With the aim of transforming sewage sludge into a P-fertiliser material in a single combustion step, the chemical processes underlying sewage sludge combustion were analysed using powder X-ray diffraction (P-XRD), infrared spectroscopy (IR), thermogravimetric (TGA) as well as elemental analyses (EA). In addition to the combustion of sewage sludge on its own ("mono-combustion"), additions of different additives prior to the combustion step were also carried out. Based on the very positive effects of the additives sodium and potassium carbonate on the obtained ashes concerning their phosphate solubilities in neutral ammonium citrate (NAC) solution, sewage sludge combustions after additions of Na