Anaerobic digestion is a series of steps where tiny living things break down materials that can be broken down without oxygen. This process helps manage waste or create fuels in factories and homes. Many types of fermentation used to make food and drinks, as well as home fermentation, rely on this method.
Anaerobic digestion reduces the amount of sludge and harmful germs. It also creates biogas rich in methane, which can be used for renewable energy. The liquid left after digestion, called digestate, can be treated and sometimes used again as a resource.
This process happens naturally in some soils and in the sediment of lakes and oceans. It is often called "anaerobic activity." The methane gas from this process, known as marsh gas, was discovered by Alessandro Volta in 1776.
Anaerobic digestion has four main stages:
1. Bacteria break down complex materials into simpler forms that other bacteria can use.
2. Acid-producing bacteria change sugars and amino acids into carbon dioxide, hydrogen, ammonia, and organic acids.
3. Acetate-producing bacteria convert these acids into acetic acid, along with ammonia, hydrogen, and carbon dioxide.
4. Methane-producing bacteria change these substances into methane and carbon dioxide. These special bacteria are important for treating wastewater.
Anaerobic digestion is used to treat biodegradable waste and sewage sludge. It helps reduce harmful gases released into the air when waste is placed in landfills. Digesters can also use crops grown specifically for energy, like corn.
This process is widely used to create renewable energy. It produces biogas, which contains methane, carbon dioxide, and small amounts of other gases. This biogas can be used directly as fuel, to power engines, or upgraded to natural gas-like biomethane. The nutrient-rich digestate can also be used as fertilizer.
Process
Many tiny organisms influence anaerobic digestion, such as bacteria that produce acetic acid and archaea that produce methane. These organisms help change organic material into biogas through several chemical reactions.
To keep oxygen out of the reactions, systems use physical barriers. Anaerobic microorganisms use substances other than oxygen to help with their chemical reactions. These substances can come from the organic material itself or from inorganic materials in the input. When oxygen comes from the organic material, the intermediate products are mostly alcohols, aldehydes, organic acids, and carbon dioxide. With the help of special methane-producing archaea, these intermediates become methane, carbon dioxide, and small amounts of hydrogen sulfide. In anaerobic systems, most of the energy from the starting material is released as methane by methanogenic archaea.
Anaerobic microorganisms usually need a long time to grow and become active. To speed this up, people often add materials that already contain these microorganisms, a process called "seeding." This is often done by adding sewage sludge or cattle waste.
Anaerobic digestion has four main steps: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. The overall process can be shown by a chemical reaction where organic material like glucose is turned into carbon dioxide and methane by anaerobic microorganisms:
C₆H₁₂O₆ → 3CO₂ + 3CH₄
How well the process works depends on factors like temperature (mesophilic at 35–37°C or thermophilic at 50–55°C), organic loading rate, pH (usually 6.8–7.2), and solids retention time. Keeping these factors controlled helps prevent problems like instability or buildup of harmful acids.
Anaerobic digesters can be built in different ways. They can operate in batch or continuous modes, use mesophilic or thermophilic temperatures, handle low or high solids content, or use single or multi-stage processes. Continuous systems are more complex but can be cheaper in the long run than batch systems, which require more space and initial costs. Thermophilic systems use more heat but produce methane faster and in larger amounts. Low solids digesters handle up to 15% solids, while high solids digesters process materials with 25–40% solids without adding water. Single-stage systems use one reactor for all steps, while multi-stage systems use multiple reactors to separate steps like hydrolysis and methanogenesis.
Anaerobic digestion can be done in batch or continuous systems. In a batch system, organic material is added at the start, and the reactor is sealed until the process finishes. Batch systems often need already processed material to start the process. If the reactor is opened too early, strong odors may occur. Some advanced batch systems reduce odors by combining digestion with composting. Batch systems are simpler and cheaper but may need multiple reactors to keep biogas production steady.
In continuous systems, organic material is added constantly or in stages, and the end products are removed regularly. This creates a steady biogas output. Examples of continuous systems include stirred-tank reactors, upflow sludge blankets, and internal circulation reactors.
Anaerobic digesters operate best at two temperature ranges: mesophilic (30–38°C) or thermophilic (49–57°C). Mesophilic systems use moderate temperatures and are more stable, while thermophilic systems use higher temperatures and produce more biogas faster. Thermophilic systems also reduce pathogens better, which is important in places with strict regulations.
In some cases, like in Bolivia, anaerobic digestion happens at very low temperatures (less than 10°C), making the process very slow. Experiments in Alaska showed that psychrophiles (cold-loving microbes) can still produce methane, though less than in warmer climates.
