In the 17th century, Jan Baptita Van Helmont first determined that decaying organic matter could result in flammable gasses. In 1859, the first digestion plant was built in Bombay, India and by 1895 anaerobic digestion was being used in England to recover gas from waste management. Anaerobic digestion remains popular in Europe where some facilities have been in operation for over 20 years, with Denmark leading the way in terms of experience.

Anaerobic digestion is becoming more available in the United States and the northeast. There are over 20 facilities currently operating in New England and numerous proposals in the works for future projects. For a more detailed history of Anaerobic Digestion, please visit: Penn State: A Short History of Anaerobic Digestion.

How does it work?

Anaerobic digestion (AD) is the natural breakdown of biodegradable material by microorganisms in the absence of oxygen. Biodegradable materials such as animal manure, food scraps, wastewater treatment solids, restaurant grease, and municipal and industrial wastewater and residuals can be put to use to help with waste management and energy needs.  Biodegradable materials are put into a digester, where there is an absence of oxygen. The naturally occurring microorganisms found within the materials break down the organic matter, producing biogas and digestate. The disgester is usually heated to a certain temperature, which depends upon what material is being digested and what type of system is used. Often times a combined heat and power (CHP) unit that runs off of biogas is used, producing both electricity and heat.  Most anaerobic digestion facilities share a similar process, but some facilities have separated the process into multiple stages.  There are different types of digesters for different types of material, but all generally convert organics into biogas and digestate.

Biogas is composed mostly of methane and carbon dioxide, with trace amounts of hydrogen sulfide, nitrogen, and other particulates.  Biogas can be combusted to generate electricity and/or heat. It is most often used to generate electricity for on-site use or to sell to local utilities. To generate electricity, biogas is used to fuel a gas engine which drives a generator to produce electricity. It is also commonly used to produce heat by burning the biogas in boilers. With a combined heat and power plant system (CHP) power is generated and the heat produced can be used to heat the digester. Biogas can also be upgraded to pure methane, also known as bio methane or renewable natural gas, by removing carbon dioxide, hydrogen sulfide, and other trace elements. Pure methane can be used as a substitute to natural gas in pipe lines and can also be used for vehicle fuel. For more information on the uses of biogas, please visit US EPA - AgSTAR: Gas Use Devices.

Digestate is the solid and liquid digested material produced in AD. Digestate is rich in nutrients such as nitrogen, phosphorus, and potassium.  It can be processed into products such as fertilizer, compost, and animal bedding.