Anaerobic digestion is a biological process by which organic matter is broken down to form a gas mixture known as biogas. This process is widely found in nature, taking place, for example, in moors or in the rumen of ruminants; the same principle is already widely used in wastewater treatment.

The resulting gas mixture consists of methane (50-70% vol) and carbon dioxide (25-50% vol), and small quantities of other gases as hydrogen, hydrogen sulphide, ammonia, etc.

The composition of the gas is essentially determined by the substrates, the fermentation process and the various technical designs of the plants.

The process by which biogas is formed can be divided into steps:

Hydrolysis:

In this phase, the complex compounds of the starting material (carbohydrates, proteins and fats) are broken down into simpler organic compounds such as amino acids, fatty acids, and monosaccharides in soluble form. The hydrolytic bacteria involved in this stage release enzymes that decompose the material by biochemical means. The intermediate products formed by this process are then broken down during acidogenesis by fermentative bacteria to form lower fatty acids (acetic, propionic and butyric acid) along with carbon dioxide and hydrogen. In addition, small quantities of lactic acid and alcohols are also formed.

Acetogenesis:

Starting from the substrates formed during the hydrolysis and acidification phase (volatile acids, such as propionate and butyrate, but also alcohols), acetogenic bacteria produce acetic acid, formic acid, carbon dioxide and hydrogen. Again, as with hydrolysis, two different mechanisms of action are distinguished, depending on whether the degradation occurs from long chain fatty acids (LCFA, long chain fatty acids) or short chain fatty acids (SCFA, short chain fatty acids, or VFA, volatile fatty acids).

An excessively high hydrogen content prevent the conversion of the intermediate products of acidogenesis, for energy-related reason; as a consequence, organic acids, such as propionic acid, isobutyric acid, isovaleric acid and hexanoic acid, accumulate and inhibit the formation of methane. For this, the acetogenic bacteria must co-exist in a close biotic community with hydrogen-consuming methanogenic archaea, which consume hydrogen together with carbon dioxide during the formation of methane, thus ensuring an acceptable environment for acetogenic bacteria.

Methanogenesis:

Methanogenesis is the final stage of biogas generation. Methane is the only non-reactive compound in anaerobic digestion process, as carbon dioxide can react with hydrogen in the acetogenesis phase.

Methanogenesis take place in two stage:

  • Hydrogenotrophic bacteria act on anaerobic oxidation of hydrogen;
  • Anaerobic dismutation of acetic acid with methane and carbon dioxide formation.

The most of methane production occurs through this second phase (so-called acetoclastic)

With their activity, methane creating bacteria carry two important functions within the anaerobic trophic chain:

  • Degrade acetic and formic acid to CH4 by eliminating acids from the substrate and thus preventing the degradation phenomena of organic substrates by excess acidity;
  • Maintain H2 concentration at low levels, allow the conversion of LCFA and alcohols into acetate and hydrogen. If the hydrogenotrophic phase is slowed, there is an accumulation of hydrogen in the substrate that inhibits methane production, while the acetoclastic phase may undergo substrate-inhibition phenomena in the presence of high concentrations of acetic acid.

The process can be summarized as follows:

Essentially, the four phases of anaerobic degradation take place simultaneously in a single-stage process. However, as the bacteria involved in the various phases of degradation have different requirements in terms of habitat (pH value, or temperature), a compromise has be found in the process technology.

Biogas production process takes place in “digesters”, where the biomass introduced (the substrate) is broken down in percentages ranging from 40% up to 60%.

 

Biogas generated by the digestion process is composed:

 

 

Chemical Components Average values
Methane 45-75 %
Carbon dioxide 25-55 %
Water vapor 3,1 %
Nitrogen 1 %
Oxygen 0,3 %
Hydrogen < 1 %
Ammonia traces

 

 

 

Biogas is recognised worldwide as renewable energy and has a good calorific value (20.000 to 25.000 J/M3)

Biogas could be burned in an engine called co-generator, to produce electricity and heat.

With a m³ of biogas it is possible to produce:

  • about 1.8-2.2 kWh of electricity
  • about 2-3 kWh of thermal energy