Sludge from wastewater treatment plants can have a range of final uses, with land application remaining one of the most prevalent strategies in many countries. However, growing concerns about the presence of contaminants, such as PFAS, in biosolids or treated sludge are leading some markets and municipalities to reconsider this practice. Consequently, utilities are increasingly turning to advanced thermal methods like pyrolysis and gasification, which have demonstrated potential for effective contaminant destruction.
This article explores the pyrolysis process, examining its applications, benefits, and challenges. It also provides insightful comparisons of pyrolysis integration scenarios to guide decision-making.
Sludge pyrolysis is a thermal treatment process used for stabilising or disposing of dewatered sludge, typically derived from sewage sludge in wastewater treatment facilities. The process involves heating the sludge to high temperatures (usually between 550–700 °C) in an oxygen-starved environment. This controlled thermal decomposition helps reduce sludge volume while converting waste into valuable by-products.
In a sludge pyrolysis system, dewatered sludge—often pre-dried to around 80-90% solids—is heated within a reactor under oxygen-limited or oxygen-free conditions. The typical operating temperature ranges from 550 to 700 °C, although higher temperatures can be applied depending on the desired outcomes.
Pyrolysis yields three primary products:
Typically, around 30–60% of the sludge is converted into gas and oil, while the remainder forms biochar. The syngas produced is often combusted on-site to generate heat, which can be utilised to maintain reactor temperatures and power sludge drying processes. This integration makes the system more energy self-sufficient. Additionally, the heat generated can be used in heat exchangers to facilitate the thermal drying required to prepare the sludge feedstock.
The pyrolysis process is comparable to gasification, which operates at higher temperatures (800 - 1200°C), but the two differ in many other aspects, as will be highlighted in a table in the following section.
Pyrolysis can be categorised based on heating rate and residence time into slow pyrolysis, fast pyrolysis or flash pyrolysis. Slow pyrolysis is ideal when biochar is the main desired product, while fast pyrolysis suits bio-oil production for fuels or chemical feedstocks. Flash pyrolysis, also meant to optimise bio-yield, remains confined primarily to research settings due to operational complexity and energy demands.
Sludge incineration remains the most commonly employed thermal technique for wastewater solids management. However, interest in pyrolysis and gasification is growing, driven by findings that both methods can significantly destroy PFAS (per- and polyfluoroalkyl substances) while being more sustainable than traditional incineration. Due to their ability to convert sludge into carbon-rich by-products, pyrolysis and gasification are often referred to as carbonisation methods.
The table below highlights key differences between sludge pyrolysis, gasification, and incineration:
Apart from pyrolysis, gasification and incineration, there are various thermal or thermochemical methods that can also treat sludge:
Among these, SCWO and potentially HTL have demonstrated effectiveness in PFAS destruction. However, they are not widely adopted due to high implementation costs and complex operational requirements. In contrast, HTC and WAO have been shown to be ineffective for PFAS destruction.
By comparing these methods, it becomes clear that sludge pyrolysis offers a balanced solution, combining energy recovery, resource utilisation, and emission control. Its growing adoption reflects the increasing focus on sustainable and energy-efficient sludge management.
One of the most compelling aspects of sludge pyrolysis is the utilisation of its by-products. The three primary output streams—biochar, bio-oil, and syngas—each offer unique applications that can enhance sustainability and resource efficiency.
This porous, nutrient-rich carbon product can enhance soil fertility and structure, serving as a slow-release fertiliser. Rich in phosphorus and other nutrients, biochar has been shown in some studies to be nearly as effective as conventional phosphate fertilisers. In addition, the carbon in biochar is very stable (having resisted combustion during pyrolysis), so it acts as a form of carbon sequestration – locking carbon away in soils for decades or centuries.
Beyond agriculture, biochar is explored as an adsorbent for pollution control, a construction material additive (e.g., cement component), and even in innovative applications like battery electrodes.
However, a key factor for using the material is the char's purity: heavy metal content and other contaminants must be within safe limits for the intended application.
Bio-oil, a viscous and hydrocarbon-rich liquid produced during pyrolysis, has promising applications in the energy and chemical sectors. It can be used as industrial heating fuel, replacing fossil fuels in boilers and engines. However, raw bio-oil typically contains impurities and is chemically unstable, requiring further processing before it can be used effectively.
Processed bio-oil can be refined into biofuels, offering a renewable alternative for power plants or industrial applications. Additionally, bio-oil contains valuable chemicals, such as phenols and fatty acids, which can be extracted and utilised in various industrial processes, enhancing the oil's economic value.
Syngas: On-Site Energy and Power Generation
Syngas, or synthetic gas, is a combustible mixture mainly composed of carbon monoxide (CO), hydrogen (H₂), methane (CH₄), and some carbon dioxide (CO₂). It is an energy-rich by-product of sludge pyrolysis, often used directly as a fuel.
The most common application of syngas in sludge pyrolysis systems is self-sustaining energy generation. The gas is typically combusted on-site to heat the pyrolysis reactor, forming a closed energy loop that minimises external fuel dependency. Excess syngas can be harnessed for other processes, such as sludge drying or electricity generation, thereby further reducing reliance on fossil fuels.
Some modern pyrolysis systems are investigating the integration of gas engines or fuel cells to make more efficient use of syngas. However, syngas often contains tar residues, which must be removed through gas cleaning before it can be used in these applications.
Sludge pyrolysis, while not yet widely adopted on a commercial scale, has shown promising potential through several pilot projects and operational plants. These initiatives demonstrate both the advantages and limitations of this technology. Below are some key benefits observed in practical applications:
✔ Sludge Volume Reduction
In typical operations, pyrolysis can decrease sludge mass by over 50%, and in optimised systems, the reduction can reach up to 90%. This substantial volume reduction makes transportation and disposal significantly easier and less costly, while also mitigating environmental impacts, such as landfill leachate and emissions.
✔ Energy Recovery and Potential Self-Sufficiency
The syngas and bio-oil from pyrolysis are typically combusted on-site to supply energy for drying or reactor heating. In well-designed facilities, the energy from these by-products can sustain operations without external fuel, allowing for near or full energy self-sufficiency.
✔ Environmental Advantages
Pyrolysis sanitises sludge by destroying pathogens and toxins at high temperatures (>500 °C). It also significantly reduces harmful contaminants, including pharmaceuticals, endocrine disruptors, and PFAS, with studies showing over 90% removal in integrated systems. Biochar produced through pyrolysis sequesters carbon long-term when used in soil, offsetting greenhouse gases and mitigating pollution risks.
✔ Reduced Odours and Emissions
Pyrolysis occurs in enclosed, low-oxygen reactors, which inherently reduces odour emissions compared to conventional sludge processing methods. Furthermore, modern pyrolysis facilities are equipped with advanced emissions control systems, such as scrubbers, which further minimise air pollution.
Despite its many benefits, sludge pyrolysis faces several challenges that can hinder widespread adoption. Recognising and addressing these challenges is crucial for implementing successful pyrolysis projects.
✖ High Energy Demand
Wet sewage sludge typically contains 70–80% water, which must be reduced to at least 65%—ideally around 90%—dryness before entering the pyrolysis reactor. Achieving this dryness requires significant energy input, and sludge drying is often energy-intensive. To offset this demand, facilities often use recovered syngas for drying, but smaller installations or those handling particularly wet sludge may still require supplementary fuel. While modern systems are more efficient due to improved heat recovery, energy consumption remains a critical barrier to cost-effectiveness.
