Copper(II) ion toxicity onto activated sludge organisms was eliminated by addition of powdered waste sludge (PWS) to the feed wastewater for removal of Cu(II) ions by biosorption before biological treatment. The synthetic feed wastewater containing 14 or 22 mg l(-1) Cu(II) was mixed with PWS in a mixing tank where Cu(11) ions were adsorbed onto PWS and the mixture was fed to a sedimentation tank to separate Cu(H) containing PWS from the feed wastewater. The activated sludge unit fed with the effluent of the sedimentation tank was operated at a hydraulic residence time (HRT) of 10h and sludge age (SRT) of 10 days. To investigate Cu(II), COD and toxicity removal performance of the activated sludge unit at different PWS loadings, the system was operated at different PWS loading rates (0.1-1 g PWS h(-1)) while the Cu(H) loading rate was constant throughout the operation. Percent copper, COD and toxicity removals increased with increasing PWS loading rate due to increased adsorption of Cu(II) onto PWS yielding low Cu(II) contents in the feed. Biomass concentration in the aeration tank increased and the sludge volume index (SVI) decreased with increasing PWS loading rate due to elimination of Cu(11) from the feed wastewater by PWS addition. PWS addition to the Cu(II) containing wastewater was proven to be effective for removal of Cu(11) by biosorption before biological treatment. Approximately, I g PWS h(-1) should be added for 28 mg Cu h(-1) loading rate for complete removal of Cu(II) from the feed wastewater to obtain high COD removals in the activated sludge unit. (C) 2007 Published by Elsevier B.V.