Biogas is a type of renewable fuel made from organic materials such as farm waste, animal manure, city trash, plants, sewage, wastewater, garden waste, and food scraps. It is created through a process called anaerobic digestion, which uses bacteria in a special container called an anaerobic digester, biodigester, or bioreactor. The gas mainly contains methane (CH₄) and carbon dioxide (CO₂), and may also include small amounts of hydrogen sulfide (H₂S), water, and siloxanes. Methane can be burned or combined with oxygen to release energy. This energy allows biogas to be used as a fuel, such as in fuel cells, for cooking, or to generate heat. It can also power engines to produce electricity and heat.
Biogas can be improved to meet the quality standards of natural gas by removing carbon dioxide and other impurities. When biogas is upgraded to match natural gas, it is called renewable natural gas (RNG). RNG can be used directly in the natural gas system or processed into compressed natural gas for vehicles.
Biogas is considered a renewable resource. In general, it is a carbon-neutral fuel because the carbon dioxide released when it is burned is balanced by the carbon dioxide absorbed from the atmosphere during the growth of the organic materials used to make it. However, the actual carbon impact of biogas can vary based on how the organic materials are produced and the methods used to create and upgrade the gas. In some cases, capturing biogas can reduce methane emissions, which helps lower overall emissions.
Production
Biogas is created when certain microorganisms, like methanogens and sulfate-reducing bacteria, perform anaerobic respiration. Biogas can be made naturally or in industrial settings.
In soil, methane is produced in areas without oxygen by methanogens. However, methane is often used up by methanotrophs in areas with oxygen. Methane emissions happen when methanogens have an advantage over methanotrophs. Wetland soils are the largest natural source of methane. Other sources include oceans, forest soils, termites, and wild ruminants.
Anaerobic digestion is a process where microorganisms break down organic material without oxygen. This process creates biogas, which can be used as fuel. Industrial biogas production can be done in specially built systems, such as anaerobic digesters that process manure and organic waste, or by capturing biogas from landfills or wastewater treatment plants.
A biogas plant is a type of anaerobic digester that processes farm waste, city organic waste, or energy crops. These plants use air-tight tanks to create oxygen-free conditions. The material is heated to either moderate temperatures (~38°C) or high temperatures (>55°C) and is kept for two to thirty days.
These plants can use energy crops, such as maize silage, or biodegradable waste like sewage sludge and food waste. Microorganisms convert biomass waste into biogas and digestate. More biogas is produced when wastewater is mixed with waste from industries like dairy, sugar, or brewing. For example, mixing 90% brewery wastewater with 10% cow whey increased biogas production by 2.5 times compared to using brewery wastewater alone.
Landfill gas is created when wet organic waste breaks down without oxygen, similar to biogas. The waste is covered and compressed by the weight of new material, blocking oxygen and allowing anaerobic microbes to grow. If a landfill is not designed to capture the gas, biogas can slowly escape into the air. Uncontrolled landfill gas can be dangerous because it may explode when mixed with oxygen. Methane becomes explosive at concentrations between 5% and 15%.
Methane in biogas is 28 times more powerful as a greenhouse gas than carbon dioxide. Uncontained landfill gas can worsen global warming. Volatile organic compounds in landfill gas also contribute to photochemical smog.
Biogas produces air pollution similar to natural gas. When methane is burned, it creates carbon dioxide, a greenhouse gas (CH₄ + 2 O₂ → CO₂ + 2 H₂O). Toxic hydrogen sulfide in biogas can cause accidents. Unburned methane leaks are also a risk because methane is a strong greenhouse gas. A facility might leak 2% of its methane.
Biogas forms an explosive mixture with air when its concentration is between 6% and 22% by volume. To prevent explosions during maintenance, digesters must be properly ventilated, cleared of gas, and monitored before entering. Systems must be designed to avoid negative pressure, which can pull air in and create an explosive atmosphere. This is ensured through technical controls, such as automatic shut-off devices, not just by avoiding low pressure.
Frequent smell checks should be done on biogas systems. If biogas is smelled, windows and doors should be opened immediately. If there is a fire, the gas should be shut off at the gate valve of the system.
