## 1 Introduction  
Management of municipal solid waste (MSW) has become a major global concern due to the increase of urbanization and consumption standards [1]. In Europe, the generation of municipal solid waste has been growing annually, reaching 502 kg per capita in 2019, of which today less than 60% is fully recycled, incirinated, or composted [2]. As a result of public health and environmental protection issues, efforts have been made towards increasing this percentage by improving recovery strategies and/or recycling useful materials according to the Waste Directive 2008/98/EC [3]. MSW typically comprises about 50-55% organic materials, principally derived from food scraps and food waste (the organic fraction of MSW, or OFMSW), and green waste (GW), originated from private yards and public greenspace (i.e., roadside edges, public lawns) management [4]. OFMSW disposal is typically associated with composting or landfilling [5], which is strongly discouraged, because if inappropriately performed, they can contaminate water sources and soil with leachate and air with greenhouse gas emissions [6]. Composing is facing a lack of public interest mainly due to the low value of the product (i.e., compost) and limited waste volume reduction [7]. Moreover, the majority of OFMSW is generally rich in nutrient substances, such as proteins, minerals, and sugars, which can be recovered in the production of green energy before composting, saving more resources and improving the circularity of biological nutrients. Anaerobic digestion (AD) is an established biological process suitable for stabilizing organic solid wastes and is coupled with the recovery of energy and nutrients. It provides a sustainable option for energy recovery, thereby contributing to a circular economy [8]. According to several recent studies [9; 10; 11], biogas/biomethane will play a key role in helping Europe's transition to a clean energy system, achieving the goal of sustainability with a special focus on the economic and environmental dimensions, and enabling a carbon-neutral (or even carbon-negative) Europe with zero pollution by 2050. As a consequence of the Paris Climate Agreement, which requires global decarbonization, the development of green energy can contribute significantly to tackling climate change [12]. The transformation of organic waste such as OFMSW and GW through the process of AD would encourage EU member states to unlock the national potential of their renewable gases, which would generate ample benefits: adequate waste management, resource-efficient agriculture, and displacement of fossil energy, triggering significant emission savings [13]. A European-wide target can help cities and municipalities to establish a local and circular bio-economy; organic residues, such as OFMSW and GW, can be separated and fermented in anaerobic digestion plants, producing biogas and biomethane that can then be used to fuel, for instance, urban garbage trucks, thereby reducing noise pollution and improving air quality by replacing diesel-powered trucks [14]. AD under controlled conditions is a proficient technique for OFMSW treatment, and it is currently a mature technology, mostly found in Europe [15]. It presents a double advantage as it recovers energy from waste-producing biogas and simultaneously treats the residues, reducing their disposal in landfills [16]. AD refers to a series of biological processes in which a microbial consortium synergistically decomposes organic matters to biogas (a mixture of CH\({}_{4}\) at 45-70% and CO\({}_{2}\) at 24-40%, on average), leaving a digestate (composed by the not-promptly fermentable fraction of organic materials). The process occurs in four sequential stages: hydrolysis, acidogenesis, acetogenesis, and methanogenesis [17]. Biogas is a promising renewable source of bioenergy that can be used for different applications including vehicular fuel, heating, and electricity production [18].