Simultaneous phosphorus and nitrogen recovery from anaerobically digested sludge using a hybrid system coupling hydrothermal pretreatment with MAP precipitation

Abstract In order to dispose excess sewage sludge and recycle its nitrogen and phosphorus resources, hydrothermal pretreatment coupling with MgNH 4 PO 3 ·6H 2 O (MAP) crystallization was investigated. Hydrothermal pretreatment at different temperature and residence time were performed on anaerobically digested sludge to maximize phosphate and ammonium release and their recovery from the resultant hydrolysate. Results showed that hydrothermal pretreatment at 200 °C for 30 min favored phosphate and ammonium release achieving phosphate and ammonium concentrations at 579.98 mg-P/L and 456.85 mg-N/L, respectively. During the subsequent MAP precipitation, the optimum condition for N and P recovery was determined at pH 9 and Mg/P molar ratio of 1, realizing P and N recovery of 91.60% and 54.88% respectively from the hydrolysate in addition to MAP purity of 84.24%. The produced MAP was confirmed by FTIR and XRD analysis, which fulfills the related standards and can be used as fertilizer with high promise.