Choice of Hydrothermal Treatment Temperature for Sewage Sludge Based on Heavy Metals Safety Assessment

Abstract:

Sewage sludge was treated in a hydrothermal reactor within temperature of 170~270 ℃ to investigate the influence of hydrothermal temperature on heavy metals’ distribution and their leaching from solid products. The balance of heavy metals was calculated; and the influence of oxygen presence in the hydrothermal process was checked. Experimental results show that after hydrothermal treatment, heavy metals remain mainly in the solid products (hydrothermal chars) with small part moving into liquids. Leaching tests of hydrothermal chars were carried out according to two Chinese standards and the results showed that heavy metals in the hydrothermal chars were relatively stable, and that for sanitary landfilling hydrothermal chars were safer than the dried sludge; but for land application purpose, proper reaction temperature should be chosen to produce chars to replace sludge itself; as chars obtained at temperatures lower than 210 ℃ were inferior to sludge itself in respect to heavy metal leaching. Generally leaching of heavy metals from hydrothermal chars decreased as the hydrothermal temperature increased. From point view of heavy metals’ stabilization, temperature of 260 ℃ corresponded to the char with the least heavy metal leaching and to the liquid with the lowest heavy metal concentrations. Therefore, 260 ℃ was the best choice of hydrothermal temperature for the tested sewage sludge. Two digestion methods were adopted to digest hydrothermal chars for their heavy metals measurement. It was found that heavy metals in the chars obtained at temperatures over 210 ℃ existed mainly in residual species, which were quite stable. Also it was difficult for the heavy metals such as Zn, As, Pb and Cu to move from solid phase into liquid phase in the hydrothermal process. The appearance of O2 in the hydrothermal process slightly enhanced the mobility of heavy metals to the liquid phase, while heavy metals left in the chars were still stable.