Pre-treatment methods, like shredding materials or using heat, can speed up biogas production. Heat treatment, such as pasteurization, also reduces harmful germs in the final product.
When managing digesters, three main solids content categories are used:
– High solids (dry—stackable materials)
– High solids (wet—pumpable materials)
– Low solids (wet—pumpable materials)
High solids (dry) digesters handle materials with 25–40% solids without adding water, unlike wet digesters that process liquid slurries.
Feedstocks
The most important starting point when using anaerobic digestion systems is the type of material, called feedstock, used in the process. Almost any organic material can be used, but if the goal is to produce biogas, the material’s ability to break down is the most important factor. The easier a material is to break down, the more biogas the system can produce.
Feedstocks include biodegradable materials like waste paper, grass clippings, leftover food, sewage, and animal waste. Woody materials are an exception because they are hard to break down, as most anaerobic bacteria cannot digest lignin, a tough part of plant material. Special bacteria or high-temperature treatments, such as pyrolysis, can help break down lignin. Anaerobic digesters can also use specially grown energy crops, like silage, for biogas production. In Germany and other parts of Europe, these systems are called "biogas" plants. A codigestion or cofermentation plant is an agricultural digester that uses two or more materials at the same time.
The time needed for anaerobic digestion depends on how complex the material is. Materials with simple sugars break down quickly, while materials rich in lignocellulose, like cellulose and hemicellulose, take longer to break down because anaerobic microorganisms cannot digest lignin.
Anaerobic digesters were first designed to process sewage sludge and manure. However, these materials are not the best for biogas production because much of their energy has already been used by the animals that produced them. Many digesters now use codigestion, combining two or more feedstocks. For example, a farm digester using dairy manure might add grass, corn, or organic waste from slaughterhouses or restaurants to increase gas production.
Digesters using dedicated energy crops, like maize or grass silage, can produce high amounts of biogas. Systems that only use slurry (liquid waste) are cheaper but produce less energy. Adding a small amount of crop material (about 30%) can increase energy output ten times compared to slurry-only systems, with only three times the cost.
Another important factor is the moisture content of the feedstock. Drier materials, like food and yard waste, can be digested in tunnel-like chambers. These systems produce little wastewater, which is helpful in areas where liquid waste is a problem. Wetter materials are easier to handle with standard pumps and take up less space relative to the gas they produce. The moisture level also determines the type of system used. If using a high-solids digester for wet materials, bulking agents like compost can be added to increase the solid content. The carbon:nitrogen (C:N) ratio of the feedstock is also important. The ideal C:N ratio is 20–30:1. Too much nitrogen can harm the digestion process by causing ammonia buildup.
The level of physical contaminants in the feedstock is important for wet or plug-flow digestion systems. If materials like plastic, glass, or metal are present, they must be removed before digestion to prevent blockages. Dry digestion or solid-state anaerobic digestion (SSAD) systems avoid this issue because they handle dry, stackable materials with high solid content in airtight chambers called fermenter boxes. More preprocessing, like sorting, shredding, and pulping, increases the surface area for microbes to work on, speeding up digestion.
The composition of the feedstock greatly affects methane production. Techniques exist to analyze feedstock characteristics, and factors like solids content, elemental analysis, and organic content help design digesters. Methane yield can be estimated based on the feedstock’s elemental composition and how much of it breaks down. Predicting biogas composition (the mix of methane and carbon dioxide) requires information about reactor temperature, pH, and substrate composition, along with chemical models. Direct measurements, like gas evolution or gravimetric tests, are also used to assess biogas potential.
Applications
Using anaerobic digestion technologies can help reduce the emission of greenhouse gases in several important ways:
- Replacing fossil fuels with other energy sources
- Reducing or removing the energy needed for waste treatment plants
- Lowering methane emissions from landfills
- Reducing the use of chemical fertilizers made in factories
- Cutting down on the number of vehicles needed for transportation
- Reducing energy lost during electricity transport
- Lowering the use of LP Gas for cooking
- An important part of Zero Waste programs
Anaerobic digestion works best with organic materials, such as industrial waste, wastewater, and sewage sludge. This simple process can greatly reduce the amount of organic waste that would otherwise be thrown into the ocean, landfills, or burned in incinerators.
Environmental laws in developed countries have led to more use of anaerobic digestion to reduce waste and create useful products. These systems can process waste that has been separated from other materials or combine with sorting systems to handle mixed waste. These facilities are called mechanical biological treatment plants.
If the waste processed in anaerobic digesters were placed in landfills instead, it would break down naturally and release methane. Methane is about 20 times more harmful to the environment than carbon dioxide, causing serious environmental harm.
