The ability of treated alum sludge to remove fluoride from aqueous solution has been investigated. The studies were carried out as functions of contact time, concentration of adsorbent and adsorbate, temperature, pH, and effect of concentrations of other anions. The data indicate that treated alum sludge surface sites are heterogeneous in nature and that fits into a heterogeneous site binding model. The optimum pH for complete removal of fluoride from aqueous solution was found to be 6. The rate of adsorption was rapid during the initial 5 minutes, and equilibrium was attained within 240 minutes. The adsorption followed first-order rate kinetics. The present system followed the Langmuir adsorption isotherm model. The loading factor (i.e., the milligram of fluoride adsorbed per gram of alum sludge) increased with initial fluoride concentration, whereas a negative trend was observed with increasing temperature. The influence of addition of anions on fluoride removal depends on the relative affinity of the anions for the surface and the relative concentrations of the anions. Copyright 1998 Academic Press.