A perspective on hydrothermal processing of sewage sludge

The greatly rising amounts of sewage sludge from different sources are causing severe concerns around the globe. Disposal of sewage sludge is exceptionally difficult and poses serious environmental problems due to the significant quantity of toxic heavy metals and organic contaminants in its constituents. This review provides an overview of different conversion methods of sewage sludge into energy, that is, pyrolysis, hydrothermal gasification, hydrothermal liquefaction, hydrothermal carbonization, anaerobic digestion, and transesterification. It also focuses on each conversion route's state‐of‐the‐art research advances, features, and drawbacks to ensure financial and ecological responsibility. The detailed descriptions of the methods and respective energy generation could help scholars and industrialists produce energy from sewage sludge to keep the environment harmless.

18 - A review on hydrothermal pretreatment of sewage sludge: Energy recovery options and major challenges

The transition toward low-carbon energy systems requires technologies capable of converting residual biomass into value-added energy carriers. In this context, hydrothermal carbonization (HTC) of sewage sludge (SS) has emerged as a promising pathway for producing solid biofuels while addressing persistent challenges in waste management and sanitation. This study evaluates the technological basis, environmental implications, and regional development potential of SS-HTC within renewable energy and decarbonization strategies. HTC enables the direct processing of wet sewage sludge, avoiding energy-intensive drying steps and generating a carbon-rich hydrochar with reduced moisture content and improved fuel properties compared to the raw material. In addition to the solid fraction, the process produces a nutrient-rich liquid stream that can be further valorized, supporting circular resource management and integrated waste-to-energy systems. Although the estimated global production potential of SS-derived hydrochar (approximately 45 million dry tons) is insufficient to replace fossil coal on a large scale, its targeted use in selected industrial sectors can partially substitute mineral coal, contributing to greenhouse gas mitigation and incremental decarbonization. Beyond its technological advantages, SS-HTC offers opportunities for regional development, particularly in areas with expanding wastewater treatment infrastructure. Integrating HTC into sanitation systems enables localized biofuel production, reduces dependence on imported coal, and supports industrial diversification. The State of Paraná, Brazil, with a population of 11.5 million and sanitation coverage of approximately 81%, exemplifies a favorable context in which SS-HTC can enhance regional competitiveness while advancing environmental sustainability and socio-economic development within the broader low-carbon transition.

Sewage sludge is a by-product of municipal wastewater treatment, and its production is increasing with population growth and urbanization. Hydrothermal carbonization (HTC) is a promising method for managing sewage sludge due to its applicability and cost-effectiveness. The HTC process produces hydrochar, a stabilized carbon material derived from sewage sludge. In this study, sewage sludge was subjected to HTC at optimized conditions of 200 °C and 6 h using the response surface methodology (RSM) statistical tool. The structural morphology of the hydrochar was analyzed using Scanning Electron Microscopy and Transmission Electron Microscopy, which revealed a fluffy and coarse surface with deeper fragments, indicating the presence of carbon microspheres. Fourier Transform Infrared (FTIR) spectra showed that the hydrochar retained similar functional groups as the original sewage sludge, such as hydroxyl, aliphatic methyl, amino, ketonic, amide, and ether groups, with minimal shifts and reductions in vibrational peaks. The higher heating value (HHV) of the sewage sludge increased from 16.24 to 19.41 MJ kg−1 after HTC, indicating energy densification with a factor of 1.07. However, when hydrochar was blended with coal, the HHV gradually decreased. Thermo-gravimetric analysis was performed to assess the co-combustion effectiveness, and it showed that a blend of 50% coal and 50% sewage sludge-derived hydrochar exhibited extended heat generation with minimal reduction in HHV compared to other blends, reaching a value of 23.16 MJ kg−1. These findings suggest that hydrochar produced from sewage sludge through HTC could serve as an alternative energy source in industries, reducing the reliance on coal. Furthermore, the combustion behavior of hydrochar was significantly altered after the HTC process, highlighting its potential for energy generation. By utilizing hydrochar, the industry can contribute to the sustainable management of sewage sludge while reducing greenhouse gas (GHG) emissions associated with coal combustion.

