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ABSTRACT: A simple experimental set-up has been proposed to study the influence of Cu(II) and Zn(II) on the activated sludge growth kinetics by following the rate of change on biomass concentration during batch growth experiments. Data on biomass were fitted to a sigmoidal equation providing us the main biokinetics parameters (Yxs, and μm). Inocula seeded to the system was obtained from a wastewater treatment plant operating at 14 days cell residence time. A synthetic feed solution containing 2000 mg/l gelatin (corresponding to 1630 mg/l COD) served as source of carbon. Different concentrations of Cu(II) and Zn(II) (1, 5, 10 and 20 mg/l) were introduced singly in the reactors keeping all environmental parameters constant (pH, Ta, basic nutrients). The combined effects of Cu(II) and Zn(II) were determined by mixing these metallic ions (5/5, 5/10, and 10/5 mg/l of Cu(II)/Zn(II) respectively). Experimental data showed that Zn(II) was less toxic than Cu(II), as expressed by a slight stimulating effect for 1 mg/l Zn(II). Moreover, biokinetic parameters were not adversely affected by the presence of Cu(II) up to a concentration of 5 mg/l. However, a concentration of 10 mg/l and higher, caused serious upsets in the system. Combined effects of copper and zinc on the activated sludge growth kinetics, indicated that these two heavy metals acted neither synergistically nor antagonistically. The sigmoidal model very well fits the experimental data and could be used in a simulation study.