The purpose of this research was to conduct a laboratory study to determine the effects of nickel on the completely-mixed activated sludge process. Continuous-flow bench-scale reactors were operated at COD:TKN ratios of approximately 1.0:1 and 0.5:1 by varying the nitrogen concentrations in the feed solutions. Each unit received a COD concentration of 400 mg/l and was dosed continuously with a nickel concentration of 0.77 mg/l. The mean cell residence time was utilized as the operational control parameter to assess the influence of nickel on organic removal efficiency, on the degree of nitrification, and on the maximum yield and the microbial maintenance energy coefficients, Y_max and k_d. The results obtained in this study indicated that the soluble COD removal efficiency of the heterotrophic microorganisms was not affected by the 0.77 mg/l nickel concentration. The low continuous nickel dose to the reactor, however, appeared to stimulate the heterotrophic growth or to cause a replacement of the bacteria with a species of nickel-tolerant microorganisms. At the same time, the nickel appeared to stress the heterotrophs and to cause an increase in their maintenance energy requirement. Nitrification was found to be inhibited by the low nickel concentration, and this inhibition was not reduced by operating the reactors at lower COD:TKN ratios. / Master of Science