ABSTRACT
The application of a variety of chemical and biological tests has enhanced our understanding of the origins and fate of toxic chemicals within mill process and treatment systems. Recent implementation of a wide variety of new process and treatment strategies, particularly at bleached kraft mills, has greatly altered the chemical composition and toxicity of final effluents. Rapid developments and improvements in cooking and prebleaching technologies that reduce the kappa number are expected to further reduce the residual lignin entering the bleach plant. The substitution of elemental chlorine for chlorine dioxide in the bleaching sequence significantly reduces the release of organochlorines and alters the chemical composition of the effluents. It seems inevitable that the industry trend toward conversion to elemental chlorine-free (ECF) bleaching will continue and will reduce the discharge of organochlorines and toxicity of mill effluents. New bleaching technologies, including hydrogen peroxide, ozone and enzymes, which are totally chlorine free (TCF), are now entering production and will continue to be developed. Unfortunately, there is relatively little known about the chemical composition and toxicity of TCF effluents and this represents a much needed area of research.
