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

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 collection 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. (2025) 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. ( 2025) 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 gCODfed -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. ( 2025) 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, 2piperidinone 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%-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. (2025) 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 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 at: http://www.ntis.gov Romero, A. D., Poli, J. V., Larson, S., Stagg-Williams, S., Carter, R., and Sturm, B. S. M. (2025). Inhibition of nitrifying bacteria from heterocyclic N-containing organic compounds from municipal sludge hydrothermal liquefaction. Frontiers in Chemical Engineering 7. doi: 10.3389/fceng.2025.1532958 Wehner, W., Norton, J., Oyler, J., Winchell, L., Willis, J., andFonoll Almansa, X. (2025).Assessing the fate of pfas during the hydrothermal liquefaction of sewage sludge., in ECO-STP (conference), (Stockholm: IWA).