Biogas Production from Sewage Sludge Methods & Techniques

Biogas production from sewage sludge uses anaerobic digestion to break down organic matter in wastewater solids (biosolids), creating a renewable, methane-rich gas. This process, accelerated by technologies like thermal hydrolysis and advanced anaerobic digestion, typically produces 350 m³ of biogas per ton of sludge, which can be used for electricity, heating, or vehicle fuel.

Ideas to Remember About Biogas Production from Sewage Sludge

• One of the most accessible and underutilised feedstocks for renewable energy is sewage sludge — it's already produced in vast quantities by centralised wastewater treatment plants in most developed cities.
• Approximately 350 m³ of biogas can be generated from a single tonne of sewage sludge, which equates to roughly 850 kWh of electricity through combined heat and power systems.
• The core process behind biogas production from sewage sludge is anaerobic digestion, but yields can be dramatically increased beyond what conventional digestion achieves alone with advanced pre-treatment methods like thermal hydrolysis.
• Co-digestion — the process of mixing sewage sludge with food waste, cheese whey, or olive mill wastewater — improves the carbon-to-nitrogen ratio and significantly boosts methane output compared to using sludge alone.
• Keep reading to find out why cities that are still landfilling sewage sludge are missing out on significant renewable energy potential and cost savings.

Renewable Energy's Goldmine: Sewage Sludge

Today, one of the most reliable and scalable renewable energy sources available is sitting untapped in wastewater treatment plants around the world.

The solid byproduct of wastewater treatment, known as sewage sludge, has been a disposal issue for a long time. It's expensive to manage, it takes up space, and when it's landfilled, it creates environmental concerns. However, when sewage sludge is processed through anaerobic digestion, it becomes a consistent source of fuel that produces biogas — a combustible gas mixture that is rich in methane. Platforms that focus on water and wastewater sustainability have played a key role in raising awareness about how this overlooked resource can be the foundation of a truly circular energy economy.

What's particularly exciting about this is the advantage of existing infrastructure. Unlike agricultural biogas plants, which require logistics for feedstock and land, municipal wastewater treatment is already centralised in nearly every major city in the developed world. The sludge is already being collected. The facilities are already there. In many cases, transitioning to biogas recovery is more about upgrading existing systems than starting from scratch.

“Anaerobic Digestion (Large-Scale)” from sswm.info and used with no modifications.

Understanding Biogas and Its Significance

Biogas is a gaseous blend that is formed when organic matter is decomposed by microorganisms in an oxygen-free environment, a process known as anaerobic digestion. The gas that is produced is primarily made up of methane (CH₄), which accounts for approximately 50–70% of the total volume, and carbon dioxide (CO₂), which makes up the majority of the rest. Raw biogas also contains small quantities of hydrogen sulphide (H₂S), water vapour, and other gases.

The most valuable part of biogas is methane, which is the same molecule found in natural gas. This means that biogas can be burned for heat, converted into electricity, upgraded to biomethane for injection into gas grids, or even used as a fuel for vehicles. The difference is that biogas from sewage sludge is renewable. It is continuously generated as long as wastewater treatment continues. Additionally, when it is burned, it is considered carbon-neutral within the biogenic carbon cycle.

Breaking it down: A single tonne of sewage sludge can produce an estimated 350 m³ of biogas. When this biogas is processed through a Combined Heat and Power (CHP) system, it can generate approximately 850 kWh of electricity — enough to power an average UK home for about two months.

Turning Sewage Sludge into a Source of Energy

Sewage sludge is the solid material that is separated at various stages during wastewater treatment. This solid material is rich in organic compounds — proteins, fats, carbohydrates — that are the main source of food for the anaerobic microorganisms that produce biogas. The more organic content in the sludge, the more potential biogas it can produce. This is why the combination of primary and secondary sludge is often considered the best mix for digestion.

Three Different Kinds of Sewage Sludge in Biogas Production

Sewage sludge is not all the same. The point at which it is removed during the treatment process decides what organic matter it contains, how much water is in it, and how much biogas it can produce in the end. For more insights on this, explore the biogas production from sewage sludge process.

