Thermal treatment of sewage sludge for phosphorus fertilizer production a model experiment

Phosphorus P resource availability and quality is declining and recycling P fertilizers from waste materials are becoming increasingly important. One important secondary P resource is sewage sludge SSL where P is often bound as aluminum phosphate Al P , iron phosphate Fe P and polyphosphate poly P , respectively. Thermal treatment in different ways is a promising way in P recycling to produce highly plant available P fertilizers. To investigate mechanisms behind transformation of hardly available P species toward plant available P forms we treated a model SSL containing different kinds of defined P sources by low temperature conversion LTC at 500 amp; 8201; C and subsequent thermochemical treatment of the LTC product with Na additives TCT at 950 amp; 8201; C, respectively. Pot experiments with ryegrass were carried out to determine the plant availability of P of the different treatments. The poly P here pyrophosphates based fertilizers had a very high plant availability after both thermal treatments. During LTC treatment the plant availability of the Fe P and Al P variants increased because of the formation of Fe II phosphates and or pyro polyphosphates. Especially the formation of Al polyphosphate shows a high plant availability. The subsequent TCT further increased strongly the plant availability of the Fe P variants because of the formation of highly plant available CaNaPO4. Thus, a direct TCT without prior LTC probably also produce CaNaPO4 and is recommended for Fe P based SSL. However, a molar Ca P ratio of amp; 8764;1 in the fertilizer is favorable for CaNaPO4 formation. Thus, the knowledge on the source of primary P in SSL is essential for choosing the accurate thermal treatment method to produce highly plant available P fertilizers from SS