Thermochemical treatment of fly ash and desulfurization wastewater from municipal sewage sludge incineration plant for phosphorus recycling

Phosphorus (P) recycling from incinerated sewage sludge ash (SSA) is a promising way to mitigate the upcoming shortage concerns of phosphate rocks. However, low P bioavailability and high heavy metals concentration limit its recycling. In this study, a novel thermochemical treatment of SSA in N2 atmosphere was proposed with the substitute of sodium-based additives with flue gas desulfurization wastewater (DW) and the introduction of CaCl2. They played the synergistic roles in P transformation and P bioavailability with the increment of positive Delta i from 9.01 to 23.36% and from 20.24 to 53.58%, respectively compared to their theoretical values. Meanwhile, the decrement of heavy metals enrichment rate on different levels was generally observed owing to the formation gaseous chlorides by chlorination reaction. In detail, calcination enriched majority of P in SSA, though the P enrichment rate reduced. Alkaline DW promoted the transformation of non-apatite inorganic phosphorus (NAIP) to apatite phosphorus (AP) and the formation of Ca-bearing P minerals. A plant-available P compound, namely CaNaPO4, was detected due to the abundant sodium in DW. Both the increased P-solubility in citric acid (CA) and heavy metals reduction were observed. Adding CaCl2 further enhanced the transformation of NAIP to AP in the drying and calcination stages along with the reduction of Cd, Zn, Cu and Pb. At 30% CaCl2 dosage, the calcined SSA had the P-solubility in CA of 81.6%, which was near the conventional single super phosphate. The heavy metals concentration in the calcined SSA could meet the requirements of EU regulation of fertilizer. The work provides a new trial for the potential P recycling from SSA with co-utilization of DW.