2.1 Anaerobic digestion historical

The use of anaerobic digestion for the treatment and the stabilization of solid waste is not new. It had been used in the 19th century. In rural parts of China and India, simple reactor constructions were used a long time ago to treat the manure and agricultural wastes in order to recover energy for cooking and lighting (Gijzen, 2002). In 1860s in France (McCarty, 2001), the anaerobic digestion of sludge waste was obtained from wastewater treatment plant, on a large scale, by means of an advanced technology. Furthermore, at the end of 1980s, co­digestion processes treating a mixture of different types of waste, were introduced (Ahring, 2003). Today, anaerobic digestion is one of the most environmentally friendly and suitable treatment methods for of solid organic waste. This technology is widely applied for bio­energy production, because of the increasing request for renewable energy. A consequence of the increasing implementation of this technology is the necessity to determine the ultimate biogas potential for several solid substrates (Angelidaki & al., 20096).

2.2 Anaerobic digestion principle
use organic pollution (biodegradable organic matter) as substrate to produce biogas which can be exploited according to several forms. Thus, anaerobic digestion allows a reduction of the dry matter from approximately 50% (OTV, 1997) and the production of a biogas, mainly methane (55-70%) and carbon dioxide (25-40%), with traces of hydrogen and of H2S, (Mata — Alvares, 2003). Methane can be developed in the form of energy (boiler producing of heat or electricity). At the same time the anaerobic micro-organisms consume little energy, which involves a limited production of muds limited (3 to 20 times lower than an aerobic treatment), (Bitton, 1994). Indeed, the micro-organisms use only approximately 10 to 15 % of the energy of the substrate for their growth (Trably, 2002 and Moletta, 1993), the remaining being used for the production of biogas. Finally, anaerobic digestion allows a reduction of the pathogenic micro-organisms.

Anaerobic digestion consists of sludge fermentation, under strict anaerobic conditions. It is made up of four stages: hydrolysis, acidogenesis, acetogenesis and the methanogenesis. To achieve an anaerobic digestion, it is necessary that the reaction kinetics for the consumed or produced component is balanced. The general diagram of anaerobic digestion is presented on Figure 1 (Edeline, 1997).