Phosphorus transformation during subcritical hydrothermal conversion of sewage sludge

Decoupling of temperature and pressure in hydrothermal process was achieved via external nitrogen gas pressurization. Effects of organic matters and initial pressure on the phosphorus transformation during subcritical hydrothermal conversion of sewage sludge were investigated, and the feasibility of vivianite formation was verified simultaneously. The results show that initial pressure can enhance the enrichment of phosphorus in hydrochar. It promotes the decomposition and conversion of glucose and protein into reducing substances, which favors the formation of vivianite confirming by its strong intensity of diffraction peaks. Phosphorus is preferred to enrich in hydrochar and further enhanced by increasing initial pressure. The increase of total phosphorus (TP) content from 31.45 mg/g (0.1 MPa) to 39.68 mg/g (1.0 MPa) is observed. At hydrothermal temperature of 110℃, increasing the initial pressure promotes the conversion of Fe(Ⅲ)-P to Fe(Ⅱ)-P. The Fe(Ⅱ)-P content in the hydrochar (1.0 MPa) is 6.31 times that of raw sludge. Increasing the initial pressure promotes the decomposition of organic compounds such as carbohydrates and proteins in sludge. The organic acids in the process water lower its value of pH, while the increasing NH+4 - N concentration is observed due to the degradation of proteins. The increase in redox potential of process water indicates that reducing substances such as organic acids are consumed, which promotes the reduction of trivalent iron with the increase of Fe(Ⅱ)-P content in hydrochar. The characteristic diffraction peaks of vivianite are observed in the XRD patterns of hydrochar samples under the pressurized conditions, indicating that it is feasible for the formation of vivianite during the decoupling temperature-pressure hydrothermal conversion of sewage sludge. This study provides a new way for the phosphorus recovery from sewage sludge. © 2024 Materials China. All rights reserved.