Most of the organic material with high dry matter content (>30% DM) is usually treated in an aerobic way like composting. Regarding process energy balance in this biological treatment at
least mechanical power has to be added for periodic turning of the substrates. Also it is impossible to use the energy accruing during the process.
Treating such substrates in conventional liquid biogas plants, high volumes of water are necessary, which then in most cases remain as wastewater. In relation to the quantity of input material, high plant and process energy costs for material transportation and aeration arise.
The development of the new 3A-biogas batch-process for solid state biowaste can reach the best available synergies of composting and fermentation technology. It combines biogas- and compost- production including sanitation of the compost.
The 3A-biogas process is a combined composting and digestion process which is divided into three phases. The first phase is aerobic, the second is anaerobic and the third is aerobic again. These changes between aerobic and anaerobic conditions gave the 3A process its name.
In the 1st phase the input material is ventilated, so the substrate is aerated and the aerobic microbiological activity causes an increase in temperature. The temperature is controlled by regulating the ventilation. During the 1st phase the output consists of carbon dioxide and water. Lightly degradable substances are reduced. The 1st phase lasts up to 3 days depending on the input substrate.
It reaches temperatures up to 50 °C.
The 1st phase has the following effects on the substrate:
- heating of the substrate for the 2nd phase without additional energy
- Reduction of lightly degradable substances in order to decrease formation of acids
The 2nd phase of the 3A-biogas process is carried out under anaerobic conditions, so the methane production can start. There are two possibilities for the temperature management. Usually the reactor temperature is regulated under mesophilic conditions (35 to 45 °C). Within this temperature range methane forming microorganisms can be described as active. The microorganisms are quite resistant against changes of acidity within the mesophilic temperature range. Under thermophilic conditions (45 to 55 °C), the activity of microorganisms is higher but they will also react more sensitively to changes of acidity. In the 2nd phase the digestion takes place and biogas is produced. A reduction of the input substrate volume proceeds parallel to this effect. This step of the 3A-Process lasts from 21 up to 35 days depending on the input substrate.
The 3rd phase of the 3A-biogas process starts with aeration of the substrate for the second time. No percolation water is added to the substrate anymore. Because of the aerobic conditions the temperature increases again up to 60 °C. During the first 1 to 3 days the substrate is stabilised and becomes quite inodorous. The 3rd phase ends after about 5 days depending on the material and the requirements for maturity and biological characteristics of the output compost.
To reach compost with a further stage of maturity for special applications, the output material of the 3A-biogas process can be composted in a conventional way outside the fermentation reactors. This composting process could also be done inside the reactor but reduces the material throughput.