The fate of heavy metal during subcritical and supercritical water gasification of sewage sludge

In this work, a sequential extraction procedure, recommended by the Tesseir, was used for fractionation of the heavy metals As, Cd, Cr, Cu, Ni, Pb and Zn in sewage sludge and its residues produced after sub- and supercritical water gasification with different residence times from 0 to 60 min. The results supported that, after sub- and supercritical water gasification, heavy metal in exchangeable and carbonate forms in the sewage sludge could be transformed into other relatively stable forms, e.g., As, Cr and Pb into residual fraction, Cu into organic matter fraction, except Ni. Moreover, a sharp increase in Ni and Cr content in the residues was detected because the corrosion was occurred under sub- and supercritical conditions since some of Ni and Cr dissolved were from autoclave. The Toxicity Characteristic Leaching Procedure (TCLP) experiments indicated that, the TCLP concentration of heavy metals dropped sharply after supercritical water gasification with residence time of 30 min, which meant that sub- and supercritical water gasification could enhance the stability of those heavy metals in residues. Consequently, it is believed that treatment of sewage sludge via sub- and supercritical gasification is an effective alternative for safety disposal of sewage sludge.