Phosphorus species transformation and recovery in deep eutectic solvent-assisted hydrothermal carbonization of sewage sludge

Abstract

The efficient release of phosphorus (P) from sewage sludge plays a crucial role in P recovery. Hydrochars derived from sewage sludge were prepared using a CaCl2-lactic acid deep eutectic solvent-assisted hydrothermal carbonization. The addition of CaCl2-lactic acid increased HTC hydrochar yields from ca. 40–50% to almost ca. 45–57%. The sewage sludge floc structure could be disrupted to release more organic matter, subsequently transforming it into inorganic-P and fixing it onto the hydrochar. The deep eutectic solvent was found to significantly disrupt the floc structure of sewage sludge, facilitating the transfer of Poly-P into Pyro-P and orthophosphate. The addition of the deep eutectic solvent increased the Ortho-P/Pyro-P content from 12.9% to 19%. The total organic values exhibited a slight increase at 180 °C (10,653 mg/L) with deep eutectic solvent addition, indicating that lactic acid reacted with organic matter during hydrothermal carbonization, altering the properties of the hydrochars. This study proposes an alternative and sustainable approach to enhance P release from sewage sludge.