Potential utilization of phosphorus in fly ash from industrial sewage sludge incineration with biomass

The recycling and reuse of phosphorus from incineration fly ash of sewage sludge are considered to be feasible and economical. Addition of biomass was proposed to increase the plant availability of phosphorus from the fly ash during industrial sewage sludge (ISS) incineration. Co-combustion experiments of ISS and four types of biomass were performed in a laboratory-scale bubbling fluidized bed. The experimental results reveal that the mass percentage of phosphorus present in fly ash increases by addition of biomass into ISS during incineration. During ISS incineration alone, phosphorus in the fly ash is mainly present in the form of apatite phosphorus (AP), exhibiting high bioavailability. After co-firing of ISS with biomass, the Ca, Cl, and Mg compounds in biomass effectively facilitate the speciation conversion of non-apatite inorganic phosphorus (NAIP) like AlPO4 to new AP, such as Ca2P2O7, Ca5(PO4)3Cl, and Ca4Mg5(PO4)6. Compared with wood, wheat straw, and corn stalk, the effect of cotton stalk is better. It is also found that increasing the cotton stalk mass ratio in mixture favors the transformation of NAIP to AP. The phosphorus solubility of the co-firing fly ash in citric acid (2%) increases to 83.30% with increasing the cotton stalk ratio to 50%, improving the phosphorus bioavailability.