Due to sewage sludge being an abundant biobased resource, and with the number of biogas plants utilizing sewage sludge increasing, digested sewage sludge (DSS) is a promising feedstock for producing bio-oil. This study uses DSS from a biogas plant to produce bio-oil in a hydrothermal liquefaction process adjusting time from 2 to 6 hours, temperature from 280 to 380°C and the presence of a base as a depolymerization agent and potential catalyst. High conversion yields are obtained, with the maximum of 58 wt% on a dry, ash free basis and an energy recovery of up to 94%. The oils contain compounds with a potential for utilization as biofuels and building blocks, especially fatty acids as biodiesel feedstock and biobased phenols, glycols and aliphatic alcohols.

Spatially informed multi-objective decision-making tool for retrofitting municipal wastewater treatment plants.

Improving the sustainability of organic waste management practices in the food-energy-water nexus: A comparative review of anaerobic digestion and composting

A review on Co-Hydrothermal carbonization of sludge: Effect of process parameters, reaction pathway, and pollutant transport

Thermo-kinetic analysis of pyrolysis of thermally pre-treated sewage sludge from the food industry

Full-scale anaerobic digestion of sewage sludges: Fate evaluation of pharmaceuticals and main metabolites

A review on hydrothermal carbonization of potential biomass wastes, characterization and environmental applications of hydrochar, and biorefinery perspectives of the process

As the result of new regulation from the German Sewage Sludge Ordinance (AbfKlärV 2017) and the future obligation to recover phosphorus, thermal treatment (mono-incineration) has become increasingly popular, whereas land-based utilization has decreased. Germany has produced 1.71 million metric tons (DM) of sewage sludge in the year 2021. Sewage sludge contains important nutrients such as phosphorus but also heavy metals and organic pollutants making the direct utilization of sewage sludge in agriculture controversial. Rural areas in particular have benefited from land-based sewage sludge utilization however the future ban on direct land-based utilization is forcing them to find alternative solutions for sewage sludge treatment and management. Hydrothermal carbonization (HTC) has developed considerably over the last 15 years and offers a viable alternative for the utilization of municipal and industrial organic waste such as sewage sludge. The process takes place in an aqueous environment without the need for pre-drying sewage sludge and thereby facilitating direct processing. HTC is especially suitable in combination with the recovery of nutrients like phosphorus. Technologies to recover this essential resource are important because phosphorus is an element that cannot be substituted and is therefore essential. HTC could make a significant contribution to sewage sludge management in combination with phosphorus recovery. However, the technology has yet to establish itself as a sewage sludge valorization process (2023) and is not yet a recognized state-of-the-art. Nevertheless, the HTC technology could gain greater relevance in the future, especially as an alternative valorization pathway for sewage sludge in rural areas of Germany.

Sewage sludge, a byproduct of wastewater treatment, poses significant risks to human health and ecosystems due to its high levels of harmful pollutants, including heavy metals, viruses, and non-biodegradable materials. To mitigate these hazards, thermochemical conversion has emerged as a sustainable strategy for recovering energy and nutrients while reducing the toxicity of sewage sludge. A comprehensive literature search across Science Direct, Scopus, and Web of Science yielded 46 peer-reviewed papers from an initial 2,715 publications. This paper presents a systematic review of the thermochemical conversion processes used to transform sewage sludge into solid fuels, focusing on pyrolysis, torrefaction, and hydrothermal carbonization. The study highlights the significance of optimizing operational parameters and investigates the physicochemical properties of the biochar produced. The results indicate that reaction temperature, time, and heating rate significantly influence the quality and yield of the resulting biochar. Higher temperatures (300–1000°C) enhance the energy content while reducing solid yield. The environmental impacts associated with thermochemical methods, including emissions and potential pollutants, are discussed along with the challenges in treating and transforming sewage sludge into solid fuels. These findings indicate that hydrothermal carbonization is a promising method for waste management and energy production, supporting global efforts to reduce greenhouse gas emissions and dependence on fossil fuels. However, challenges remain in scaling up these technologies for commercial implementation due to high capital and operational costs This review contributes to the understanding of thermochemical processes and their potential applications in sustainable waste-management practices. Future research should focus on pilot and industrial-scale validation, cost-effective pretreatment strategies, and standardized analytical methods. Supportive policy frameworks and investment in demonstration projects are crucial for promoting thermochemical conversion as a viable waste-to-energy solution, contributing to sustainable development and climate change mitigation.

