Resource efficient bio-chemical production and waste treatment

Project Information

REBOOT

Grant agreement ID: 849841

DOI

10.3030/849841

Project closed

EC signature date 18 September 2019

Start date 1 January 2020

End date 31 December 2024

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 494 622,00

EU contribution

€ 1 494 622,00

Coordinated by

AARHUS UNIVERSITET
Denmark

Periodic Reporting for period 4 - REBOOT (Resource efficient bio-chemical production and waste treatment)

Reporting period: 2024-07-01 to 2024-12-31

Summary of the context and overall objectives of the project

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.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

A study was conducted to evaluate temperature effects on phosphorus (P) partitioning across different HTL product phases. Various wet wastes, including manure, sewage sludge, anaerobic digestate, and slaughterhouse waste, were tested. The results confirmed that HTL recovers over 95% of P in the solid fraction, aligning with the goals of novel separation technologies designed for continuous flow reactors.

To advance this, a dedicated continuous HTL reactor was designed and built, where in-line separation devices were tested and optimized. The findings from the continuous reactor were compared to batch systems, commonly used in literature, where solids are separated after cooling. This work led to the publication of two articles on continuous solid separation in HTL and three on P recovery and its application as fertilizer.
Research was also conducted on catalysts for HTL bio-crude upgrading. Catalyst supports TiO2, ZrO2, and TiO2+ZrO2 were doped with transition metals (Ni, Co, Fe) and screened for activity and stability. All supports remained stable under hydrothermal conditions and contributed to increased bio-crude yields. However, oxygen (O) and nitrogen (N) content in bio-crudes was not significantly reduced, leading to the decision that post-reaction catalytic upgrading was more viable than in-situ catalysis within HTL reactors.

A fixed-bed hydrogenation reactor was designed and constructed to upgrade manure-derived bio-crudes into diesel and kerosene. This bespoke catalytic hydrotreating reactor was successfully applied to diverse bio-crudes from different wet waste sources.

The challenge of HTL process water treatment was initially explored using three technologies:
- Microbial Electrolysis Cells (MEC) – Evaluated for water cleaning and hydrogen production but proved unsuitable due to the complexity of HTL process water.
- Electrochemical Oxidation (EO) – Effectively cleaned water and produced hydrogen, but high energy consumption limited its industrial feasibility.
- Wet Oxidation (WO) – Developed as an alternative, converting soluble organics into heat and CO2. This technology can be integrated into HTL to meet the reactor's heat demand while purifying water.
Both EO and WO were experimentally validated in continuous flow reactors, leading to one publication on EO and two on WO. WO was identified as the most promising solution and is now the focus of further scaling in a Horizon Europe project, in collaboration with industrial partners.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The most significant advancement against the state of the art lies in HTL process water management and valorization. Wet oxidation has proven to be an effective, low-cost, and process-enhancing solution. As an exothermic reaction, it generates heat that can be reintegrated into the HTL process, while simultaneously cleaning the wastewater. Our research demonstrated high carbon conversion efficiencies, nitrogen transformation into ammonium, and residual volatile fatty acid production, which may hold additional value. We published results on wet oxidation of HTL process water from sewage sludge, detailing the effects of temperature and residence time. These findings guided the design of continuous flow experiments, incorporating heat exchangers for energy recovery. This represents a major step forward in bringing HTL process water treatment closer to industrial implementation. We anticipate that further optimization and integration of wet oxidation with HTL will enable near-autothermal operation, establishing it as the new state-of-the-art solution for HTL process water management.

A new continuous flow reactor was constructed to optimize P recovery under different process conditions using in-line separation devices. The results revealed significant differences in inorganic recovery between state-of-the-art continuous reactors and batch systems. Additionally, we demonstrated that inorganics can be effectively removed via in-line separation in continuous flow reactors. However, secondary char formation in the reactor poses challenges for bio-crude utilization, as this char must be removed before upgrading to high-quality biofuels.

Biofuel cuts produced in REBOOT

Wet oxidation of HTL process water in a continuous reactor concept graphic

Conecptual graphic of REBOOT objective 1, to recover P from wet wastes during continuous HTL