmental sustainability and the principles of the circular economy. This trend is also in line with the commitments set by the European Union on renewable energy. Directive (EU) 2023/2413 sets a binding target of 42.5% of energy consumption from renewable sources by 2030, with an indicative supplement of 2.5% to reach 45% [4]. This policy framework reinforces the need to adopt technologies that enable a realistic and effective transition to a cleaner, more resilient, and self-sufficient energy model. Within this European roadmap, the recovery of organic waste represents an interesting strategy for integrating the principles of the circular economy into the energy sector [5]. By converting by-products such as slurry or sludge from wastewater treatment into renewable energy sources, the environmental impact of key sectors such as agriculture and water treatment can be significantly reduced [6], while at the same time generating new economic opportunities, optimising waste management, and promoting rural development [7]. In this context, anaerobic digestion represents an effective measure to avoid fugitive emissions of greenhouse gases through the controlled putrefraction of waste. It is a biochemical process that converts organic substrate into biogas--a mixture made of two major gases, methane (CH\({}_{4}\)) and carbon dioxide (CO\({}_{2}\))--and a residual slurry called digestate [8]. It mimics natural anaerobic conditions where organic matter decomposes, although uncontrolled environments like neglected sewage systems can generate odours and health hazards [9]. Methane stores energy, while CO\({}_{2}\) results from endothermic oxidation processes. The absence of oxygen leads to reducing conditions, forming by-products such as hydrogen sulphide [10]. An anaerobic digestion is optimal for wet waste, as removing and transporting diluting water are costly when aiming at the thermochemical route [11]. A wide range of organic materials, e.g., the organic fraction of municipal solid waste, faeces, and industrial sludge, can be used [12]. Wastewater treatment plants improve feasibility by centralising sludge [13], but agricultural systems can face economic limitations. Industrial streams may be suitable, although heavy metals may inhibit microbial activity or contaminate digestate [14]. The biodegradable fraction of organic waste, termed Volatile Solids (VS), is determined via heating tests [15], representing the portion of matter biodegradable by microbial action [16]. In parallel, green hydrogen has gained prominence as an energy source with great potential to decarbonise emission-intensive sectors [8]. However, efficient and safe storage remains one of the main challenges for large-scale adoption. Underground storage, especially in structures such as depleted reservoirs, aquifers, or salt caverns, offers promising solutions [17]. In contrast to above-ground tanks, it stands out for its significantly higher capacity, density, and storage pressure [18].