Effects of a common heavy metal (zinc) on activated sludge were studied with a sequencing batch reactor (SBR). Organic removal in activated sludge was postulated to proceed by a rapid adsorption of organics on the sludge, followed by a slower metabolic assimilation mechanism. Metal-laden waste water at a subtoxic level affected the SBR performance to different extents depending on the hydraulic retention time (HRT). Heavy metal acted as a strong competitor for active sites on the sludge,hampering organic adsorption, and affected the COD reduction efficiency under a short HRT.