This study presents a valorisation approach that converts waste sludge from a South African paper mill into activated biocarbon, therefore meeting environmental and sustainability goals. A two-step synthesis using hydrothermal carbonization and NaOH activation of paper mill waste at 700 °C yielded activated biocarbon. N₂ at 77k adsorption revealed the development of a carbon material with high porosity and a surface area of 1139 m²/g. The three-electrode cell in alkali (3M KOH) and neutral (1M Na₂SO₄) electrolyte shows the synthesised activated biocarbon electrode exhibits relatively good specific capacitance (C_sp) of 233 and 244.2 F/g respectively. In both electrolytes, symmetric triangle curves in galvanostatic charge-discharge indicate a quick charge-discharge process. Full cells (symmetric two electrodes) with 3M KOH and 1M Na₂SO₄ electrolytes had specific capacitances of 125 and 152 F/g, respectively, and energy densities of 17.4 and 21.1 Wh/kg, with a capacity retention efficiency of 96% after 5000 cycles in both electrolytes. Thus, this study has demonstrated that a porous carbon material made from paper sludge in alkaline and neutral electrolytes is a waste valorisation strategy for developing promising sustainable electrode materials for high-energy, greener supercapacitor applications.