What is a Biogas

Biogas is a renewable fuel produced through the anaerobic decomposition of organic matter such as sewage sludge, food waste, agricultural residues and industrial effluents. It consists mainly of methane and carbon dioxide, with smaller amounts of hydrogen sulphide, ammonia and other trace gases. The process that generates biogas occurs naturally wherever organic materials are broken down by microorganisms in the absence of oxygen, such as in marshes, landfills and wastewater treatment plants.

In controlled conditions, this biological process can be harnessed to produce a valuable source of renewable energy. Biogas can be used directly for heating, electricity generation or as a vehicle fuel once purified. The digestate left after gas production can also be recycled as a nutrient-rich fertiliser. As such, biogas technology represents both an energy recovery solution and a sustainable waste management practice that contributes to carbon reduction goals.

Composition and characteristics of biogas

Biogas is a mixture of several gases, with methane (CH₄) as its main energy component. On average, its composition is:

  • Methane (CH₄): 55–70%

  • Carbon dioxide (CO₂): 25–40%

  • Water vapour (H₂O): 2–7%

  • Hydrogen sulphide (H₂S): 0.1–0.5%

  • Nitrogen, oxygen and trace gases: less than 2%

The exact composition depends on the type of feedstock, process conditions and the design of the digestion system.

Methane is the combustible element that gives biogas its fuel value. With a calorific value of around 21–25 megajoules per cubic metre, biogas has about 60% of the energy content of natural gas. However, unlike fossil fuels, it is renewable and carbon-neutral because the carbon released during combustion is part of the short-term biological carbon cycle.

The presence of carbon dioxide and other impurities means that raw biogas must often be cleaned or upgraded before use, particularly when it is injected into gas grids or used as a transport fuel.

How biogas is produced

Biogas is generated through a process called anaerobic digestion, in which microorganisms break down biodegradable materials in the absence of oxygen. The process occurs in four key stages:

  1. Hydrolysis: Complex organic materials such as carbohydrates, fats and proteins are broken down into simpler soluble compounds like sugars, fatty acids and amino acids.

  2. Acidogenesis: The soluble compounds are further converted into volatile fatty acids, alcohols, hydrogen and carbon dioxide by acid-forming bacteria.

  3. Acetogenesis: Intermediate products are converted into acetic acid, hydrogen and carbon dioxide, which serve as substrates for the final stage.

  4. Methanogenesis: Methane-producing archaea transform acetic acid, hydrogen and carbon dioxide into methane and water, forming the main components of biogas.

These stages occur simultaneously and require careful environmental control. The process typically operates at mesophilic (around 35°C) or thermophilic (around 55°C) temperatures, with neutral pH and complete oxygen exclusion.

The time required for digestion, known as hydraulic retention time (HRT), can range from 15 to 40 days depending on feedstock type, temperature and reactor configuration.

Feedstocks used for biogas production

Almost any biodegradable organic material can be used as a feedstock for biogas production. Common sources include:

  • Municipal wastewater sludge: One of the oldest and most widespread applications of anaerobic digestion. Sewage treatment plants use digesters to stabilise sludge, reduce odour and generate energy for onsite operations.

  • Agricultural waste: Manure, crop residues and silage are commonly used to produce biogas on farms. The process reduces greenhouse gas emissions and provides nutrient-rich digestate for fertilising fields.

  • Food waste: Household, commercial and industrial food residues are ideal for biogas generation due to their high organic content and biodegradability.

  • Industrial effluents: Wastes from breweries, dairies, distilleries and food processing plants are frequently digested to recover energy while reducing treatment costs.

  • Landfill gas: Although not produced in a controlled reactor, methane generated from decomposing waste in landfills can be captured and utilised as biogas.

Selecting the right feedstock mixture helps maintain a balanced nutrient ratio, providing the microorganisms with the carbon, nitrogen and trace elements they need for efficient digestion.

Types of biogas reactors

Several reactor designs are used for anaerobic digestion, each suited to specific feedstocks and operational conditions. Common types include:

  • Continuous stirred tank reactor (CSTR): A fully mixed tank where feedstock is continuously added and digested material is removed. Common in municipal sludge treatment.

  • Plug flow reactor: A long, horizontal reactor in which material moves slowly in one direction with minimal mixing, often used for animal manure.

  • Upflow anaerobic sludge blanket (UASB): Wastewater passes upward through a dense bed of granulated sludge, promoting efficient contact between microbes and substrate.

  • Fixed film and hybrid reactors: Employ packing materials or biofilms to retain microorganisms and improve process stability.

  • Covered lagoons and digesters: Simpler systems suitable for agricultural applications where space is available and operational control is less critical.

Each type balances capital cost, operational simplicity and efficiency differently. The choice depends on the scale of operation, feedstock type and energy recovery objectives.

