Sludge-to-Energy: Plants Boosting Energy Recovery with THP

Wastewater utilities are increasingly looking to sewage sludge as a resource that can support renewable energy production while lowering operating costs and reducing the volume of material requiring handling. This article explains how energy is recovered from sewage sludge and explores how thermal hydrolysis and its configuration within the sludge line can influence biogas and renewable energy outcomes. Selected plants using thermal hydrolysis provide real-world examples of the resulting improvements in biogas production and overall plant energy balance.

What is Sludge-to-Energy?

Sludge-to-energy refers to recovering energy from sewage sludge, the organic solids separated during wastewater treatment. Rather than treating this material as a residual to be disposed of, wastewater treatment plants can use it as a feedstock to produce biogas, heat, electricity, and biomethane.

The most established sludge-to-energy pathway is anaerobic digestion. During this process, microorganisms break down biodegradable organic matter without oxygen. This produces methane-rich biogas and stabilised biosolids. The biogas from wastewater sludge can fuel boilers, generate heat and electricity in combined heat and power (CHP) engines, or be upgraded to biomethane for gas-to-grid injection or vehicle fuel.

Sludge is central to wastewater energy recovery because primary settlement and biological treatment capture much of the wastewater's organic material as primary sludge and waste activated sludge. Concentrating these solids creates a practical feedstock for digestion, but higher biogas production alone does not define a successful energy outcome.

Wastewater treatment plants use energy throughout the process, including for aeration, pumping, mixing, heating, dewatering, nutrient removal, odour control, and, at some sites, sludge drying or incineration. Pretreatment, gas cleaning, and energy-conversion equipment also have their own energy demands. The goal is therefore to improve the site's overall energy balance: recovering more useful energy from sludge while reducing avoidable energy use across the treatment train.

There are several ways to improve sludge-to-energy performance at plants with anaerobic digestion, with thermal hydrolysis being a leading option.

Thermal Hydrolysis and How it Improves Sludge-to-Energy Systems

Thermal hydrolysis (THP) uses steam and pressure to break down sewage sludge in conjunction with anaerobic digestion. It makes more organic material accessible to the bacteria that produce biogas, thereby improving volatile solids destruction and reducing the volume of biosolids requiring downstream handling. THP is particularly effective on waste activated sludge (WAS), which is generally harder to digest than primary sludge.

For sludge-to-energy systems, the additional biogas produced with THP can support electricity and heat generation, biomethane production for vehicle fuel, or gas-grid injection. While THP requires steam, the additional biogas production typically more than covers this requirement.

THP also improves pathogen reduction and sludge dewaterability, helping to further reduce energy and costs along the sludge line. Regional sludge centres can be especially well suited to THP, as larger, more consistent sludge volumes enable efficient operation and better integration of digestion, steam production, and energy recovery.

Sludge-to-Energy Potential Across THP Configurations

The energy performance of thermal hydrolysis depends partly on how it is integrated into the sludge treatment process. Different THP configurations can influence both biogas yields and the energy required to operate the system.

• Full THP (THP before anaerobic digestion): Treating all sludge before digestion makes more organic material accessible to the digesters. This is generally the best option when a plant also needs more digestion capacity. CambiTHP sites using this configuration report biogas increases of 20–50%. Because all sludge is treated, the energy required for steam production is correspondingly higher than in other configurations.
• WAS-only THP before anaerobic digestion: Treating only waste activated sludge, then blending it with primary sludge before digestion, focuses THP on the sludge fraction that responds most strongly to hydrolysis. It can deliver much of the biogas benefit with a smaller, more energy-efficient THP system.
• I-THP (Intermediate THP or THP between digesters): Sludge is digested, dewatered, and hydrolysed before a second digestion stage. Because the first digester removes the more readily biodegradable material, THP treats a smaller volume and targets the remaining harder-to-digest solids. This can increase biogas while using less THP capacity and energy than a full pre-digestion arrangement.
• SolidStream (THP after anaerobic digestion): Here, sludge is digested first and hydrolysed before final dewatering. The energy-rich liquid released during dewatering is returned to the digesters, creating an additional biogas-production route. Cambi identifies this configuration as having the highest biogas production potential (up to around 50%), while also improving cake dryness and reducing the final biosolids volume.

CambiTHP Plants with Exemplary Sludge-to-Energy Outcomes

Many wastewater treatment plants using CambiTHP have reported positive outcomes in biogas production, renewable energy recovery, and sludge handling. Several facilities have shared more detailed operational data and offered clearer insight into how thermal hydrolysis and their overall strategy have improved energy balance.

The plants below illustrate different routes to stronger sludge-to-energy performance.

Greece: Psyttalia Wastewater Treatment Plant

Sludge Challenge: Psyttalia serves a population of approximately 3.5 million and is one of Europe's largest wastewater treatment plants. Before its upgrade, mixed primary sludge and waste activated sludge (WAS) were digested, dewatered and dried in four rotary drum dryers before being sent to cement kilns. Over 70% of the biogas produced on-site was used for the sludge dryers. In 2014, the operator at the time, Aktor, was required to reduce the plant's total energy footprint by more than 20%.

