Phosphorus Transformation from Municipal Sewage Sludge Incineration with Biomass: Formation of Apatite Phosphorus with High Bioavailability

Phosphorus (P) is an essential and limited nutrient element for all life. The recovery and reuse of P from municipal sewage sludge (MSS) incineration fly ash are considered to be practical and economical. Addition of biomass into MSS was proposed to enhance the P bioavailability during incineration. The speciation conversion of P during MSS incineration with different types of biomass was studied in this work. The chemical reactions between P-containing model compound (AlPO4) and mineral model compounds in biomass (CaO and KCl) were investigated to simulate the conversion mechanism of nonapatite inorganic phosphorus (NAIP) to apatite phosphorus (AP) during MSS incineration with biomass. It is shown that the addition of biomass increases the P mass percentage and facilitates the transformation of NAIP to AP in fly ash. Cotton stalk has the most positive effect on the P transformation in the four biomass samples. Ca, Cl, K, and/or Mg compounds in biomass promote the conversion of NAIP (such as AlPO4) to AP (such as Ca2P2O7, Ca5(PO4)3Cl, and Ca10K(PO4)7) during MSS incineration. Higher temperature stimulates the transformation of NAIP to stable AP. The primary reaction pathway between phosphorus and the main components in biomass is revealed. AlPO4 can react with CaO to form Ca2P2O7 and Ca3(PO4)2 at 900 °C, and two new P-containing compounds, Ca5(PO4)3Cl and Ca10K(PO4)7, are formed in the presence of KCl.