This study investigated the feasibility of using anaerobic granules as a novel type of biosorbent for lead, copper, cadmium, and nickel removal from aqueous solutions using continuous column operation. Calcium treated anaerobic biomass was shown to be highly effective. The average biosorption column capacity for lead, copper, cadmium, and nickel before breakthrough occurred was 1.55, 0.88, 0.89, and 0.51 meq/g, respectively. The removal of Pb2+, Cu2+, Cd2+, and Ni2+ ions from the bed was accompanied by the elution of Ca2+ ions. While the feed pH was 4.0 for the four metals studied, the exit solution pH increased slightly (pH 5.0 to 5.5) for most of the experiments before the breakthrough point. At the breakthrough point, the pH value decreased to the value of the feed solution. The adsorbed metal ions were desorbed from the biomass by feeding into the column a 0.5 M of CaCl, solution. Ion exchange was identified to be the dominant mechanism for the biosorption of nickel by the anaerobic biomass. For copper and cadmium, 77% and 82% of the total amount adsorbed was attributed to ion exchange, respectively. Eighteen percent and 15% of the total amount adsorbed of copper and cadmium was attributed to a complexation process, respectively. For the case of Pb ions, it was found that ion exchange accounted for almost 50% of the total uptake mechanism. Thirty percent of the total uptake mechanism was attributed to a precipitation mechanism. The remaining 20% was due to complexation. The results of this study indicate that the anaerobic sludge could be used as an effective biosorbent for the removal of heavy metals in a continuous flow system. (C) 2005 Elsevier Ltd. All rights reserved.