## 6 Practical Cases of AD Biogas Plants  
Throughout the studied period for this review (2004-2024), it was possible to observe that the older research concentrated on comprehending the principles of the AD process [21; 25; 54], whereas more recent studies targeted increasing efficiency, looking for new methods to improve biogas production and/or its quality, including digestate valorisation [28; 48; 55]. Among the articles obtained, 14 were selected for a deep examination since they focused their research on practical case studies at small- and lab-scale [25; 27; 43; 47; 48; 56] or farm- and full-scale [25; 28; 52; 53; 57; 58; 59; 60] trials under mesophilic conditions, all between 2009 and 2022, with the majority of incidences (64% of the referred articles) between 2011 and 2018. The retrieved studies were mostly located in Europe (half of them), three in China, two in the United States of America, one in Brazil, and one in South Korea. Pilot-scale studies are important since they can mimic the operational conditions of full-scale biogas plants and help overcome setbacks of implementation on a larger scale. Moreover, these studies often present interesting results that can be considered the \"worst-case scenario\", since there are pre-treatments only applied due to the constraints of the laboratory-scale digesesters. However, laboratory-scale studies are also of great importance,since they enable quick and affordable access to the possibility of scaling up, and even test several different methods at the same time. Thus, lab-scale testing can be used to optimise the process and acquire designing data once an appropriate process configuration has been identified. This can reduce the requirement for more costly large-scale testing. Regarding this, it has become increasingly important to study manure treatment systems that can be easily implemented at full scale [28]. [PERSON] et al. [49] concluded from the comparison of ten PS treatment technologies that AD is a good choice for large-scale farms, considering the environmental and economic benefits. Additionally, it is important to investigate the application of pre-treatments that can be applied to modify the substrates' physicochemical structures and improve their biodegradability, potentially enhancing the anaerobic process and improving biogas production [6,45,47]. From the studies presented in Table 2, most studies were conducted with HRTs above 20 days. [PERSON] et al. [58] also studied an HRT of 40 days for manure from feeder-to-finisher in North Carolina in a 7600 m\({}^{3}\) digester, achieving 65% v/v of CH\({}_{4}\), which is aligned with the quality of biogas reported by other authors with lower HRTs [58, 59, 60]. However, that study [58] did not report values of biogas production, since its purpose was to assess the removal of efficiencies from a full-scale innovative swine waste-to-energy management system, studying a combined system that links AD with a microturbine for electricity generation and an aeration basin where intermittent ventilation occurs for nitrogen management purposes. Six studies were conducted on reactors on small scales (with volumes lower than 10 L), with HRTs varying from 15 [27,60] to 30 days [25]. Curiously, the highest biogas productions (0.31 m\({}^{3}\) CH\({}_{4}\)/kg VS) are presented by the highest HRT of 30 days [25] and the lowest by an HRT of 15 days (0.38-0.52 m\({}^{3}\) CH\({}_{4}\)/kg VS) [27], with similar organic loading rates (2.0 kg VS/(m\({}^{3}\)-d) and 1.5 kg VS/(m\({}^{3}\)-d), respectively). [PERSON] et al. [27] evaluated the impact of varying feeding regimes at an OLR of 1.5 g VS/(L\({}_{\text{reactor}}\cdot\)d), achieving specific methane productions of 0.38 L CH\({}_{4}\)/g VS for daily feeding and 0.52 L CH\({}_{4}\)/g VS for intermittent starvation periods of two days. On the other hand, the value presented by [PERSON] and [PERSON] [25] refers to the mean CH\({}_{4}\) yields during a stable gas production period of 30 days, under an OLR of 2.0 kg VS/(m\({}^{3}\)-d).