Anaerobic Digestion (AD) converts 30-60% of the digestible solids in the feedstock into biogas by bacterial fermentation. The biogas is then burned to generate renewable energy. When used for heat only, the biogas is burned in a modified gas boiler to provide heat energy to heat the digester and for export. Alternatively, the biogas can be used to fuel engines of vehicles and other machinery. When biogas is used as a fuel for a combined heat and power (CHP) unit, electricity and heat are produced. The heat from the CHP unit can be used to maintain digester temperature and supply heat energy for export.
The products of AD are:
- Biogas - mixture of methane (55-80%), carbon dioxide (20-40%), hydrogen sulphide (0-3%) plus ammonia and nitrogen (0-5%). Calorific value is 17-25 MJ/m3 (21.4 MJ/m3 at 60% methane content). The biogas needs “cleaning” to remove hydrogen sulphide and water prior to use as a vehicle fuel.
- Digestate – The pollution potential (BOD) of digestate is considerably less than the feedstock (60 to >80% reduction), though it is still very high. Most of the nitrogen (N), phosphorus (P), and potassium (K) present in the feedstock remain in the digestate.
Digestate can be applied to land as an organic fertiliser, or can be mechanically separated to produce:
- Fibre – high in organic matter and plant nutrients. Fibre can be used as a nutrient-rich soil conditioner or further processed to produce granular organic fertiliser or a combustible fuel.
- Separated liquid – contains plant nutrients and can be applied to land as an organic fertiliser.
The quantity of biogas produced depends on type of feedstock, temperature of the digester and the hydraulic retention time (HRT) of feedstock in the digester. In temperate climates heating of the digester is normally required.
There are two main types of AD system:
- Mesophilic: The digester is heated to 25-35oC. HRT of feedstock is typically 15-30 days. Most on-farm biogas plants are mesophilic.
- Thermophilic: The digester is heated to 49-60oC. HRT of feedstock is typically 12-14 days.
Compared with mesophilic digestion, thermophilic digestion produces more biogas per unit time, requires smaller digestion tanks for a given volume of feedstock and gives better pathogen ‘kill’. However, thermophilic digestion is less robust and less tolerant than mesophilic digestion, requires more expensive technology, greater energy input and a higher degree of operation and monitoring.