Anaerobic digestion is a biological process by which, in the absence of oxygen, the organic substance contained in materials of plant and animal origin is transformed into biogas, consisting mainly of methane (CH4) and carbon dioxide (CO2).
The percentage of methane varies, depending on the type of digested organic substance and the process conditions, from a minimum of 50 to about 80%.
The anaerobic microorganisms that carry out this transformation exhibit low growth rates and low reaction rates; hence the need to maintain, as far as possible, optimal conditions of the reaction environment to promote its metabolism.
Anaerobic digestion can be carried out in mesophilic conditions (temperatures of about 35 ° C), thermophilic conditions (about 55° C) or, more rarely, in cold conditions (psychrophilic digestion). The reaction temperature also generally determines the duration of the process (residence or retention time).
The times are on average between 15 and 50 days if the process occurs in mesophilia, between 14 and 16 if it occurs in thermophilia and 60-120 days in psychrophilia. Anaerobic digestion is a very complex process operated by different groups of bacteria acting in series. The transformation takes place with a sequence of successive phases which, to a small extent, tend to overlap. The first two phases can be considered preparation and only in the third phase there is production of biogas. More specifically, in the first phase, the hydrolytic bacteria “break up” the complex organic compounds (ie carbohydrates, proteins and fats) into simpler substances (hydrolysis phase). In the second phase these substances are transformed into
a first stage, in organic acids through acidogenesis reactions and, subsequently, in acetate (COOH-CH3), carbon dioxide (CO2) and hydrogen (H2), through acetogenesis processes (fermentation phase). In the last phase, the most delicate one, the methanogenic bacteria transform the products formed in the previous phase into methane (CH4) and carbon dioxide, the main constituents of biogas (methanogenesis) The organic substance is then degraded releasing biogas, the energy vector of the process, to a variable extent from 30 to 85%. Low levels of biogas yields can be attributed to several factors: low temperatures; retention times too short for a given temperature; incorrect hydrodynamic management of the reactor (dead zones); significant presence of antibiotic substances.
The biogas yield also depends on the type of biomass used. The next chapter reports an extensive review of organic matrices and the related functional characteristics to anaerobic digestion (AD). In the first instance, the biogas yield and the percentage of methane contained in it are indicated, correlated to the organic composition of the starting materials. The greater methanogenic capacity is attributable to fats (≈ 0.85 m3 / kg), followed by proteins (≈ 0.5 m3 / kg) and finally by carbohydrates (≈ 0.4 m3 / kg).
