Using the Unused: Energy from Waste
Anaerobic Digestion (AD) is a process that converts waste, like animal manure and crop residue, to biogas—a combination of methane and carbon dioxide. In multiple industries, AD is used as a practicable waste management solution that allows factories, farms, and municipalities to manage their waste and create a source of renewable energy.
AD digesters use an oxygen-free environment, which allows microbes to digest organic materials and release gases that are captured and stored. Biogas can be used on site to meet energy demands of the facility, or be upgraded to Renewable Natural Gas (RNG) and supplied directly to consumers at pipeline-level quality.
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Introduction to AD
Feedstock is a term used to describe the input for Anaerobic Digestion (AD). There are multiple types of feedstock—manure, wastewater sludge, crop residue, food waste—but each can undergo very similar AD processes. These materials are suitable for AD because they contain organic materials, like proteins and sugars, that can be broken down by microbes. Before being applied to AD systems, feedstocks are usually treated with water, separated by solids and liquids, or heated.
AD is also referred to as “biogas recovery” because most of the feedstock types would typically be thought of as waste. Rather than being left to rot in landfills or unused on farms, these materials can be used as a source of renewable natural gas, horticultural products, and bioproducts.
Types of Feedstock
(click each to learn more)
What is Agricultural Feedstock?
Agricultural facilities typically use animal waste as a source of feedstock for anaerobic digestion. Materials like wood chips that are utilized for manure cleanup can also be added to the feedstock.
Why is it good for AD?
Animal waste is full of nutrients, like nitrogen and phosphorus, that are used by bacteria which create biogas. AD makes use of the manure and acts as a waste management method.
What is Wastewater Feedstock?
Wastewater treatment facilities separate solid waste collected through sewer systems, resulting in the production of wastewater sludge. This sludge can be used as a feedstock for anaerobic digestion at wastewater treatment cites.
Why is it good for AD?
Anaerobic digestion offers a treatment solution for sludge at wastewater facilities. AD can convert half of the organic materials in sludge to biogas, thus reducing the volume of sludge for disposal.
What is Food Waste Feedstock?
Food Waste feedstock is exactly what it sounds like—food that has gone bad or unused that can be repurposed as a source of organic materials and nutrients for anaerobic digestion.
Why is it good for AD?
In the US, 40% of all food goes to waste, with much of it ending up in landfills. Food is easily degraded by microbes, so rather then let it rot in an open environment, AD is used as a waste management solution that also converts the organic material to biogas.
What is Industrial Waste Feedstock?
Industrial Waste feedstock is usually high-energy wastewater facilities that produce candy, potato chips, dairy products, beverages, or beer. Wastewater from these locations can be quite variable, but each have high concentrations of organic materials.
Why is it good for AD?
Recent advancements in AD reactors has made it possible for both high- and lower-energy industrial wastewater to be treated. Therefore, AD offers facilities a way to treat their waste and produce biogas.
Some feedstock types require transportation to AD facilities while others can be used on site. For example, AD of wastewater sludge occurs onsite at water treatment plants. This, however, is not always the case. Because agricultural facilities are spaced out, transportation and delivery of AD feedstock is an important factor. First is the collection and transportation of feedstock. This service is usually provided by a transportation company which takes feedstock from the collection site to the AD facility. Those who own the site where the feedstock is collected are often tasked with paying for the transportation. AD facilities also charge a tipping fee dependent on the weight of feedstock being dropped off. Agricultural AD facilities typically accept feedstock from multiple different locations, each under a different contract pertaining to volume, weight, quality, and frequency.
AD System Types
There are numerous types of AD systems with variations in optimal feedstock type, temperature, size, and microbial species. Even with these variations, AD is possible because of underlying, fundamental microbial processes that have been harnessed by scientists and engineers.
Types of AD Systems
(click each to learn more)
Common Uses: Agriculture, food, industry
Digestion Time: 15 to 35 days
Percent Solid Feedstock: 3 to 6
A continuous stirred tank reactor (CSTR) is exactly what it sounds like: a tank filled with feedstock that is being continuously stirred as digestion takes place. CSTR systems are ideal for 3 to 6 percent solids and require feedstocks to be digested for between 15-35 days depending on the temperature of the tank. After the feedstock has been digested, it will be released as new feedstock is added to the tank. CSTRs are widely applicable to various industries, and can be adapted for agricultural waste, food waste, and industrial waste.
