A novel method to remove nitrogen from sewage sludge during hydrothermal carbonization via inhibiting Maillard reaction

In order to prepare low nitrogen content hydrochar, deep eutectic solvents (DES) have been utilized as a green solvent to remove nitrogen from sewage sludge (SS) during the hydrothermal carbonization (HTC) processing. Compared to without DES, the maximum removal efficiency of nitrogen in hydrochar can increase as high as 69.49% at 210 oC when choline chloride and oxalic acid are employed as DES. Results demonstrate that different DES systems possess different capability to remove nitrogen, and DES offers the advantages to degrade floc structure of SS, thus inhibiting Maillard reaction during the HTC. It is revealed that organic matter releasing from floc structure within SS, low polysaccharide content, as well as fast caramelization under basic condition are vital factors to retard Maillard reaction to obtain low nitrogen content hydrochar from SS. In addition, the possible removal pathways are also analyzed and proposed in this investigation.