Different phosphorus reclamation strategies of solid biowastes after thermal treatment: Insights into phosphorus bioavailability and metal species

This study compared phosphorus (P) bioavailability of char and ash products derived from pyrolysis, hydrothermal carbonization, and incineration of sewage sludge (SS) and pig manure (PM). Char and ash products derived from PM consistently exhibited higher total P and bioavailable P compared to SS regardless of thermal treatment method applied. NaOH-P (Fe/Al-P; 54.4–80.0 % of total P) dominated in SS derived products while HCl-P (Ca/Mg-P; 62.2–93.6 % of total P) prevailed in PM derived products. Non-apatite P (NAIP) dominated in SS derived products (57.1–75.8 % of inorganic P (IP)), whereas apatite P (AP) prevailed in PM derived products (77.5–96.5 % of IP). The relatively high Fe and Al concentrations in SS may promote NAIP dominance via the formation of FePO₄/AlPO₄, while the relatively high Ca concentrations in PM may facilitate AP crystallization into Ca₅(PO₄)₃F/Ca₅(PO₄)₃Cl during thermal treatment. The concentration of bioavailable P showed positive correlations with NaHCO₃-P (regression coefficient (r) = 0.71, probability (p) < 0.001), HCl-P (r = 0.85, p < 0.001), IP (r = 0.90, p < 0.001), AP (r = 0.91, p < 0.01), and microbial-P (r = 0.67, p < 0.001). Malic and oxalic acids secreted by phosphate solubilizing microorganism showed higher binding capabilities with Ca and Mg over Fe and Al. Struvite recovered from PM ash (incinerated at 850 °C) achieved both higher net profit (47.0 versus 16.5 USD/ton) and P recovery efficiency (40.8 % versus 27.5 %) than those of aluminum and iron hydroxyphosphate recovered from SS hydrochar (hydrothermal treatment at 100 °C). This study proposes a different P reclamation strategy for char and ash products derived from thermal treatment of solid biowastes by delving into P speciation regulated by metal species.