Composition
The makeup of biogas depends on the type of material used and the conditions in the anaerobic digester, such as temperature, pH, and how much material is present. A study from 2025 in Ethiopia found that factors like agroecology (which affects temperature), the design of biogas plants, and temperature strongly influence the quality of biogas produced. Landfill gas usually contains about 50% methane. Advanced waste treatment methods can create biogas with 55–75% methane. For digesters with liquid, in-situ purification techniques can increase methane levels to 80–90%. Biogas naturally contains water vapor. The amount of water vapor depends on the temperature of the gas. Simple math can adjust measured gas volume to account for water vapor and thermal expansion, providing the standardized volume of dry biogas.
For 1,000 kg (wet weight) of material entering a typical biodigester, total solids may make up 30% of the wet weight. Of these, volatile suspended solids may account for 90% of the total solids. Protein might be 20% of the volatile solids, carbohydrates 70% of the volatile solids, and fats 10% of the volatile solids. Biochemical oxygen demand (BOD) measures the oxygen needed by aerobic microorganisms to break down organic matter in the biodigester. BOD levels in liquid discharge help calculate the daily energy output of a biodigester.
Hydrogen sulfide (H₂S), which is toxic, corrosive, and has a strong odor, is the most common contaminant in biogas. If not removed, burning biogas produces sulfur dioxide (SO₂) and sulfuric acid (H₂SO₄), both of which are corrosive and harmful to the environment. Other sulfur-containing compounds, such as thiols, may also be present.
Ammonia (NH₃) forms from nitrogen-containing organic compounds, like amino acids in proteins. If not removed, burning biogas releases NOₓ emissions.
In some cases, biogas contains siloxanes, which come from the anaerobic breakdown of materials in soaps and detergents. When biogas with siloxanes is burned, silicon is released and may combine with oxygen or other elements in the combustion gas. This creates deposits mainly made of silica (SiO₂) or silicates (SiₓOᵧ), which may also include calcium, sulfur, zinc, or phosphorus. These white mineral deposits can build up to several millimeters thick and must be removed using chemical or mechanical methods.
Debate
High levels of methane are created when manure is stored without oxygen. During storage and when manure is spread on land, nitrous oxide is also made as a result of a process called denitrification. Nitrous oxide (N₂O) is 273 times more powerful as a greenhouse gas than carbon dioxide, and methane is 27 times more powerful than carbon dioxide. By turning cow manure into methane biogas through a process called anaerobic digestion, the millions of cattle in the United States could produce 100 billion kilowatt hours of electricity. This amount of energy would be enough to power millions of homes across the United States. One cow can create enough manure in one day to generate 3 kilowatt hours of electricity. Additionally, using manure to make biogas instead of letting it break down naturally could reduce global warming gases by 99 million metric tons, or 4%.
Some environmental groups say manure-based biogas is a form of greenwashing. They argue that it supports large-scale animal farming operations and releases other harmful substances, such as hydrogen sulfide. In 2022, six U.S. senators, including Bernie Sanders and Elizabeth Warren, stated that biogas would need government money to succeed and that this funding could be better used for other energy methods. They also claimed that biogas might lead to larger farms growing to qualify for biogas subsidies. They cited examples from California, where farmers expanded their operations after biogas incentive programs were introduced.
Others say biogas already receives too much financial support. For example, in Wisconsin, spending on biogas in two years (2022–2023) was greater than spending on solar energy over 12 years. Producing biogas from crops like maize grown specifically for this purpose has been criticized as harmful because these crops are planted in very dense areas that can damage soil and cause erosion.
Applications
Biogas can be used to produce electricity at sewage treatment plants through a combined heat and power (CHP) gas engine. The heat from the engine is used to warm the digester, cook food, heat buildings, warm water, and provide heat for industrial processes. When compressed, biogas can replace compressed natural gas in vehicles. It can power internal combustion engines or fuel cells and reduces carbon dioxide emissions more effectively than using biogas in on-site CHP plants.
Raw biogas from digestion contains about 60% methane, 39% carbon dioxide, and small amounts of hydrogen sulfide. This mixture is not suitable for machinery because hydrogen sulfide is harmful and can damage equipment. Methane can be purified using a biogas upgrader to match the quality of fossil natural gas. This process removes carbon dioxide, water, hydrogen sulfide, and other impurities. If the local gas network allows, the purified biogas, now called biomethane, can be sent through the network.
Four main methods are used to upgrade biogas: water washing, pressure swing absorption, selexol absorption, and amine gas treating. Membrane separation technology is also becoming more common, with plants in Europe and the USA using it. The most common method is water washing, where high-pressure gas flows through a column. Water removes carbon dioxide and other impurities, leaving gas with up to 98% methane. This process uses about 3% to 6% of the total energy produced to operate.
