Hydrothermal liquefaction of sewage sludge for biofuel application: A review on fundamentals, current challenges and strategies

Scientific studies have demonstrated that hydrothermal liquefaction (HTL) can be suggested as a cost-effective and eco-friendly solution for the valorization of sewage sludge in the fields of waste management and renewable energy production. On-going efforts are continued extensively to improve conversion efficiency in order to transform this technology into industrial application. The present review focuses on state-of-the-art of HTL of sewage sludge, systematically, the HTL fundamentals, the influences of the essential operation parameters, main existing challenges and integrated upgrading are evaluated in detail. Opportunities and strategies including pretreatment, co-liquefaction, post-application of aqueous and solid phases are addressed. Additionally, some available results regarding the continuous-flow tests of sewage sludge are summarized. Finally, further considerations and recommendations are made for improving or completing the economic feasibility of HTL process.

Automated summary

This article reviews the latest research progress of hydrothermal liquefaction (HTL) technology in the fields of sewage sludge treatment and renewable energy production. It evaluates the fundamentals of HTL, the influence of key operation parameters, and the main challenges and upgrading methods. It also discusses opportunities and strategies such as pretreatment, co-liquefaction, and post-application. The results of continuous-flow tests of sewage sludge are summarized. Finally, considerations and recommendations for improving the economic feasibility of the HTL process are proposed.