Following pilot studies, Aktor and Cambi selected thermal hydrolysis to treat half of the WAS stream: an approach that limited upfront investment while targeting the sludge fraction with the greatest potential for improved digestion and dewatering.

THP and Transformation: A CambiTHP B6-4 system entered operation in 2015. It initially treated half of the plant's WAS, while primary sludge bypassed THP and was blended with the hydrolysed WAS before digestion. The remaining sludge was treated in a separate, conventional digestion line. The two digested streams were dewatered separately and dried together.

Biogas is used primarily to support the rotary drum dryers, with surplus gas available for combined heat and power (CHP). In 2023, Psyttalia added another THP train, enabling treatment of 100% of the site's WAS.

Energy Outcome and Biogas Use: In the initial half-WAS configuration, the THP-fed digestion line achieved a 16% increase in biogas production. Dewatered-cake dryness increased from 20–22% to 29–31% dry solids, while greater volatile-solids destruction reduced the amount of material requiring drying. Together, these improvements cut sludge-dryer energy demand by 19%.

Modelling for full-WAS treatment indicated up to 160% more energy available for electricity or biomethane production and a near-40% reduction in dryer energy demand compared with operation without THP. In 2023, Psyttalia added a further THP train, enabling treatment of 100% of the site's WAS.

Psyttalia shows how sludge-to-energy performance can be improved not only by producing more biogas, but also by reducing one of the plant's largest downstream energy demands.

The Netherlands: Hengelo Energy Factory

Sludge Challenge: Waterschap Vechtstromen serves around 800,000 people through 23 wastewater treatment plants in the Twente region. Its sludge was formerly treated at three locations, but ageing digesters at one site needed major renewal, and another site had no spare capacity.

Rather than reinvest in several dispersed facilities, the utility chose to centralise sludge treatment at Hengelo. The objective was to create long-term regional capacity while recovering more renewable energy from waste activated sludge.

THP and Transformation: Cambi commissioned a CambiTHP B6-2 system at Hengelo in 2016. The system initially treated the site's own WAS before progressively introducing imported sludge. By 2022, the renamed Hengelo Energy Factory was receiving sludge from 20 wastewater treatment plants, allowing a neighbouring ageing digestion facility to be decommissioned. An additional THP reactor was added in 2023.

At Hengelo, hydrolysed WAS is digested with primary sludge. Thermal hydrolysis increases the amount of sludge that can be treated within the available digestion volume, helping the utility centralise treatment without building equivalent new conventional digester capacity.

Energy Outcome and Biogas Use: Hengelo now produces approximately 14 million kWh of electricity a year from biogas. Around 6 million kWh is used on site, with the balance exported to the national grid.

Treating WAS before digestion doubled Hengelo's digestion capacity. THP also improved volatile-solids reduction and dewaterability, cutting cake production by around 10,000 wet tonnes per year and reducing transport requirements. Vechtstromen reports annual operating-cost savings of €1.5 million from its centralised treatment strategy.

Hengelo demonstrates how establishing a regional sludge hub can be a practical sludge-to-energy advantage: more efficient treatment, greater renewable power generation, and less material to transport.

Poland: Południe Wastewater Treatment Plant

Sludge Challenge: Południe, also known as Warsaw's South Plant, serves around one-quarter of the city and discharges treated effluent to the Vistula River. During modernisation, the municipal utility MPWiK Warszawa sought to increase the recovery of renewable energy from sludge and improve biosolids handling without replacing the site's existing digestion infrastructure.

THP and Transformation: Commissioned in 2023, the CambiTHP B2 operates in an intermediate thermal hydrolysis (I-THP) configuration between two digestion stages. Sludge is first conventionally digested and dewatered, then thermally hydrolysed before entering a second, advanced-digestion stage.

This arrangement removes part of the solids load before hydrolysis, reducing the required THP capacity. The second digester then converts newly solubilised organics into additional biogas for CHP. Centrate from final dewatering is treated using deammonification and phosphorus recovery systems.

Energy Outcome and Biogas Use: Initial test-period results showed more than 40% improvement in biogas yield and biosolids dewaterability compared with conventional digestion. Preliminary guarantee-period data presented in 2025 reported approximately 550 Nm³ of biogas per tonne of volatile solids fed, a reduction in volatile solids of around 60%, and a final cake dryness increasing from a pre-I-THP average of 17.7% to 25%. Higher cake dryness also reduces the mass that requires transport, disposal, or thermal treatment.

Południe offers valuable insight into the potential of the ITHP configuration and is a useful example of how sludge-to-energy upgrades can be integrated into existing assets.

Scotland: Seafield Wastewater Treatment Works

Sludge Challenge: Seafield is Scotland's largest wastewater treatment works. It treats around 300 million litres of wastewater a day for approximately 850,000 people, while meeting a sensitive coastal discharge permit. Scottish Water and Veolia began a major renovation in 2011 to improve odour control and overall facility performance.

