Enhanced biological treatment of bleached kraft mill effluents — I. Removal of chlorinated organic compounds and toxicity

Laboratory-scale biological treatment studies with bleached kraft mill effluents assessed and optimized the removal of chlorinated organic compounds as well as aquatic and microbial toxicity. Three conventional biological processes, namely activated sludge and two stabilization basin treatment systems, were compared under conservative conditions of elevated sludge age (i.e. 15–25 days) and hydraulic retention times (i.e. 0.5–1 day and 15 days for the respective systems) as well as moderate temperatures for the treatment of softwood kraft bleachery effluents from a modernized mill using oxygen delignification and 60% chlorine dioxide substitution. Enhanced treatment performance was achieved with the optimized activated sludge, facultative stabilization basin and aerated stabilization basin processes as indicated by consistently high removals of conventional organic contaminants, polychlorinated phenolics (i.e. 85–93%), adsorbable organic halogen (i.e. 43–58%) and toxicity. The high level of polychlorinated phenolics removal was considered representative of an enhanced treatment capability for hard-to-degrade organics. The ultimate degree of biotreatability of various in-plant bleaching and non-bleaching process effluents from the kraft mill was also tested.