Enhancement of the Plant‐Accessible Phosphate Fraction in Sewage Sludge Ashes by Na⁺ or K⁺ Addition Prior to Combustion

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 Na₂CO₃ or K₂CO₃ were investigated in detail. We found that these additions altered the main phosphate-containing product found in the ashes from whitlockite (Ca₉(Mg,Fe)(PO₃OH)(PO₄)₆), a hardly plant-accessible species, to other phosphate containing compounds such as buchwaldite (CaNaPO₄), which is known for a long time as a very good P-source for plants. Consecutive greenhouse experiments with maize (Zea mays L.) as test plant confirmed the results of the chemical analyses and demonstrated that Na- or K-ashes obtained from a "co-combustion" of sewage sludge mixed with alkali carbonates exhibit relative P-fertilising efficiencies of up to ~80 % in comparison to commercial superphosphate, making these materials promising surrogate phosphate mineral sources for agricultural production.