Common Uses: Agriculture and food
Digestion Time: 30 to 40 days
Percent Solid Feedstock: 0.5 to 2
Covered lagoons are large, in-ground pits that are best suited for feedstocks of 0.5 to 2 percent solid. The pits are lined and covered to prevent leaking and ensure that biogas is captured. Covered lagoons are not heated, meaning their operation largely depends on the surrounding temperature. Because of this, they have a longer digestion time of 30 to 40 days. Covered lagoons are most commonly used for agricultural settings, and can be adapted to co-digest food waste.
Common Uses: Agriculture
Digestion Time: 15 to 25 days
Percent Solid Feedstock: 11 to 14
Plug flow digesters are similar to CSTRs in that as digested feedstock is removed, fresh feedstock will be added. As opposed to CSTRs, however, plug flow digesters do not mix the feedstock and can handle 11-14 percent solid. Plug flow digesters take between 15 and 25 days to digestate feedstock and are primarily used on farming facilities to manage manure.
Common Uses: Agriculture
Digestion Time: 5 or less days
Percent Solid Feedstock: 3 to 8
Anaerobic sequencing batch reactors (ASBRs) use four stages of processing to complete digestion: filling, reacting, settling, and decanting. ASBR systems are most effective at treating feedstocks that range from 3 to 8 percent solid, and are most commonly used for agricultural feedstock. While further implementation of ASBR systems have yet to be realized, the low digestion time of 5 or less days is attractive to researchers and industry members.
Common Uses: Wastewater and industry
Digestion Time: 5 or less days
Percent Solid Feedstock: 3 or less
Up-flow anaerobic sludget blanket (UASB) digesters are best equipped to handle feedstock that is 3 percent or less solid. Digestion times average 5 days or less, which is much shorter compared to other systems. UASB systems are most commonly used to manage wastewater from municipalities or private companies like breweries.
The Science of AD
Digesters can range in size from dozens of gallons to entire, covered lagoons. Regardless of size, each facility type utilizes similar microbes that convert organic material into methane gas. These microbes thrive in temperatures ranging from 90℉ to 140℉ and require an oxygen-free environment—hence the name anaerobic digestion. There are 4 steps that occur between depositing the feedstock and collection of biogas.
Four steps of Anaerobic Digestion
(click to learn more)
Starting Materials: polysaccharides, proteins, lipids
Ending Materials: monosaccharides, amino acids, long-chain fatty acids
Details: Enzymes produced by microbes, like micro-sized machines, break down the large organic molecules found in the feedstock. These enzymes use water molecules to break apart bonds that keep the larger molecules together, resulting in smaller, more manageable sources of energy and carbon for the microbes. Hydrolysis is typically rate-limiting for AD, meaning the pace of this reaction sets the pace for the rest of the steps.
Starting Materials: monosaccharides, amino acids, long-chain fatty acids
Ending Materials: Volatile Fatty Acids (VFAs)
Details: The smaller molecules created from hydrolysis — the amino acids, fattys acids, and sugars — are then converted by microbes into VFAs such as acetic acid, propionic acid, and butyric acid. Microbes called acidogens use fermentation, a process similar to alcohol production, to produce VFAs.
This step results in the production of different types of acids which can lower the overall pH of the digester.
Starting Materials: VFAs
Ending Materials: hydrogen gas and carbon dioxide
Details: Two types of bacteria are involved in acetogenesis, the third step of AD. First, hydrogen-producing acetogens use fatty acids to produce hydrogen gas and carbon dioxide. Then, homoacetogens use the carbon dioxide and hydrogen gas to produce acetate.
Similar to the second step, Acetogenesis results in the production of acetate — an acid. This means that acetogenesis can impact the overall pH of the reactor.
Starting Materials: hydrogen gas, carbon dioxide, and acetate
Ending Materials: methane and carbon dioxide
Details: The last step involves methanogens, methane-producing bacteria. The majority of methane is produced through the conversion of acetate, but the hydrogen gas and carbon dioxide can also be utilized.
