[The pH-dependent solubility of heavy metals from sewage sludge of different compositions]

Sludge suspensions of anaerobically digested sewage sludge, limed anaerobically digested sewage sludge, precipitation sludge of the tertiary waste water treatment and limed precipitation sludge were incubated for 8 days with continuous rotation at nine defined pH-values (pH 1, 3-10). After membrane filtration (0.45 micron) solution concentrations of Zn, Cd, Cu, Pb and Cr were determined by atomic absorption spectroscopy. Results show that solubilities of Cd, Zn and Cu depend not only on pH-value but also in a high degree on the sludge type. The solubility is by far the lowest in the anaerobically digested sludge. Most obvious differences can be found for Cd: While the concentrations of soluble Cd in anaerobically digested sludge only increase at pH values lower than pH 4, the solubility of Cd in precipitation sludge and limed sludges already show rapid increases at pH values lower than 7. The solubility of Pb and Cr remains low in all sludges at all pH-values ranging from pH 3 to pH 10. These results show that when judging ecological effects of heavy metals from sewage sludges the physical and chemical composition of these sludges should be considered.