With the ability to turn organic waste into a gas that can be used to produce electricity and thermal energy (heat), anaerobic digestion is an increasingly important technology in the Commonwealth's renewable energy portfolio. Anaerobic digestion (AD) can also play an important role in diverting some of the organic waste disposed of in landfills across the Commonwealth - thereby reducing landfill emissions of methane (a potent greenhouse gas) and mitigating the need for landfill expansions. After decades of successful use - in Massachusetts, the United States and around the world - plus recent technical advances, there is growing interest in AD as an alternative to conventional power generation. With that interest comes questions regarding the benefits of AD technologies, as well as potential impacts on local communities and the environment.
What is Anaerobic Digestion (AD) and how does it work?
Anaerobic digestion (AD) is a biological process in which micro-organisms break down or "digest" organic materials in the absence of oxygen and form biogas. In the "wet" version of AD, pumpable organic feedstocks (such as food processing waste or animal manure) are placed in an enclosed vessel that is maintained at temperatures of 95 - 140 degrees Fahrenheit, typically for about three to four weeks. Naturally-occurring micro-organisms that thrive in this heated environment break down the organic solids and produce biogas, comprised primarily of methane (also known as natural gas) and carbon dioxide. "Dry" AD systems operate in a similar way, but can handle feedstocks with lower moisture content, such as table scraps or yard waste.
Is there anything that remains after the digestion?
While the digestion process greatly reduces the volume of solids, the degree of reduction varies depending on the feed source, temperature, and amount of time the solids are retained in the digester. Some digesters see up to an 85 percent reduction in organic solids. The digestate (material that remains after most of the solids have been broken down) is rich in nutrients and may be used directly as a fertilizer or soil amendment, or mixed with other materials and composted. Sometimes, excess liquid must be sent to a wastewater treatment plant.
How is the biogas used?
While it used to be common simply to capture and "flare" surplus biogas (burn it off into the air), there are several ways in which biogas can be used to produce energy. At a minimum, it can be used as fuel to heat the digester itself to maintain proper temperature. Biogas can also be used to create heat for other industrial processes. Alternatively, the biogas can be fed into a generator to create electricity, or used in a combined heat and power (CHP) system, also known as a co-generator, to simultaneously produce both electricity and heat. Finally, biogas can be converted to compressed natural gas (CNG) and used to fuel vehicles, such as buses or trucks.
What types of facilities use AD?
Since the 1940s, Massachusetts wastewater treatment plants have used AD to reduce solids that would otherwise be landfilled or incinerated. The process also sufficiently reduces pathogens to make the solids safe for application to the land as a fertilizer. In addition to wastewater treatment plants, AD with CHP has applications on farms, at industrial and food processing facilities, and at stand-alone organics recycling centers. AD with CHP is used widely in Europe for waste reduction and renewable energy production. Biogas production accounts for a significant portion of clean energy production in Europe.
What questions should a community consider when evaluating the siting of an AD facility?
Communities should understand the types and quantities of organic materials that will be accepted at the facility; the types of vehicles that will be used to bring the materials and/or deliver finished products; truck traffic anticipated; and use of proven technology and practices to control odors, noise, or other potential local impacts.
What is Massachusetts' interest in AD?
The use of AD, along with combined heat and power, to reduce organic waste and generate renewable energy is common in Europe and on the rise here in the US. As a nationally-recognized environmental and clean energy leader, Massachusetts is investigating additional applications for this technology to deliver environmental and economic benefits to our municipalities, farms, and other businesses. Several types of financial assistance are available to help develop AD facilities.
To further explore the topics outlined above, see the following case studies on existing AD projects and other online resources: