The chemical oxygen demand (COD) removal and respiration rates of several samples of activated sludge which were dosed with micronutrient supplements have been compared in this study. Six trace metals and three vitamins were used as chemical additives dosed into the mixed liquor. It was confirmed experimentally that a wastewater stream from a chemicals manufacturing plant did not contain a sufficient supply of macro- or micronutrients for efficient biological treatment. Some of the supplements increased the metabolic rate of the sludge while some decreased it, indicating a range of stimulatory and toxic effects; all of the supplements resulted in increased COD removal rates. Complex interactions between micronutrients that were dosed simultaneously were evident. Control sludge batches (receiving no micronutrient supplements) attained an average COD removal rate of 1.335 kg COD.kg MLSS-1 d(-1). Dosing micronutrients into the mixed liquor produced COD removal rates of up to 4.239 kg COD.kg MLSS-1 d(-1). It was concluded that micronutrient supplements have the potential to optimize biological treatment of industrial wastewaters which are nutrient limited or changeable.