The influence of process parameters on the removal of heavy metals in activated sludge

The behavior of six heavy metals in an activated sludge pilot plant under conditions of normal and elevated influent metal concentrations has been studied. Percentage heavy metal removals at sludge ages of 4, 9, and 12 d were more consistent when metals were added to the system, possibly as a result of lesser variations in heavy metal speciation in the settled sewage, and the greatest degree of accumulation of heavy metals by the biomass occurred at the 12 d sludge age. When shock loads of heavy metals were introduced into the system, their concentrations in the effluent did not immediately rise in proportion; however, heavy metal removal efficiencies were similar under steady state conditions of normal and elevated influent metal concentrations. Cadmium, Cu, and Ni were largely soluble in the effluent samples, but Cr, Pb, and Zn were predominantly insoluble, indicating that particulate forms of these metals may escape removal in secondary sedimentation. The high degree of Ni solubility in the settled sewage and final effluent reflected its consistently poor removal in the process. The two most soluble metals, Cu and Ni, appeared to be associated to a large degree with high molecular weight substances in the settled sewage and final effluent, possibly influencing their availability for uptake by the biomass.