Compared to the other types of microbes used in AD, Methanogens are the most sensitive to fluxuations in the acidity. The two steps prior to methanogenesis both produce acids and can inhibit methane-producing archaea in this step. In order to maintain biogas production, pH levels must be kept between 6.5 – 7.5
After the feedstock has been fully digested, two different outputs — biogas and digestate — can be used in a variety of ways.
Types of Output
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Biogas generally consists of 60 percent methane (CH4), 40 percent carbon dioxide (CO2), and small amounts of other gases. Biogas that have been captured can be used on-site with very little processing required. This biogas can be burned directly to heat facilities and overall decrease heating expenses. In addition to heating, biogas can be used as fuel for combustion engines to produce electricity. This helps facilities meet their energy demands, and even offers a stream of revenue if excess electricity is sold back to the grid.
Biogas can also be upgraded to Renewable Natural Gas (RNG) by separating removing non-methane gas. There are multiple upgrading techniques that can meet RNG requirements of 98% methane. This concentrated methane is pipeline-quality RNG, and can be supplied directly to consumers or used to produce electricity to supply the grid. RNG can be further converted to compressed natural gas or liquid natural gas, both of which can be used as vehicle fuel. Because of limited natural gas fueling infrastructure for vehicles, compressed and liquid natural gas is commonly used for fleet vehicles that have refuel at the same location each time.
Digestate is one of the two products of AD and consists of high-nutrient solids and liquids. After being removed from the AD system, liquids and solids are often separated using one of several methods, such as evaporating liquids, filtering out solids, and biodrying. These materials also require treatment to remove pathogens and other unwanted materials. After separation and treatment, solid and liquid digestate can be used differently.
Liquids can be easily incorporated into irrigation systems to offer aqueous fertilizer treatment for agricultural fields. Because it is harder to transport than solids, liquid digestate is often used on-site when applicable. Solid digestate often have high levels of nutrients like nitrogen, phosphorus, and potassium, and can therefore be used in solid fertilizers, which is more readily transported to for off-site sales. Solids can also be used in other value-added materials, like animal bedding and construction materials.
Public Policy for Renewable Energy Sources
What is a Renewable Energy Source?
Non-renewable energy sources, like fossil fuels, are limited in quantity and are carbon-intensive, meaning they emit large quantities of greenhouse gasses into the atmosphere. Renewable energy sources, however, are naturally replenished and are much less carbon-intensive.
Wind, solar, and hydroelectric energy sources are more well-known among the public and given primary attention in public policy. Biomass conversion technologies like AD, however, have significant potential to add to renewable markets. For example, 21% of Wisconsin’s renewable energy in 2015 came from biogas, more than solar, wind, and hydro combined. Certain states will have greater potential to implement AD, especially those with significant agricultural industries.
How does Public Policy Support it?
Incentives for producing and using renewable energy is broken into federal policy and state policy. Some of the strongest policy tools at the federal level consist of grants, loans, and tax credits for constructing and maintaining renewable energy facilities.
States are left to construct what are called Renewable Portfolio Standards (RPS). These standards are a regulatory mechanism that require certain amounts of a states total energy production and usage to come from renewable sources. Currently, 30 states have an RPS and 7 have voluntary renewable goals. Some states have included RPSs that require 100% carbon-free production by the years 2045 or 2050 while others have slower transitions, such as achieving 15% by 2025.
AD: Waste Management vs. Biogas Production
AD is primarily considered as a method to manage different types of waste. Ranging from municipalities treating wastewater to farms managing animal manure, AD offers both the private and public sector a way to handle waste.
Beyond the waste, AD is capable of producing enough biogas to be captured, and eventually used or sold. In the public sector, such as wastewater treatment plants, biogas production is more of a secondary consideration and is most often used to power on-site processes if the biogas is collected at all. The private sector often looks to use investments for AD systems that promise the most returns in the form of biogas and bioproducts
Here, we consider AD’s potential as a renewable source of biogas and the impact of public policy on production, prices, and emission reductions.
Where is AD used?
AD is used for waste management and biogas production at wastewater treatment plants, industrial locations (food and beverage production facilities), and farming facilities. This means that AD is used by facilities in both the private and public sector. Both sectors consider AD as a solution for managing waste, but private facilities are more driven to the potential revenue streams like biogas and bioproducts. Most public facilities consider AD as part of the services provided to residents and biogas production a secondary benefit.