First Stage Sludge

First-stage sludge is gathered during the initial stage of wastewater treatment, where raw solids simply sink out of the incoming sewage due to gravity. It has a high organic content and a relatively low water-to-solids ratio compared to sludges from later stages, making it one of the most productive feedstocks for biogas. Its easily degradable organic matter means that anaerobic microorganisms can break it down efficiently during digestion.

Biological Sludge

Biological sludge, also known as activated or secondary sludge, is generated during the biological treatment phase. This is the stage where microorganisms feed on the dissolved organic matter in the wastewater. The microbial biomass that results is then settled and gathered. Although biological sludge has a higher water content and may be more difficult to digest than primary sludge, it is still high in organic nitrogen and contributes significantly to overall biogas production.

The industry standard for biogas production is a combined mix of primary and secondary sludge. The two types of sludge have organic profiles that complement each other, which enhances the performance of the digester and optimises the production of methane.

Third-Stage Sludge

Third-stage sludge is a byproduct of advanced treatment steps, like chemical precipitation for phosphorus removal. It typically contains lower organic concentrations and higher inorganic and chemical content. This type of sludge contributes less to biogas yields and, in some situations, can introduce compounds that hinder anaerobic digestion if not properly handled.

How Anaerobic Digestion Drives Biogas Production

Almost all sewage sludge biogas programs today rely on anaerobic digestion (AD) as their biological foundation. This is a tried-and-true process that not only generates renewable energy but also lessens the amount of sludge that needs to be disposed of in the end. This double whammy of benefits makes it an attractive choice for wastewater utilities from both a financial and environmental standpoint.

Four Phases of Anaerobic Digestion

Anaerobic digestion is not a one-step process. It occurs in four successive microbial phases, each one performed by unique groups of microorganisms. To understand more about this process, you can explore the biogas production from wastewater techniques.

1. Hydrolysis — This is the process by which complex organic polymers like proteins, fats, and carbohydrates are broken down into simpler soluble compounds. These include amino acids, fatty acids, and sugars.
2. Acidogenesis — In this process, acidogenic bacteria ferment the simpler compounds into volatile fatty acids, alcohols, carbon dioxide, and hydrogen.
3. Acetogenesis — Acetogenic bacteria convert the volatile fatty acids into acetic acid, hydrogen, and CO₂. These are the direct precursors to methane production.
4. Methanogenesis — Methanogenic archaea convert the acetic acid and hydrogen into methane and carbon dioxide. This produces the biogas that is captured and used.

When sewage sludge is the feedstock, hydrolysis is typically the rate-limiting step. This is especially true with waste-activated sludge because the cell walls of microbial biomass resist rapid enzymatic breakdown. This is why pre-treatment technologies have become a major focus of innovation in the sector.

The Reasons Behind Anaerobic Digestion Being the Go-To Method for Sewage Sludge Treatment

Apart from producing biogas, AD provides a variety of practical benefits, making it the go-to method for large-scale sludge management. This process stabilises sludge by decomposing volatile solids, significantly reducing the smell and getting rid of most pathogens. As a result, a safer, easier-to-handle biosolid is produced. The digestate left after the process is rich in nutrients and can be used as a fertiliser on farmland, further promoting the circular economy.

When it comes to the financial side of things, every kilowatt-hour of electricity produced on-site from biogas directly reduces the need to buy electricity from the grid. For large wastewater treatment plants, this could mean saving millions of pounds each year on energy, which could significantly alter the economics of sludge management.

Drawbacks of Traditional Anaerobic Digestion of Wastewater Sludges

Even with its benefits, traditional mesophilic anaerobic digestion — usually run at about 35–37°C — is known to have drawbacks when used solely for sewage sludge. The volatile solids destruction rate is frequently capped at about 40–50%, indicating that a substantial portion of the organic matter goes through the digester without being converted.

Waste-activated sludge is particularly difficult to break down because microbial cell walls are structurally strong and decompose slowly during hydrolysis. This limits the amount of methane produced and means that more digestate still needs to be managed after treatment, which somewhat undermines the benefit of reducing volume.

Mesophilic vs Thermophilic Temperature Range Digestion

Traditional digesters generally need long hydraulic retention times (HRTs), usually between 15 and 30 days, which implies that large reactor volumes are needed. This increases the capital cost and land footprint. These limitations have led to the creation of pre-treatment and co-digestion methods that are specifically intended to address these issues.

