Individual and combined effects of Ni2+ and Cu2+ concentrations on the nitrification process were investigated in a submerged biofilter. The activity of nitrification organisms was inhibited by high concentrations of Cu2+. However, low concentrations of Cu2+ promoted NOB activity. There were no observable stimulation effects of Ni2+. The presence of 0.2-4.0 mg/L Ni2+ caused a decrease in the rate of nitrification. Although the inhibition of Cu2+ on the nitrification organisms started at a concentration of 0.5 mg/L, it started at 0.2 mg/L for Ni2+. Ni2+ had stronger inhibitory effect on the nitrification organisms than Cu2+. Using a mixture of Ni2+ and Cu2+, inhibition level drastically increased to 32% and 49% at concentrations of 2.0 and 3.0 mg/L, respectively. At the same concentrations, individual inhibition by Ni2+ and Cu2+ was much lower, about 12-15% and 18-24%, respectively. (C) 2016 Elsevier B.V. All rights reserved.