Phase analysis of sewage sludge ash thermochemically treated with sodium and potassium sulfates
Source: BAM, division Thermochemical Residues Treatment and Resource Recovery
Phosphorus is an essential element for plants, animals and humans. Phosphorus fertilizers from farmyard manures and slurries as well as mineral fertilizers based on the fossil raw material phosphate rock are used to ensure the phosphorus supply for plants in agriculture. To reduce dependence on fossil raw materials and to close the phosphorus nutrient cycle, phosphorus is recovered from phosphorus-rich sewage sludge from wastewater treatment plants. Most of the sewage sludge is incinerated in mono-sludge incineration plants. The ashes from mono-incineration plants may have high phosphorus content (12%) and are suitable for recovering phosphorus.
A promising process to recover phosphorus is thermochemical treatment in a rotary kiln (AshDec®-process). A large-scale plant located in Altenstadt (Bavaria) starts operation in 2024. This process transforms poorly plant-available phosphorus phases in the sewage sludge ash (mainly the calcium phosphate whitlockite and aluminium phosphate) to highly plant-available calcium alkali phosphates Ca(Na,K)PO4 in the thermochemical product.
In this study, potassium sulfate was tested as an additive to integrate the nutrient potassium into the process. The use of potassium sulfate required reaction temperatures higher than 1100 °C above the melting point of the ash. In order to incorporate potassium sulfate into the process at lower temperatures, sodium sulfate and potassium sulfate were mixed before treatment. This resulted in suitable reaction temperatures between 900 °C and 1000 °C. To identify the incorporation of sodium and potassium into the phosphate phases, methods of phase analysis and chemical extraction were combined. Thus, different modifications of the calcium alkali phosphates and their composition could be determined.
Crystalline phase analysis and phosphorus availability after thermochemical treatment of sewage sludge ash with sodium and potassium sulfates for fertilizer production
Hannes Herzel, Zeynep Aydin, Christian Adam
Journal of Material Cycles and Waste Management, Heft 23, Seiten 2242 - 2254, 2021
BAM, division Thermochemical Residues Treatment and Resource Recovery