Most wastewater treatment works digesters are run as mesophilic reactors.

Advanced Pre-Treatment Methods That Increase Biogas Yields

Due to the inherent limitations of traditional digestion, pre-treatment technologies have become the most effective tool for increasing biogas output from sewage sludge. By breaking down cell walls and releasing intracellular organic matter before sludge enters the digester, pre-treatment speeds up hydrolysis — the rate-limiting step — and makes far more organic substrate available to methane-producing microorganisms.

There are a variety of pre-treatment methods currently in use or under development, including thermal and mechanical methods as well as chemical and advanced oxidation techniques. Each of these has its own efficiency profile, energy requirements, and capital costs.

Heat Treatment

Heat treatment is the most commercially developed and widely used pre-treatment method in the sewage sludge biogas industry. The method exposes sludge to high temperatures — usually 150–180°C — and high pressures for a brief period before quickly reducing the pressure. This combination breaks down microbial cell walls and dissolves complex organic compounds, making them much more accessible to anaerobic microorganisms. Scandinavian and UK wastewater facilities have shown that heat treatment combined with anaerobic digestion can allow plants to achieve full energy neutrality — producing enough biogas to power the entire treatment plant.

Using Microwave Irradiation and Steam Explosion

When we use microwave irradiation, we aim electromagnetic energy at the sludge. This heats the sludge quickly and evenly. It disrupts the cell structures in the sludge through both thermal and non-thermal mechanisms. Steam explosion does something similar. It pressurises the sludge with steam and then releases the pressure quickly. This sudden decompression physically tears apart the cell structures in the sludge. Both methods have shown promising results in studies done at the pilot scale. They both improve volatile solids breakdown and methane yields. However, neither method has reached the widespread commercial adoption that thermal hydrolysis has.

• Power and exposure time can be adjusted in microwave pre-treatment to target specific sludge characteristics
• Compared to full thermal hydrolysis systems, steam explosion requires a relatively small capital investment
• Both methods decrease the viscosity of the sludge, which enhances the efficiency of pumping and mixing within the digester
• The energy input requirements must be carefully weighed against the gains in biogas yield to ensure a net energy benefit

The practical challenge with both technologies is their energy consumption. Pre-treatment is only truly beneficial when the additional biogas produced is more than the energy required to run the pre-treatment process itself — a calculation that is heavily dependent on the composition of the sludge and the performance of the digester.

There is still a lot of research being done to improve the operation of both microwave and steam explosion systems. Most of this research is centred around finding the best energy balance in full-scale operations, not just in controlled laboratory conditions.

Ozone and Chemical Pre-treatment

The process of ozonolysis involves exposing sewage sludge to ozone gas. This gas is a strong oxidising agent that can attack and break down the complex organic molecules that make up microbial cell walls. The oxidation reaction that takes place as a result of this exposure solubilises a significant amount of the sludge's organic content. This releases intracellular material that anaerobic microorganisms can then convert into methane much more easily. Studies have shown that pre-treatment with ozone can significantly increase the rate of volatile solids destruction and reduce the overall amount of digestate that is produced.

Using chemicals for pre-treatment, such as sodium hydroxide (NaOH), can break down the structure of microbial cells by raising the pH level. The process of alkaline hydrolysis at regulated temperatures breaks down the protein and lipid bonds in the cell walls, essentially pre-digesting the sludge before it goes into the anaerobic reactor. While using chemicals can increase biogas production, the cost of the chemicals and the need to neutralise the pH level before digestion can add operational challenges that must be balanced with the energy benefits.

Breaking Down Sludge Mechanically

There are several mechanical pre-treatment methods that can be used to physically break down sludge flocs and microbial cells. These methods include high-pressure homogenisation, ultrasonic disintegration, and ball milling. These methods work by using shear forces, cavitation, or impact energy to break down the sludge. Ultrasonic disintegration is a method that uses high-frequency sound waves to create tiny cavitation bubbles. When these bubbles burst, they create enough force to break cell membranes. These methods do require careful energy optimisation at scale. However, if they are applied correctly, they can significantly speed up hydrolysis and reduce the amount of time that the sludge needs to be retained in the digester. This can help to make better use of the existing reactor capacity without the need for costly infrastructure expansion.

