## 1 Introduction  
The European Green Deal solidifies ambitions for a green transition, encompassing climate targets that aim for net zero by 2050 [1]. Additionally, it emphasizes the need for cutting-edge technologies in key industrial sectors by 2030, with priority areas including alternative fuels and energy storage [2]. The European Union's reliance on external energy supplies has hindered energy security and resulted in increased energy costs for households and industries, with Russia and Norway being the top two suppliers [3]. Growing concerns about securing the energy supply led the European Energy Security Strategy and the Energy Union to focus on reducing energy dependency from abroad [4, 5]. This external dependency entails economic, social, ecological, and physical risks [6]. The tension in energy markets due to geopolitical uncertainty is escalating. Wars are reshaping the global energy system, prompting governments to bolster energy security. In 2021, a quarter of the EU's energy came from Russia, underscoring the importance of investing in robust gas network infrastructure to better integrate regional markets [7]. The EU is committed to reducing its dependence on imported gas, accelerating the production and integration of renewable energies to mitigate climate change, and fostering energy and agroecologicaltransitions, in addition to embracing the circular economy [8]. The REPowerEU plan exemplifies the proactive measures taken by policymakers to enhance internal energy security following the 2022 crisis and advance towards climate objectives, promoting the sustainable production of biomethane to 35 billion cubic meters by 2030 as a cost-effective means of reducing natural gas (NG) imports from Russia [9]. The security of the energy supply encompasses two dimensions: an external one and an internal one [4]. In the internal dimension of securing energy, efforts are made to reduce the energy demand, increase renewable energy sources, and create a common energy market. Therefore, the security of the energy supply is closely related to the EU's climate strategies and targets, which aim to progressively reduce greenhouse gas (GHG) emissions up to 2050 and achieve the transformation towards a low-carbon economy [10]. Facing this energy transition, NG will play a key role in the EU's energy mix [11] (it is promoted as a transition fuel from coal to renewable resources), but it is necessary to diversify imports by country and by route to increasingly interconnect domestic gas networks in order to overcome supply disruptions [5; 11]. Additionally, there is a lack of reliable data regarding its real sustainability due to methane emissions into the atmosphere throughout its lifecycle [12]. This variability is influenced by factors such as the method of extraction, mode of transport, and distance, among others [13; 14]. Methane is the second most significant greenhouse gas, after carbon dioxide, with an even greater capacity for retaining heat in the atmosphere. On a 100-year timescale, methane exhibits a global warming potential 28 times greater than that of carbon dioxide, and its potency increases to 84 times on a 20-year scale [15]. However, considerable uncertainties surround the total emissions, and results vary significantly among studies, consistently demonstrating that methane emissions are likely to be significantly underestimated [13; 16]. Therefore, they remain a significant challenge in key sectors [17]. Anaerobic digestion (AD) is recognized both as a technology for treating organic waste and as a renewable source of biogas; it is composed predominantly of methane (CH\({}_{4}\)) and carbon dioxide (CO\({}_{2}\)). It is notable that the presence of CO\({}_{2}\) significantly diminishes the heating value of the biogas, which typically consists of 55-65% methane (CH\({}_{4}\)) and 35-45% carbon dioxide (CO\({}_{2}\)) [18]. Raw biogas may also contain trace amounts of H\({}_{2}\)S, water vapor, NH\({}_{3}\), and siloxane.