**Manfredi Picciotto Maniscalco, Maurizio Volpe and Antonio Messineo \***

Faculty of Engineering and Architecture, Kore University of Enna, Cittadella Universitaria, 94100 Enna, Italy; manfredi.maniscalco@unikore.it (M.P.M.); maurizio.volpe@unikore.it (M.V.) **\*** Correspondence: antonio.messineo@unikore.it

Received: 6 July 2020; Accepted: 4 August 2020; Published: 7 August 2020

**Abstract:** Hydrothermal carbonization (HTC) represents an efficient and valuable pre-treatment technology to convert waste biomass into highly dense carbonaceous materials that could be used in a wide range of applications between energy, environment, soil improvement and nutrients recovery fields. HTC converts residual organic materials into a solid high energy dense material (hydrochar) and a liquid residue where the most volatile and oxygenated compounds (mainly furans and organic acids) concentrate during reaction. Pristine hydrochar is mainly used for direct combustion, to generate heat or electricity, but highly porous carbonaceous media for energy storage or for adsorption of pollutants applications can be also obtained through a further activation stage. HTC process can be used to enhance recovery of nutrients as nitrogen and phosphorous in particular and can be used as soil conditioner, to favor plant growth and mitigate desertification of soils. The present review proposes an outlook of the several possible applications of hydrochar produced from any sort of waste biomass sources. For each of the applications proposed, the main operative parameters that mostly affect the hydrochar properties and characteristics are highlighted, in order to match the needs for the specific application.

**Keywords:** HTC; waste biomass; energy recovery; environmental remediation; nutrients recovery; activated carbon
