In this study, immobilized activated sludge from a municipal wastewater aerobic treatment plant has been evaluated as a potential biosorbent for the removal of heavy metals from liquid waste streams. The method of immobilization consists of entrapment of the microorganisms in calcium alginate beads.

The affinity of immobilized activated sludge to cadmium, lead, chromium and copper has been evaluated under well defined laboratory conditions.

Initially, the time course of metallic ions uptake and the effect of pH and temperature on biosorption were studied for each metal. Then the biosorption pattern and the effect of biosorbent concentration on binding capacity were investigated. It was found that biosorption by the bacterial beads could be described by the Freundlich Isotherm.

A bench-scale system has been developed and tested to optimize the biosorption process. Promising results have been obtained for lead and chromium.

Results indicate that alginate-immobilized activated sludge has a capacity to adsorb a maximum of 240 mg Pb²⁺/g dry biobeads; 36.9 mg Cr³⁺/g dry biobeads; 18.9 mg Cu²⁺/g dry biobeads, and 7.2 mg Cd/g dry biobeads. A residual Pb²⁺concentration of 0.04 mg/l, which is below the drinking water limit, has been obtained with biobeads in a bench- scale biosorption treatment of lead from an initial concentration of 300 mg/l. This is a very encouraging result, because at a concentrations of Pb²⁺as high as 300 mg/l the metabolic activity of the sludge is not inhibited. Therefore, biodegradation of organic contaminants, which might be found along with lead in some industrial effluents, can occur at the same time.

Other results suggest that the sludge biosorbent can also be used to selectively remove specific metals from solutions containing various metal ions. This may be done by adjusting the conditions of biosorption, especially the pH.