In countries where household waste is collected, local anaerobic digestion systems can reduce the need to transport waste to landfills or incinerators. This lowers the carbon emissions from garbage trucks. If these systems are connected to electricity networks, they can also reduce energy loss from long-distance power transmission.
Anaerobic digestion can help clean up sludge polluted with PFAS. A 2024 study showed that combining anaerobic digestion with activated carbon and voltage can remove up to 61% of PFAS from sewage sludge.
In developing countries, simple anaerobic digestion systems at homes or farms can provide low-cost energy for cooking and lighting. Since 1975, both China and India have supported small biogas plants for household use. Today, projects in developing countries can get financial help through the United Nations Clean Development Mechanism if they show they reduce carbon emissions.
Methane and electricity from anaerobic digesters can replace fossil fuel energy, reducing greenhouse gas emissions. This is because the carbon in biodegradable materials is part of a natural cycle. When biogas is burned, the carbon released was recently taken in by plants. If plants are regrown, the system becomes carbon neutral. Fossil fuels, however, have stored carbon underground for millions of years, and burning them increases carbon dioxide in the atmosphere. Large-scale operations are better suited for anaerobic digestion because they need large amounts of waste to be financially viable.
Biogas from sewage sludge is sometimes used to power engines that generate electricity for wastewater treatment plants. Some heat from the engines is used to warm the digesters. While the power generated from sewage plants is limited, biogas from other sources like food waste, energy crops, and animal waste has greater potential. In the UK, farm biogas plants are expected to reduce CO₂ emissions and support the energy grid while helping farmers earn extra income.
Some countries offer financial support, such as feed-in tariffs, to encourage the use of renewable energy by paying for electricity fed into the power grid.
In Oakland, California, the East Bay Municipal Utility District (EBMUD) mixes food waste with wastewater solids and other waste for digestion. This process produces more energy than digesting wastewater solids alone. For example, digesting food waste can generate 730 to 1,300 kWh of energy per ton, compared to 560 to 940 kWh for wastewater solids.
The methane in biogas can be purified into biomethane, which is similar to natural gas. This process removes impurities like hydrogen sulfide and carbon dioxide. Technologies like pressure swing adsorption, water scrubbing, and membrane separation are used for this.
After purification, biomethane can be compressed and used as fuel for natural gas vehicles. In Sweden, over 38,600 vehicles use natural gas, with 60% of the supply coming from biomethane produced through anaerobic digestion.
The solid material left after digestion can be used as a soil enhancer to improve soil quality. The liquid from the digester can replace chemical fertilizers, which are more energy-intensive to produce and transport. In countries like Spain, where soil quality is poor, the demand for digested solids can be as valuable as the biogas itself.
Anaerobic digestion can also be used on a small scale to produce cooking gas. Organic waste like leaves, kitchen scraps, and food waste is crushed and mixed with water. This mixture is placed in a digester, where bacteria break it down to create gas. The gas is then used for cooking. A 2 cubic meter digester can produce 2 cubic meters of gas, which is equal to 1 kg of LPG.
Products
The three main products of anaerobic digestion are biogas, digestate, and water.
Biogas is the main waste product created when microbes break down biodegradable materials. It is mostly methane and carbon dioxide, with small amounts of hydrogen (H₂), water vapor (H₂O, the amount depends on temperature), and trace amounts of hydrogen sulfide (H₂S). Most biogas is produced during the middle of the digestion process, after the bacteria have grown, and production slows as the decaying material is used up. The gas is usually stored in an inflatable bubble on top of the digester or in a gas holder near the facility.
The methane in biogas can be burned to create heat and electricity, often using a reciprocating engine or microturbine. This is typically done in a system where both electricity and heat are used to warm digesters or buildings. Extra electricity can be sold to suppliers or added to the local power grid. Energy from anaerobic digesters is considered renewable and may receive financial support. Biogas does not increase atmospheric carbon dioxide levels because the gas is not released directly into the air, and the carbon dioxide comes from organic sources with a short carbon cycle.
Biogas may need treatment to remove impurities before being used as fuel. Hydrogen sulfide, a harmful gas formed from sulfates in the feedstock, is present in small amounts. Environmental agencies, such as the U.S. Environmental Protection Agency or the English and Welsh Environment Agency, set strict limits on hydrogen sulfide levels. If levels are too high, equipment like amine gas treating systems is used to clean the gas. Adding ferrous chloride (FeCl₂) to the digester can also reduce hydrogen sulfide production.
Volatile siloxanes, which are found in household waste and wastewater, can contaminate biogas. These compounds can form silicon dioxide (SiO₂) when burned, which builds up inside machines and causes damage. Technologies to remove siloxanes and other contaminants are available. In some cases, in situ treatment can increase methane purity by reducing carbon dioxide in the gas.
