The current state-of-the-art solution for managing wet waste, such as sewage sludge and manure, is anaerobic digestion. However, this process still leaves behind digested sludge, which is either applied to land for fertilizer recovery or incinerated. Due to growing concerns about pollutants, land application is declining, while incineration results in nutrient loss.
Hydrothermal liquefaction (HTL) is an emerging alternative that converts wet waste into bio-crude by heating it in water under pressure (around 350°C for 15 minutes). This process mimics natural fossil fuel formation, creating a renewable bio-crude that can be refined into sustainable aviation and marine fuels.
The REBOOT project advanced HTL technology, focusing on phosphorus recovery—a finite resource crucial for nutrient recycling. The project developed new separation technologies that extract inorganics within the HTL reactor while producing a low-metal bio-crude, improving its suitability for refining. Additionally, improved catalysts were designed to enhance bio-crude quality, making downstream refining more efficient and cost-effective.
Another major challenge addressed was the treatment of process water, a byproduct containing high organic loads. Since HTL operates without prior drying, large volumes of wastewater are generated. The most significant breakthrough of the project was the discovery of an innovative wastewater treatment solution using wet oxidation. This process successfully oxidized organics into CO2, heat, and volatile fatty acids (VFAs), first in batch reactors and later in a continuous flow system.
This solution offers several advantages:
Heat recovery, enabling an autothermal HTL process.
VFA production, which can serve as platform chemicals or be further refined.
CO2 utilization for Power-to-X applications.
In addition, REBOOT developed a bespoke continuous HTL reactor to study hydrochar formation and separation. This led to improved solid separation techniques at a pilot scale and new insights into hydrochar formation pathways, which are critical for scaling up the technology.
Overall, REBOOT made key advancements in HTL technology, improving reactor design, nutrient recovery, bio-crude quality, and wastewater treatment—paving the way for more sustainable and scalable implementation.
