Anaerobic digestion is a series of steps in which tiny living things called microorganisms break down materials that can be decomposed in the absence of oxygen. This process is used in industries and homes to manage waste or create fuels. Many types of fermentation used to make food and drinks, as well as home-based fermentation, rely on anaerobic digestion.

Anaerobic digestion reduces the amount of sludge and harmful germs while producing biogas rich in methane, which can be used to generate renewable energy. The liquid left after digestion, called digestate, can be treated further and sometimes used as a resource.

Anaerobic digestion happens naturally in some soils and in sediments at the bottom of lakes and oceans. This natural process is often called "anaerobic activity." It is the source of marsh gas, methane, which was discovered by Alessandro Volta in 1776.

Anaerobic digestion has four main stages:
1. Bacteria break down complex materials, like carbohydrates, into simpler substances 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, called methanogens, turn these substances into methane and carbon dioxide. These bacteria are very important in treating wastewater.

Anaerobic digestion is used to manage biodegradable waste and sewage sludge. It helps reduce the release of landfill gas into the atmosphere when part of a waste management system. Anaerobic digesters can also process crops grown specifically for energy, such as maize.

Anaerobic digestion is widely used to create renewable energy. The process produces biogas, which is mainly methane and carbon dioxide, along with small amounts of other gases. This biogas can be used directly as fuel, in engines that produce heat and power, or upgraded to a form similar to natural gas. The nutrient-rich digestate produced can also be used as fertilizer.

Process

Many tiny living things, such as bacteria that make acetic acid and archaea that make methane, help in the process of anaerobic digestion. These microorganisms help change organic materials into biogas through chemical reactions.

Oxygen gas is kept out of the reactions by using containers that block it. Anaerobic microorganisms use other substances, like organic materials or inorganic oxides, as electron acceptors instead of oxygen. When the oxygen comes from the organic material itself, the main intermediate products are alcohols, aldehydes, organic acids, and carbon dioxide. When specialized methane-making archaea are present, these intermediates become methane, carbon dioxide, and small amounts of hydrogen sulfide. In an anaerobic system, most of the energy in the starting material is released as methane by methanogenic archaea.

It takes time for anaerobic microorganisms to grow and become active. To speed this up, people often add materials that already have these microorganisms, a process called "seeding." This is usually done by adding sewage sludge or cattle waste.

The four main steps in anaerobic digestion are hydrolysis, acidogenesis, acetogenesis, and methanogenesis. The overall process can be shown by the chemical reaction where organic material like glucose is broken down into carbon dioxide (CO₂) and methane (CH₄) by anaerobic microorganisms:

C₆H₁₂O₆ → 3CO₂ + 3CH₄

How well the process works depends on factors like temperature (mesophilic at about 35–37°C or thermophilic at 50–55°C), the amount of organic material added, the pH level (usually between 6.8 and 7.2), and how long the material stays in the digester. Keeping these factors controlled is important to avoid problems like instability or buildup of harmful substances.

Anaerobic digesters can be built in different ways. They can operate in batch or continuous modes, use mesophilic or thermophilic temperatures, handle high or low solid content, or use single or multiple stages. Continuous processes require more complex designs but may cost less over time than batch processes because batch systems need more space and initial investment. Thermophilic systems use more heat energy but produce more methane faster. Low solid digesters handle up to 15% solids, while high solid digesters (dry or wet) process materials with higher solid content. Single-stage digesters complete all four steps in one reactor, while multistage digesters use separate reactors for different steps.

Anaerobic digestion can happen 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, leading to steady biogas production. Examples of continuous systems include reactors like continuous stirred-tank reactors and upflow anaerobic sludge blankets.

Anaerobic digesters operate at two main temperature levels, which affect the types of methane-making microorganisms:
– Mesophilic digestion happens best at 30–38°C (or 20–45°C), where mesophiles (moderate-temperature microbes) are most active.
– Thermophilic digestion happens best at 49–57°C (or up to 70°C), where thermophiles (heat-loving microbes) are most active.

In Bolivia, anaerobic digestion has been tested at very low temperatures (less than 10°C), but it works much slower. In Alaska, a digester using cold-adapted microbes produced about 20–30% of the methane from warmer systems. Mesophilic systems are more stable and better at handling changes in conditions, while thermophilic systems produce more biogas quickly and reduce pathogens more effectively.

To improve biogas production, some digesters use pre-treatment methods, like cutting materials into smaller pieces or using heat (like pasteurization) to increase efficiency. Pasteurization also helps reduce harmful germs in the final product.