Wastewater Treatment Plants
(food and beverage production facilities)
Biogas as a Qualifying Renewable Energy Source
What is Biogas?
Animal manure, wastewater, and industrial waste can be converted into a combination of methane and carbon dioxide, called biogas, in a renewable way through AD. Biogas can be used on-site to heat facilities or help complete other processes, or it can be upgraded to Renewable Natural Gas (RNG) by removing all non-methane gases. RNG can be injected directly into gas pipelines for consumer use or used as vehicle fuel.
How does biogas fit into public policy?
Biogas and RNG are considered a renewable energy source, and qualify under many existing programs to incentivize production. At the federal level, most programs are designed to provide payments, loans, or grants to existing and proposed biogas production sites.
Federal Grants and Payment Programs
Advanced Biofuel Payment Program
- Direct, quarterly payments to biofuel producers; amount depends on qualifying BTU produced
Rural Energy for America Program Guaranteed Loan Program
- Payments to be used for eligible activities of biofuel producers, including energy audits, renewable energy technical assistance, and renewable energy site assessments
Value Added Producer Grants
- Priority given to small and medium sized farm/ranches that are producing value-added products
- Up to 50% of total project costs; maximum for planning Grants at $75,000, and maximum for working Capital Grants at $250,000; funds can be used for processing costs and marketing
Federal Loan Programs
Biorefinery Assistance Program
- Provides up 80% loan guarantee for qualifying projects, up to $250 million
Rural Energy for America Program
- Maximum $100,000 loan per fiscal year for energy audits, technical assistance, and energy site assessments
Biogas in California
California’s Renewable Portfolio Standard (RPS) mandates 50% of statewide electricity to be produced by renewable energy sources by 2025, ultimately reaching zero-carbon production by 2045. Biogas is RPS-eligible, meaning methane from AD technology counts towards utilities requirements for renewable energy.
The state’s Low Carbon Fuel Standard (LCFS) also encourages development of multiple renewable energy types, including biogas. The LCFS measures the amount of carbon emitted along the supply chain of different fuel types and ranks them with a Carbon Intensity (CI) score. Negative CI scores indicate low emissions and generate credits that can be sold to other utility provides that have not met the required standards. There are multiple biogas-generating facilities that have CI scores ranging from -100 to -400 and earn credits that can be solid at high prices to other energy and fuel companies. In addition, the California Public Utilities Commission allocats up to 50% funding for biogas facility development. Capped at $3 million for each project, funding encourages new facilities that connect upgraded biogas directly to the utilities gas supply.
As a result of the many policies and investment in biogas infrastructure, California will have 160 RNG-producing facilities by 2024, largely to produce vehicle fuel for natural gas trucks. It is estimated that these facilities will have a combined impact of replacing almost 120 million gallons of diesel fuel, the equivalent to eliminating the emissions of 730,000 cars.
Biogas in Wisconsin
The Wisconsin Department of Revenue offers renewable energy producers, including biogas, an exemption from property taxes to commercial, industrial, residential, and agricultural facilities. The Renewable Energy Competitive Incentive Program (RECIP), administered by Focus on Energy, allows businesses to submit request for proposals three time throughout the year. If accepted, these projects are awarded funding the program’s total funds of $700,000. The Focus on Energy Program also accepts proposals for funding feasibility studies, which assist facilities with understanding if biogas production is right for them and how they could benefit from project development.
Wisconsin’s agricultural industry contributes over $100 billion each year to the state’s economy. AD is used on 34 different agricultural facilities, both for waste management and biogas production. Statewide, there are 81 municipal wastewater digesters, 35 landfills with gas capture, and 21 industrial wastewater digesters. In 2019, biomass-to-energy, such as AD, accounted for 25% of Wisconsin’s renewable energy.
