Preparation of phosphorus containing products by co-incineration of sludge ash and calcium-based additives: Focusing one-step and multi-step method

Due to the irreplaceable nature of phosphorus (P) in biological growth and the shortage of P rock, it is necessary to recover P from waste, such as sludge ash. P-containing products were prepared using sludge ash and calcium-based additives (CaCO3 and eggshell). In addition, the effects of different incineration methods (one-step method (OSM) and multi-step method (MSM)), additive doses, and incineration temperature on the P content and species in the products were investigated. The results indicated that as the dose of calcium-based additives increased, total P (TP) content in P-containing products reduced, apatite P (AP) content increased, non-apatite P (NAIP) content declined, and P solubility in citric acid content decreased. The amount of AP increased, NAIP reduced, and P solubility in citric acid decreased as the incineration temperature climbed. Although P in P-containing products prepared by OSM and MSM changed in a similar way at different additive doses and temperatures, P-containing products prepared by MSM had at least a 6.1% increase in P solubility in citric acid. Compared with OSM, MSM could save 10% of calcium-based additives when reaching the maximum AP value. Additionally, pure materials were employed to investigate how P species changed during the incineration procedure. The advantage of the MSM-prepared product over the OSM-prepared product may be explained by the high concentration of Ca3(PO4)2 and low concentration of amorphous calcium bound P (Ca-P). Overall, MSM is an effective method to reduce the dose of calcium-based additives and increase the bioavailability of P in P-containing products.