Optimising Incineration Parameters of Wastewater Sludge Ash Production for a Sustainable Environment and Construction Materials

South Africa produces approximately 639,000 dry tonnes of wastewater sludge annually, creating disposal challenges and opportunities for resource recovery through Wastewater Sludge Ash (WSA). This study optimised and interpreted the thermal conversion of Polokwane Wastewater Sludge (WWS) to WSA by evaluating atmosphere and heating rate-dependent decomposition, as well as kinetic and thermodynamic parameters. Thermogravimetric analyses were conducted to 900 °C at 10, 20 and 40 °C/min under air and nitrogen (N2). Three decomposition stages were identified: moisture loss (25-150 °C), organic decomposition (200-500 °C) and carbonate decomposition (600-900 °C), producing approximately 70% WSA in air. Coats-Redfern analysis showed higher effective activation energies in air (118.6 and 142.1 kJ/mol) than in N2 (94.8 and 107.5 kJ/mol), with strong linear fits (R2 = 0.986-0.994). Decomposition was endothermic, with a higher ΔH in air, while positive ΔG indicated non-spontaneous thermal decomposition. XRD identified quartz, calcite and haematite, with increased crystallinity at 600 °C. FTIR confirmed organic burnout at 600 °C through the disappearance of C-H and C=O/N-H bands.