Co-Digestion: Blending Sewage Sludge With Other Organic Waste

Co-digestion, the process of adding other organic waste streams to sewage sludge, is a highly effective and economical way to increase biogas production from existing anaerobic digestion facilities. Sewage sludge on its own tends to have a relatively low carbon-to-nitrogen (C:N) ratio, which can limit microbial activity and constrain methane yields. By adding carbon-rich co-substrates, this imbalance is corrected, creating conditions that truly promote methanogenesis. A study published in Waste Management (2018) specifically showed that blending sewage sludge with a thermally dried mixture of food waste, cheese whey, and olive mill wastewater significantly improved both biogas yields and overall digestion performance.

Using Food Waste in Co-Digestion

Food waste is one of the most energy-rich co-substrates that can be used in anaerobic co-digestion with sewage sludge. The high lipid and carbohydrate content of food waste leads to high methane yields. Municipal wastewater treatment plants are increasingly able to receive segregated food waste from local authorities. This is part of a broader strategy to manage urban waste. Centralised sewage treatment and food waste collection routes are often in the same areas. This makes it easy for cities to recover renewable energy from organic waste streams without having to invest in significant additional infrastructure.

Using Combined Heat and Power Systems for Biogas

“Combined Heat and Power (CHP) | WBDG …” from www.wbdg.org and used with no modifications.

Creating biogas is just the first step. The next is turning it into useful energy in an efficient way. This is where Combined Heat and Power (CHP) systems come in. They are now the go-to technology for wastewater treatment plants all over the world. CHP, also known as cogeneration, captures both the electrical and thermal energy that is released when biogas is burned. This makes it much more efficient than traditional electricity-only generation.

How Biogas is Transformed into Electricity and Heat Through CHP Systems

A CHP system powered by biogas functions by burning the methane-rich gas in a gas engine or turbine, which then drives a generator to produce electricity. This electricity can be used directly to power the treatment works, thus reducing the cost of grid electricity. The heat produced by the engine's cooling systems and exhaust gases is not wasted but rather recovered through heat exchangers. This recovered thermal energy is usually used to heat the anaerobic digesters to their optimal operating temperature. This is particularly useful during the winter months when maintaining mesophilic or thermophilic conditions requires a lot of energy input.

When you take into account both the electrical and thermal outputs, a modern biogas CHP unit generally has a total conversion efficiency in the 80-90% range of the fuel's energy content. This is in stark contrast to electricity generation alone, which typically has an efficiency of around 35-40%. This is why CHP is the standard configuration at any significant biogas-to-energy installation at a wastewater treatment plant.

Scandinavian and UK Facilities Reach Energy Neutrality

Scandinavian wastewater utilities are at the forefront of showing what can be achieved when thermal hydrolysis pre-treatment is paired with high-efficiency CHP systems. Numerous facilities in Norway and Sweden, including large municipal treatment works, have reached full energy neutrality. They generate enough electricity and heat from biogas to power all on-site operations without needing to draw from the national grid. In some instances, they even export surplus electricity, effectively transforming the treatment plant into a net energy producer.

In the UK, facilities that have adopted the Cambi thermal hydrolysis process — a commercially proven system widely deployed at sites including Beckton Sewage Treatment Works in London — have seen dramatic increases in biogas production and significant reductions in the volume of biosolids requiring disposal. These real-world outcomes demonstrate that energy neutrality in sewage sludge biogas systems is not a theoretical aspiration but an achievable operational reality for well-designed plants.

Preventing Methane Leakage from Biogas Systems

Biogas systems are designed to trap methane and put it to good use. However, no system is completely airtight. Methane leaks, or “fugitive emissions,” that slip through digesters, gas storage tanks, pipes, and processing equipment without being trapped are both an environmental hazard and a loss of potential energy.

Methane is a powerful greenhouse gas that has a global warming potential that is 28–36 times higher than CO₂ over a 100-year period. Even minor, ongoing leaks from biogas infrastructure can significantly reduce the climate advantages of a biogas programme if they are not addressed. This is why managing fugitive emissions has become an increasingly critical operational and regulatory issue for wastewater utilities.

