Sulfite enhancing nitrogen removal from sewage sludge during hydrothermal carbonization

In order to prepare low nitrogen content hydrochar, Na2SO3, and NaHSO3 have been utilized respectively as an additive to remove nitrogen from sewage sludge (SS) during hydrothermal carbonization (HTC). Compared to without additives, the maximum removal efficiency of nitrogen in hydrochar can increase as high as 68.68% at 210 °C when 10% of Na2SO3 was added. Results demonstrate that Na2SO3 has a greater capability than NaHSO3 to remove nitrogen. The key role of NS during HTC of SS was the degradation of extracellular polymeric substances (EPS) to release polysaccharides to the aqueous phase and destroying proteins structure, thus inhibiting Maillard reaction to reduce nitrogen content in hydrochar. This gave a new insight to remove nitrogen during HTC via the release of polysaccharides to the aqueous phase to avoid interaction and subsequent reaction between polysaccharides and proteins. In addition, the possible removal pathways are also analyzed and proposed in this investigation. © 2023 Elsevier B.V.