Anaerobic digestion is a biological process of organic substrate degradation in absence of free oxygen. The degradation is done by bacteria that obtain the oxygen necessary for their vital functions from biomass. There is no better technological solution in the absolute sense. Each plant has some peculiarities that must be considered in the feasibility study and design phases.
For these reasons, IES Biogas has developed more technologies and has the necessary know-how to identify the best configuration on the basis of project specificities.
At the technological level, there are three categories of anaerobic digestion:
- Wet digestion, characterized by a dry matter content in the digester of about 10%;
- Semi-dry digestion, characterized by a dry matter content in the digester of about 25% and at the limit of pumpability;
- Dry digestion (Dry digestion) characterized by a very high dry matter content that involves the possibility of biomass handling only with the use of mechanical tools (normally wheel loaders).
WET Process – Dry material approx. 10%
In wet processes, anaerobic digestion takes place in heated and mixed CSTR (Continuous Flow Stirred Tank Reactor) cylindrical tanks in an environment with a dry matter content of 10% or less. The process can be conducted under thermophilic or mesophilic conditions and retention times usually range from 15 to 30 days.
The low dry matter content, which characterizes this technology, requires an efficient waste pre-treatment system, which purpose is to remove plastics and sand. Otherwise, they would tend to accumulate on the surface of the digester on the bottom.
Each digester is equipped with a mixing system that guarantees perfect homogenization of the digester and heat distribution.
In a wet anaerobic digestion plant the following functional areas are identified:
- Mechanical pre-treatment for plastic separation and biomass size reduction;
- De-sanding for the removal of sand, glass fragments and sedimentable materials
- Homogenization pre-tank
- Anaerobic digester CSTR
- Dehydration of digestate
SEMI-DRY Process – Dry material between 20 and 40%
In semi-dry processes anaerobic digestion takes place in an environment featured by a dry matter content between 20 and 40%, under thermophilic conditions and with retention times of about 15-20 days.
These systems are known as plug-flow digesters: the advancement of biomass is only related to the input of fresh biomass and the extraction of exhausted digestate.
Unlike common opinion, these digesters are not mixed, but are equipped with a digester movement system, whose only purpose is to allow the biogas to rise and the biomass to heat up.
The high viscosity of the digestant keeps plastics and sand in suspension: the pre-treatment system associated with this type of plants is, therefore, less complex than that required on wet plants.
The outgoing digestate has a dry matter generally higher than 20% and lends itself well to be mixed as it is, with structuring materials to be sent directly to the aerobic composting treatment.