Injecting biogas into the natural gas grid is possible only after upgrading it to biomethane. Before this, biogas undergoes cleaning to remove harmful substances. This allows the gas to be transported to consumers for energy production, reducing energy loss during transport. Natural gas transmission systems typically lose 1% to 2% of energy, while electricity transmission systems lose 5% to 8%.
Compressed biogas is used in vehicles, especially in Sweden, Switzerland, and Germany. A biogas-powered train called Biogaståget Amanda has operated in Sweden since 2005. Biogas also powers cars. In 1974, a British documentary showed how biogas from pig manure fueled a custom engine. In 2007, about 12,000 vehicles worldwide used upgraded biogas, mostly in Europe.
Biogas is part of a group of gases that includes water vapor in the gas stream. This vapor can condense inside pipes and stacks, forming mist or fog. Biogas is found in places like wastewater digesters, landfills, and animal farming operations. Ultrasonic flow meters are the best tools for measuring biogas flow because moisture in the gas can interfere with other devices.
Biogas can power internal combustion engines like Jenbacher or Caterpillar engines. It can also be used in gas turbines and other systems to produce electricity and heat. The leftover material from biogas production, called digestate, is used as fertilizer. Biogas is a reliable energy source that can be used quickly when needed. Using biogas instead of fossil fuels reduces global warming potential. However, biogas can increase acidification and eutrophication effects 25 and 12 times more than fossil fuels, respectively. These effects can be reduced by using the right mix of materials, covering digesters, and managing waste properly. Overall, biogas still reduces most environmental impacts compared to fossil fuels, but balancing these effects is important when using biogas systems.
Technological advancements
Projects like NANOCLEAN are creating new methods to produce biogas more efficiently by using iron oxide nanoparticles during the treatment of organic waste. This method increases biogas production by three times.
Faecal sludge is a result of onsite sanitation systems. After being collected and transported, faecal sludge can be treated with sewage in a traditional treatment plant or processed separately in a faecal sludge treatment plant. It can also be treated together with organic solid waste through composting or anaerobic digestion. Biogas can be made by anaerobic digestion when treating faecal sludge. Properly managing excreta and turning it into biogas from faecal sludge helps reduce problems caused by poor excreta management, such as waterborne diseases and pollution of water and the environment.
Resource Recovery and Reuse is a subprogram of the CGIAR Research Program on Water, Land and Ecosystems. It focuses on research to safely recover water, nutrients, and energy from waste from homes and industries. This program believes using waste to create energy is financially beneficial and can help solve sanitation, health, and environmental challenges.
Legislation
The European Union (EU) has laws about managing waste and landfill areas known as the Landfill Directive. Countries like the United Kingdom and Germany now have laws that help farmers earn steady income and ensure energy supplies. The EU requires that engines using biogas have enough gas pressure to work efficiently. In the EU, ATEX centrifugal fan units made following the European directive 2014–34/EU (previously 94/9/EG) are required. Examples of these units include Combimac, Meidinger AG, and Witt & Sohn AG. These units can be used in Zone 1 and Zone 2 areas.
In the United States, laws address landfill gas because it contains harmful chemicals called VOCs. The Clean Air Act and Title 40 of the Code of Federal Regulations (CFR) require landfill owners to calculate how much non-methane organic compounds (NMOCs) are released. If the estimated NMOC emissions are more than 50 tonnes per year, the landfill owner must collect the gas and treat it to remove the NMOCs. This often involves burning the gas. Because many landfills are in remote areas, it may not be cost-effective to use the gas to make electricity. Several programs in the United States offer grants and loans to support the development of anaerobic digester systems. These include the Rural Energy for America Program, the Environmental Quality Incentives Program, the Conservation Stewardship Program, and the Conservation Loan Program.
Global developments
Biogas is becoming a popular energy source in the United States. In 2003, the U.S. used 43 terawatt-hours (147 trillion BTU) of energy from landfill gas, which was about 0.6% of the total natural gas used in the country. Methane biogas made from cow manure is being tested in the U.S. A 2008 study found that methane biogas from cow manure could produce 100 billion kilowatt-hours of energy, enough to power millions of homes. This biogas could also reduce 99 million metric tons of greenhouse gas emissions, or about 4% of the total greenhouse gases produced by the U.S.
