Frequently Asked Questions

What is an anaerobic digester?

“Anaerobic digestion,” or  “AD,” refers to the process of using anaerobic bacteria to decompose organic waste. This process happens naturally when organic waste is trapped in an oxygen free environment.  An “anaerobic digester” is basically a mechanical stomach. It is a machine used to intentionally replicate this process and capture the biogas produced by the anaerobic bacteria as a byproduct of digestion. Anaerobic digesters are also called “biodigesters” to retain the reference to a biological digestion process with fewer words.

How do anaerobic digesters work?

Anaerobic digesters are tools used to break down and recycle organic matter. Anything that is biodegradable, or “biomass,” that comes from a plant or animal can be put into a digester, and is called a “feedstock.” Anaerobic digesters manage feedstocks from farms, municipalities, and institutions– wastes that otherwise would not have a purpose.

Though each digester is a little different, they are all designed to break down organic matter and put it toward another purpose. Digesters enlist the help of anaerobic bacteria, which exist in environments deprived of oxygen. As the bacteria feed on organic matter, they release methane gas. When this process occurs in an open system, like a landfill or a manure slurry pit, the methane is released into the atmosphere as a greenhouse gas. Digesters allow this biological process to unfold in a closed system where the methane is captured and put to use as an alternative to fossil fuels.

Digesters can be large or small– they range from “microdigesters” used to process the organic waste from a single home to industrial scale digesters handling hundreds of tons of material at a time. Generally, a digester is made up of a series of tanks. Some of these tanks mix organic wastes up, some are just chambers that hold the feedstock as it gets eaten by the bacteria. And others are storage for either the biogas or the digestate. Biodigesters might only take one feedstock, like cow manure, or might be designed to handle a mix of commercial food waste and other materials. Most digesters have three end products: biogas, a nutrient-rich liquid called digestate, and a solid by-product similar to compost. Different operations put these outputs to use in a variety of ways.

What is biogas?

Biogas is produced as a result of bacteria decomposing organic matter in the absence of oxygen. The anaerobic bacteria that produce biogas are called “methanogens.” Methane is flammable gas and is the primary component of traditional fossil fuel natural gas. This means biogas can be converted into electricity, heat, and even transportation fuel.

What feedstocks can be used as fuel?

In the United States, the majority of anaerobic digesters are located on livestock operations due to the supply of manure. Other systems complement the processing of sewage water at wastewater treatment facilities. There is increasing interest in digesting food waste and food waste byproducts as more cities and states institute waste diversion laws. Other opportunities include food and beverage processing residuals like whey, beer waste, glyercin and unconverted oils, and distillery waste. Depending on the design of the digester, and the intended end-use for its products, one or more of these feedstocks will be appropriate.

Each feedstock has its own biogas potential– that means some materials are expected to generate more biogas than others. Finding the right combination of feedstocks to make a desirable amount of biogas and digestate can be tricky, though generally a feedstock that is high in sugars and oils will produce more biogas. Other traits, such as consistency and availability, may be more important than biogas potential to ensure that a system stays balanced.

What other systems take advantage of AD technology?

Major cities, like Philadelphia, use anaerobic digestion to offset heating and electricity needs of their wastewater treatment plants. Other places have used biogas generated from the anaerobic digestion of food waste to fuel their fleet of food residuals collection trucks. Many on-farm systems deposit manure into the digester and spread the process’s nutrient-rich liquid digestate on their fields as fertilizer. Yet, other systems inject the upgraded biogas into a natural gas pipeline system to be used as  “renewable natural gas.”

Why is anaerobic digestion particularly beneficial for the food and beverage industry?

The food and beverage industry may generate waste that is not appropriate to flush into a municipal wastewater treatment system. Most municipal systems aren’t designed to handle byproducts with high organic loads, which means instead of sending their waste down the drain through their municipal system, companies must transport the waste offsite to be disposed. Treating the high strength organic wastes on-site in an anaerobic digester, before any waste gets sent to the municipal treatment plant, can reduce the overall cost of byproduct disposal. This is because wastewater treatment plants have trouble with “high strength” organic waste, like 10,000 pounds of melted ice cream. They have to put extra oxygen into their system, to make sure their aerobic bacteria don’t die. Anaerobic systems don’t need oxygen to break down organic waste, which makes it cheaper and easier to help manage high strength organics.

By treating onsite, companies can reduce municipal wastewater surcharges, reduce trucking costs, comply with wastewater permitting, and actually increase production capacity through the elimination of wastewater permitting constraints. These cost-savings allow certain AD systems to actually pay for themselves over time.

Why are so many AD systems installed on concentrated animal feeding operations (CAFOs)?

Concentrated animal feeding operations generate organic waste in the form of manure. Manure is rich in phosphorous and nitrogen, two nutrients that are often used as fertilizer. The volume of manure generated on CAFOs is often too large to be able to be responsibly applied back on the land. CAFOs often store manure in large slurry pits, rather than spreading it on the land. Manure decomposes in the slurry pits and releases methane gas directly into the atmosphere. Or farmers choose to spread the manure on the land. Agricultural runoff can occur if the land is unable to absorb the nutrients applied. The nutrients found in the manure make their way to rivers and lakes. Nutrient pollution leads to an explosive growth of algae and bacteria in the water. As the algae breaks down, the oxygen dissolved in the water is used up, and anything in the water that needs oxygen to survive starts to suffocate.

