## 1 Introduction  
Livestock farming is increasing worldwide, and intensive farming is under pressure to ensure food security given the growing world population [1]. However, an increase in livestock production brings large volumes of biowaste streams that need appropriate management; otherwise, severe environmental problems can occur, such as an increase in greenhouse gas (GHG) emissions [2]. Furthermore, the world's energy demand is rising along with the human population, which heightens the need for innovative renewable energy sources [3]. Several routes can be taken into consideration regarding livestock management, but anaerobic digestion (AD) stands out as one of the most promising [4]. Due to its ability to simultaneously recover energy (biogas or biomethane) and create a digestate that may be used for agronomic valorisation, AD technology aligns with the circular economy framework [1] and contributes to the search for new alternative energy sources. Being a biowaste-to-energy technology, it contributes to biowaste management and nutrient recovery, reduces GHG emissions, and decreases dependency on fossil fuels. Therefore, it is a step forward in the energy transition the European Union aims to achieve [5], while also meeting the UN Sustainable Development Goals (SDGs), particularly \"Affordable and Clean Energy\", \"Industry, Innovation and Infrastructure\", and \"Responsible Consumption and Production\". Even though AD has been a well-known and well-established technology for several years and its advantages for biowaste treatment (especially livestock management) are well recognised as a bio-valorisation pathway [2; 6], its implementation at a real scale still has potential for improvement. For this reason, this literature review covers twenty years of research in this area, from 2004 to 2024, to reinforce the potential role of biowaste-to-energy technology in meeting the EU's goals for energy transition towards GHG reduction and renewable energy production. Adding to that, due to its innovative and flexible approach, AD can be widely applied. Though the 20 years of literature in this review article highlight that the problems with biowaste management have been known for several years, it seems now that addressing the issue requires a more realistic approach to face the future challenges, as it continues to be a key topic on the current agendas of many countries, especially as part of efforts to address climate change, waste management, and renewable energy [7; 8]. For a few years now, new strategies for upgrading AD have been studied. Among them, there is the use of additives, which can improve process stability, reduce the impact of inhibitors, and increase methane yield [9; 10; 11]. According to [PERSON] et al. [10], several additives can be considered for improving the AD process, depending on the purpose of the additive's incorporation, for example biological additives for enzymatic action [12]; zeolite application for introducing cation exchange properties in anaerobic digestion [13]; trace metal application for metabolic activities [14]; nanoparticle application for catalytic activity, metabolism, and symbiosis during AD [15; 16]; and carbon-based materials application for syntrophic activity improvement [17; 18]. Carbon-based additives can promote syntrophic relationships in the AD reactor and strengthen direct interspecies electron transfer (DIET) in the system [19]. Materials like activated carbon, granular activated carbon, biochar, or hydrochar facilitate the consumption of volatile fatty acids (VFAs) more quickly and provide substrates to methanogens [10]. One of the most promising is biochar, a carbonaceous material that can also be obtained from by-products of other industries (agro-industrial biomass, digestate from AD process, slurries, etc.) and thus contribute to the pursued economic circularity, adding value to wastes [4].