Valorization of animal husbandry waste into slow-release phosphorus fertilizers through pyrolysis

Intensive animal husbandry generates substantial farming sludge from on-site wastewater treatment as primary solid waste instead of manure. This study valorized farming sludge into slow-release phosphorus (P) fertilizers through pyrolysis. The P transformation upon pyrolysis was examined using sequential P extraction and ³¹P nuclear magnetic resonance spectroscopy. The results revealed the transformation of labile P (e.g., water-extractable P and organic phosphate) to metal-P (e.g., Al-P and Ca-P) during pyrolysis, depending on the metal profile of feedstock and pyrolysis temperature. Three-month soil incubation and pot experiments showed that biochar produced at relatively low temperature (300 °C and 450 °C) exhibited better trade-off between P retention and bioavailability. The mineral fertilizer equivalent of the farming sludge biochar progressively increased over time, e.g., from 8% to 17% in three months. Correlation analysis and partial least squares path modeling implied that P fertilizer efficiency was positively correlated to water-extractable P, organic phosphate, and Fe-P but negatively to Al-P. This necessitates optimization of metal salts employed in farming wastewater treatment, e.g., using Fe salts instead of Al salts. Life cycle assessment showed that pyrolysis of farming sludge into fertilizers significantly minimized its eutrophication risk and climate change impact by 50-89% compared with direct land application.