Basic Information about Anaerobic Digestion
How does AD work?
Anaerobic digestion is the process by which microorganisms break down organic materials in the absence of oxygen. In this instance, “organic” means coming from or made of plants or animals. Anaerobic digestion happens in spaces where there is no air (or oxygen) and can occur naturally in wetlands, rice fields, and landfills, as well as in enclosed, controlled environments. The initials “AD” are used to refer to the process of anaerobic digestion, and the built system where anaerobic digestion takes place is called a digester.
The following materials are generally considered “organic.” These materials can be processed in a digester:
- Animal manures.
- Food scraps.
- Fats, oils, and greases.
- Industrial organic residuals.
- Wastewater solids (sewage sludge).
All anaerobic digestion systems adhere to the same basic principles whether the feedstock is food waste, animal manures or wastewater solids. The systems may have some differences in design but the process is basically the same.
Learn more about how anaerobic digestion works.
What is made during the AD process?
Biogas is generated during anaerobic digestion when microorganisms break down (eat) organic materials in the absence of air (or oxygen). Biogas is mostly methane (CH4) and carbon dioxide (CO2), with very small amounts of water vapor and other gases. The carbon dioxide and other gases can be removed, leaving only the methane. Methane is the primary component of natural gas.
The material that is left after anaerobic digestion is called “digestate.” Digestate is a wet mixture that is usually separated into solid and liquid portions. Digestate is rich in nutrients and can be used as fertilizer for crops. When anaerobic digestion takes place at a water resource recovery facility, the digestate is called "biosolids."
How are the products of AD used?
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Power engines, produce mechanical power, heat and/or electricity (including combined heat and power systems).
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Fuel boilers and furnaces, heating digesters and other spaces.
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Run alternative-fuel vehicles.
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Supply homes and business through the natural gas pipeline.
Low quality biogas is typically used in tougher, less efficient engines, such as internal combustion engines. Higher quality biogas cleaned of trace contaminants can be used in more efficient, but also more sensitive engines.
Biogas treated to meet pipeline quality standards can be distributed through the natural gas pipeline and used in homes and businesses. Biogas can also be cleaned and upgraded to produce compressed natural gas or liquefied natural gas. CNG and LNG can be used to fuel cars and trucks.
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Bedding for livestock.
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Flowerpots.
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Soil amendments.
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Fertilizers.