Towards sustainable bio-oil Production: Recycling and separation techniques applied to the aqueous phase byproduct of algae biomass hydrothermal liquefaction

Valorizing or recycling the hydrothermal liquefaction aqueous phase (HTL-AP) remains a considerable challenge in commercializing the HTL process for wet biomass, especially for algae, due to the high concentration of organics and nutrients. This review focuses on separation methods for HTL-AP, with particular emphasis on membrane filtration technologies. Most HTL-AP primary separation efforts have been limited to laboratory-scale solvent extraction, which requires a secondary separation/polishing step, and upscaling these processes thus remains a challenge. Various secondary treatment processes have recently been proposed to support the development of water-closed-loop HTL systems, including the recovery of value-added products such as struvite from HTL-AP. This could improve the overall economic viability of HTL. Membrane-based separation offers a scalable and environmental friendly approach, enabling nutrient concentration, valorization, and resource recovery. However, additional research, including fouling mitigation and cost-effective cleaning strategies, is needed to assess the techno-economic feasibility of membrane applications. Integrated membrane cascades combining microfiltration, ultrafiltration, nanofiltration, and/or reverse osmosis could improve process sustainability by enhancing separation efficiency, reducing fouling, minimizing inhibitory organics, and lowering freshwater demand in algae cultivation. While a variety of secondary treatment methods have been proposed, techno-economic analyses and life cycle assessments comparing their efficiency and impacts are lacking. Such assessments should be based on identical process conditions, thereby isolating the effects of feed composition, HTL operating parameters, and primary separation techniques. Moreover, treated HTL-AP should be reused as a growth medium with the same algae strains used as HTL feed to ensure consistency across cycles.