Biodegradable materials can be put to good use through anaerobic digestion creating digestate and biogas, a fuel that can be processed and replace natural gas.
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.
There are many types of biodegradable materials (a.k.a. feedstock) that can be used in anaerobic digestion and each holds varying amounts of energy. To account for variables in feedstock, and to attain the best efficiency, there are different types of anaerobic digestion technologies. Some digesters operate at mesophilic temperatures, and others at thermophilic temperatures. There are also dry/high solids and wet/low solids digesters. Digesters are usually designed to accomodate the anticipated feedstocks. Unlike most other renewable energy technologies in this section, Anaerobic Digestion is unique as it produces both biogas and digestate. More information on AD, composting, and organics recycling can be found at DEEP: Large Scale Organics Management. For more information on large scale food scrap recycling technologies, see this EPA primer.
There are several generators of biological materials that may be good candidates for either an on-site AD system, or for participation in an off-site AD facility. These include dairy farms, businesses and institutions that manufacture, distribute, prepare, or otherwise handle food, and wastewater treatment plants/water pollution control facilities.
For more information on different digester technologies, please visit American Biogas Council - Digester Technologies.
A great example of AD in our region can be found at Barstow's Longview Farm in Hadley, Massachusetts.
AD is a technology that can be highly customized to an individual project and the needs of the user. Processes and concerns differ widely among projects, and the technology is constantly evolving. If you have specific questions, please visit the links below.
- DEEP: Large-Scale Organics Management - Anaerobic Digestion
- US EPA: Anaerobic Digestion
- US EPA: AgSTAR- Anaerobic Digestion
- American Biogas Council
- Alternative Energy: Anaerobic Digestion of Biomass
- Anaerobic Digestion - Waste to Energy & Biomass in California
- Anaerobic Digestion of Food Waste in New England (PDF)
AD as Renewable Energy
Connecticut’s Renewable Portfolio Standard (RPS) requirements mandate that electricity providers “obtain a minimum percentage of their retail load by using renewable energy.” Anaerobic Digestion is listed as a Class I renewable energy source as defined in the Connecticut General Statutes (CGS) Section 16-1(a)(26)(v).
AD as Responsible Waste Management
Per Connecticut law (CGS 22a-226e), as of January 1, 2014, certain Connecticut businesses that generate an average projected volume of 104 tons or more per year of source-separated organic materials and are located within 20 miles of an authorized source-separated organic material composting facility must separate such materials from other solid waste and insure that it gets recycled.