✖ Feedstock Variability and Contaminants
The composition of sewage sludge can vary significantly depending on its source, affecting the consistency and quality of pyrolysis products. Factors such as organic content, fat, minerals, and the presence of contaminants like heavy metals can influence the heating value of syngas, the composition of bio-oil, and the quality of biochar. In particular, elevated levels of toxic metals (e.g., lead, mercury, cadmium, arsenic) can make biochar unsuitable for agricultural use. To meet safety standards, extensive cleaning and post-treatment of syngas and bio-oil may be required, including processes like gas scrubbing and char washing.
✖ Economic and Financial Barriers
The high capital investment required for pyrolysis facilities, including specialised reactors, emissions controls, and drying equipment, presents a financial challenge. Maintenance costs, such as managing tar build-up, also add to operational expenses. The economic viability of sludge pyrolysis often hinges on generating multiple revenue streams, such as selling energy and biochar, as well as reducing disposal costs. Economic feasibility can vary significantly by site, influenced by local energy prices, government grants, and carbon credits.
✖ Regulatory and Public Acceptance Issues
Regulatory frameworks for sludge pyrolysis are still evolving, and stringent permitting processes can delay project implementation. A significant challenge is the classification of biochar—whether it is treated as a waste product or a marketable good—which affects both regulatory compliance and economic feasibility. Moreover, public perceptions of thermal treatment methods can be sceptical, making transparent communication and demonstrable safety measures essential for gaining community support.
One of the primary challenges of sludge pyrolysis is the high energy demand associated with drying the sludge before pyrolysis. Therefore, technologies that enhance sludge drying or improve the overall energy balance of wastewater treatment plants can significantly complement a pyrolysis strategy.
Anaerobic digestion (AD) can be used to stabilise sludge prior to pyrolysis, reducing sludge volume and decreasing the investment and expenses needed for both drying and pyrolysis. This approach can make pyrolysis more economically and operationally viable. The thermal hydrolysis process (THP) is an advanced digestion technology that has been modelled to positively affect the energy balance of carbonisation strategies like pyrolysis and gasification.
To understand the potential benefits of combining THP with pyrolysis, Cambi’s Julien Chauzy developed a comparative model assessing the energy balance, operational costs, and capital investment for pyrolysis and gasification under four distinct scenarios:
Below is a snapshot of the operational expenses comparison of the four scenarios, considering two possible uses for the biogas produced: cogeneration or biomethane upgrading.
The results demonstrate that integrating THP with anaerobic digestion can significantly lower operational expenses associated with carbonisation. In fact, THP with AD results in a savings scenario in this model.
CambiTHP® technology currently has over 90 references around the globe. It is used by wastewater treatment plants that incinerate and land-apply biosolids or enhanced treated sludge. In 2025, Cambi will commission its first reference where gasification serves as the final biosolids treatment method at Secunda – Sasol in South Africa.
The future of sludge pyrolysis looks promising, with ongoing advancements aimed at improving efficiency and fostering integration within broader waste management systems.
Innovation and research within the area are currently investigating hybrid systems combining pyrolysis with hydrothermal processing or gasification to maximise energy recovery, as well as variations of the process such as catalytic pyrolysis, microwave pyrolysis and plasma-assisted pyrolysis.
As with any sludge treatment solution, sludge pyrolysis is not a one-size-fits-all answer; it comes with its own set of considerations in terms of cost and operational complexity. Yet, with ongoing technological improvements and supportive policies (such as mandates for nutrient recovery as in the recently revised EU wastewater directive), the momentum behind pyrolysis and similar thermal treatments is building.
Looking ahead, we can expect to see pyrolysis increasingly integrated into modern wastewater treatment infrastructure, complementing other processes to achieve zero waste and net-zero emissions goals.
Want to know more about how thermal hydrolysis reduces the cost of pyrolysis? Watch our 2025 webinar, “Combining THP with Pyrolysis and Gasification: What are the Expected Benefits?” where author Julien Chauzy compares the material and energy requirements of various scenarios of pyrolysis or gasification, with and without THP.