The effects of a continuous dose of 0.5 mg/l nickel on activated sludge performance at varying COD:TKN ratios were investigated. Continuous flow, complete mix, bench-scale reactors were operated over a range of mean cell residence times, and COD removal efficiency, biokinetic coefficients, extent of nitrification, and nickel removal evaluated at each. Data from two earlier studies, in which 0.5 and 1 mg/l nickel doses were applied to similar units, were included for comparison. Organic removal efficiency was not impaired for the nickel doses considered. Biokinetic coefficients and nitrate production were also unaffected by 0.5 mg/l nickel. In contrast, one mg/l nickel sharply inhibited nitrification, caused an apparent decrease in reactor solids concentration, and related biokinetic changes in coefficients. Nickel removal was erratic. / Master of Science