Recovery of Nutrients from the Aqueous Phase of Hydrothermal Liquefaction—A Review

Hydrothermal liquefaction (HTL) is a thermochemical conversion process that converts wet biomass into biocrude oil, a gas phase, a solid phase, and an aqueous phase (HTL-AP). An obstacle to the development and scaling of HTL is the volume of HTL-AP produced during the process, which has high concentrations of nitrogen and carbon and cannot be disposed of in the environment without treatment. The HTL-AP is enriched with organic compounds, particularly light polar organics and nitrogenous compounds, which are inhibitory to microbial treatment in wastewater treatment plants. For this reason, the valorization of the HTL-AP is significant for the circular economy of HTL. This review synthesizes published findings on different types of treatment of the HTL-AP for the recovery of valuable nutrients and the removal of toxic compounds. This work outlines the trade-offs of the treatments to serve as a guide for future research to address these weaknesses and improve the valorization of the HTL-AP. Furthermore, this work uniquely focuses on HTL-AP treatment for recovering plant-available nitrogen, targeting its potential use as a fertilizer. The literature highlights the importance of increasing nitrogen bioavailability in HTL-AP through two-step treatments and by selecting HTL-AP derived from protein-rich feedstocks, which offer higher initial nitrogen content. According to the current state of research, further work is needed to optimize chemical and biological treatments for nutrient recovery from HTL-AP, particularly regarding treatment scale and duration. Additionally, economic analyses across different treatment types are currently lacking, but are essential to evaluate their feasibility and practicality.