## 3 Processes for Bioenergy Production
### Biochemical Methods
#### 3.2.4 Anaerobic Digestion  
This biochemical conversion process takes place in the absence of oxygen and entails the microbial degradation of complex organic substances (lipids, proteins, carbohydrates) contained in plants and animal by-products, with the production of gas (biogas). The raw biogas is usually constituted mainly of methane (about 50%-75%), CO\({}_{2}\) (25%-50%), H\({}_{2}\)O (1%-5%), and other minor volatile components, such as N\({}_{2}\), H\({}_{2}\)S, NH\({}_{3}\) and siloxanes or other organic hydrocarbons [5]. During AD, degradation and stabilization of complex organic matter are performed by microorganisms, leading to the production of an energy-rich biogas that can be used as renewable energy to replace fossil energy sources [37; 38]. The digestion of the biomass takes place in four phases (Figure 3): hydrolysis, acidogenesis, acetogenesis, and methanogenesis, each one carried out by specific microorganisms [5]. In the hydrolysis phase, complex organic substrates, such as proteins, fats, and carbohydrates, are degraded by different bacterial groups, with the formation of simple compounds, such as amino acids, fattyacids, monosaccharides, and alcohols. The acidogenic phase oxidates microbially the hydrolyzed simple organic substrates to low molecular weight intermediate compounds (e.g., lactic, propionic, butyric, formic, and acetic acids, ethanol, and H\({}_{2}\)), which are converted into acetate in the subsequent acetogenic phase by acetogenic bacteria. Then, methanogenic archaea are responsible for the final degradation and the formation of CH\({}_{4}\)[37]. Normally, the biogas produced is used in an endothermic engine to produce electricity. The electricity produced can be directly fed into the distribution network and sold at a remunerative price as a renewable energy source. Frequently, the digested material is applied to agricultural soil as a source of nutrients (due to its high concentration of nutrients, mainly N as NH\({}_{4}\)+ that is directly available to plants), partially substituting mineral fertilizers [39]. The most widely used biomass is maize silage (about 10 m\({}^{3}\) of methane can be obtained from 1 t of maize silage; [23]). However, other organic materials are also used frequently as feedstocks for AD, like animal manure, crop residues, sewage sludge, industrial wastewater, food waste, and municipal solid waste [5; 29]. The calorific value of the biogas obtained varies according to the methane content. A typical value can be about 23,000 kJ Nm\({}^{-3}\)[11]. The biogas can be treated, accumulated, and used as fuel for gas boilers or internal combustion engines. However, it can also be purified (i.e., removal of CO\({}_{2}\), H\({}_{2}\)O, H\({}_{2}\)S- or S-containing compounds (mercaptoethanol), siloxanes, and other minor components) [5; 40] and upgraded to produce biomethane (97% CH\({}_{4}\)), with a quality equivalent to natural gas, which can be used as a biofuel for transport and for introduction into the natural gas network. Upgrading the biogas by removing CO\({}_{2}\) and increasing the CH\({}_{4}\) concentration increases the energy content of the biogas, but also the costs of biogas production. Some technologies for biogas upgrading are already available, and others are in the pilot or demonstration plant stage [41]. Four methods are in use for biogas upgrading through CO\({}_{2}\) removal: washing on a temperature or pressure swing, in which a liquid removes CO\({}_{2}\); cryogenic, based on cooling and compressing for removing liquid CO\({}_{2}\); vacuum pressure swing adsorption (VPSA), in which a packed bed absorbs CO\({}_{2}\); selective membranes, which permeate through the CO\({}_{2}\) or methane, depending on the applied membrane [42]. Anaerobic digestion is, therefore, considered a competitive source for the production of renewable energy in  
Figure 3: Phases of the anaerobic digestion process [8; 11]. terms of efficiency and cost [5; 37].