In typical operations, three types of solid content are used in digesters:
– High solids (dry—stackable materials) with 25–40% solids.
– High solids (wet—pumpable materials) with high solids but liquid enough to pump.
– Low solids (wet—pumpable materials) with lower solids content.

High solids (dry) digesters handle solid materials without adding water, unlike wet digesters that process liquid slurries.

Feedstocks

The most important first step when using anaerobic digestion systems is choosing the right feedstock, or starting material. Many organic materials can be used in these systems, but if the goal is to produce biogas, the material’s ability to break down, called putrescibility, is the most important factor. The easier a material is to break down, the more gas the system can produce.

Feedstocks can include biodegradable materials like waste paper, grass clippings, leftover food, sewage, and animal waste. Woody materials, such as wood chips, are an exception because most anaerobic bacteria cannot break down lignin, a tough part of wood. Special bacteria called xylophagous anaerobes or high-temperature treatments, like pyrolysis, can help break down lignin. Anaerobic digesters can also use specially grown energy crops, such as silage, for biogas production. In Germany and parts of Europe, these systems are called "biogas" plants. A codigestion or cofermentation plant is an agricultural digester that uses two or more types of materials at the same time.

The time needed for anaerobic digestion depends on how complex the material’s chemical structure is. Materials with simple sugars break down quickly, while materials rich in lignocellulosic components, like cellulose and hemicellulose, take longer to break down. Anaerobic microorganisms generally cannot break down lignin, the tough, aromatic part of plant material.

Anaerobic digesters were originally designed to process sewage sludge and manure. However, these materials are not the best for biogas production because the energy-rich parts have 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 as the main material can produce more gas by adding other materials like grass, corn, slaughterhouse waste, restaurant fats, oils, or household waste.

Digesters that use dedicated energy crops, like maize or grass silage, can produce high levels of biogas. Systems that only use liquid waste, such as slurry-only systems, are cheaper but produce much less energy. Adding a small amount of crop material (about 30%) to a slurry-only system can increase energy output ten times for only three times the cost.

Another important factor is the moisture content of the feedstock. Drier materials, like food and yard waste, can be processed in tunnel-like chambers. These systems produce little to no wastewater, which is helpful in areas where wastewater discharge is a problem. Wetter materials are easier to handle with standard pumps and take up less space compared to the gas they produce. The moisture level also determines the type of system used. If a high-solids digester is used for wet feedstocks, bulking agents like compost may be added to increase the solid content. The carbon-to-nitrogen ratio of the feedstock is also important. This ratio represents the balance of nutrients microbes need to grow, and the ideal range is 20–30:1. Too much nitrogen can cause ammonia to build up and harm the digestion process.

The level of physical contamination in the feedstock is important for wet or plug-flow digesters. If the material contains plastic, glass, or metal, it must be cleaned before use. Otherwise, these materials can block the digester and reduce efficiency. Dry digestion or solid-state anaerobic digestion (SSAD) systems do not face this issue because they process dry, stackable biomass with high solid content (40–60%) in sealed containers called fermenter boxes. The more processing required for a feedstock, the more machinery is needed, which increases the project’s cost.

After removing physical contaminants, the feedstock is often shredded, minced, or pulped to increase the surface area available to microbes, speeding up digestion. A chopper pump can transfer the material into an airtight digester where anaerobic treatment occurs.

The composition of the feedstock greatly affects the amount of methane produced and how quickly it is made. Techniques exist to analyze the feedstock’s characteristics, and factors like solids content, elemental composition, and organic analysis are important for designing and operating digesters. Methane yield can be estimated based on the feedstock’s elemental composition and how much of it can be converted to biogas. To predict the mix of methane and carbon dioxide in the biogas, information about reactor temperature, pH, and substrate composition is needed, along with a chemical model. Direct measurements of biogas potential are also made using gas evolution tests or newer gravimetric methods.

Applications

Using anaerobic digestion technologies can help reduce the emission of greenhouse gases in several important ways:

• Replacing fossil fuels
• Reducing or removing the energy use of waste treatment plants
• Lowering methane emissions from landfills
• Replacing chemical fertilizers made in factories
• Reducing the number of vehicles needed for transport
• Lowering energy losses during electricity transport
• Using less LP Gas for cooking
• A key part of Zero Waste programs.

Anaerobic digestion works best with organic materials and is often used to treat industrial waste, wastewater, and sewage sludge. This process can greatly reduce the amount of organic waste that would otherwise be dumped in the ocean, landfills, or burned in incinerators.

