Editorial: Hydrothermal liquefaction: aqueous phase treatment, product recovery, and downstream implications

Editorial on the Research Topic
Hydrothermal liquefaction: aqueous phase treatment, product recovery, and downstream implications

Hydrothermal Liquefaction (HTL) represents a promising pathway for bio-crude oil production, a precursor for transportation fuel, from a wide variety of wastes (e.g., sludge, food waste, certain plastic materials). HTL is particularly promising for sludge management in water resource recovery facilities (WRRFs), as it recovers resources in the form of biocrude and phosphorus, reduces sludge volume by 80%–90% in 15–30 min, and shows promise for handling emerging contaminants (Basar et al., 2021; Abbott et al., 2023; Li et al., 2024; Wehner et al., 2025). During the process, a complex aqueous by-product phase is produced containing valuable chemicals but also posing significant disposal challenges. The treatment of this aqueous phase, the recovery of valuable products, and understanding the downstream implications are crucial for the sustainable deployment of HTL. This article Research Topic focuses on these critical areas, comprising four original articles dedicated to recent advances in the treatment and impact of HTL aqueous by-product (HTL-aq). The contributions span from technologies to reduce the toxicity of HTL-aq and recover resources, to the impact that HTL-aq can have on WRRF processes and microorganisms.

Two of the original articles focused on the treatment of HTL-aq. In their study, Kenney et al. studied supercritical upgrading as a new technology that reduces the organic content of HTL-aq and co-produces supplemental biocrude. Using a ZSM-5 catalyst bound with silica sol, the aqueous carbon content was reduced by 64%–73% with corresponding production of aromatic hydrocarbons including phenol and 2-pentanone. Total nitrogen was reduced by 10%, and the remaining nitrogen compounds were channeled into a single class of compounds: pyridines. The results show that 50% carbon yield as biocrude could be obtained with this process. In the second original article, Fonoll Almansa et al. used a novel anaerobic digestion configuration, the recirculating anaerobic dynamic membrane bioreactor, with the goal of growing biofilms, which have been shown to be effective in degrading HTL-aq. The HTL-aq came from sewage sludge treated in an HTL process that recovered high amounts of phosphorus in the biochar, leaving the HTL-aq without sufficient P for the growth of methanogens. After supplementing the HTL-aq with a phosphorus-rich solution, the bioreactor was able to degrade 65% of the COD and produce 0.19 ± 0.02 L CH₄ gCOD_fed^-1. According to the microbial analyses performed, the biofilm had a higher relative abundance of methanogens than the suspended biomass.

The other two original articles focused on the impact that HTL-aq has on WRRFs biological processed and microbes. Romero et al. evaluated the inhibitory effect of HTL-aq on activated sludge nitrifiers. The researchers collected mixed liquor from the biological reactor of a WRRF and performed specific ammonia uptake tests to assess inhibition due to 2-pyrrolidinone, pyrazine, 2-piperidinone, and HTL-aq stored for 1 week and 15 weeks. In this study, 2-piperidinone was not strongly inhibitory to nitrifiers, while 2-pyrrolidinone and pyrazine caused inhibition. The IC₅₀ of the HTL-aq was determined to be 0.08% (v/v), and if all HTL-aq were recycled to the influent of the WRRF, that would represent a percentage ranging from 0.029% to 0.108% (v/v). Based on this batch experiment, it is feasible to recycle the HTL-aq generated, which is a key factor for full-scale implementation of HTL. In addition, the data demonstrate that storage of HTL-aq can be used to reduce the concentration of inhibitory nitrogen-containing organic compounds. Blackwell et al. performed acute and chronic inhibition tests using batch and continuous systems, respectively, with non-nitrifying mixed liquor microorganisms. The inhibition during acute and chronic exposure studies was evaluated by using different HTL-aq dilutions that are expected in a WRRFs implementing HTL at the pilot- or full-scale. The results from the continuous experiments showed that the specific oxygen uptake rates were not affected by the addition of HTL-aq, but linear-mixed models showed statistically significant lower specific dissolved organic carbon removal rates associated with higher HTL-aq concentration in the influent. The results from the model suggest that HTL-aq treatment is required before discharge to the headworks. The authors also examined HTL-aq impact on two additional critical WRRF functions: nitrification and UV disinfection. Nitrifying activated sludge cultures exhibited inhibition when exposed to HTL-aq, demonstrating that WRRFs with nitrogen discharge limits or nitrification-dependent processes must treat the aqueous phase prior to returning it to the headworks. Furthermore, UV disinfection experiments revealed a dramatic 93% reduction in disinfection efficiency in the presence of HTL-aq, attributed to UV-absorbing compounds such as phenol and 4-methylphenol. Collectively, these results suggest that while pilot-scale headworks discharge of HTL-aq may be feasible under certain conditions, full-scale implementation will require pretreatment of HTL-aq, particularly at facilities employing ultraviolet disinfection or operating under stringent nitrogen removal requirements.

We are pleased to present novel discoveries on the treatment of HTL-aq and its effects on WRRF processes and microorganisms. The studies presented here are useful to accelerate the implementation of HTL, especially in WRRFs.

Author contributions

XF: Writing – review and editing, Supervision, Writing – original draft. BS: Writing – review and editing. DC: Writing – review and editing. MT: Writing – review and editing.

Funding

The author(s) declared that financial support was not received for this work and/or its publication.

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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The author(s) declared that generative AI was not used in the creation of this manuscript.

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References

Abbott, T., Yuzik, J., Islam, M., Kadota, P., and Eskicioglu, C. (2023). “Fate and partitioning of contaminants of emerging concerns (CECs) through the hydrothermal liquefaction of wastewater sludge,” in Water environment association of Ontario annual conference.

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Basar, I. A., Liu, H., Carrere, H., Trably, E., and Eskicioglu, C. (2021). A review on key design and operational parameters to optimize and develop hydrothermal liquefaction of biomass for biorefinery applications. Green Chem. 23, 1404–1446. doi:10.1039/d0gc04092d

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Li, S., Jiang, Y., Seiple, T., Snowden-Swan, L., Ou, L., Cai, H., et al. (2024). Site-specific design case study for wet waste hydrothermal liquefaction and biocrude upgrading to hydrocarbon fuels. Available online at: http://www.ntis.gov. (Accessed December 26, 2025)

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Wehner, W., Norton, J., Oyler, J., Winchell, L., Willis, J., and Fonoll Almansa, X. (2025). “Assessing the fate of pfas during the hydrothermal liquefaction of sewage sludge,” in ECO-STP (conference). (Stockholm: IWA).

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