ABSTRACT
During the last decade, a rapid development and introduction to mill-scale of new and modified techniques for production of bleached kraft pulp has occurred in the kraft pulping industry. The main objectives of this development have been to reduce the amount of residual lignin entering the bleach plant and to reduce and subsequently eliminate the use of chlorine in the bleaching process, thereby minimizing the discharge of chlorinated organic matter. This development will also facilitate the closure of the water system in the bleach plant. Results from chemical characterization of effluents from Swedish bleached kraft mills show that the combination of extended delignification in the cooking stage, oxygen delignification and ECF bleaching is capable of bringing the discharge of AOX down to such a low level as 0.2 kg ptp before secondary effluent treatment and to 0.1 kg ptp after secondary treatment. The reduction in the quantity of chlorinated phenolic compounds, resulting from the introduction of ECF bleaching, is even more pronounced than the AOX reduction. TCF bleaching practically eliminates the discharges of chlorinated compounds. Thus, the chlorine content of high molecular weight effluent materials from TCF bleaching of softwood and hardwood kraft pulps was extremely low and fully comparable to the chlorine content found in naturally occurring humic materials. The contents of extractives such as fatty acids, resin acids and sterols in untreated kraft mill effluents were found to vary considerably between different mills. Other factors than the type of bleaching process used may be of large importance for these variations. An effective removal of extractives was observed both in aerated lagoons and in activated sludge plants.
