The apparatus or implement or even the place where Biodigestion occurs, is called Biodigester.

In addition to the countless types of natural biodigesters, like soil, stagnant waters and rivers, we have those biodigesters developed and implanted by man, with distinct finalities, whose objective is to accelerate the process of Biodigestion.

Tipical examples are the biodigesters used to produce antibiotics in pharmaceutic laboratories, the vats for the fermentation of sugar cane juice to produce brandy and alcohol, the stone vats for the fermentation of grape must in the production of wines, the septic tanks for sanitation of human feces, and many others.

The biodigesters can be classified into one of three major generic categories depending upon its use:

1 - Industrial biodigesters
2 - Urban biodigesters
3 - Agricultural biodigesters

Here we will speak only about agricultural biodigesters, whose purpose is not only de decomposition of organic matter, but also the production of a product with a high concentration of nutrients for vegetables, wich is used as a fertilizer, and is called Biofertilizer.

This isn't the place to tell the history of biodigesters, as many of its facets are well known, but it is interesting to provide an overview of the evolution of those which are currently in use.

When man evolved to the stage of needing to plant vegetables for his subsistence, he slowly noted that these vegetables developed better, when planted in soils that had or appeared to have signs of residues of organic matter in a state of putrefaction or even completly putrefied.

From these observations, the man developed the idea of digging pits (or cesspools) to deposit those decomposed matters, (the dung-hills or manure-heaps), constituted not only by vegetable residues, but also by animal ones, and even human residues.

These residues fermented, and produced a dark product (the compost), which when added to soil produced better crops.

In handling the cesspools, it became apparent that it was more practical to make them impermeable, and, as they produced bad odors they found that it was better to cover them.

Soon, it became apparent that from those enclosed pits, emanated combustible gases, that eventualy could burn, producing a blue flame, similar to that of of the fatuous fire of the swamps and cemeteries.

We know today that those gases are essentially composed of methane (CH4), carbonic gas or carbon dioxide (CO2), and sulfide gas or sulfide acid (Hydrogen Sulfide - H2S).

It has been verified too, that on the impermeable bottom of dung-hills a viscous and concentrated liquid was accumulated, and when this liquid was sprayed over soil, it "burned" plants.

Through trial and error, experiments were performed in using diluted forms of this liquid before spreading it over soil.

And the result was very good, as plants watered with the diluted liquid developed very well, strong and healthy.

Today we know that this liquid is the result of an Anaerobic Biodigestion, which has been named Biofertilizer.

Keeping stride with the scientific developments, including the understanding of the of different phases of Biodigestion, biodigesters evolved to incorporate this new knowledge.

Now, armed with this knowledge, it is possible to plan and build Biodigesters to fulfill our various needs.

Within a biodigester, the biodigestion of organic matter is processed in various phases.   The principal ones can be summarized as follows:

1 - ENZYMATIC REACTIONS - Assisted by mechanical actions, enzymes act as catalysts in those reactions that transform complex organic compounds, changing them into simpler ones, hydrolyzing them, thus facilitating the action of acids and bacteria, making digestion more simple and easy, as it happens within gastric system of animals.

It is useful to recall here, that the food digestion process in animal, including man, is a biodigestion, as nearly all of it is processed by bacteria.

One of the most important reactions in this phase, is the transformation of amylaceous compounds (ex.: starch) present in organic matter, into sugars.

2 - ACTION OF OXYGEN AND BACTERIA - In this phase, organic matter (partially digested and partially not digested), yet treated by enzymes, is expoosed to the accion of Oxygen, aerobic bacteria and aerobic/anaerobic bacteria, proceeding from animal manure, resulting in the formation of water soluble composts with single chains.

3 - THE BACTERIAL ATTACK - In this phase, partially decomposed organic matter, undergoes the action of alcoholic leaven, and all sugars proceeding from amylaceous compounds during the first phase, are transformed into alcohols.

Through bacterial action, these alcohols are transformed into acetic acid.   This acid, reacts with various compounds which are present, and form acetates.

This phase is still predominantly an aerobic one, as it occurs in the presence of Oxygen from the air.

4 - THE MIXED ACID PHASE - During this phase, we have an essetialy bacterian work, with aerobic and some anaerobic bacteria, in an environment that changes periodically from acid to neutral and from neutral to acid, and is one of the most critical during the process.

