Syngas production and heavy metal dynamics during supercritical water gasification of sewage sludge

The rising production of sewage sludge, characterized by high organic content and excessive heavy metals, necessitates an effective treatment method. This study investigated the production of syngas and the migration and transformation behavior of heavy metals such as Zn, Ni, Cr, Cu, and As during supercritical water gasification (SCWG) of sewage sludge. The experiments were conducted without or with alkaline additives at temperatures between 380 to 420 °C and retention time from 15 to 60 min. The results revealed that the highest syngas yield reached 10.9 mol/kg with an H₂ concentration of 44.7% at 420 °C and 60 min. In this process, heavy metals were effectively immobilized and converted into a more stable form, whereas higher temperatures and longer retention time enhanced this effect. The introduction of alkaline additives (NaOH, KOH, Ca(OH)₂, Na₂CO₃, and K₂CO₃) led to the redistribution of heavy metals, further promoting the stabilization of Zn, Cr, and Cu. An environmental risk assessment showed that SCWG could significantly lower the risk associated with heavy metals to a low or negligible level.