Practical Advantages of Biogas Production from Sewage Sludge

Wastewater utilities that decide to produce biogas from their sewage sludge stand to gain more than just electricity. The reasons for producing biogas are truly multifaceted — impacting energy costs, sludge management costs, regulatory compliance, carbon reporting, and the resilience of long-term infrastructure.

Decreased Sludge Volumes and Cost Savings

When sewage sludge undergoes anaerobic digestion, the volume of volatile solids can decrease by 40–60%, depending on the sludge composition and the pre-treatment methods employed. This results in a direct reduction in the mass and volume of digestate that needs to be managed post-digestion, which can lead to significant savings in transportation, processing, and final disposal costs. These costs can often make up a large chunk of a treatment plant's operating budget. For more insights on biogas production, visit Save the Water.

Advanced anaerobic digestion can lead to significant savings for facilities that were previously landfilling raw or conventionally treated sludge. This is due to the fact that landfill costs in the UK and EU are continuously rising, thanks to escalating landfill taxes and stricter disposal regulations. As a result, these savings increase in value each year.

The nutrient-rich byproduct of the digestion process, known as digestate, adds another layer to the cost-effectiveness of the process. When treated to meet biosolids quality standards, it can be used as a substitute for fertiliser on agricultural land. This not only reduces the cost of synthetic fertiliser for farmers but also provides a revenue stream for the utility. At the very least, it provides a cost-neutral disposal method.

Reducing Greenhouse Gas Emissions

Biogas from sewage sludge has a multi-faceted impact on the climate. Firstly, every cubic meter of methane that is trapped and burned in a combined heat and power (CHP) unit is methane that would have been released into the atmosphere either directly from uncontrolled sludge decomposition or through less efficient treatment processes. When methane is burned in a controlled manner, it is converted to CO₂, which has a much lower global warming potential.

Second, the electric power produced from biogas directly replaces electricity from the grid, which in most nations still has a significant carbon intensity. Each unit of renewable electricity produced on-site from sewage sludge biogas avoids the emissions associated with fossil fuel-derived grid power — a benefit that increases in value as carbon pricing mechanisms tighten globally.

Third, reducing the volume of sludge reduces the emissions associated with the transportation and disposal of sludge. This is often overlooked but is a significant part of the overall carbon footprint of a wastewater treatment plant. When all three of these pathways to reduce greenhouse gases are considered together, a well-run biogas program can provide significant verified carbon savings. This supports both corporate sustainability reporting and regulatory compliance.

Circular Economy Applications: From Waste to Vehicle Fuel

Biogas doesn't have to be used on-site as electricity. When biogas is upgraded to biomethane — a process that removes the CO₂ and trace contaminants to leave a gas that is over 95% methane — it becomes functionally the same as fossil natural gas and can be directly injected into the national gas grid or compressed for use as vehicle fuel. Several cities in Europe already operate entire bus fleets on biomethane derived from sewage sludge, closing the loop between the waste generated by their residents and the fuel that transports them. This application is one of the most compelling examples of circular economy thinking in the urban infrastructure context — turning a byproduct of sewage into clean fuel for public transport.

The cycle of a circular economy is completed by the digestate co-product. The treated digestate, which is rich in nitrogen, phosphorus, and potassium, is applied to agricultural land, returning the nutrients that were originally consumed in food production back to the soil. This process reduces the dependence on synthetic fertilisers that require a lot of energy to produce and supports a truly regenerative resource cycle that goes from the plate to the field and back to the plate again.

Challenges with Rules and Safety

Biogas, a combustible, pressurised gas mixture that includes hydrogen sulphide, a toxic compound that can have severe health and safety consequences at high concentrations. Running a biogas plant, as a result, necessitates adherence to a complicated set of health and safety rules, environmental permit requirements, and planning controls that differ greatly from one jurisdiction to the next. In the United Kingdom, for instance, biogas facilities must comply with Environment Agency permits, Health and Safety Executive requirements for gas handling, and grid connection contracts for any electricity or biomethane export — each of which involves different regulatory agencies with different timelines and criteria.

The digestate, which can be used as an agricultural fertiliser, must meet strict quality standards set by the biosolids regulations. It can be difficult to meet these standards, especially when it comes to reducing pathogens and limiting heavy metals. This requires a consistent process control and regular testing by a third party. The ongoing costs and administrative burden of compliance can be especially challenging for smaller operators to manage.

