Hydrothermal liquefaction of municipal sludge: Biocrude production and fuel quality upgrading

Hydrothermal liquefaction (HTL) has emerged as a transformative technology for wet municipal sludge (hereafter referred to as "sludge") treatment, effectively bypassing energy-intensive drying processes while converting organic matter into biocrude. Although extensive studies over the past two decades have validated sludge-derived biocrude as a viable biofuel precursor, three critical barriers hinder its industrial implementation: complex formation process, suboptimal fuel quality and undefined upgrading methods. In this review, over 100 peer-reviewed sludge HTL articles (2008-2025) were evaluated to analyze the biofuel production process from three key perspectives: (1) characterization of sludge, (2) formation of hydrothermal biocrude, and (3) post-processing biocrude upgrading technologies. The primary objectives of this study were: (a) delineating physicochemical attributes and reaction pathways of sludge-derived biocrude under varying operational parameters, (b) conducting comparative analysis of catalytic upgrading techniques (including pretreatment, catalytic HTL, fractional distillation, catalytic cracking, and catalytic hydrotreating), and (c) identifying the gaps associated with sludge HTL and highlight areas necessitating further investigation. Based on the three-stage upgrading theory proposed in this study, we further discuss two practical implementation pathways: direct combustion application and blending with petroleum fuels. Overall, hydrothermal liquefaction technology for sludge represents a highly promising approach for harmless disposal and energy recovery. To accelerate commercial deployment, future research should prioritize by-products utilization, pilot-scale blended process integration, and catalyst innovation.