Environmental laws in developed countries have increased the use of anaerobic digestion to reduce waste and create useful byproducts. These systems can process waste that has been separated from other materials or be combined with sorting machines to handle mixed waste. These facilities are called mechanical biological treatment plants.

If the organic waste processed in anaerobic digesters were sent to landfills, it would break down naturally and release methane. Methane is about 20 times more harmful to the environment than carbon dioxide, which causes serious problems.

In countries that collect household waste, local anaerobic digestion plants can reduce the need to transport waste to landfills or incinerators. This reduces emissions from garbage trucks. If these plants are connected to electricity networks, they can also lower energy losses from long-distance power transmission.

Anaerobic digestion can help clean sludge polluted with PFAS. A 2024 study found that combining anaerobic digestion with activated carbon and electricity 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, China and India have supported small biogas plants for household use. Today, projects in developing countries can receive funding through the United Nations Clean Development Mechanism if they reduce carbon emissions.

Methane and electricity from anaerobic digesters can replace fossil fuels, reducing greenhouse gas emissions. The carbon in biodegradable materials is part of a natural cycle. Plants absorb carbon from the atmosphere, and when biogas is burned, that carbon is released. If plants are regrown, the cycle continues, making the system carbon neutral. Fossil fuels, however, trap carbon underground for millions of years, increasing carbon dioxide in the atmosphere. Large-scale operations, not small farms, are best for producing biogas because they need large amounts of waste to be cost-effective.

Biogas from sewage sludge is sometimes used to power engines that generate electricity for wastewater treatment plants. Heat from the engines can warm the digesters. While this provides some energy, the total power from sewage plants is small compared to overall energy needs. Biogas from other sources, like food waste or farm waste, has greater potential. In the UK, farm biogas plants can reduce carbon emissions and help the electricity grid while giving farmers extra income.

Some countries offer financial support, such as feed-in tariffs, to encourage renewable energy production.

In Oakland, California, the East Bay Municipal Utility District (EBMUD) mixes food waste with wastewater solids and other waste for digestion. Combining food waste with wastewater solids produces more energy than digesting wastewater alone. For example, digesting one dry ton of food waste can generate 730 to 1,300 kWh of energy, compared to 560 to 940 kWh from wastewater solids.

Methane from biogas can be purified into biomethane, which is similar to natural gas. This process removes harmful substances like hydrogen sulfide and carbon dioxide. Technologies like pressure swing adsorption, water scrubbing, and membrane separation are used for purification.

After purification, biomethane can be compressed and used as fuel for natural gas vehicles. In Sweden, over 38,600 vehicles use natural gas, and 60% of their fuel comes from biomethane produced through anaerobic digestion.

The solid, fibrous material left after digestion can be used as soil conditioner to improve soil quality. The liquid from digesters can also be used as fertilizer, reducing the need for chemical fertilizers, which are more energy-intensive to make and transport. In countries like Spain, where soil is often poor in organic matter, the value of digested solids can be as important as the biogas itself.

Anaerobic digestion can be used on a small scale to produce cooking gas. Organic waste, such as 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 enough gas to cook for one day, equivalent to 1 kg of LPG.

Products

The three main products of anaerobic digestion are biogas, digestate, and water.

Biogas is the gas produced when microbes break down organic material. It is mostly made of methane and carbon dioxide, with small amounts of hydrogen, water vapor, and a tiny amount of hydrogen sulfide. Most biogas is created during the middle of the digestion process, after the microbes have grown, and production decreases when the organic material is used up. The gas is often stored in a bubble on top of the digester or kept in a separate container next to the facility.

The methane in biogas can be burned to create heat and electricity, usually using an engine or turbine. This process often uses both the electricity and heat, with the heat used to warm digesters or buildings. Extra electricity can be sold to power companies or added to the local power grid. Energy from anaerobic digesters is considered renewable and may receive financial support. Biogas does not add carbon dioxide to the atmosphere because the gas is not released directly, and the carbon dioxide comes from organic material that has a short cycle.

Biogas may need cleaning to be used as fuel. Hydrogen sulfide, a harmful gas created from sulfates in the feedstock, is present in small amounts. Environmental agencies set strict limits on hydrogen sulfide levels. If levels are too high, special equipment is needed to remove it. Adding ferrous chloride to the digester can also reduce hydrogen sulfide production.

Volatile siloxanes, chemicals found in household waste and wastewater, can also contaminate biogas. These chemicals can form silicon dioxide when burned, which damages equipment. Technologies to remove siloxanes and other contaminants are available. In some cases, additional processing can increase methane purity by removing carbon dioxide.