Though the anaerobic process does not remove phosphorous, nitrogen, or other nutrients from the decomposed organic waste digesters are able to capture those nutrients and divert them from waterways. Anaerobic digester systems on CAFOs act as nutrient management tools for the watershed.

Does AD make us more reliant on infrastructure that supports fossil fuels?

The United States has a large network of existing natural gas pipelines, which move natural gas to power plants and to homes and businesses that use gas. Once it meets certain standards, biogas is called “renewable natural gas” and can be put into those existing pipelines for the same uses.

There are gas utility companies that purchase renewable natural gas and offer it as a more sustainable option for their customers. However, in a 2014 study, the National Renewable Energy Laboratory estimated that the methane potential from anaerobic digestion in the United States could displace about 5% of the natural gas currently used for electricity and 56% of the natural gas used for transportation. So, biogas cannot completely eliminate the current demand for natural gas and since gas pipelines are all “open access,” they must be open to any kind of natural gas, regardless of source. Any expansion of pipeline infrastructure could lead to further natural gas development, even if it is intended to support biogas.

Additionally, federal incentives for renewable transportation fuel reward companies for injecting their biogas into the natural gas grid. The Renewable Fuel Standard, or RFS, was developed by the EPA to encourage converting wastes, like manure and food scraps, into fuels, and requires that a certain volume of renewable fuel is used to replace gasoline and diesel each year. Oil refineries and fuel importers can comply with the RFS by either blending renewable fuel into the existing fuel supply or by purchasing enough RINs to cover their obligation. RINs are Renewable Identification Numbers, and are certificates or ID cards that identify renewable fuel. They can be bought and sold separately from the fuel itself. RINs help the EPA’s track and enforce the Renewable Fuel Standard.

The EPA has approved pathways to generate RINs. Biogas made from agricultural digesters, wastewater treatment plants, and landfills, and used as compressed or liquid natural gas generates RINs. In order to be turned into compressed or liquid natural gas, biogas typically must travel through a pipeline to get to the companies that will process it to be used in the transportation sector. Companies are more likely to develop biogas projects that will qualify for RINs because they can generate another revenue stream.

California has an additional policy called the Low Carbon Fuel Standard. This policy works similarly to the RFS. For renewable natural gas produced out of state to qualify for California credits, the renewable natural gas has to be sold to a company with dispensing facilities or a vehicle fleet in California. Or the gas can be injected into a natural gas pipeline with the ability to flow to California. Again, the LCFS incentivizes pipeline injection and transportation uses, even if that means transporting the renewable natural gas itself over really long distances.

Biogas that is used onsite for heat or electricity does not help meet the Renewable Fuel Standard or Low Carbon Fuel Standard, and therefore does not generate RINs or credits in California. These projects, while highly valuable in a locality, are not supported by federal policy in the same way that biogas projects supporting the transportation sector are incentivized. The higher revenue potential for a biogas project that supports transportation incents the use and development of fossil fuel infrastructure, like pipelines.

Do anaerobic digesters reduce greenhouse gas emissions?

Yes. Anaerobic digesters work in two ways to reduce greenhouse gas emissions. First, they reduce emissions that would have occurred if their feedstocks had not been collected and processed. Manure and food scraps that are not digested are responsible for greenhouse gas emissions on agricultural operations and landfills, respectively.

Second, biodigesters displace energy needs that would have come from fossil fuels. If an AD system is generating electricity, it is offsetting generation that likely would have been coal or natural gas, depending upon where it is located. If the system is generating renewable natural gas for use in the transportation sector, biogas is substituted for diesel or natural gas. Using biogas for heat would largely displace propane, heating oil, and natural gas. Each of these sectors has different considerations. For example, coal releases the most carbon dioxide when burned, yet the pathway from wellhead to end use for natural gas has been shown to leak methane, a more potent greenhouse gas than CO2. The specific use of the biogas can significantly change the emissions profile of digesters, from both the choice feedstocks to the choice of flaring biogas, combustion for heat or electricity, or upgrading for use as a transportation fuel.

It is also important to note that the carbon dioxide emissions that occur when the biogas is burned, whether for electricity, heating, or transportation, are from the terrestrial carbon cycle. Unlike fossil fuels, which are part of the global carbon cycle over millions of years, the terrestrial carbon cycle occurs through the photosynthesis, respiration, decay, and decomposition of plants in daily and seasonal flows. Anaerobic digesters shepherd this biological process in a closed system, though the breakdown of organic matter and its associated carbon dioxide emissions would have occurred with or without the AD system. Because of the technology’s prevention of emissions and displacement of fossil fuel energy sources, some even call anaerobic digestion a carbon-negative technology.

How is anaerobic digestion different from other renewable energy technologies?

