Hydrothermal carbonization (HTC) has been proposed and applied to process the residual stream exuding from the dark fermentation (DF) treatment of cheese whey (CW), which is the liquid effluent from the precipitation and removal of milk casein during the cheese-making process. This is considered one of the most problematic agro-industrial biowaste streams to manage, considering how dairy activities play an essential role in the agrarian economy of the European Union. DF produces an H2-rich biogas for further uses as fuel cells, combustion, biofuels as well as general industrial uses, while the slurry effluent – the fermentate – remains rich in organic content and can be further exploited by other steps. In this case a thermochemical treatment, namely HTC, is proposed for producing hydrochar for further use. The reaction parameters of the hydrothermal carbonization process concerns different temperatures as the main influencing factor, namely 180, 200, and 220 °C. The mass and energy balances of the process will be determined. The physical and chemical properties of hydrochar and postprocessing water will be studied. The elemental analysis of solid product will be investigated. The postprocessing water will be analyzed taking into account pH, conductivity, total organic carbon, chemical oxygen demand, phenol index, nitrogen and phosphates contents, as well as the main heavy metals content to estimate the environmental impact of the liquid phase and evaluate the potential nutrient recovery and wastewater recycling.