Mechanism of Metal Removal in Activated Sludge

Abstract

Cadmium, copper, and nickel removals during the activated sludge process are investigated using a pilot scale plant. Cadmium and copper are predominantly insoluble in the settled sewage and have high percentage removal efficiencies. Nickel is mostly soluble in the influent and is poorly removed. Batch studies undertaken on mixed liquor samples at different solids concentrations indicate that a major removal mechanism for cadmium and copper is the interaction of particulate associated metal with the settleable biological solids, whereas uptake of soluble metal dominates for nickel. Titration of free metal ions with the mixed liquor solids and extracellular polymer indicates that the latter is responsible for some of the metal immobilization. The low resolubilization of copper by ethylenediaminetetraacetic acid and the calculated metal:ligand stoichiometry of less than unity indicate copper's high affinity for mixed liquor. Stoichiometrics greater than unity for cadmium and nickel suggests that their removal is mechanistically dissimilar to copper.