Recent advances on treatment and valorization of hydrothermal liquefaction aqueous phase: A comprehensive review

• First comprehensive review on HTL-AP separation and valorization technologies • Ozonation improves HTL-AP biodegradability by degrading toxic organics • Nanofiltration and adsorption enable efficient nutrient and phenol recovery • Electrochemical oxidation removes 99% COD while producing green hydrogen • Hybrid systems enhance COD removal up to 97% and energy recovery efficiency Hydrothermal liquefaction (HTL) is an emerging thermochemical process that converts wet biomass into biofuels and value-added chemicals. A major byproduct, the HTL aqueous phase (HTL-AP), contains water-soluble organics, inorganic salts, residual bio-oil, and trace metals, creating both environmental risks and opportunities for resource recovery. This review examines HTL-AP composition, highlighting key constituents such as carboxylic acids, phenolics, and nitrogen-containing compounds, which largely govern toxicity and biodegradability. Composition varies significantly with feedstock and operating conditions. Advanced treatment technologies—including oxidation processes, membrane filtration, adsorption, liquid–liquid extraction, and electrochemical methods—are evaluated for contaminant removal and nutrient recovery. Integrated treatment trains consistently outperform single-unit systems; for example, wet air oxidation and electro-oxidation can achieve up to ∼95% and 99% chemical oxygen demand (COD) reduction, respectively, while some electrochemical systems also enable hydrogen co-production and selective nutrient recovery. Emphasis is placed on hybrid approaches that mitigate ecological risks while improving process economics by valorizing waste streams. Recent advances, remaining technical challenges, and research needs are discussed, with particular attention to scalable modular systems, durable materials, pilot-scale validation, and robust techno-economic and life cycle assessments. Effective HTL-AP management is essential for sustainable biofuel production and circular resource utilization.