The pulp and paper industry generates significant amounts of waste from various processes consisting of organic and inorganic matter. Due to its composition, the management of pulp and paper mill sludge presents an economic burden for wastewater treatment plants. In South Africa, paper mill sludge is mainly incinerated, landfilled or used for land application. These practices cause environmental and health hazards. In this study, a process of stabilization of paper mill sludge through hydrothermal carbonization is proposed. The quality of biochar produced and energy yield will depend on the elemental composition, calorific value and ash content of the sludge. Results revealed samples are composed of flat fibrous cellulosic morphology, with high volatile matter and little fixed carbon percentage. The main decomposition occurred between 220-400 °C. Sludge from primary clarifier contains the highest ash percentage, hence it was found to be unsuitable for energy recovery through hydrothermal carbonization. Sludge from kraft and recycling paper mills exhibited lower ash content and higher caloric values, thus more preferable for coal conversion through hydrothermal carbonization. The sludges have a sulfur composition (2.33%) compared to heavy fuel oils and bituminous coal.