Phosphorus migration during sewage sludge thermal conversion with a focus on the role of temperature and atmosphere

Among the various sewage sludge disposal technologies, thermal treatment has emerged as a promising method for the effective recovery of energy and nutrients. It is well established that sewage sludge contains a large amount of phosphorus (P), which can be further concentrated in the solid products upon thermal conversion, highlighting its potential for P recovery. This study investigated the migration of P during the thermal conversion of sewage sludge across a wide temperature range (500-900 °C) and under various atmospheric conditions (nitrogen, oxygen, and steam). Below 700 °C, the majority of P was retained in the sewage sludge char or ash (SSC/A), with the atmosphere exerting minimal influence on P behavior. However, at elevated temperatures (700-900 °C), the P retention rate in nitrogen atmosphere significantly decreased from 86.1% to 60.2%. Furthermore, the presence of oxygen and steam had a pronounced effect on P retention. At 900 °C, the P retention rates in oxygen and steam atmospheres were 70.5% and 76.2%, respectively, both of which were significantly higher than that in nitrogen atmosphere (60.2%). Notably, among the three atmospheric conditions examined, the steam atmosphere demonstrated the most substantial enhancement in the conversion of non-apatite phosphorus to apatite phosphorus, emphasizing its potential to improve the phosphorus bioavailability in SSC/A. The results obtained in the present study could provide theoretical support for the directional regulation of phosphorus during sewage sludge thermal conversion, aiming to achieve efficient phosphorus recovery.