Efficient recovery of phosphorus in sewage sludge through hydroxylapatite enhancement formation aided by...

Reclaiming abundant phosphorus from sewage sludge (SS) via pyrolysis for use as a fertilizer has gained increasing attention owing to the rapid depletion of global P reserves. In this study, the enhancement effect of Ca-based additives on sludge P transformation to hydroxylapatite through pyrolysis was systematically investigated. Three Ca-based additives were added in the pyrolysis of SS, and they were found to promote the conversion of sludge P to hydroxylapatite, which is bioavailable to plants. The characterization of the sludge-derived pyrochars indicated that the addition of 10% CaO, 5% Ca(OH)₂, or 10% Ca₃(PO₄)₂ facilitated peak hydroxylapatite production. The thermodynamic simulation of the production of hydroxylapatite during pyrolysis showed that these additives increased the enthalpy of the pyrolysis system. Furthermore, the pyrolysis with CaO addition had the lowest enthalpy, thereby suggesting that the addition of CaO during sludge pyrolysis was preferable for recovering sludge P in the form of hydroxylapatite. Moreover, the hydroxylapatite produced with these additives was unstable when the pyrolysis temperature was above 900 °C. The pot experiment demonstrated the feasibility using the sludge-derived pyrochars as P fertilizer for plant growth. Therefore, changing the Ca form and/or Ca/P ratio with the addition of Ca-based additives could be an effective strategy for reclaiming P from SS in the form of hydroxylapatite, and this demonstrates a pathway for global P sustainability by recycling P from P-abundant wastes.