Anaerobic digestion

For many years the producing systems for the electrical energy based on the exploitation of the alternative energy sources is improved. The biogas is one of these energetic sources.

The anaerobic digestion is a complex biological system, through which, in the absence of the oxygen, the organic substance is transformed into biogas (or biological gas), constituted of methane and carbonic anhydride. The percent of methane varies depending on the type of the digested organic substance and the process conditions, from a minimum of 50% to approximately 80%.

For the process to take place, the action of some different microorganisms is needed, microorganisms which are capable to transform the organic substance into intermediary compounds, such as acetic acid, carbonic anhydride and hydrogen. These substances are used by the microorganisms that conclude the process, producing the methane. The anaerobic microorganisms have a relatively low growing speed and a low reaction speed, so it’s necessary for the environment conditions to be optimum. With all these abridgments the process times are relatively high, if we compare them with the ones from other biological processes; still, the advantage of this process is that the complex organic matter is transformed into methane and into carbonic anhydrides, resulting an alternative energy source, in the guise of a gas with a high caloric power.

The next types of bacteria participate to the process:

1) Bacteria obtained through the hydrolysis process, which decompose the biodegradable macromolecules into more simple substances;

2) Bacteria that produce acids, which use the simple organic compounds produced by the first type of bacteria as a substratum and produce short chain organic acids, which, at their time, represent the substratum for the next groups of bacteria;

3) Obligate Hydrogen Producing Acetones bacteria (OPHA), which use the products from the second type of bacteria as a substratum, giving birth to acetate, hydrogen and carbonic anhydrides;

4) Bacteria which synthesize acetate starting with the carbonic anhydrites and with the hydrogen;

5) Methane bacteria, differentiated into two groups;

a) The ones which produce methane by transforming the acetic acid, named acetoclastics.

b) The ones which produce methane starting with the hydrogen and with the carbonic anhydrides, named hydrogenotrophes. While the methane is completely liberated as a gas, the carbonic anhydride participates at the balance of the carbonates which exist into the reacting biomass. The interactions between different species of bacteria are very close and the products of some species can be used by other species as a substratum or as a growing factor.  

Into a context of continuous energetic necessity, the anaerobic digestion followed by the recuperation of the produced biogas proves out to be a high interest system, capable to offer multiple advantages:

1) Production of energy: the anaerobic treatment in controlled conditions leads to the degradation of the organic substance and then to the producing of biogas. The cogeneration of electrical energy and heat by burning the biogas proves out to be advantageous either for own consumption or for mass consumption.

2) The elimination of the smells and contaminant emissions (NH3 and CH4): bad smelling substances which eventually form during the process are set in motion along with the biogas, at its ignition.

3) The stabilization of the dejections: the removed organic matter which contains carbon obtained through the process of anaerobic digestion offers a high stability, even in the ulterior storing periods; there is a deceleration of the degrading and fermentation processes.