Much emphasis has been placed on the adverse environmental effects of effluents from chemical pulp mills utilizing chlorine-based bleaching. Bleached kraft pulp mills discharge relatively bioaccumulative and potentially toxic and persistent chlorinated compounds. Elimination of toxic contaminants through source control is the most satisfactory corrective measure, but will require massive investment for in-plant modifications. A parallel approach is to utilize the capability of existing end-of-pipe treatment technology to minimize the discharge of toxic contaminants. The Wastewater Technology Centre (WTC) has completed a 2-year laboratory-scale comparison of the potential of three biological processes to remove persistent toxic substances and AOX compounds, and improve the consistency of acutely lethal and sublethal toxicity elimination. The treatment processes examined were an activated sludge (AS) system with a 100% aerobic biodegradative environment, a facultative stabilization basin (FSB) incorporating anaerobic and aerobic zones of equal volume, and an aerated stabilization basin (ASB) that was completely aerobic. The systems were exposed to bleached kraft combined mill effluent under conditions of variable solids retention times (SRT) and operating temperature. The AS process, operated in an extended aeration mode, is considered a technically effective and practical biological treatment process for consistent control of conventional and persistent toxic contaminates. Toxic contaminant data have been used to calibrate a fate model that can predict the destination of a range of contaminants through the AS process.