Recent advances in hydrothermal liquefaction of manure wastes into value-added products

The rapid human development is requiring extensive energy and meat demands, leading to an increasingly serious energy and environment crisis caused by the large consumption of fossil fuels and the massive production of manure wastes. Hydrothermal liquefaction (HTL) technology provides a promising "waste-to-energy" route to efficiently convert manure wastes into value-added products. In this regard, this paper reviews the recent HTL studies for the valorization of manure wastes from several points of view, starting from characteristics of manure wastes, conventional HTL of manure wastes, and then concentrating on the advanced HTL methods to valorize manure wastes. Recent progress in the batch and continuous studies on the both conventional and advanced HTL technologies including fast hydrothermal liquefaction (fast-HTL), hydrothermal co-liquefaction (co-HTL), and catalytic hydrothermal liquefaction (ca-HTL) were summarized, along with the emphasis on the production and application of valuable products (e.g., biocrude and hydrochar). In addition, techno-economic analysis (TEA) and life-cycle assessment (LCA) pertaining to HTL of manure wastes were also demonstrated, presenting the huge commercial potential of HTL technologies for manure wastes valorization. Consequently, this review illustrates the milestones that are necessitated to be achieved in the future, simply put, this review will offer basis for future sustainable and efficient treatment of manure wastes.