The combined uptake Of Cu2+ and Zn2+ by activated sludge (biomass) was investigated at steady state in an activated sludge process (ASP) without recycle (namely, once-thro ASP), for different values of influent metal concentrations and hydraulic residence time in the range of 2.5-40 h. The experiments were performed at a constant pH of 7 and temperature of 25 degreesC. The results showed that the percentage removal of both copper and zinc by activated sludge increased with the increasing residence time; moreover, the percent metal uptakes were higher for both metals at the low level of influent metal concentrations [rho(Cu2+) = 1.5 mg/L and rho(Zn2+) = 9 mg/L], than those at the higher level of concentrations [rho(Cu2+) = 4.5 mg/L and rho(Zn2+) = 27 mg/L]. The least represented forms of both Cu2+ and Zn2+ in activated sludge were their sulfides, and the dominant metal forms were found to be 'organically bound' for both of the metals, Zn2+ and Cu2+. (C) 2004 Elsevier B.V. All rights reserved.