High concentration of heavy metals is toxic for most microorganisms and cause strict damage in wastewater treatment operations and often a physico-chemical pretreatment prior to biological treatment is considered necessary. However, in this study it has been shown that biological systems can adapt to Ni (II) and Cr (VI) when their concentration is below 10 and 20 mg/L, respectively. The aim of this study was to evaluate the effect of Ni (II) and Cr (VI) on the lab-scale rotating biological contactor process. It was found that, addition of Ni (II) up to 10 mg/L did not reduce the chemical oxygen demand removal efficiency and on the contrary concentrations below 10 mg/L improved the performance. The influent Ni (II) concentration of 1 mg/L was the concentration where the treatment efficiency produced a maximum COD removal of 86.5%. Moreover, Ni (II) concentration above 10 mg/L was relatively toxic to the system and produced lower treatment efficiencies than the baseline study without Ni (II). Turbidity and suspended solids removals were not stimulated to a great extent with nickel. Addition of Ni (II) did not seem to affect the pH of the system during treatment. The dissolved oxygen concentration did not drop below 4 mg/L at all concentrations of Ni (II) indicating aerobic conditions prevailed in the system. Experiments conducted with Cr (VI) revealed that addition of Cr (VI) up to 20 mg/L did not reduce the COD removal efficiency and on the contrary concentrations below 20 mg/L improved the performance. The influent Cr (VI) concentration of 1 mg/L was the concentration where the treatment efficiency produced a maximum COD removal of 88%. Turbidity and SS removals were more efficient at 5 mg/L Cr (VI) concentration, rather than 1 mg/L, which lead to the conclusion that 5 mg/L Cr (VI) concentration is the optimum concentration, in terms of COD, turbidity and SS removals. Similar with Ni (II) experiments, addition of Cr (VI) did not significantly affect the pH value of the effluent. The DO concentration remained above 5 mg/L.