The number of farm-based digesters increased by 21% in 2021, according to the American Biogas Council. In Vermont, biogas from dairy farms is part of the GMP Cow Power Program. Customers can pay a higher price on their electric bills, and that money goes directly to farms in the program. In Sheldon, Vermont, Green Mountain Dairy uses an anaerobic digester to process waste from 950 cows. This system produces electricity for about 300 to 350 homes and creates fertilizer and bedding material. The system has a generator capacity of about 300 kilowatts.
In Hereford, Texas, cow manure powers an ethanol plant. This plant saves 1,000 barrels of oil each day and reduces transportation costs. It also plans to create more jobs for future biogas plants. In Oakley, Kansas, an ethanol plant uses manure, food waste, and ethanol waste to produce heat. At full capacity, this plant could replace 90% of the fossil fuel used in making ethanol and methanol. In California, the Southern California Gas Company wants to mix biogas into natural gas pipelines. However, state officials believe biogas is better used in industries that are hard to electrify, like aviation and trucking.
Biogas development varies in Europe. Countries like Germany, Austria, Sweden, and Italy use biogas widely, but other parts of Europe have untapped potential, especially in Eastern Europe. MT-Energie, a German company, works on biogas technology. Legal rules, education, and technology availability affect biogas growth. Public opinion also influences its progress.
In 2009, the European Biogas Association (EBA) was created in Brussels to promote biogas use in Europe. EBA focuses on three goals: making biogas a key energy source, encouraging waste separation to increase gas production, and supporting biomethane as vehicle fuel. By 2013, EBA had 60 members from 24 European countries.
In the UK, there are about 130 non-sewage biogas plants, mostly on farms. Some larger plants use food waste. In 2010, biogas was first injected into the UK gas grid. Sewage from 30,000 Oxfordshire homes is turned into biogas, which powers about 200 homes. In 2015, Ecotricity planned to build three biogas plants connected to the gas grid.
In Italy, biogas production began in 2008 with government incentives. These were later replaced by different support programs, which slowed growth after 2012. By 2018, Italy had over 200 biogas plants producing about 1.2 gigawatts of energy.
Germany is Europe’s largest biogas producer. In 2010, there were 5,905 biogas plants in Germany, mostly in Lower Saxony, Bavaria, and eastern states. Most plants are connected to combined heat and power systems, which generate electricity and heat. In 2010, these plants produced 12.8 terawatt-hours of electricity, or 12.6% of all renewable energy in the country. German biogas comes from mixing energy crops like corn with manure and organic waste.
Germany’s biogas growth started in 1991 with the Electricity Feed-in Act, which required power companies to buy renewable energy. In 2000, the Renewable Energy Sources Act guaranteed fixed payments for 20 years, helping farmers produce energy. In 2004, the NawaRo-Bonus encouraged using energy crops. By 2011, energy crops for biogas covered about 800,000 hectares in Germany, creating competition for agricultural land.
Household level and decentralized systems
Many biogas projects in low- and middle-income countries focus on small, local systems rather than large industrial plants found in wealthier nations. These systems use household digesters to create energy that does not rely on large power grids. They also produce nutrient-rich material that can be used as fertilizer. These systems are common in areas with many farmers who raise livestock, such as parts of South Asia, East Africa, and China. Decentralized biogas systems help achieve several goals at once, including providing better access to energy, improving waste management, reducing harmful indoor air pollution from burning traditional fuels, and increasing a region’s control over its energy needs.
Geographic and social/economic factors
The way well biogas systems work can change a lot based on where they are located and the local economy. Important factors that affect how well they work include the climate, how much water is available, the amount of material used to make the gas, and how much money families earn. Microbes that help create biogas work better in warmer temperatures, which means less extra heating is needed in hot areas. The number of animals in an area is also important because animal waste is often the main material used in small biogas systems. Places where people live far apart or have few animals may struggle to keep producing energy regularly. Getting help with technical problems and repairs can also affect how long the systems last and how widely they are used.
Feedstocks and Agricultural systems
In low- and middle-income countries, biogas is often made from organic waste that is already available locally, instead of crops grown specifically for energy. Common materials used include cow manure, chicken waste, leftover plant parts from farming, food scraps, and other materials that can break down naturally. Using these waste materials helps protect the environment by capturing methane, a gas that would otherwise be released when waste breaks down without control. It also improves sanitation and helps farming by using the leftover material from the process, which can be used as fertilizer. In high-income countries, more energy is made from crops like corn and soy that are grown for this purpose. Some studies show that systems using waste are better suited for areas with limited land, water, and money.
