Additional options for the sustainable treatment of municipal sludge are required due to the significant amounts of sludge, high levels of nutrients (e.g., C, N, and P), and trace constituents it contains. Hydrothermal processing of municipal sludge has recently been recognized as a promising technology to efficiently reduce waste volume, recover bioenergy, destroy organic contaminants, and eliminate pathogens. However, a considerable amount of solid residue, called hydrochar, could remain after hydrothermal treatment. This hydrochar can contain abundant amounts of energy (with a higher heating value up to 24 MJ/kg, dry basis), nutrients, and trace elements, as well as surface functional groups. The valorization of sludge-derived hydrochar can facilitate the development and application of hydrothermal technologies. This review summarizes the formation pathways from municipal sludge to hydrochar, specifically, the impact of hydrothermal conditions on reaction mechanisms and product distribution. Moreover, this study comprehensively encapsulates the described characteristics of hydrochar produced under a wide range of conditions: Yield, energy density, physicochemical properties, elemental distribution, contaminants of concern, surface functionality, and morphology. More importantly, this review compares and evaluates the current state of applications of hydrochar: Energy production, agricultural application, adsorption, heterogeneous catalysis, and nutrient recovery. Ultimately, along with the identified challenges and prospects of valorization approaches for sludge-derived hydrochar, conceptual designs of sustainable municipal sludge management are proposed.

由于大量污泥，高水平的养分（例如，C，N和P）及其所含的微量成分，因此需要对城市污泥进行可持续处理的其他选择。最近，城市污泥的水热处理已被公认为是一种有前途的技术，可有效减少废物量，回收生物能，破坏有机污染物并消除病原体。但是，经过水热处理后，可能会残留大量的固体残渣（称为炭）。这种碳氢化合物可包含大量能量（以干基计，具有更高的发热量，最高可达24 MJ / kg），营养物质和微量元素，以及表面官能团。污泥衍生的碳氢化合物的增值可以促进水热技术的发展和应用。这篇综述总结了从城市污泥到水炭的形成途径，特别是水热条件对反应机理和产物分布的影响。此外，这项研究全面封装了在各种条件下生产的水煤油的描述特性：产量，能量密度，理化特性，元素分布，所关注的污染物，表面功能和形态。更重要的是，本综述比较并评估了水焦的应用现状：能源生产，农业应用，吸附，非均相催化和养分回收。最终，伴随着已确定的挑战和来自污泥的水焦的增值方法的前景，提出了可持续城市污泥管理的概念设计。