Advancing insights into phosphorus transformation during thermochemical treatment of sewage sludge

This review examines the fate of phosphorus (P) during the thermochemical treatment of sewage sludge (SS). It begins with a brief discussion on P transformation during municipal wastewater treatment, affecting the distribution of P species in SS. Subsequently, a brief assessment of analytical methodologies for P characterization is presented, comparing established approaches while highlighting emerging tools that remain underutilized in this field. Thereafter, a comprehensive analysis of the influence of process parameters (such as temperature, retention time, reactor configuration, reactor atmosphere, and solid-to-liquid ratio) and feedstock characteristics (such as chemical composition, co-treatment with additives, particle size, and pH) on P transformation during incineration, pyrolysis, hydrothermal treatment, and gasification is presented. Assessment of multiple studies indicated that the operational temperature and the chemical composition of SS are the most critical factors influencing the fate of P. Generally, organic-P decomposes first, followed by pyrophosphate (whose relative fraction in ash/char peaks at ∼300-600^oC), both of which convert to orthophosphate above 600^oC. The apatite P/non-apatite inorganic P ratio in the ash/char increases with rising operational temperatures. Additives influence the metal/P molar ratio, thereby increasing the fraction of mineral P, with the effect depending on the specific type of additive used. P volatilization generally occurs at elevated temperatures, typically above 1100°C. However, SS with a high organic-P content may induce volatilization at temperatures below 1000°C. This understanding of P transformation is crucial for optimizing P recovery strategies from SS ash/char, with a broader goal of closing the anthropogenic P cycle along with SS treatment.