Biogas is a combustible mixture of gases mainly consisting of methane and carbon dioxide.
Anaerobic digestion is a microbiological process of decomposition of organic matter, in the absence of oxygen, largely applied today to produce biogas in air-proof reactor tanks named digesters. A wide range of micro-organisms are involved in the anaerobic process which has two main end products: biogas and digestate. Biogas is a combustible gas consisting of methane, carbon dioxide and small amounts of other gases. Digestate is the decomposed substrate, rich in nutrients and suitable to be used as plant fertilizer.
The biogas process
The biogas process requires the following to work:
- Anaerobic environment – an oxygen-free environment
- Methane bacteria – the collected waste
- Fixed temperature – 0 to 65 degrees
- Continuous supply of new manure – to keep the process going and ensuring an ongoing production of biogas (as new material is added, more biogas is produced).
The complete biological decomposition of organic matter to methane and carbon dioxide under oxygen-depleted condition (anaerobic) is a complicated process from an interaction between a number of different bacteria. What may be a waste product from some bacteria could be food for others, and in this way the bacteria are interdependent.
- Anaerobic environment
- Acidity (pH)
- Substrate (feedstock)
- Dry matter content
- Carbon/ nitrogen ratio
- Organic load
Reduction of GHG and mitigation of global warming
Biogas production reduces the emissions of methane, which has higher greenhouse gazes emission than carbon dioxide (CO2), and nitrous oxide by storing and using animal manure as fertilizer. The substitution of fossil fuels by biogas for energy production leads to a reduction of CO2, CH4 and N2O emissions which contributes to global warming mitigation.
One of the main advantages of biogas production is the ability to transform waste material into a valuable resource. Many European countries are facing enormous problems associated with overproduction of organic wastes from industry, agriculture and households. Biogas production is an excellent way to comply with increasingly restrictive national and European regulations and to utilize organic wastes for energy production, followed by recycling of the digested substrate as fertilizer. Anaerobic digestion can also contribute to reducing the volume of waste and costs for waste disposal.
Reduction of dependency on imported fossil fuels
Contribution to EU energy and environmental targets
Flexible and efficient end use of biogas
Low water input
Benefits for the farmers
Additional income for the farmers involved
Production of feedstock in combination with operation of biogas plants makes biogas technologies economically attractive for farmers, providing them with an additional income. The farmers also become important energy providers and waste treatment operators.
Digestate is an excellent fertiliser
A biogas plant is not only a supplier of energy. The digested substrate, usually named digestate, is a valuable soil fertiliser, rich in nitrogen, phosphorus, potassium and micronutrients, which can be applied on soils. Compared to raw animal manure, digestate has improved fertilizer efficiency due to higher homogeneity and nutrient availability.
Flexibility to use different livestocks
Various types of feedstock can be used for the production of biogas: animal manure and slurries, crop residues, organic wastes from dairy production, food industries and agro- industries, wastewater sludge, organic fraction of municipal solid wastes, organic wastes from households and from catering business as well as energy crops. Biogas can also be collected from landfill sites.
One main advantage of biogas production is the ability to use “wet biomass” types as feedstock, characterized by moisture content higher than 60–70% (e.g. sewage sludge, animal slurries, flotation sludge from food processing etc.). Energy crops as well as all kinds of agricultural residues (damaged crops, unsuitable for food or resulting from unfavourable growing and weather conditions) can be used to produce biogas and fertiliser. A number of animal by-products unsuitable for human consumption can also be processed in biogas plants.
Reduced odour and flies
Storage of liquid manure, animal dung and many organic wastes are sources of persistent, unpleasant odours and attract flies. The biogas process reduces these odours by up to 80%.