Limitations and Challenges
Although there are many possible advantages, small biogas systems still have challenges that need to be overcome before they can be widely used. High costs for installing these systems can be a problem, especially for families with low incomes, and these costs become harder to manage without help from government programs or grants. Changes in the availability of materials needed for the system and a lack of water can sometimes lower the amount of gas produced. These issues are often influenced by weather patterns and other climate-related conditions. If the systems are not properly maintained due to a lack of technical help, repairs become harder. Studies in sub-Saharan Africa and South Asia show that even if a system works well technically, it may not be used long-term without community support. These systems also need to be accepted by local people and supported through training, help from organizations, and clear rules. Areas that involve local communities in planning and using the systems often have more success in getting people to use them and keeping the systems running long-term.
Emerging alternative energy feedstocks
Biogas is made from organic materials like manure, food waste, and certain plants. Scientists have studied plants such as switchgrass, miscanthus, and Napier grass because they grow well on less fertile land and produce a lot of biomass. These plants can be used in anaerobic digestion, a process that breaks down organic matter without oxygen to create biogas.
In India, biogas has been made from cow manure for many years. These systems, called "gobar gas" plants, have been used in rural areas since the mid-20th century. Dr. Ram Bux Singh helped develop one of the first successful biogas plants in 1957. His work supported the use of biogas as a renewable energy source in rural India. Over time, new designs like the Deenabandhu model have been created. This model is small, usually holding 2 to 3 cubic meters of gas, and can be built with bricks or a mix of cement and other materials. The Indian government provides some financial help to build these systems.
Biogas, which is mostly methane, can also be used to make animal feed by growing Methylococcus capsulatus bacteria. This process uses little land and water. The carbon dioxide left over from biogas production can be used to grow algae or spirulina, which may replace crude oil in the future. The Indian government has programs to use agricultural waste to improve rural economies and create jobs. These systems turn waste into useful products without polluting water or releasing greenhouse gases.
Liquefied Petroleum Gas (LPG) is a common cooking fuel in urban India, but its cost has risen. Because of this, biogas is being explored as an alternative. New technologies, like the Biourja process, have improved the efficiency of biogas systems for larger-scale use.
In India, Nepal, Pakistan, and Bangladesh, biogas made from manure is called "gobar gas." In India, over 2 million households use this system, while Bangladesh has about 50,000 and Pakistan has thousands. These systems use airtight pits filled with manure and wastewater. The gas is then used for cooking. One type of system is the Sintex Digester. Some designs use worms to help turn waste into compost. In Pakistan, a program has built over 5,000 biogas plants and trained workers. In Nepal, the government helps pay for biogas systems.
China is the largest producer and user of household biogas in the world. They started using biogas in 1958 and built over 6 million digesters by 1970. By 2010, biogas production had grown to 248 billion cubic meters. The Chinese government supports rural biogas projects, and over 30 million households use biogas digesters. However, in colder areas, biogas production is lower because digesters lack heating systems.
In Zambia, the capital city of Lusaka has over 2 million people, with more than half living in areas near the city. Many use pit latrines, creating about 22,680 tons of waste each year. This waste is not properly managed, harming the environment and public health.
Although biogas research started in the 1980s, Zambia has not widely adopted it. Challenges include lack of funding, poor policies, high costs, and cultural resistance. Other issues include limited research, poor management of biogas systems, and lack of support for using biogas in the region.
Associations
- World Biogas Association (https://www.worldbiogasassociation.org/)
- Anaerobic Digestion and Bioresources Association (UK) (https://adbioresources.org/)
- American Biogas Council (https://americanbiogascouncil.org/)
- Canadian Biogas Association (https://www.biogasassociation.ca/)
- European Biogas Association
- German Biogas Association
- Indian Biogas Association
Society and culture
In the 1985 Australian movie Mad Max Beyond Thunderdome, a post-apocalyptic community called Barter Town uses a central biogas system powered by pig waste to generate electricity. This system also provides fuel for vehicles in the town. A story titled Cow Town, written in the 1940s, describes a city that relies heavily on cow manure, which produces methane gas. This gas creates challenges for the city’s residents. An engineer named Carter McCormick, from a nearby town, is sent to find ways to use the methane gas to help power the city instead of causing harm. Today, biogas production offers new job opportunities as new technologies continue to develop.