Training is Essential for Plant Workers

Operating an anaerobic digestion facility is a complicated task that requires a specific skill set not always found in traditional wastewater treatment teams. Ensuring the digester runs smoothly – keeping an eye on volatile fatty acid buildup, monitoring alkalinity, adjusting the feedstock ratios, and troubleshooting any issues – necessitates a solid understanding of the microbiology involved and hands-on experience with the plant's specific behaviour. As the industry has grown, a real shortage of skills has become apparent, with the need for qualified AD plant operators regularly exceeding the number of trained workers available in many countries.

Adoption Driven by Government Subsidies and Industry Partnerships

Biogas production from sewage sludge is heavily influenced by the financial and regulatory landscape. The role of government policy is crucial in determining the commercial viability of biogas projects, especially for utilities that are yet to make full returns from energy sales.

Biogas Plant Tax Credits and Renewable Energy Incentives

In the UK, the Renewable Heat Incentive (RHI) was a long-term tariff support for biomethane injection into the gas grid, making it more economically viable to upgrade biogas to grid-quality biomethane at wastewater treatment works. The RHI has since evolved into the Green Gas Support Scheme (GGSS), but the concept of government-backed tariff support for renewable gas production is still in place and continues to be a key factor in investment decisions. In the United States, the Investment Tax Credit (ITC) and Production Tax Credit (PTC) — which were extended and expanded under the Inflation Reduction Act — have also helped to speed up the development of biogas projects at municipal wastewater facilities by lowering the effective capital cost of anaerobic digestion infrastructure.

How Biogas Projects Are Being Scaled Through Public-Private Partnerships

Public-private partnership (PPP) structure is one of the most efficient models for increasing biogas capacity without putting the full capital burden on public water utilities. In this structure, a private developer finances, builds, and operates the biogas facility and in return, gets long-term offtake agreements for the gas or electricity produced. This arrangement transfers construction and operational risk to a specialist private operator and provides the utility with a guaranteed cost reduction on its energy and sludge disposal bills. This is often done without any upfront capital expenditure from the public body.

In the UK, major water companies such as Severn Trent Water and Thames Water have employed versions of this model to speed up the implementation of biogas across their treatment works networks. In the US, energy companies that specialize in renewable natural gas have joined forces with municipal wastewater authorities to create biomethane upgrading facilities that feed directly into local gas distribution networks. In return for long-term agreements to supply sludge, the utility receives a portion of the gas revenues.

Co-digestion models are especially beneficial for public-private partnerships. When a wastewater utility consents to accept food waste, cheese whey, or other organic co-substrates from private food processors, the gate fee income — which is essentially a tipping fee charged for accepting industrial organic waste — can significantly contribute to the financial model, making projects viable that would not be feasible with sewage sludge alone. This aligns the interests of food industry waste managers, wastewater utilities, and renewable energy developers towards a shared commercial and environmental goal.

Public-Private Biogas Partnership Model — How It Works:

Step 1: Private developer finances and builds AD and CHP infrastructure at the utility's treatment works
Step 2: Utility provides guaranteed sludge supply under a long-term feedstock agreement
Step 3: Co-substrate gate fees from food industry partners supplement project revenue
Step 4: Biogas-derived electricity powers the treatment works, with surplus exported to the grid
Step 5: Revenue from energy sales and Renewable Energy Incentives is shared between the developer and utility under agreed commercial terms
Step 6: At end of contract, infrastructure ownership typically transfers to the utility

Biogas From Sewage Sludge Is Ready to Scale — Cities Need to Act Now

Every day that a wastewater treatment plant landfills its sewage sludge without recovering biogas is a day of renewable energy, cost savings, and carbon reductions that cannot be reclaimed. The technology is proven, the economics are increasingly favourable, and the regulatory frameworks — while complex — are navigable. The question for most cities is no longer whether biogas from sewage sludge is viable, but how quickly they can move from intention to implementation.

Biomethane Produced at WWTWs Compared with Natural Gas

If biogas is compressed, it can be used as a vehicle fuel. As a replacement for natural gas – if biogas is cleaned up and upgraded to natural gas standards, it's then known as biomethane and can be used in a similar way to methane.