In countries like Switzerland, Germany, and Sweden, methane from biogas is often compressed for use as vehicle fuel or added to gas pipelines. In other countries, where renewable energy subsidies focus on electricity, this process is less common because it uses energy and reduces the amount of electricity available for sale.
Digestate is the non-gaseous material left after digestion. It includes parts of the original feedstock that microbes cannot use and the remains of dead bacteria. Digestate is produced in two forms: fibrous (from the acidogenic stage) and liquid (from the methanogenic stage). In two-stage systems, these forms come from different tanks. In single-stage systems, the two types mix into a sludge and can be separated if needed.
Fibrous digestate is a stable, organic material made mostly of lignin and cellulose, along with minerals and dead bacterial cells. It resembles compost and can be used as fertilizer or as a raw material for building products like fiberboard. Liquid digestate is rich in plant nutrients, such as nitrogen and potassium, and can be used as fertilizer if it meets quality standards. Levels of potentially toxic elements (PTEs) must be tested, as they depend on the original feedstock. Industrial waste may contain higher PTE levels, requiring careful handling.
Fibrous digestate contains biopolymers like lignin, which microbes cannot break down. Liquid digestate may have ammonia at levels harmful to plants. For these reasons, digestate may undergo composting to break down lignin and convert ammonia into nitrates, improving its quality as a soil amendment. Composting also reduces the volume of material, making it easier to transport. Composting is often used in dry digesters, while wet digesters may need bulking agents to support the process. Most digesters cannot break down compostable packaging, so secondary composting is needed for such materials.
The final product of anaerobic digestion is water, which comes from the moisture in the original waste and water produced during microbial reactions. This water may be separated from digestate during dewatering or remain separate.
Wastewater from anaerobic digestion facilities often has high levels of biochemical oxygen demand (BOD) and chemical oxygen demand (COD), which measure the potential for pollution. Some of this material, called "hard COD," cannot be broken down by microbes into biogas. If released into waterways, this effluent could cause eutrophication. Additional treatment, such as oxidation using air in sequencing batch reactors or reverse osmosis units, is often required to reduce pollution risks.
History
Scientific interest in making gas from the natural breakdown of organic matter began in the 17th century. Scientists Robert Boyle and Stephen Hales observed that stirring sediment in streams and lakes released flammable gas. In 1778, Alessandro Volta, an Italian scientist, discovered that the gas was methane.
In 1808, Sir Humphry Davy proved methane was present in gases from cow manure. The first known anaerobic digester was built in 1859 at a leper colony in Bombay, India. In 1895, a septic tank in Exeter, England, was used to produce gas for lighting. In 1904, a dual-purpose tank for treating waste was installed in London, England.
By the early 1900s, anaerobic digestion systems looked similar to modern versions. In 1906, Karl Imhoff created the Imhoff tank, an early model of an anaerobic digester and wastewater treatment system. After 1920, closed tank systems replaced open lagoons for treating waste. Research on anaerobic digestion started seriously in the 1930s.
During World War I, production of biofuels slowed as oil became more widely used. Fuel shortages during World War II revived interest in anaerobic digestion, but interest dropped after the war. The 1970s energy crisis increased interest in small-scale and rural systems. In India, Dr. Ram Bux Singh developed early biogas plants and promoted their use. His work was mentioned in publications like Mother Earth News, which called him “perhaps the father of methane development in the United States” during early biogas experiments. Factors affecting the use of anaerobic systems include willingness to try new ideas, pollution penalties, government support, and access to funding.
Today, anaerobic digesters are often used on farms to reduce pollution from manure or at wastewater treatment plants to lower disposal costs. Germany has the most agricultural digesters, with 8,625 in 2014. The United Kingdom had 259 facilities in 2014, with 500 planned by 2019. The United States had 191 operational plants in 34 states in 2012. Differences in adoption rates may be due to government policies.
Germany introduced feed-in tariffs in 1991, which offered long-term contracts for renewable energy investments. This led to a rise in anaerobic digester plants, from 20 in 1991 to 517 by 1998. After 2000, the German government adjusted these tariffs to improve profitability, leading to steady growth in biogas production.
Anaerobic digesters have caused fish kills in rivers like the River Mole, River Teifi, and others. They have also led to explosions, such as in Avonmouth, England, and at wastewater treatment plants in the United States, including in Maine, Oregon, and Pennsylvania. In Europe, about 800 accidents occurred on biogas plants between 2005 and 2015, though few caused serious harm to people. A 2016 study recorded 169 accidents involving anaerobic digesters.