Hydrothermal liquefaction of sewage sludge for circular bioeconomy: Focus on lignocellulose wastes, microplastics, and pharmaceuticals

The rapid increase in sewage sludge (SS) generation from wastewater treatment plants (WWTPs) has become a pressing global environmental challenge. The SS contains a wide variety of pollutants, including lignocellulose from plants and paper wastes, microplastics (MPs) from plastic wastes, and pharmaceutical residues (PRs), all of which pose substantial risks to ecosystems and human health. To address these waste management issues while also meeting rising energy demands, a shift towards a circular bioeconomy is essential. Hydrothermal liquefaction (HTL) of SS (SS-HTL) presents a sustainable solution by converting waste into renewable biofuels and mitigating environmental hazards. This review addresses five key areas: (1) an in-depth analysis of current advancements in SS-HTL technology; (2) factors influencing bio-oil production; (3) transformation pathways of lignocellulose, MPs, and PRs during HTL; (4) advanced methods for upgrading SS, including chemical, mechanical, and in situ liquefaction techniques; and (5) future perspectives on enhancing SS-HTL technology. Additionally, the review evaluates the potential applications of byproducts like the aqueous (AQ) phase, solid residues (SRs), and gases. By addressing the challenges in SS-HTL research and implementation, this article aims to improve economic feasibility and expand industrial applications. It serves as a valuable resource for researchers and innovators committed to advancing waste management technologies and accelerating the transition to a sustainable circular bioeconomy.