Hydrothermal Carbonization of Sewage Sludge

Hydrothermal carbonization (HTC) of sewage sludge is a thermochemical treatment that transforms wet organic waste into a carbon-rich solid (hydrochar), an aqueous phase and minor gaseous by-products under moderate temperatures (180–260 °C) and autogenous pressures. By simulating geological coalification within hours, HTC enhances the dewaterability of sludge, stabilises nutrients and inactivates pathogens. The resulting hydrochar exhibits increased fixed carbon content, improved energy density and tailored surface functionalities, making it suitable as a solid fuel, soil amendment or precursor for activated carbon. Meanwhile, the liquid fraction retains dissolved organics, nutrients and metals, requiring further purification to meet environmental discharge standards.

Key process parameters—temperature, residence time, feedstock composition and pretreatment strategies such as solvent assistance or co-carbonization—dictate product yields and characteristics. Greater severity (higher temperature and longer reaction time) typically reduces H/C and O/C ratios in hydrochar while promoting phosphorus immobilisation and ash accumulation. Innovations have focused on combining HTC with selective leaching, machine-learning optimisation and downstream gasification to recover nutrients, generate syngas and mitigate harmful emissions. Globally, HTC aligns with circular economy goals by converting a challenging waste stream into valuable materials and fostering decentralised, low-carbon sludge management.

Research from Nature Portfolio

No recent Nature Portfolio content available.

Hydrothermal Carbonization of Sewage Sludge publication trend

The graph below shows the total number of articles in hydrothermal carbonization of sewage sludge across all publications each year (not limited to Nature Index journals).

Technical terms

Hydrothermal Carbonization: A wet thermochemical process converting biomass into carbon-rich solids, liquids and gases under elevated temperature and pressure.

Hydrochar: The solid, carbonaceous product of hydrothermal carbonization, enriched in fixed carbon and modified porosity.

Dewaterability: The capacity to remove free and bound water from sludge or hydrochar, influencing drying efficiency and handling.

Deep Eutectic Solvent: A eutectic mixture of hydrogen-bond donors and acceptors that lowers melting point and dissolves biomass components.

Syngas: A synthesis gas chiefly composed of carbon monoxide and hydrogen, produced by gasification of carbonaceous feedstocks.

References

1. Hydrothermal carbonization process: Fundamentals, main parameter characteristics and possible applications including an effective method of SARS-CoV-2 mitigation in sewage sludge. A review. Renewable and Sustainable Energy Reviews (2022).
2. Hydrothermal carbonization of sewage sludge: Hydrochar properties and processing water treatment by distillation and wet oxidation. Energy Reports (2023).
3. Effects on the Physicochemical Properties of Hydrochar Originating from Deep Eutectic Solvent (Urea and ZnCl2)‑Assisted Hydrothermal Carbonization of Sewage Sludge. ACS Sustainable Chemistry & Engineering (2022).
4. Sequential hydrothermal carbonization and CO2 gasification of sewage sludge for improved syngas production with mitigated emissions of NOx precursors. Chemical Engineering Journal (2023).
5. Applied machine learning for predicting the properties and carbon and phosphorus fate of pristine and engineered hydrochar. Biochar (2025).
6. Acid-Induced Phosphorus Release from Hydrothermally Carbonized Sewage Sludge. Waste and Biomass Valorization (2021).