Existing Biogas Policy in Minnesota
Minnesota’s most notable policy for biofuel production is the Bioincentive Program. This policy requires certain levels of production, and that at least 80% of the biomass feedstock come from Minnesota and that it be derived from agricultural sources, forestry sources, or organic solid waste. The policy also breaks down biofuel production into three subsections with Advanced Biofuel being most relevant to AD technology. The Advanced Biofuel requirements are designed to assist commercial-scale operations because of the higher production quotas necessary to receive payments. This program uses criteria set by the federal Renewable Fuel Standard (RFS) to define qualifying biofuel. If a facility meets these requirements, they will be paid based on the amount of biogas produced. Minnesota’s own RFS mandates 25% renewable energy by 2025 and includes biogas as a qualifying renewable energy source for additional incentive payments based on production.
Biogas Facilities in Minnesota
Minnesota’s agricultural industry in the fifth largest in the nation, generating $112 billion each year. Given the size of the industry, there is no shortage of agricultural waste feedstock, with millions of gallons of manure generated each day. Yet there are only 7 biogas systems located on farms, with an estimated the potential for an additional 324 biogas facilities on swine farms and 108 on dairy farms.
With over 300,000 tons of food gone to waste in Minnesota each year, there are currently no biogas systems designed for food waste, with the potential for 6 systems throughout the state. Municipalities are beginning to develop organics recycling programs, which could also lead to demand for organic management programs like AD systems.
25 wastewater treatment plants currently produce biogas with an additional 9 that could be developed. AD is considerably developed on wastewater treatment facilities compared to other industries. However, these numbers do not account for the potential development of wastewater AD system on private, industrial facilities.
Lack of Policy Support in Minnesota
The cost of biogas production on dairy farms range from $15 to $30 per dekatherm, which is enough gas to supply the average household for 4 days. In Minnesota, residential natural gas prices have hovered around $9 per dekatherm, meaning that the cost of producing biogas is far higher than what it can be sold at. In order to bolster biogas production, some states have adopted policies and laws that make AD and RNG more viable.
For example, California instituted carbon credit programs that incentivize production of biogas on dairy facilities. In California, dairy carbon credits were valued at $68 per dekatherm, outweighing the costs of biogas production and making it a more viable option. California also has a strong Renewable Portfolio Standard and helps to accelerate the development of emerging biogas markets and facilities. In Oregon, Senate Bill 98 creates a clear path towards 30 percent of all natural gas to be sourced from RNG-producing facilities. The bill also recognized the different needs and goals of larger utilities versus smaller utilities, helping to ensure optimal development of RNG production across the board.
Minnesota does not have the same level of policy infrastructure to support biogas production. There are some grant and loan programs, but much of the support stops here. California biogas markets support production much more than Minnesota, resulting in an incentive for Minnesota biogas producers to sell to out-of-state utilities.
Supporting RNG in Minnesota
Minnesota is often thought of as a hot-spot for progressive agricultural and environmental policies, but it appears to be lagging behind in the newly developing biogas markets. To strengthen the policy landscape, Minnesota could adopt a series of policies and programs that have proven successful in other states, such as:
RNG goals similar to Oregon as part of the Renewable Portfolio Standard
A carbon credit program that could help create a market landscape similar to California so that Minnesota biogas producers could inject RNG into local pipelines and markets
Considering the agricultural similarities to Wisconsin, Minnesota could match certain funding programs or policies that exempt biogas producers from property taxes.
Animal farms are not the only facilities types that benefit from AD. The least developed industries for AD are those involving food waste. There are currently no AD sites that offer waste management for the more than 300,000 tons of food gone to waste each year in Minnesota.
Recently, Minnesota established the goal of composting and recycling solid waste at a rate of 75 percent by 2030. In 2019, 38 percent of solid waste was diverted from landfills to be recycled or composted. As more and more solid waste is diverted, developing new AD facilities has the potential to help municipalities reach their goals.
Private industrial facilities could also greatly benefit from AD technology. Breweries, distilleries, canning facilities, and candy manufacturers all produce high-strength wastewater that could easily be treated and managed through AD systems. Research partnerships between scientists and industry leaders, like those established through MnDRIVE Seed and Demonstration Grants, are an important part in assisting further development of biogas systems on farms and industrial sites.
This site was designed and created by Reed Grumann and Rose Lam, members of the Science Communication Lab at the University of Minnesota.
Special thanks to Dr. Bo Hu, Dr. Paige Novak, and John Jaimez for their input and assistance with the development of content.