Phosphorus Recovery from Sewage Sludge Digestate and Pig Manure by Hydrothermal Treatments

Sewage sludge and pig manure are renewable sources of phosphorus that can be recovered and valorised for the production of fertilizers. This study aims at contributing to the development of hydrothermal treatment processes which integrate the recovery of phosphorus from sewage sludge and pig manure, by concentrating phosphorus in a solid phase mainly composed of Ca phosphates that can be reused as fertilizer. The effect of different process parameters such as temperature and reaction time on phosphorus conversion pathways was investigated by performing comparative experiments in a batch reactor, by using real samples of sewage sludge digestate and pig manure. The results indicate that increasing the temperature and reaction time leads to the increase of P recovery yields in solid products. Moreover, increasing the temperature and reaction time seems to promote the conversion of non-apatite inorganic phosphorus (e.g. Fe and/or Al phosphates) to apatite inorganic phosphorus (e.g. Ca phosphates). Overall, the results of this study allow to propose and validate a mechanistic model describing phosphorus conversion during hydrothermal treatment of residual biomass and provide crucial data to contribute to the development of alternative valorisation chains for sewage sludge and pig manure.