## 3 Anaerobic Digestion Plant Technologies  
The EU is the world leader in biogas electricity production. The installed electric capacity raised in Europe from 4158 MW in 2010 to more than 10,000 MW in 2017 reaching the total number of 17,662 biogas plants [16]. The growth has been mainly due to the building of plants running on agricultural substrates. Agricultural plants are then followed by biogas plants running on sewage sludge (2838 units), landfill waste (1604 units) and various other types of waste (688 units) [17]. Plant technology depends strongly on the properties of the organic substrates and on the sector in which it is operated (biomasses, waste, sludge). The amount of available substrate affects the feeding system (i.e., continuous, semi-continuous, batch). A first classification of the process can be based on the dry matter content of the organic matrix and on the temperature of the process. According to the dry matter content, the process may be divided into dry, semi-dry and wet, with solid substances higher than 20%, between 20-10% and lower than 10%, respectively. Dry anaerobic digestion may be the most appropriate process for the degradation of by-products with high total solid content, especially those coming from agriculture and food waste. However, this process is not widely used yet, compared with the wet process. Dry fermentation may potentially achieve a biogas yield similar to that obtained using the wet digestion process, although design and management of the plants can profoundly affect the efficiency of the process. Rheology, substrate heterogeneity, origin and maturity of inoculum, inhibition, co-digestion, pre-treatments and microbial dynamics are the main aspects to be considered when the dry process is applied [18]. Other relevant features include retention time, loading rate, and recirculation of digestate and leachate. Moreover, the operation of the plant requires relevant man labor, mainly related to loading and unloading of the anaerobic cells [19]. As a consequence, anaerobic digestion plants relying on wet anaerobic digestion are more common and preferred over those based on dry processes. Anaerobic digestion can be carried out under mesophilic (about 35 \({}^{\circ}\)C), thermophilic (about 55 \({}^{\circ}\)C) or, more rarely, psychrophilic conditions (below 20 \({}^{\circ}\)C). The reaction temperature may affect the kinetics, the specific biogas production and the overall efficiency (Table 1). The duration of the process, in terms of hydraulic retention time, is 15-50 days, if the process takes place in mesophilic conditions, 14-16 days, if it takes place in thermophilic conditions, and 60-120 days in psychrophilic conditions. According to several experiences at full-scale level, the mesophilic process is easier to control, for that reason it is still the most common [20]. The AD process can be carried out in one or two stages, depending on the combination or separation of hydrolysis/acidogenesis and acetogenesis/methanogenesis phases, respectively. A two-phase configuration process in the case of food waste digestion significantly improves the methanogenic process as the fermentation plays the role of \"pre-treatment\", thus promoting the conversion efficiency of the volatile fraction to biogas. According to [21], the payback time for introducing a first digester can be less than 1.5 year. Therefore, the optimal conditions for AD would be thermophilic hydrolysis/acidogenesis and mesophilic methanogenesis, which is consistent with a two-phase anaerobic digestion process [22]. Anaerobic digestion systems were strongly improved in recent decades, through the introduction of accelerants (metabolic stimulants for microorganisms) in order to accelerate and increase gas production [22; 23]. The AD plants can be divided into categories according to the reactor type. The main groups, ordered according to increasing innovation and complexity, are covered lagoons, conventional anaerobic reactors, sludge retention reactors and anaerobic membrane reactors.