Analysis of the effects of prepyrolysis hydrothermal treatment on phosphorus recovery from sewage sludge using a life cycle assessment

Sludge phosphorus (P) recovery is an effective method for alleviating P depletion. This study presents a convenient hydrothermal (HT) treatment step before the pyrolysis of sewage sludge to improve biochar-P availability. The results demonstrate that sludge P can be stabilized during this HT treatment process. The biochar-P availability decreased gradually as the HT treatment temperature increased from 130 to 280 °C. The biochar-P release behavior indicated that a considerable portion of fast-release P was present in the biochar pyrolyzed from sludge when pretreated with the HT process. The environmental conditions (i.e., environmental pH, ionic strength of the solution, and low-molecular-weight acids) had almost negligible effects on the biochar-P release behavior. The seed germination rate was improved from 40% without adding biochar to 73.3% with the addition of biochar pyrolyzed from sludge with prepyrolysis HT treatment at 130 °C. The environmental impact was minimized by HT treatment of the sludge at 160 °C for 1 h according to the results of the comparative life cycle assessment of various P recovery alternatives. As shown in the present study, the prepyrolysis HT treatment of sludge at 160 °C for 1 h potentially represents a promising P recovery strategy by improving biochar-P availability.