Hydrothermal conversion of dewatered sewage sludge: Focusing on the transformation mechanism and recovery of phosphorus.

The recovery of phosphorus from sewage sludge was critical due to the depletion of phosphate ore. The present research aims to identify the phosphorus speciation and reveal the phosphorus transformation mechanism of dewatered sewage sludge during hydrothermal conversion (HTC) process, as well as to achieve the high efficiency recovery of phosphorus. Multiple analysis of SMT method, VK diagram, XANES and NMR showed that most phosphorus (>80%) was transferred to the hydrochar and presented as inorganic phosphorus (IP) after the HTC process. A dehydration trend was observed of the HTC process with the increase of sub-critical temperature. Ca-associated phosphorus increased significantly as the temperature increased. The Pyro-P gradually transformed to Ortho-P with the increase of HTC temperature and disappeared at 320 °C. The addition of HCl (6.13 and 12.3 mmol/g) in the HTC process resulted in a high percentage (>80%) of phosphorus transferred to the aqueous phase, and the bioavailability of the residual phosphorus increased significantly. The recovery rate of phosphorus could achieve 98.37% at the pH of 7.52, with the struvite purity of 90.41%. The results of this study provide new insights into the selective transfer of phosphorus in dewatered sludge by HTC process, in addition to some efficient ways for the utilisation of the HTC products.