## 1 Introduction
The Environmental Protection Agency (EPA) [16] defines a hierarchy in FW management: (a) Source reduction, (b) feed hungry people, (c) feed animals, (d) industrial uses, (e) composting, and (f) incineration or landfilling. In the context of the EU, already in 1995 the Council Directive 1999/31/EC of 26 April 1999 on the landfill of waste [17] was launched, with the objective for 2016: \"_not later than 15 years after the date laid down in Article 18(1), biodegradable municipal waste going to landfills must be reduced to 35% of the total amount (by weight) of biodegradable municipal waste produced in 1995 or the later year before 1995 for which standardised Eurostat data is available_\". As a follow-up on the Council Directive 1999/33/CE [17], the DIRECTIVE 2008/98/EC OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL, of 19 November 2008, on waste and repealing certain Directives [18] has the scope of establishing \"_measures to protect the environment and human health by preventing or reducing the adverse impacts of the generation and management of waste and by reducing overall impacts of resource use and improving the efficiency of such use_\". According to this, a waste hierarchy has been established in waste management: (a) prevention; (b) preparing for re-use;(c) recycling; (d) other recovery, e.g., energy recovery; and (e) disposal, as shown in Figure 1. This hierarchy must be followed, both by consumers (a, b) and by waste managers (c, d and e) [18,19]. Some remarkable objectives for 2020, set in this directive, are:  
_\"by 2020, the preparing for re-use and the recycling of waste materials [... ], shall be increased to a minimum of overall 50% by weight\"_. _\"by 2020, the preparing for re-use, recycling and other material recovery, [... ] shall be increased to a minimum of 70% by weight\"_. Due to the above, the main scope around those legal instruments is to progressively change the traditional point of view from residues as \"waste to eliminate\", to perceive them as a new resource for other uses, including industrial ones. FW can be utilized for production of highly valued materials such as biodegradable plastics, enzymes, and organic acids [20]. It can be also used for developing value-added bioproducts [21], such as methane [22; 23; 24], biopolymers and organic acids [25; 26; 27], ethanol [22; 28; 29], and hydrogen [30; 31]. Energy and fuel applications are the ones creating more added value, along with animal feed and electricity generation [32], to be composed of ethanol, hydrogen, methane, and biodiesel. Anaerobic Digestion (AD) is a process enabling the creation of biogas (methane-rich gas) via microbiological digestion of the organic fraction of residues in anaerobic conditions (FW in this case). The result of the anaerobic digestion, known as digestate, can be treated as a residue (with a smaller volume), or as a by-product to be used as an agricultural fertilizer. AD process represents a way of recycling, recovery (in form of energy and industrial use as fertilizer), and a reduction system of landfill disposal; partial if digestate is eliminated, or complete if digestate is re-used [4; 33]. In addition to prevention and the preparation for reuse, obligations for generators and citizens, AD is a process that accomplishes all of the steps of the European Waste Hierarchy, previously shown in Figure 1.  
In light of this, AD is one of the most remarkable and adequate solutions for recovering residues. This paper is intended to review in general terms the recovery of FW and in particular, the AD process as a biofuel or energetic recovery in form of biogas, the anaerobic process, its characteristics, and factors implicated in biogas production. Furthermore, experiences with anaerobic digestion and co-digestion are reviewed, along with pre-treatments to enhance biogas production.