What Is Anaerobic Digestion?

Anaerobic digestion is a biological process whereby 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 approximately 3,800,000,000 years ago and were the dominant form of life on the planet before plants appeared. As plant life arose around 3,200,000,000 years ago, anaerobic digestion continued in natural environments where oxygen was absent such as swamps, water-logged soils, and in ground constantly covered by water such as lakes and rivers. The biological processes of anaerobic digestion require that several types of bacteria decompose organic matter in a series of four steps, including hydrolysis, fermentation, acetogenesis, and methanogenesis.

As of 2011, the main use for anaerobic digestion by human industry is to produce methane gas for fuel and electricity generation. This is done in waste treatment facilities that process agricultural waste such as manure or municipal waste. The brewing industry also relies on anaerobic digestion to break down organic byproducts of beer production into methane fuel which would otherwise have to be disposed of by municipal waste water treatment systems.

The process of anaerobic digestion in nature is also instrumental in generating a form of renewable energy known as natural gas. Though natural gas is a fossil fuel, it consists of about 80% methane along with other related gasses such as propane and butane, and is more readily generated by the earth than other fossil fuels such as petroleum. It is a fossil fuel that is often deposited alongside other fossil fuels as well such as coal and oil.

Industrial biomass reactors that process biomass waste like manure to generate fuel generally produce less methane gas as a percentage by volume than what is contained in natural gas. The typical output of a set volume of biogas from a digester is 50% to 80% methane with a significant amount of waste gas in the form of carbon dioxide at 20% to 50%. Other trace gasses are also generated in the process that have some commercial value such as hydrogen, nitrogen, and oxygen, and toxic gasses of which must be safely disposed are also generated including hydrogen sulfide and carbon monoxide.

The biological processes that are necessary for waste digestion to effectively take place can be complex and rely on strictly-controlled conditions. Temperature is a major concern in the process as the bacteria that break down the waste 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 and thrive at a higher optimal temperature of 130° Fahrenheit (54.4° Celsius).

Conditions must be altered for temperature, pH, and other factors like the water versus solid ratio of the biomass mixture and the carbon/nitrogen ratio as the organic material is chemically degraded as well. The two main types of bacteria that are used in anaerobic digestion are acetogenic and methanogenic bacteria, and, though they are used in tandem, each has unique living conditions under which they thrive. Acetogenic bacteria produce the chemical acetate during anaerobic digestion and methanogenic bacteria produce methane.

Biomass material is taken through four stages for effective methane recovery. The hydrolysis stage uses water to decompose solids or semi-solids into simpler compounds, and then either fermentation or acidogenesis is used to break down carbohydrate chain structures into more basic compounds like ammonia, hydrogen, and organic acids. Acetogenesis is then used as the third step in the process, where acetogenic bacteria convert the organic acids into acetic acid along with further byproducts like hydrogen and carbon dioxide. The final step of methanogenesis uses methanogenic bacteria to combine these primary end products of acetate, hydrogen, and carbon dioxide into methane, which can then be used for fuel.