Transformation and distribution of nitrogen and phosphorus in sewage sludge during supercritical water gasification

In this study, the comprehensive investigation of the transformation and distribution of nitrogen (N) and phosphorus (P) during supercritical water gasification (SCWG) of sewage sludge were carried out. The nitrogen in raw sludge is mainly in form of organic nitrogen, metal nitrides and silicon oxynitride (NSi₂O). During SCWG, most of organic nitrogen was converted to ammonium nitrogen. With the increase of reaction temperature and time, nitrogen gradually migrated from solid phase to liquid phase. Phosphorus in raw sludge mainly existed in the form of calcium phosphide (Ca₃P₂), triphenylethylene phosphine (Ph₃P) and cyclopentyl phosphide (Ph₂PSH). During SCWG, Ca₃P₂, Ph₃P and Ph₂PSH were decomposed and degraded into phosphates, enriched in the solid phase. Phosphides in the liquid phase are rarely detected, as more phosphorus was deposited into the solid product with increasing reaction temperatures and time. Therefore, the liquid product can be potentially used as nitrogen containing fertilizer, and P can be recovered from solid product after SCWG.