Recovery of phosphorus from sewage sludge in combination with the supercritical water process

Abstract

In this paper, the fraction transformation and recovering of phosphorus (P) from sewage sludge (SS) residues, derived from supercritical water process, was investigated by extraction and precipitation processes. In addition, the form of heavy metals existing during the recovery process is also discussed. First, P in the solid residues was recovered by acid leaching with HCl, and then the derived P was adsorbed by activated alumina (Al2O3). Finally, the Al2O3 was desorbed with low concentration of NaOH. Results showed that 80% organic P was converted into HCl-P. The total P (the chief ingredient of HCl-P) in solid residue increased from 86.1 to 95.6% as temperature increased from 350 to 400 °C. The amount of P in the solid residue that was dissolved by 1 M HCl was 97.8%, and over 95% of P in the leaching solution (15 mg/L for P concentration) was adsorbed after 5.0 g of Al2O3 powder was added. The amount of P desorbed from Al2O3 with 0.1 M NaOH was 98.7%. Ultimately, over 85% of TP in SS was recovered. Moreover, the proportion of Cu, Zn and Pb in the extracted P products was lower than 5%.