What is anaerobic digestion?
Anaerobic digestion is a process in which bacteria breaks down the organic matter in the absence of oxygen, such as animal manure, wastewater biosolids, and food wastes. Anaerobic digestion for biogas production occurs in a sealed vessel known as a reactor, which is designed and built in a variety of shapes and sizes based on the site and feedstock conditions. These reactors contain complex microbial communities that break down (or digest) waste to produce biogas and digestate (the solid and liquid material end-products of the AD process), which are discharged from the digester.
Co-digestion is the practice of combining multiple organic materials in a single digester. Manure, food waste (processing, distribution, and consumer generated materials), energy crops, crop residues, and fats, oils, and greases (FOG) from restaurant grease traps are all examples of co-digested materials. Co-digestion of low-yielding or difficult-to-digest organic waste can boost biogas production.
The term "anaerobic" literally means "without oxygen." The biogas produced naturally in the sealed tanks is used as a fuel in a CHP (combined heat and power) unit to generate renewable energy in the form of electricity and heat. What remains after the process is a nutrient-rich biofertilizer that is pasteurized to kill any pathogens and then stored in large covered tanks to be applied twice a year on farmland in place of fossil fuel-derived fertilizers.
One of the many advantages of anaerobic digestion is that every tons of food waste recycled as an alternative to landfill prevents between 0.5 and 1.0 tone of CO2 from entering the atmosphere.
Outputs of anaerobic digesters:
Biogas and digestate are two valuable byproducts of anaerobic digestion.
Biogas is primarily composed of methane (CH4), the primary component of natural gas (50 to 75 percent), carbon dioxide (CO2), hydrogen sulphide (H2S), water vapor, and trace amounts of other gases. Biogas energy can be used similarly to natural gas to provide heat, generate electricity, and power cooling systems, among other things.
Biogas can also be purified to generate renewable natural gas by removing inert or low-value constituents (CO2, water, H2S, etc).This can then be sold and injected into the natural gas distribution system, compressed and used as vehicle fuel, or further processed to make alternative transportation fuel, energy products, or other advanced biochemicals and bioproducts.
Digestate is the waste product of the digestion process. It is composed of both liquid and solid elements. These are frequently separated and handled separately because each has value that can be realized through varying degrees of post-processing.
Both the solid and liquid components of digestate can be used in a variety of beneficial applications, such as animal bedding (solids), nutrient-rich fertilizer (liquids and solids), a foundation material for bio-based products (e.g., bioplastics), organic-rich compost (solids), and/or simply as soil amendment (solids), the latter of which could include the farm spreading the digestate on the field as fertilizer. Digestate products can generate revenue or save money, and they are frequently pursued to increase the financial and net-environmental benefit of an AD/biogas project.
Hydrolysis, acidogenesis, acetogenesis, and methanogenesis are the four key stages of anaerobic digestion. The overall process can be described by a chemical reaction in which organic material such as glucose is biochemically digested by anaerobic microorganisms into carbon dioxide (CO2) and methane (CH4).
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