Phosphorus Transformation in Hydrothermal Pretreatment and Steam Gasification of Sewage Sludge

Sewage sludge is an important pool of phosphorus (P), while thermal disposal results in the enrichment of P into a final solid product. Hydrothermal treatment combined with steam gasification is an effective method to produce high-quality syngas from sewage sludge. This study discussed the transformation of P during this integrated thermochemical process based on thermodynamic calculation and the Standards, Measurements and Testing Programme analysis of solid products. The results showed that P was enriched in hydrochar with the concentration above 14 mg/g. By the hydrothermal pretreatment, organic phosphorus (OP) was significantly removed from the solid as a result of destruction, while inorganic phosphorus (IP) was mostly recovered. Part of OP was converted to IP in disposal from 220 to 240 degrees C. At 260 degrees C, destructed OP was emitted to the gaseous phase in the form of phosphorus oxide and, thus, not captured by minerals. In addition, inorganic phosphorus associated with oxides and hydroxides of Al, Fe, and Mn (NAIP) tended to be transformed into Ca-associated forms (AP) with the increase of pH, and CaO addition significantly promoted this transformation. In the gasification process, OP was completely removed, while IP was almost recovered with the substantial conversion from NAIP to AP. The highest total P recovery (84.92%) was achieved from gasification ash of hydrochar pretreated at 200 degrees C with CaO addition, while P was the most concentrated in gasification ash of hydrochar pretreated at 240 degrees C (30.39 mg/g). This study provided basic data for P recovery from sewage sludge using thermochemical conversion.