A combination of flocculation and settling is the mechanism by which metal removal is achieved in activated sludge. Bacterial extracellular polymers appear to play an important role in flocculation; metal cations may also be involved in this process. Extracellular polymers in activated sludge are mainly of a polysaccharide nature, although protein and nucleic acid from autolysis are constituents of the polymer matrix. Precipitated metals may be removed either by independent settling or by physical trapping in the sludge floc matrix. Metals present in the ionic form may be removed from solution by adsorption to sites on bacterial extracellular polymers. Metal ions may also be accumulated in the cytoplasm of a bacterial cell, or adsorbed on to the cell wall. If activated sludge plants are overloaded with metals, toxic effects on bacteria and other microorganisms may severely inhibit the treatment process, resulting in poor quality effluents. Acclimated bacterial cultures can tolerate much higher metal concentrations than non-acclimated cultures; these are advantageous in the treatment of metal-laden wastes.