Abstract Effects of hydraulic residence time (HRT) and the sludge age (solids retention time, SRT) on the performance of an activated sludge unit treating synthetic wastewater containing 14 mg l −1 Cu(II) ion were investigated. Percent COD removal increased with increasing sludge age for both Cu-free and Cu(II)-containing wastewater. However, percent COD removal was lower in the presence of Cu(II) due to Cu(II) ion toxicity on the microorganisms at all sludge ages. Cu(II) ion toxicity was eliminated at high sludge ages (30 days) due to high biomass concentrations. Percent toxicity removals and biomass concentrations increased, but the sludge volume index (SVI) decreased with increasing sludge age. Increases in hydraulic residence time also resulted in increases in percent COD and toxicity removals and also in biomass concentrations, but decreases in the sludge volume index (SVI) for both in the absence and presence of Cu(II) ions. Percent COD removals were much higher in the absence of Cu(II) ions for all HRT levels tested. Copper ion toxicity on COD removal performance of the activated sludge unit was partially eliminated by operation at high sludge ages (30 days) and HRTs (25 h).