Anaerobic digestion is a process where bacteria break down organic material into basic compounds without oxygen. It has been present on Earth for 3.8 billion years and is used to produce methane gas for fuel and electricity. Natural gas is also generated through this process, and industrial biomass reactors produce a lower volume percentage of methane gas. The process requires strictly controlled conditions, and temperature is a major concern. The four stages of anaerobic digestion are hydrolysis, fermentation, acetogenesis, and methanogenesis.
Anaerobic digestion is a biological process in which bacteria break down organic material into more basic compounds without requiring oxygen as a component of the process. These bacteria are believed to have appeared on Earth around 3,800,000,000 years ago and were the dominant life form on the planet before the emergence of plants. When plant life arose about 3,200,000,000 years ago, anaerobic digestion continued in natural environments where oxygen was absent such as swamps, wetlands, and in constantly waterlogged terrain such as lakes and rivers. The biological processes of anaerobic digestion require different types of bacteria to decompose organic matter in a series of four stages, including hydrolysis, fermentation, acetogenesis and methanogenesis.
As of 2011, the primary use for anaerobic digestion by human industry is the production of methane gas for fuel and electricity generation. This is done in waste treatment plants that process agricultural waste such as manure or municipal waste. The brewing industry also relies on anaerobic digestion to break down the organic byproducts of brewing into methane fuel that would otherwise have to be disposed of by municipal wastewater treatment systems.
The anaerobic digestion process in nature is also instrumental in generating a form of renewable energy known as natural gas. Although natural gas is a fossil fuel, it is made up of about 80% methane along with other related gases such as propane and butane, and is more easily generated from the earth than other fossil fuels such as oil. It is a fossil fuel that is often deposited alongside other fossil fuels such as coal and oil.
Industrial biomass reactors that process biomass waste such as manure to generate fuel generally produce a lower volume percentage of methane gas than is contained in natural gas. Typical production of a given volume of biogas from a digester is 50% to 80% methane with a significant amount of offgas in the form of 20% to 50% carbon dioxide. Other trace gases that have some commercial value such as hydrogen, nitrogen and oxygen are also generated in the process, and toxic gases are also generated that must be disposed of safely, including hydrogen sulphide and carbon monoxide.
The biological processes required for effective waste digestion can be complex and depend on strictly controlled conditions. Temperature is a major concern of the process as waste-destroying bacteria thrive best at different levels. Some of the bacteria are mesophilic, thriving at a moderate temperature of 98° Fahrenheit (36.7° Celsius), and some are thermophilic, thriving at a higher optimal temperature of 130° Fahrenheit (54.4° Celsius).
Conditions need to be modified for temperature, pH and other factors such as the water/solid ratio of the biomass mix and the carbon/nitrogen ratio as the organic material is also chemically degraded. The two main types of bacteria used in anaerobic digestion are acetogenic and methanogenic bacteria, and while they are used in tandem, each has unique living conditions in which they thrive. Acetogenic bacteria produce the chemical acetate during anaerobic digestion and methanogenic bacteria produce methane.
Biomass material is withdrawn through four stages for effective methane recovery. The hydrolysis step uses water to decompose solids or semi-solids into simpler compounds, then fermentation or acidogenesis is used to break down the carbohydrate chain structures into more basic compounds such as ammonia, hydrogen, and organic acids. Acetogenesis is then used as the third step of the process, in which acetogenic bacteria convert organic acids into acetic acid along with further by-products such as hydrogen and carbon dioxide. The final stage of methanogenesis uses methanogenic bacteria to combine these primary end products of acetate, hydrogen and carbon dioxide into methane, which can then be used as fuel.
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