Effects of thermal and acid treatment on the distribution of heavy metals in sewage sludge.

Anaerobically treated sludge from a wastewater treatment plant (Psittalia) was found to contain significant amounts of metals (Cr, Cu, Fe, Ni, Pb and Zn). The sludge samples were subjected to thermal drying at 105°C and acid treatment was applied before and after thermal drying. Different acid concentrations and contact times were used in order to find the combination that caused the highest metal removal. The metal distribution in raw sewage and in thermally dried sludge as well as the chemical forms of the remaining metal load after acid treatment were determined by the application of a sequential chemical extraction procedure. It was found that thermal drying causes a shift of the metal content to more stable phases. A sufficient metal amount was removed by acid treatment and the remaining metal content was further stabilized. [ABSTRACT FROM PUBLISHER]