The purpose of this study was to evaluate the toxic effects of various heavy metals on activated sludge process. Different concentrations of Cu(II) (0.5, 1.0, 5.0, and 10.0 mg/l), Ni(II) (5.0, 10.0, 25.0 mg/l), and Cr(VI) (1.0, 5.0, 10.0, 25.0, 50,0 mg/l) were maintained in a laboratory-seale completely mixed activated sludge unit, without recycle, treating simulated wastewater. The feed solution contained 650 mg/l protein (corresponding to 1300 mg/l COD) as a source of carbon. Experimental results indicated that the treatment efficiency was not adversely affected by the presence of these heavy metals up to 10.0 mg/l Cu(II), 10.0 mg/l Ni(II) and 50,0 mg/l Cr(II) concentrations. Moreover, some stimulatory effects of these heavy metals on the biokinetic parameters; maximum specific growth rate and half-substrate saturation coefficient were observed.

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