Livestock manure is a significant source of methane and nitrous oxide emissions, posing serious environmental challenges. While conventional management often exacerbates these impacts, thermochemical technologies such as pyrolysis, gasification, and hydrothermal processing offer promising pathways to convert manure into energy, biochar, and other valuable products within a circular bioeconomy. However, these processes can still emit carbon, limiting their environmental benefits. Integrating carbon capture technologies can mitigate these emissions, enabling net-negative outcomes and enhancing overall energy efficiency. This review explores the technical, economic, and environmental aspects of thermochemical manure valorization and emphasizes the transformative potential of coupling these processes with carbon capture. Combustion and co-firing reduce greenhouse gas emissions compared to fossil fuels, but are hindered by manure’s high ash and moisture content. Advanced thermochemical methods such as pyrolysis and gasification yield biochar, bio-oil, and syngas, yet face similar limitations. Hydrothermal processing, especially hydrothermal liquefaction and carbonization, effectively addresses moisture-related challenges and is particularly effective for pig manure. Catalytic gasification further improves conversion efficiency and product quality but remains costly due to expensive catalysts and process complexity. Evidence suggests that integrating carbon capture with thermochemical conversion offers a viable solution for achieving net-negative emissions. However, the widespread deployment of these integrated systems is constrained by high capital costs and infrastructure requirements. Realizing their full potential requires targeted investment, technological innovation, and robust policy support to overcome existing barriers and drive scalable, sustainable implementation.

Hydrothermal liquefaction (HTL) is a thermochemical process in which high water containing feedstocks (95%-75%) such as food waste or sewage sludge is converted into fuels. There are four different phases generated during HTL, namely biocrude (oil phase), biochar (solid phase), aqueous phase and gaseous phase. The biocrude phase needs to be processed with hydrogen (H2) via catalytic hydrotreating to produce gasoline, diesel, jet oil. The aqueous phase (AP) contains up to 5 wt% organic compounds mostly in the form of carboxylic acids and amines and inorganics (e.g., Na, K) and cannot be treated with traditional wastewater treatment technologies (e.g., anaerobic digestion). Hence, it is necessary to treat this wastewater so it can be released safely.Electrocatalysis represents a novel approach to treat this AP waste stream while simultaneously generating the H2 needed for hydrotreating. Instead of using traditional water electrolysis (H2O → 2 H+ + 2 e- + ½ O2, E° = − 1.18 V, ∆H° = 275 kJ/mol H2), we explore electrocatalytic oxidation (ECO) of organic molecules (e.g., CH3COOH + 2 H2O → 2 CO2 + 8 e- + 8 H+, E° = − 0.02 V, ∆H° = 53 kJ/mol H2) to simultaneously remove organics from the HTL-derived AP while simultaneously producing H2. The HTL-derived AP can have different concentration and composition of both organics as well as inorganics, which depend on the source feedstock. The ECO of different HTL-derived wastewaters was evaluated in this study. It was studied, how the concentration and source of AP as well as the electrochemical reaction conditions affect the organic removal as well as the H2 production in both batch cell and continuous flow-cell configuration. Our results show that the presence of organics decreases the current density as well as causes anode deactivation after 40 h of operation in the continuous flow cell. However, the cathode performance and stability remained unaffected for more than 1,000 h. Different strategies were evaluated to enhance the life of the electrode and running the system under alkaline conditions improved the anode stability 5 folds References Andrews, E.; Lopez-Ruiz, J. A.; Egbert, J. D.; Koh, K.; Sanyal, U.; Song, M.; Li, D.; Karkamkar, A. J.; Derewinski, M. A.; Holladay, J.; Gutiérrez, O. Y.; Holladay, J. D., Performance of Base and Noble Metals for Electrocatalytic Hydrogenation of Bio-Oil-Derived Oxygenated Compounds. ACS Sustainable Chemistry & Engineering 2020, 8 (11), 4407-4418.Lopez-Ruiz, J. A.; Qiu, Y.; Andrews, E.; Gutiérrez, O. Y.; Holladay, J. D., Electrocatalytic valorization into H2 and hydrocarbons of an aqueous stream derived from hydrothermal liquefaction. Journal of Applied Electrochemistry 2021, 51 (1), 107-118.Lopez-Ruiz, J. A.; Andrews, E.; Akhade, S. A.; Lee, M.-S.; Koh, K.; Sanyal, U.; Yuk, S. F.; Karkamkar, A. J.; Derewinski, M. A.; Holladay, J.; Glezakou, V.-A.; Rousseau, R.; Gutiérrez, O. Y.; Holladay, J. D., Understanding the Role of Metal and Molecular Structure on the Electrocatalytic Hydrogenation of Oxygenated Organic Compounds. ACS Catalysis 2019, 9 (11), 9964-9972. Figure 1