Biodigesters must be designed according to their purpose, the availability of livestock and type, and the temperature at which they will work. A biodigester can be designed to eliminate all the manure produced on a pig farm, or as basic sanitation tools at a school. Another objective would be to provide a family with five hours of combustion in a kitchen, for which we already know that 20 kilos of fresh manure are required daily. As mentioned previously, the liquid fertilizer obtained is very precious, and a biodigester designed for this purpose must allow the raw material to spend a longer time inside the hermetic chamber as well as reduce the mixture with water to 1:3.

The ambient temperature at which the biodigester will work indicates the retention time necessary for the bacteria to digest the matter. In environments of 30 °C, about 10 days are required, at 20 °C about 25, and in the highlands, with a greenhouse, the working temperature is about 10 °C on average, and 55 days of retention time are required. This is why, for the same amount of incoming raw material, a volume five times greater is required for the hermetic chamber in the highlands than in the tropics.

Batch flow biodigesters

The loading of all the material to be fermented is done at the beginning of the process and the discharge of the effluent is done at the end of the process; They generally require more labor and a space to store the raw material if it is produced continuously and a gas tank (Due to the great variation in the amount of gas produced during the process, having its peak in the middle phase of the process) or alternative sources to supply it.

Semi-continuous flow biodigesters

The loading of the material to be fermented and the discharge of the effluent is carried out continuously or in small bursts (e.g. once a day, every 12 hours) during the process, which extends indefinitely over time; They generally require less labor, but a more fluid or mechanically mobilized mixture and a gas tank (if it is not used in its entirety continuously). Semi-continuous biodigesters serve to purify water contaminated by different pits. There are three types of semi-continuous flow biodigesters:

Continuous flow biodigesters

They are generally used for wastewater treatment, they tend to be large industrial in nature, with commercial systems to control and manage the process. Biogas production is much greater. They can be:

Low-cost family biodigesters have been developed and are widely implemented in Southeast Asian countries, but in South America, only countries such as Cuba, Colombia, Brazil and Costa Rica have developed this technology. These models of family biodigesters, built from tubular polyethylene sleeves, are characterized by their low cost, easy installation and maintenance, as well as requiring only local materials for their construction. For this reason they are considered an "appropriate technology."

The tubular polyethylene film is tied at its ends to conduit pipes, about six inches in diameter, with rubber bands recycled from car tire tubes. With this system, by conveniently calculating the inclination of said pipes, a hermetic tank is obtained. Since tubular polyethylene is flexible, it is necessary to build a 'cradle' to house it, either by digging a trench or building two parallel walls.

One of the pipes will serve as raw material input (mixture of manure with water 1:4). In the biodigester, a hydraulic level balance is finally reached, by which, as much manure mixed with water is added, as much fertilizer comes out through the pipe at the other end.

Due to the absence of oxygen inside the hermetic chamber, the anaerobic bacteria contained in the manure itself begin to digest it. First, a hydrolysis and fermentation phase occurs, then acetogenesis and finally methanogenesis, by which methane is produced. The gaseous product called biogas actually has other gases in its composition such as carbon dioxide (20-40%), molecular nitrogen (2-3%) and hydrogen sulfide (0.5-2%), with methane being the most abundant with 60-80%.

The biogas conduction to the kitchen is done directly, keeping the entire system at the same pressure: between 8 and 13 cm of water column depending on the height and type of stove. This pressure is achieved by incorporating a safety valve built from a soda bottle into the pipeline. A 'tee' is included in the pipe, and while the gas line follows, the third end of the pipe is introduced into the water contained in the bottle from 8 to 13 cm. A biogas reservoir or warehouse is also added to the pipeline, allowing about 2 to 3 cubic meters of biogas to be stored.

These systems adapted for the highlands must be placed in buried 'cradles' to take advantage of the thermal inertia of the soil, or two thick adobe walls in case it is not possible to dig. Furthermore, the biodigesters can be enclosed in a single-water greenhouse supported on adobe side walls. In the case of biodigesters in the tropics or valley, the greenhouse is unnecessary but the plastic must be protected with semi-shade.

The material costs of a biodigester can vary from 110 dollars for the tropics to 170 dollars for the highlands, since at altitude they have larger dimensions and require a solar tent.

Biodigesters must be designed according to their purpose, the availability of livestock and type, and the temperature at which they will work. A biodigester can be designed to eliminate all the manure produced on a pig farm, or as basic sanitation tools at a school. Another objective would be to provide a family with five hours of combustion in a kitchen, for which we already know that 20 kilos of fresh manure are required daily. As mentioned previously, the liquid fertilizer obtained is very precious, and a biodigester designed for this purpose must allow the raw material to spend a longer time inside the hermetic chamber as well as reduce the mixture with water to 1:3.

The ambient temperature at which the biodigester will work indicates the retention time necessary for the bacteria to digest the matter. In environments of 30 °C, about 10 days are required, at 20 °C about 25, and in the highlands, with a greenhouse, the working temperature is about 10 °C on average, and 55 days of retention time are required. This is why, for the same amount of incoming raw material, a volume five times greater is required for the hermetic chamber in the highlands than in the tropics.