SELECTIVE PRECIPITATION OF BIOLOGICALLY SOLUBILIZED METALS FROM AEROBICALLY DIGESTED-SLUDGE

The selective and successive precipitation resulting from the biological solubilization (Thiobacillus ferrooxidans) of sewage sludge metals was studied. At the same time, theoretical diagrams representing the solubility of hydroxides and metal carbonates (Ca, Mg, Fe, Al, Cu, Zn, Ni, Cd, Mn) and their complexes are compared to the experimental values. The pH was progressively increased by means of Ca(OH)2. The iron hydroxide precipitate, which was the least contaminated, was obtained at pH of 3.5. The theoretical pH's for acceptable metal precipitation were identified as: Fe+3 at pH 3.2, Al+3 at pH 4.7, Cu+2 at pH 6.9, Zn+2 at pH 8.15, Ni+2 at pH 8.4 and Mn+2 at pH 9.1, and were closed to theoretical values. The hydroxide ion controls Ca, Mg and Cu solubilities. A precipitate with about 20% of Cu can be obtained at a pH of 7 after iron hydroxide was precipitated. Recovery of other metals than iron is thus possible.