Small biogas plants often consist of a horizontal steel digester. In the first digesters of this type, the tanks were recycled and cleaned, equipped with a central axis and mixing arms, and with a dome to collect the gas, and then re-used as fermenters.
Today the digesters are built specifically for the production of gas. They have a volume of 50-150m ³ and a width of 3.20-3.50m to allow for easy transportation. Final assembly takes place within the plant.
After being heated by a heat exchanger, the substrate is fed into the digester, where it remains 40-50 days. Once it reaches the temperature suitable for the mesophilic process, the digestated product is mixed with the internal mixer .
A horizontal digester is suitable for manure because it is best at mixing solid components. It is recommended for small farms that do not have large quantities to process.
Standard Digester for Agriculture
The most common standard digester is a reinforced concrete cylinder, 5-6 mt. tall, with a diameter of 10-20m and volume of 500-1500m³ .
This type of digester is equipped with a water heating system along the inner walls and one or more fully immersed mixers. A motor placed outside of the tank moves the digester. The upper part of the cylinder is covered by a double membrane; the inner accumulates gas and varies in height depending on the pressure exerted by the gas itself; the external membrane functions as shelter from the weather, and always maintains tension via a vent that blows air between the two membranes.
Depending on the type of substrate, storage time of the slurry ranges from 40-80 days. This type of tank is suitable to treat all kinds of biomass in moderate quantities. It may be used for a maximum total quantity of 10.000m ³ per year.
Vertical Digester Dimension
For large amounts of biomass, such as more than 30.000m³, vertical digesters in tempered steel are generally used, because steel is able to resist the corrosion by bacteria. The standard measures vary from 1.500-5.000 m³ per cylinder, with diameter from 10-18 meters, and height from 15-20 mt.
The continuous mixing of the substrate, which is heated before entering the tank, is carried out by a mixer mounted centrally on the cover. The substrate is generally stored only 20 days. This type of plant can process up to 90.000m³ of substrate in a year. To fully exploit the material, large plants often consist of two or more fermenters, and are equipped with covers to accumulate gas .