Sludge samples were found to contain significant amounts of heavy metals (Cr, Cu, Ni, Pb and Zn) and were subjected to acid treatment using either hydrochloric, sulfuric, nitric or phosphoric acid. The optimum combination, in terms of metal removal efficiency and environmental impacts was sought through a variety of tests by applying a ratio of 1:5 of sludge quantity (g) per volume of acid (ml). The concentrations of the different acids used, were in the range of 5%-20% and the contact times ranged between 15 and 60 minutes. The optimum combination was achieved when the sludge samples were in contact with H2SO4 20% v/v for 60 minutes. In order to estimate the metal leachability, the heavy metal content as well as the metal distribution in the residue were investigated and it was found that most of the heavy metal content was extracted while the remaining was removed from the initial mobile phases to the more stable ones.
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 1741 1742 NAOUM ET AL. phases. A sufficient metal amount was removed by acid treatment and the remaining metal content was further stabilized.
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