Frequently Asked Questions: Biogas Production from Sewage Sludge

Q1 . What is biogas and how is it produced from sewage sludge?

Biogas is a renewable energy source made up primarily of methane and carbon dioxide, produced when bacteria break down organic matter in the absence of oxygen — a process called anaerobic digestion. At sewage treatment works, sludge is fed into sealed digester tanks where microbes decompose the organic material through several biological stages, ultimately releasing methane-rich gas that can be captured and used for energy.

Q2. Can smaller sewage treatment works benefit from biogas production?

Yes. While large city wastewater plants have used anaerobic digestion for many years, companies like WELTEC BIOPOWER are now making the technology accessible to smaller, local sewage works. Their modular stainless-steel plant design allows for customer-specific layouts, meaning facilities serving populations as small as 13,000 residents — like the Burgebrach plant in Bavaria — can generate their own energy from sludge digestion.

Q3. What are the environmental benefits of producing biogas from sewage sludge?

There are several key benefits. Capturing methane from sludge prevents it from escaping into the atmosphere, which matters greatly since methane is over 80 times more potent than carbon dioxide as a greenhouse gas. The digestion process also reduces the volume of sludge requiring disposal, eliminates unpleasant odour emissions around treatment sites, and replaces the need for fossil-fuel-derived energy at the facility.

Q4. Can sewage sludge be combined with other waste materials to produce more biogas?

Yes — this is known as co-digestion. Food waste, which is particularly energy-rich, can be processed alongside sewage sludge in the same digesters to significantly increase biogas output. In the UK alone, an estimated 10 million tonnes of food waste are produced annually across households, hospitality, food manufacture, and retail sectors, representing a substantial untapped energy resource.

Q5. What can the biogas produced from sewage sludge actually be used for?

The gas has several practical applications. It can be burned in a combined heat and power (CHP) plant to generate electricity and heat for use on-site — the Burgebrach plant saves around 100,000 kWh of electricity costs per year this way. If cleaned and upgraded to natural gas standards, it becomes biomethane, which can be injected into the gas grid or used as a vehicle fuel, further reducing dependence on fossil fuels.

Q6. How does thermal hydrolysis boost biogas production?

Thermal Hydrolysis Process — Before and After Digestion:

Without Thermal Hydrolysis: Sludge enters digester with intact microbial cell walls → Hydrolysis is rate-limited → Volatile solids destruction: 40–50% → Moderate biogas yield

With Thermal Hydrolysis (150–180°C, elevated pressure): Cell walls ruptured before digestion → Organic content solubilised and readily accessible → Volatile solids destruction: up to 60%+ → Significantly increased methane yield → Reduced digestate volume

Thermal hydrolysis enhances biogas production by tackling hydrolysis — the slowest and most limiting phase of anaerobic digestion for sewage sludge. By exposing sludge to temperatures of 150–180°C and elevated pressure before it enters the digester, the process physically ruptures the robust cell walls of microbial biomass that would otherwise resist rapid enzymatic breakdown. This releases intracellular organic material directly into the liquid phase, where it becomes immediately available to acidogenic and ultimately methanogenic microorganisms. For more insights on optimizing biogas yield, explore effective biogas yield optimization techniques.

This method results in a quicker and more thorough digestion process. More of the organic content in the sludge is turned into methane instead of passing through the digester unaltered. This increases the yield of biogas, reduces the amount of time the hydraulic retention needs for effective digestion, and decreases the volume and weight of the digestate that needs to be managed after digestion. All three of these results improve both the energy economics and the costs of sludge disposal at the treatment works.

“Cambi Thermal Hydrolysis Process” from www.cambi.com and used with no modifications.

The Cambi thermal hydrolysis process is the most widespread commercial system of this kind worldwide. It has been implemented at significant facilities, including Beckton Sewage Treatment Works in London, one of the largest in Europe. It has led to a substantial increase in biogas production capacity and a significant decrease in biosolids volumes, showing real-world performance at the scale needed to serve a large metropolitan area.

Further Reading

For the basic science behind the Biogas Production Process, you may like to read this.

[First published: April 2018: Updated: February 2022. Rewritten April 2026.]