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Anaerobic digestion process

Biogas and biofertilizer production from agricultural waste is based on the quality of organic substances to produce biogas when decomposing in anaerobic conditions, i.e. without air. This process is called methane fermentation and can be divided into three steps as a result of decomposition of organic substances by two main groups of microorganisms – acidic and methane bacteria.

Three steps of biogas production
The whole biogas-process can be divided into three steps: hydrolysis, acidification, and methane formation. Many microorganisms take part in this complex transformation with the main role given to 3 types of methane-producing bacteria.

In the first step (hydrolysis), the organic matter is enzymolyzed externally by extracellular
enzymes (cellulose, amylase, protease and lipase) of microorganisms. Bacteria decompose
the long chains of the complex carbohydrates, proteins and lipids into shorter parts. For
example, polysaccharides are converted into monosaccharides. Proteins are split into
peptides and amino acids.

Acid-producing bacteria, involved in the second step, convert the intermediates of fermenting
bacteria – complex organic compositions (proteins, fats and carbohydrates) into more simple compounds. At the same time in fermentation environment there appear primary products of fermentation – volatile organic acids, alcohols, amino acids, carbon dioxide, and hydrogen sulphide. These organic substances act as nutrients for methane-producing bacteria that convert organic acids into biogas.

Methane formation
Methane-producing bacteria, involved in the third step, decompose compounds with
a low molecular weight. For example, they utilize hydrogen, carbon dioxide and acetic acid to form methane and carbon dioxide. Under natural conditions, methane producing microorganisms occur to the extent that anaerobic conditions are provided, e.g. under water (for example in marine sediments), in ruminant stomachs and in marshes. They are obligatory anaerobic and very sensitive to environmental changes therefore from the condition created for them depends how intensively they produce gas.

Symbiosis of bacteria
Methane- and acid-producing bacteria act in a symbiotical way. On the one hand, acid producing bacteria create an atmosphere with ideal parameters for methane-producing bacteria (anaerobic conditions, compounds with a low molecular weight). On the other hand, methane-producing microorganisms use the intermediates of the acid-producing bacteria. If not for this symbiosis the environment inside digester would be unsuitable for both types of microorganisms.

Acid- and methane-producing bacteria can be easily found in natural conditions, particularly in animal manure. For example in cattle digestive system there is a full spectrum of microorganisms necessary for fermentation of manure and the process of fermentation starts in cattle bowels.
That is why cattle manure is often used as primary substrate which is loaded into new digester where for the fermentation to begin it is enough to create following conditions:
• maintenance of anaerobic condition in digester;
• maintenance of temperature regime;
• availability of nutrients for bacteria;
• correct choice of digestion time;
• timely load and unload of substrate;
• observance of appropriate C/N ratio;
• correctly chosen proportion of solids content and proper agitation;
• absence of inhibitors of the digestion process.
These parameters influence every type of bacteria that take part in the three stages of methane production very differently. There is also an interdependence between some parameters (for example, choice of digestion time depends n the temperature regime used) and therefore it is difficult to determine exactly the influence of each factor on the volume of biogas produced.