The chemical conversion processes include [3, 36]:

• anaerobic digestion;

• aerobic digestion;

• alcohol fermentation;

• extraction from oils and bio-diesel production.

2.1 Anaerobic digestion

The fermentation of methane, also known as anaerobic digestion, is a complex natural process that involves bio-degradation of the organic substance in the absence of oxygen (anaerobiosis), resulting in the formation of bio-gas.

As we can see from Fig. 2, the anaerobic digestion process comprises three sequential steps involving different bacterial groups that act in series. In the first step (hydrolysis), the hydrolytic bacterium breaks the complex organic compounds (carbohydrates, proteins and fats) into simpler substances. Subse­quently (in the fermentation step), these simpler substances are transformed, first, into organic acids, through acid-genesis reactions, and then into acetate, carbon dioxide and hydrogen, through vinegar-genesis processes. In the last step, the most important step (methane genesis), the methanogenic bacterium transforms the products that are formed in the previous step into methane (CH4) and carbon dioxide (CO2), the main constituents of bio-gas. Therefore, the organic component is degraded releasing the chemical energy it contains in the form of bio-gas.

As evident from the above description, bio-gas production depends on the coordinated and sequential action of all the microbic groups involved. To achieve this goal, it is essential that the reaction environment is the result of a compromise between the requirements of each individual group involved, by a strict control of the process parameters [2, 31]. The anaerobic digestion processes can be classified based on the mass fraction of the dry substance to be digested: if it is lower than 10%, the process is called wet digestion; if it is between 10% and 20%, the process is called semi-dry digestion, and for values higher than 20%, it is called dry digestion [31, 32].

Depending on the temperature range in which the process takes place, anaerobic digestion is called:

• psychrophilous: if the process temperature is kept below 20°C; the systems that work in such conditions are also called ‘cold’;

• mesophily: if the temperature is between 20°C and 40°C;

• thermophile: if the temperature of the process is between 50°C and 65°C.

The processes described above can be reproduced in confined environments such as an anaerobic methane digester (for the digestion of liquid manures with a high organic load) or in a controlled dump (for the digestion of the organic component of solid rejections) [2, 31, 32].