Biosorption of Cu(II) ions onto pre-treated powdered waste sludge (PWS) was investigated using a completely mixed reactor operating in fed-batch mode. Experiments were performed with PWS of different particle sizes between 53 and 231 mu m while the feed flow rate (0.25 1 h(-1)), feed copper ion concentrations (200 mg l(-1)) and the amount of adsorbent (2 g PWS) were constant. Breakthrough curves describing variations of the aqueous phase copper ion concentrations with time were determined for different particle sizes of the adsorbent. Percent copper removal from the aqueous phase and the biosorbed copper ion concentrations increased with decreasing particle size or increasing surface area of the PWS. A modified Bohart-Adams equation was used to determine the biosorption capacity of PWS and the rate constant for Cu(II) ion biosorption. The biosorption capacity of PWS based on per unit mass of PWS (g Cu gPWS(-1)) increased with decreasing particle size due to increased external surface area of PWS. The biosorption capacity of PWS for Cu(II) ions in completely mixed fed-batch reactor was found to be comparable with the powdered activated (PAC) adsorption columns However, the adsorption rate constant in a completely mixed fed-batch reactor was an order of magnitude larger than those obtained in adsorption columns because of the elimination of mass transfer limitations encountered in the column operations. Therefore, it was proven that the adsorption capacity and the rate of adsorption can be improved by reducing the particle size of the adsorbent (PWS) and using a completely mixed fed-batch reactor due to a larger adsorbent surface area and better contact between the phases.