REMOVAL OF METALS AND FATE OF N AND P IN THE BACTERIAL LEACHING OF AEROBICALLY DIGESTED SEWAGE-SLUDGE

The solubilization of metals from aerobically digested sewage sludge was performed biologically using Thiobacillus ferrooxidans, in a continuously stirred tank reactor (CSTR). Experiments involved the preacidification of the sludge to pH 4.0 and the addition of 4 g FeSO4-7H2O per liter of sludge. Reactors were operated with 20 g/l total solids (2% TS), 0.5 volume of air per reactor volume per minute, agitation at 300 rpm and a 20% recirculation rate. A minimal mean hydraulic residence time (t) of 0.75 day, one half the time required in a batch system, resulted in solubilization of 52% of the Cu, 62% of the Zn and 78% of the Mn. The leached sludge was more easily dewatered than untreated sludge, and would not likely require the addition of polymer if a plate filter press were used. Metals did not reprecipitate during filtration. A small quantity of added lime was sufficient to precipitate the metals; selective precipitation could permit the separation of ferric hydroxide from valuable metals (Cu, Zn, Ni, Cd), which may then be recovered. No significant changes in the total nitrogen and phosphorus content occurred during solubilization. Biological solubilization produced a better quality sludge for agriculture, as well as avoiding the dispersal of metals into the environment.