THP and Transformation: Two CambiTHP B6 thermal-hydrolysis trains entered operation around 2015 as pretreatment for anaerobic digestion. The THP installation was paired with an additional CHP unit to better utilise the available biogas. The wider improvement programme also focused on energy demand, asset availability, reliability, and process control.

Energy Outcome and Biogas Use: Earlier reporting attributed an approximately 10% increase in biogas production to THP. On-site electricity generation rose from 55% of demand in 2015 to around 85% in 2017–18, with periods of full self-sufficiency. By 2022, Seafield had become self-sufficient in electricity and a net exporter to the grid. It now generates approximately 108–125% of its own electricity requirement.

Seafield shows that energy-positive wastewater treatment can be achieved progressively by optimising the full sludge-treatment line and investing in effective biogas utilisation.

United Kingdom: Minworth Sewage Treatment Works

Sludge Challenge: Minworth is Severn Trent's largest wastewater treatment plant and one of the largest in Europe. In 2024, it served a population equivalent of approximately 2.5 million and processed sludge from Birmingham and surrounding areas.

An upgrade was needed to expand treatment capacity, improve biosolids quality, and increase renewable energy production.

THP and Transformation: Three CambiTHP B6 streams were installed ahead of anaerobic digestion, with a design capacity of 70,000 tonnes of dry solids a year. The compact footprint was particularly valuable at the space-constrained site.

The biogas system gives the operator flexibility in how recovered energy is used. Gas can supply CHP engines with more than 8 MW of continuous generating capacity or be upgraded to biomethane for injection into the national gas grid at rates of up to 750 m³ per hour. Two 4,000 m³ gas biodomes provide storage for the site's increased biogas volumes.

Energy Outcome and Biogas Use: In 2021, Minworth was producing enough renewable gas to meet the equivalent annual demand of up to 28,000 local homes. The site represents around 30% of the green energy produced across Severn Trent's wastewater operations.

Minworth shows that sludge-to-energy resilience includes commercial and operational flexibility. The ability to use biogas for electricity and heat, or upgrade it to biomethane, gives the operator options as site demand and energy markets change.

Beijing: Gaoantun Water Reclamation Plant

Sludge Challenge: Beijing needed a regional solution for large volumes of dewatered sludge from several water-reclamation plants. Gaoantun was one of five sludge centres built or upgraded by Beijing Drainage Group (BDG) to receive, stabilise and recover resources from this material. The renovated site began using thermal hydrolysis in 2018 and receives sludge from two other plants.

THP and Transformation: Beijing Drainage Group installed four CambiTHP B12 trains to treat mixed sludge before anaerobic digestion. The hydrolysed sludge is co-digested with food waste, increasing the biodegradable feedstock available for biogas production. Cambi process gas units were added in 2021 to improve thermal hydrolysis efficiency. The wider site also incorporates solar panels, water-source heat pumps, heat recovery, and efficient nutrient removal.

Energy Outcome and Biogas Use: Biogas is used in CHP engines to produce approximately 50 million kWh of energy annually, compared with the site's annual demand of around 40 million kWh. The project is estimated to avoid approximately 29,000 tonnes of CO₂ emissions annually compared with conventional treatment methods.

Gaoantun demonstrates the value of combining advanced sludge treatment with co-digestion, efficient process design, and complementary renewable energy technologies.

Beyond these examples, many other customer stories have shown the value of utilising thermal hydrolysis in the sludge line. The Tarnów facility in Poland used THP to increase the amount of imported sludge and organic waste, thereby increasing energy production and revenue for the site. Cog Moors in the UK is close to energy self-sufficiency. Medina in the US has more than halved its electricity bill while producing Class A Exceptional Quality biosolids. Hamar in Norway, which hosts the first-ever THP system installed in 1996, covers 70% of its electricity demand with on-site biogas. In 2025, Deurne-Schijnpoort replaced its sludge dryer with a THP set in this post-AD configuration to target higher biogas production and improved energy efficiency.

Turning Sludge into Energy Requires a Whole-System View

Successful sludge-to-energy projects are not defined by biogas yield alone. Their performance depends on the full treatment train: effective conversion of organics through anaerobic digestion, efficient biogas utilisation, heat integration, dewatering performance, sidestream management and reliable day-to-day operation.

Thermal hydrolysis can increase the value recovered from sewage sludge, particularly where waste activated sludge is a significant part of the feedstock. However, the greatest benefits are achieved when the wider system is designed to maximise useful energy recovery and minimise energy demand across the entire sludge treatment line and even the full wastewater treatment plant.

For plants assessing a sludge-to-energy project, the starting point is to understand the available feedstock, the site's main constraint, and how additional energy can be used. A robust business case should also account for heat demand, asset availability, liquid-stream impacts, and the long-term biosolids strategy. With the right configuration and operational approach, sludge can become a dependable source of renewable energy and resource recovery.

Want to explore how thermal hydrolysis can strengthen sludge-to-energy performance? Watch the Cambi Academy webinar, Maximising Biogas Yield: The Real Impact of Thermal Hydrolysis, for a closer look at its effect on biogas production.