As the culture medium, gradually becomes neutral, it is favorable to the action and total predominance of anaerobe bacteria, like methanogenic bacteria, still in an acid ambient.

The continuous action of anaerobe bacteria, neutralize the culture medium.

As the ambient changes completely to neutral, it receives a new charge of biomass from the former phase, and becomes acid again, becoming once more favorable to the action of aerobic bacteria.

This new charge of biomass, is very little, compared to the total biomass in this phase, and the aerobic phase here, is very rapid.

The part of the biomass that goes to the next phase, will begin a total anaerobic bacterial phase.

5 - THE NEUTRAL ANAEROBIC PHASE - The principal characteristic of this phase, is the complete predominance of anaerobic bacteria, as the medium conditions are more favorable to them than to the aerobic or aerobic/anaerobic bacteria.

Through the action of anaerobe bacteria, our culture medium is converted into a biomass essentially composed of Humus coloidal solutions, and ionized molecular solutions.

In this phase, the biomass becomes completly neutral, and its "DBO" (Demand of Biological Oxygen) is minimum (generally zero).

6 - OXYGENATION OR AERATION PHASE - This is the last phase of the process, and with it, we reduce or eliminate the "DCO" (Demand of Chemical Oxygen).

Beyond the fact that it  disciplinates or puts in order and accelerates Biodigestion, the biodigester also stores the gases proceeding from it, wich can be used to generate thermal energy, and even for bottling for future uses.

An agricultural biodigester, must be designed as to constitute a series of interconnected chambers, in such a way that inside each chamber, one individual phase of biodigestion can take place independently, as we can see in the scheme below.

We can describe the functions of each chamber, as follows:

1 - RECHARGE CHAMBER - In this chamber organic matter is loaded, shredded and mixed with additional water as needed.

In this chamber the mass of organic matter is aerated and undergoes phisical and chemical changes, catalyzed by enzymes, carried out by bacteria.

The most important rection in this chamber, is the transformation of amylaceous compounds into sugar.

Generally, this chamber can retain organic matter during 5 days. 

2 - AEROBIC TREATMENT CHAMBER - Here continues the process of aeration of organic matter, and through the action of alcoholic leaven, in a process of aerobe acid fermentation, all sugars are transformed into alcohols, and then into acetic acid.   This  acid reacts with other compounds, forming acetates, and at the same time, "DCO" (Demand of Chemical Oxygen) is drastically reduced.

This chamber is generally designed for the whole mass to be retained here for a period of 15 days.

3 - AEROBIC/ANAEROBIC TREATMENT CHAMBER - In this chamber we have the action of aerobic and aerobic/anaerobic bacteria, in an acidic environment, wich beguins shifting into neutral anaerobic digestion.

"DBO" (Demand of Biological Oxygen) beguins lowering, and we have the initial formation of Biogas and Humus.

This chamber is generally designed for the whole mass to be retained here for a duration of 15 days.

4 - ANAEROBIC TREATMENT CHAMBER - In this chamber, anaerobic digestion is completely dominant, and we have a high level production of Biofertilizer, Humus and Biogas.

The biomass is retained here for at least 15 days.

5 - ANAEROBIC TREATMENT CHAMBER - This chamber ir identical to the previous one, and anaerobe digestion continues in it.

6 - AERATION TANK - In this tank, the biomass is subjected to an intensive aeration, to encourage the action of nitrifying bacteria.   At the same time, "DCO" (Demand of Chemical Oxygen) is reduced to zero.

7 - PURE BIOFERTILIZER STORAGE TANK - In this tank we store the finished Biofertilizer.   It needs to have a capacity for about 30 days' production of the biodigester.   It is connected to the mineralization tanks.

8 - MINERALIZATION TANK - They are generally designed with a capacity of 10 m³.   Here, when necessary, we correct the chemical composition of Biofertilizer.

9 - MINERALIZATION TANK - This tank is the same as the previous one, and has the same function.


A Biodigester is a complex equipment, and to build it, we need a well designed project, wich requires the participation of an engineer, and must be carried out by professionals with a high degree of experience in this area.

So, it is recommended, if not necessary, that those who want to build such an equipment, consult with professionals with training in this area.

An Agricultural Biodigester is a piece of equipment like a tractor, a harvesting machine or an irrigation equipment.

It requires a trained operator and ongoing maintenance, and for that, we need people who have been trained by qualified professionals.