In countries like Switzerland, Germany, and Sweden, methane from biogas is sometimes compressed for use as vehicle fuel or sent into gas pipelines. In places where financial support for renewable energy is available, this process may be less common because it requires energy and reduces the amount of electricity that can be sold.

Digestate is the part of the process that is not gas. It includes leftover material from the original waste that microbes could not use and the remains of dead bacteria. The acidogenic and methanogenic stages each produce different types of digestate: fibrous and liquid, respectively. In two-stage systems, these types come from different tanks. In single-stage systems, they mix together and can be separated if needed.

Acidogenic digestate is a stable, fibrous material made mostly of lignin and cellulose, along with some minerals and dead bacteria. It resembles compost and can be used as fertilizer or for making building materials. Methanogenic digestate is a liquid rich in plant nutrients like nitrogen and potassium. It can be used as fertilizer, but the levels of harmful substances depend on the original waste. Industrial waste may have higher levels of harmful materials, which must be considered when deciding how to use the digestate.

Acidogenic digestate contains materials like lignin that microbes cannot break down. Methanogenic digestate may have high ammonia levels that can harm plants. For these reasons, digestate may be composted after digestion to break down harmful substances and improve its quality. Composting also reduces the amount of material that needs to be transported.

The final product of anaerobic digestion is water, which comes from the moisture in the original waste and from reactions during digestion. This water may be separated from the digestate or released during processing.

Water from the digestion process often has high levels of biochemical oxygen demand (BOD) and chemical oxygen demand (COD), which measure how much pollution the water can cause. Some of this pollution cannot be broken down by microbes. If this water were released directly into waterways, it could harm ecosystems by causing excessive plant growth. Therefore, further treatment is often needed. This treatment usually involves adding air to the water in special equipment to reduce pollution.

History

Scientific interest in creating gas from the natural breakdown of organic matter began in the 17th century. Robert Boyle (1627–1691) and Stephen Hales (1677–1761) observed that disturbing sediment in streams and lakes released flammable gas. In 1778, Alessandro Volta (1745–1827), an Italian scientist known as the father of electrochemistry, identified this gas as methane.

In 1808, Sir Humphry Davy proved methane was present in gases from cattle 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 sedimentation and sludge treatment was installed in Hampton, London.

By the early 20th century, anaerobic digestion systems resembled modern designs. In 1906, Karl Imhoff created the Imhoff tank, an early version of an anaerobic digester and a model for wastewater treatment. After 1920, closed tank systems replaced open lagoons for treating waste. Research on anaerobic digestion increased significantly in the 1930s.

During World War I, biofuel production slowed as petroleum became more widely used. Fuel shortages during World War II revived interest in anaerobic digestion, but interest declined after the war. The 1970s energy crisis renewed global interest in small-scale and rural anaerobic systems. In India, engineer Dr. Ram Bux Singh developed and promoted early biogas plants, helping spread anaerobic digestion technology for rural energy use. His work was noted in publications like Mother Earth News, which called him “perhaps the father of methane development in the United States” during early biogas experiments. Factors influencing the adoption of anaerobic systems include willingness to try new ideas, pollution penalties, government policies, and access to funding.

Today, anaerobic digesters are often used on farms to reduce nitrogen runoff from manure or at wastewater treatment facilities to lower sludge disposal costs. Agricultural anaerobic digestion for energy production is most common in Germany, which had 8,625 digesters in 2014. The United Kingdom had 259 facilities by 2014, with 500 planned by 2019. The United States had 191 operational plants across 34 states in 2012. Differences in adoption rates may be explained by government policies, such as Germany’s feed-in tariffs (FIT), which provided long-term contracts for renewable energy investments. Between 1991 and 1998, Germany’s anaerobic digester count grew from 20 to 517. Adjustments to FIT between 2000 and 2011 improved profitability and ensured steady growth in biogas production.

Anaerobic digesters have caused fish deaths in rivers such as the River Mole, Devon, River Teifi, and Afon Llynfi. They have also led to human fatalities, such as the Avonmouth explosion. In the United States, explosions occurred at facilities in Jay, Maine; Pensacola, Florida; Aumsville, Oregon; and Pennsylvania. In the United Kingdom, explosions happened at Avonmouth and Harper Adams College. Between 2005 and 2015, about 800 accidents occurred at biogas plants in Europe, including incidents in France (Saint-Fargeau) and Germany (Rhadereistedt, where four people died). A 2016 study collected data on 169 accidents involving anaerobic digesters.