Between the considered as renewable energy sources it is also included the anaerobic digestion or methanization, a natural biologic process of organic matter degradation where, as metabolite of the process, gas methane is generated and from which combustion is possible to obtain energy. There are three potential advantages: it is a natural process (it does not need an energetic or material input to be produced plus than the organic matter), it degrades the organic wastes completing the matter cycle, and it generates a fuel gas (methane) useful to obtain energy in heat way. The equilibrium between the organic matter degradation efficiency by anaerobic digestion and the economical rentability of the produced energy is a polemic in the industrial application of this process.
The main defined characteristic of this biological process is its production only in anaerobic conditions (in absence of oxygen). For this reason, it is incorrect call it “anaerobic composting”. During composting aerobic micro organisms act, needing the presence of oxygen to develop its activity. By the opposite, the oxygen presence is lethal for the bacteria that produce methane.

The atmosphere of our planet has approximately a 21% of oxygen, so for methanization production is necessary that the medium will be isolated from the atmosphere. So, in Nature, we almost find it in marshes and marine funds rich in organic matter, liquid mediums where is difficult for the oxygen arrives and if it does it, it is quickly consumed. When we want reproduce this process in an artificial way, the first difficulty is the necessity to isolate it from the atmosphere. The recipient where we isolated from the atmosphere the organic matter that we want to methanize is denominated anaerobic digester.
The organic matter degradation by anaerobic digestion has several consecutive phases that integrate different groups of bacteria that during their activity go consuming the organic matter and generating different metabolites and new individuals. Some of the metabolites are used by the next step microorganisms, although others like CO and methane (CH4) are liberated to the medium constituting a biogas. Commonly there are three steps well defined in the process, but to simplify it is often referred as only two steps process. In total, there are four different bacteria trophic groups for obtaining biogas.

The first step is an organic matter hydrolysis, up to make it soluble in more simple components that can be absorbed by the microorganisms. In this process act hydrolytic bacteria, that break the molecules of carbohydrates, proteins, lipids, etc... and produce fat acids. The biologic activity rate of these microorganisms is high and tends to make the medium more acid and not being a limitation for them.

The second step is an acetogenesis where two groups of bacteria are implicated: the acetogenic ones, that mainly catabolize the fat acids generated in the hydrolysis and that produce hydrogen (H); and the homoacetogenic ones, that catabolize the CO and the H and hydrolyze polysaccharides to obtain acetic acid (CH3COOH), but also butyric acid, propionic acid and ethanol are produced.

The final step is the methanogenesis process that produces CO and CH4 from the degradation of the acetic acid and the reduction of CO by the H. The methanogenic bacteria belong to the most antique group (Archeas), this implies that, between other qualities, their CO fixation mechanism would be different than the Calvin cycle. Other important differential characteristic is that their cellular walls (between the capsule and the cellular membrane) have not muramic acid. For this reason, they present a high sensitivity to the presence of fat acids. This is a key aspect of the biology of these microorganisms and of all the methanization process.
The methanogenic bacteria groups do not support acid environments, so it is necessary an equilibrium between their biologic activity and the hydrolytic bacteria biologic activity, in such a way that the high activity of the last ones do not acidify the medium, because it could be lethal for the methanogenic ones. The methanogenic bacteria are classified in three groups: Hydrogenotrophics, Acetotrophics and Methylotrophics.

Depending on the organic matter composition that is being degraded and depending on the medium conditions, other kind of bacteria that use the metabolites of the methanization process are developed. It is the case of the sulphate differential bacteria, than generate sulphuric acid (HS), product that will be part of the biogas composition.

One of the most important factors of the process is the temperature, because it influences on the methanogenic bacteria's biological activity, and in consequence in the yield and quantity of generated CH4. There are three specific temperature intervals: psichrophylic (lower than 30ºC), mesophylic (between 35 and 37ºC) and thermophylic (between 50 and 55ºC). Depending on the research and technical literature that we consult, those margins could be more or less ample, but those are the values that are often used at an industrial scale, mainly those that belong to the mesophylic interval. The advantage of this in comparison with the others is that it allows the development of high variety of hydrolytic and methanogenic bacteria, increasing the efficiency, but their CH4 productivity is lower than in the case of the thermophylic bacteria. In certain manner, the lower energetic consume that suppose to maintain the medium at thermophylic temperatures compensates this diminution in the potential production of CH4.

To be a process where coexist and act in cascade three trophic groups of bacteria with different biological activity rate and whose medium optimum conditions are also different, it is essential to arise an equilibrium between the different populations to guarantee a yield in biogas production.
There is a sequence of factors or environmental parameters that limits the process, so its monitorization will be essential to be able to act over the medium and correct any disequilibrium of the adequate conditions.

The control ability of the process by ourselves is partially limited, because we will mainly act regulating the organic matter flux or caudal that we give to the process and, since certain point, its “quality”, that are the most influent parameters on the biological process. When the nutrition caudal is regulated to the digester the hydraulic retention time (HRT) is modified, this is not more that the quotient between the digester volume and the input. The HRT is a key factor in the sizing of any methanization facility, because it is calculated to obtain a retention time and, of course, of biological process that allows guarantee a minimum biogas production to promote energetically (it means, economically) the facility. The composition and characteristics of the organic matter to methanize must also be considered at the moment of calculate the HRT to obtain a reduction in the desired organic charge of the digester’s output material.

About the biogas, as we commented before, it is mainly composed by CO2 (25 – 45%) and CH4 (50 – 75%), presenting also other gases in lower proportion and that they depend on the degraded organic matter composition, like nitrogen (N2), sulphuric (H2S),... From those two gases, the CH4 is who allows a calorific energy production from its combustion, so the interesting focus normally is to generate a biogas as rich as possible in CH4, that it would be equivalent to the natural gas. But we also need to consider the H2S concentration that the biogas can have, because it is highly corrosive and it limits the possibilities of the biogas energetic use if it is not eliminated. When we talk about the richness or quality of the biogas, we are really talking about the percentage of CH4 that it has in its composition. The problem is that sometimes we assume erroneous concepts, because we do not consider key aspects of the anaerobic digestion process:

- we are talking about biological processes, this means, that very concrete groups of microorganisms are acting on the organic matter degradation. We need to control a biological process, not a common chemical reaction.

- The original kind of organic matter, by its origin, composition and characteristics, is going to directly affect the quality and composition of the biogas generated, especially its methane richness or concentration. We could not forget the importance of the different groups of organisms responsible of the process and the necessity to choose a system or technology in accordance with the kind of microorganism, characteristics and necessities of the material to be methanized.

- If the objective is to obtain a very high energetic production, the most important is not to have a big facility, if not that before doing nothing, we need to have an enough waste (or wastes) quantity that in its anaerobic degradation would produce an important biogas quantity (production) and that it would be rich in methane (productivity). The production is expressed as Nm³ of biogas and the productivity in Nm³ of biogas (or CH4) by ton of organic matter (or volatiles solids), but sometimes this value is expressed in dry matter (total solids).

Another important aspect of the methanization but not very often considered, is the final product , the digestate. At the final of the process, depending on the initial organic matter characteristics and composition, the HRT and the process conditions, we will obtain an organic matter more or less stabilized that it must be adequately managed to ensure that it does nor produce any kind of environmental affections. The anaerobic digestion, excepting if the HRTs are being very longs or if we are working with very easily degradable organic matter, is not the final treatment, so the digestate would need a posterior process, generally an aerobic one, to arise the stability, to guarantee its hygienization and to can be storaged without problems.