## 6 Practical Cases of AD Biogas Plants
The percentage of CH\({}_{4}\) presented in the studies shown in Table 2 is in line with the average values reported by other studies (55-75% \(v/v\))[26,46]. Data presented by [PERSON] et al. [52] is particularly interesting since it refers to a biogas plant in operation for a long time, more exactly, 20 years, treating an average of 3700 m\({}^{3}\)/year of pig slurry. This biogas plant ended its operation due to changes in the farm's production goals and the type of livestock bred. While in operation, this plant provided an annual average of 45.9% of the total energy required to heat the houses on the swine farm, situated near the Pyrenees, in the northern region of Catalonia (Spain). Some of the factors that led to this success were that the plant: (a) was built at the same time as the rest of the farm, leading to lower investment costs and effective integration as part of the farm; (b) was simple by design; (c) ensured maintenance and supervision; and (d) had an integrated vision of the livestock farm and biogas plant, considering pig slurry manure as an energy source and nutrient resource. [PERSON] and [PERSON] [25] studied the impact of AD on mitigating GHG emissions and concluded that GHG emissions reduction could be achieved by directly substituting fossil fuels (meaning that replacing fossil fuels with the biogas produced contributed almost 60% to the reduction of GHG emissions on pig farms) and indirectly by lowering the production and consumption of mineral N fertilisers. In that study, the authors showed that by using an energy balance, pig farms could reach self-sufficiency in terms of their energy needs and still sell around 80% of their produced heat and electricity. [PERSON] et al. [60] monitored a biogas plant for a year and the data collected allowed them to draw conclusions about the seasonality that a full-scale biogas plant would experience. In that work, a waste-to-energy system was proposed, where the waste progressed through the following route: solid manure was sent for composting to produce a fertiliser for agricultural use, while the \"flushed slurry\" obtained from washing the barn floors went to biogas production. The biogas produced was stored and then supplied to households, and the digestate produced was stored and then forwarded to agronomic applications. In the end, the authors concluded that even with a variety of seasonal climate conditions, mesophilic swine manure can be effectively used in waste-to-energy systems. [PERSON] et al. [60] raised a crucial issue regarding large-scale biogas plants. With increased pork production and pig slurry, there is a need for adequate waste management and for resource recovery technologies to ensure the sustainable growth of the industry. To achieve this, renewable energy production through AD can be of great importance. Another important consideration is the need for hierarchisation of pig slurry from the pigs' different stages of life according to the end-use for energy recovery. In [PERSON] et al. 's work [27], a new strategy was presented based on the use of pig slurry from the fattening/finishing phase. This selection was made considering the higher organic content of this specific breeding stream, which was expected to optimise the potential of biogas production. Table 2 shows other studies that have been carried out with slurry from the fattening/finishing phase [25; 57; 58; 60]. Over the years (Table 2), several studies have mentioned the need to valorise digestate, and some of them already present plans for agronomic valorisation, presenting waste-to-energy systems where nutrient recovery is also framed [52; 53; 57]. An overall conclusion after observing the studies about biogas plants is that there is a lack of policies protecting farmers and of subsidies to encourage infrastructure implementation and maintenance. Across all the listed works in Tables 2 and 3, some management factors stand out for the success of biogas plants: the importance of planning and monitoring and the need for policies and subsidies.