Biogas upgrading and purification

Raw biogas, known as digester gas, often contains impurities such as hydrogen sulphide, moisture and carbon dioxide that must be removed before the gas can be used in advanced applications. The purification process typically includes:

  • Desulphurisation: Removal of hydrogen sulphide to prevent corrosion and reduce sulphur dioxide emissions during combustion.

  • Dehumidification: Cooling and drying the gas to remove water vapour.

  • Carbon dioxide removal: Physical or chemical scrubbing, membrane separation or pressure swing adsorption are used to increase methane concentration to around 95%.

Once upgraded, biogas becomes biomethane, which has a similar composition and energy content to natural gas. Biomethane can be injected into existing gas networks or compressed for use as a vehicle fuel (bio-CNG).

The upgrading process not only improves combustion efficiency but also enables the gas to meet regulatory standards for use in transport and power generation.

Energy recovery and utilisation

Biogas can be used directly as a fuel or converted into other useful forms of energy. The main utilisation routes are:

  1. Combined heat and power (CHP): The most common use of biogas, where it is burned in an engine or turbine to produce electricity and recoverable heat. CHP systems can achieve overall efficiencies of up to 80%.

  2. Boiler or furnace fuel: Biogas can be burned directly for process heating, space heating or drying operations, especially in agricultural or industrial facilities.

  3. Vehicle fuel: After upgrading to biomethane, it can be compressed and used in natural gas vehicles, providing a renewable alternative to petrol and diesel.

  4. Grid injection: High-quality biomethane can be injected into natural gas grids, expanding its use for residential and industrial customers.

  5. Hydrogen production: Emerging technologies can reform biogas to produce hydrogen for use in fuel cells or as a chemical feedstock.

The versatility of biogas makes it a key player in the transition to renewable energy systems.

Environmental benefits of biogas

Biogas production offers multiple environmental and social benefits. It helps close the loop between waste generation and energy recovery, turning organic waste into valuable products. Key advantages include:

  • Reduction of greenhouse gas emissions: Capturing methane from waste prevents its uncontrolled release into the atmosphere, where it is a potent greenhouse gas.

  • Waste minimisation: Organic waste is stabilised and reduced in volume, lowering disposal costs and environmental impact.

  • Renewable energy generation: Biogas replaces fossil fuels in power generation, heating and transport.

  • Nutrient recycling: The digestate produced after gas extraction can be used as an organic fertiliser, returning nutrients to the soil.

  • Odour control and sanitation: Anaerobic digestion reduces odours and pathogens in organic waste, improving hygiene around agricultural and wastewater facilities.

Together, these benefits make biogas one of the most sustainable technologies for managing organic waste streams.

Challenges and limitations

Despite its advantages, biogas technology faces several challenges that affect its widespread adoption. These include:

  • Economic viability: High initial investment and operational costs can be a barrier, especially for small-scale installations.

  • Process stability: Maintaining the right temperature, pH and nutrient balance is crucial, as the microorganisms involved are sensitive to fluctuations.

  • Feedstock availability: Seasonal or inconsistent waste supplies can limit continuous operation.

  • Gas quality: Impurities such as hydrogen sulphide and moisture require treatment, increasing complexity and cost.

  • Public perception: In some regions, biogas plants face opposition due to odour concerns or misconceptions about safety.

Technological innovation and supportive policy frameworks are helping to overcome these issues, making biogas increasingly competitive with conventional energy sources.

Integration with wastewater treatment

In wastewater treatment plants, anaerobic digesters are used to stabilise sludge while producing biogas as a by-product. The generated gas is often used on-site to power CHP units that supply electricity and heat for the plant’s operations. Excess electricity can be exported to the grid, improving the energy balance of the facility.

Many modern treatment plants are now designed as energy recovery centres, where biogas production forms part of a circular resource strategy. Combined with advanced sludge management and nutrient recovery, these systems exemplify sustainable wastewater treatment.

The role of biogas in the circular economy

Biogas plays a central role in the circular economy by transforming organic waste into clean energy and valuable by-products. Instead of treating waste as a disposal problem, it becomes a renewable resource that contributes to energy security and environmental protection.

The integration of biogas plants with agriculture, food industries and municipal waste management creates synergies that reduce waste transport, lower emissions and support rural economies. Furthermore, as nations move toward decarbonisation, biogas and biomethane offer practical pathways for achieving net-zero energy systems.

Conclusion

Biogas represents a proven and sustainable solution for renewable energy generation and waste management. Produced through the natural process of anaerobic digestion, it converts organic materials into a versatile fuel that can power homes, vehicles and industries while reducing environmental pollution.

By capturing methane that would otherwise escape into the atmosphere, biogas technology helps mitigate climate change and supports circular resource use. As innovations in digestion efficiency, gas upgrading and system integration continue to advance, biogas is poised to become an even more important component of a low-carbon future, linking clean energy production with responsible waste management.