Anaerobic digestion makes use of stocks of materials to generate power. Other technologies like wind and solar photovoltaics harness flows of energy to generate electricity. Because AD operates on stocks, it can overcome the limitation of intermittency that other renewables face. This allows digester operations and output to be quite consistent. They are able to run 24/7, produce a stable amount of biogas, and reliably generate an expected amount of electricity. Additionally, digesters are a form of distributed generation, which means using many smaller, local forms of renewable energy to power the grid, instead of larger more centralized power plants. Distributed generation helps to increase reliability because a diversified electricity supply includes hundreds of small generators, that if even a few are out of operation, their generating capacity can be substituted with other sources. And decentralized generation keeps the production of electricity close to where it is being used, which is helpful in efficiently managing the grid’s overall electric load.

Digesters, because of their ability to generate around the clock, can be a source of baseload generation. Baseload generation is the minimum amount of electricity that is needed to meet typical demand. Baseload generation is important, especially as more intermittent generation sources like solar panels and wind turbines are integrated into the grid.

What are the potential benefits of AD?

At their best, digesters can do a world of good. Digesters eat waste products and turn them into energy and fertilizer. They avoid greenhouse gas emissions from decaying organic matter and instead generate renewable electricity, heat buildings, or fuel transport. They help to manage nutrients and keep them in productive cycles, instead of polluting lakes, streams, and rivers. The fertilizer from the digestate can help make soils healthier, reduce farmers’ expenses, and improve crop yields. Healthier soil means better water retention, less need for irrigation, and reduced soil erosion and nutrient runoff. And, related to question 9, digesters have the potential to increase the grid’s reliability, reduce electric demand, and lower everyone’s costs for electricity.

How are anaerobic digesters regulated?

The specific feedstocks and the end-use of the biogas can significantly change how the technology is regulated. A digester that processes post-consumer food scraps, like food waste from homes, and food processing waste from industrial sources will be regulated differently than an on-farm digester accepting solely manure.

Generally speaking in Vermont, if a digester is accepting post-consumer food scraps, which are defined as  solid waste, they are required to have a solid waste facility permit. Digesters accepting food and beverage industry waste, or food processing residuals, are regulated similarly to wastewater treatment plants. They must abide by the indirect discharge program. On-farm digesters must set nutrient load limits and stick to an approved nutrient management plan.

Depending on the use of the biogas, there will be other permits. For example, in Vermont, you need a certificate of public good from the state public utility commission if you want to generate electricity. Digesters may also need air pollution control permits that require different emissions control technology depending on how the biogas is used.

What are the major controversies surrounding AD?

The major controversy surrounding anaerobic digesters in the US is their relationship with CAFOs. Most anaerobic digesters in the US are used for manure management at concentrated animal feeding operations. Because so many animals are concentrated in one place, the introduction of anaerobic digesters to manage their waste was once seen as a solution. Critics don’t see ADs as a true solution to the problem of factory farming, and point out that digesters may support the continuation of these sorts of operations if the “waste” issue is solved. The human and animal rights issues that CAFOs bring up necessitate that more work be done to address the problem.

While digesters are not the full solution for factory farms, but they can make the industry pollute less. Many see a potential role for digesters both at CAFOs and smaller dairy or livestock farms. If CAFOs exist, the manure must be managed in a way that keeps it from polluting streams, causing algal blooms, and kicking off the cycle of eutrophication. For smaller dairies and some other smaller operations, digesters truly make their dairy farming slightly more economically viable by providing an additional revenue stream for electricity generation.

Why isn't AD used everywhere?

As standalone operations, anaerobic digesters are vulnerable to increasing capital costs, new regulations, and labor shortages. When digesters are only seen as a waste management tool, their utility is not recognized beyond one sector. The most successful digesters are part of the infrastructure of a community, they are permanent, and they’re supported. When left as a private entity, investment in maintenance, job training, operations, feedstock security, or other issues can easily overcome the public good that a digester provides.

What considerations should guide sustainable use of AD technology?

Because biodigesters are so varied, we have developed some basic parameters to determine what we believe an efficient and appropriate use of AD technology looks like.

What makes the use of biogas efficient or inefficient, and sustainable or unsustainable, depends upon the needs of each system. Both the system inputs (source and transportation of feedstocks) and the system outputs (use of biogas and digestate) need to be thoughtfully considered.

Since each digester is part of a larger system of waste management and energy production, there are several guiding principles for creating a viable application of anaerobic digestion.

• Appropriate feedstocks: are feedstocks being diverted from another beneficial use? Are they coming from nearby or are they being transported long distances? Can feedstocks be secured for the longterm?
• Appropriate scale: does the scale of the digester meet the needs of the operation? Will it be over/under-utilized with the current availability of feedstocks or future availability? Is the scale proposed able to be financed in the longterm?
• Appropriate site: are there special zoning regulations to consider? Who are neighbors? Will noise or odor be an issue? Are there sensitive landforms nearby like lakes or cultural sites?
• Appropriate end-uses: what will the products be used for? Do those end-uses close loops in the system? Once established, does the system allow for sustained operations or will there continue to be outside influences required?