Enhanced biological treatment of bleached kraft mill effluents — II. Reduction of mixed function oxygenase (MFO) induction in fish

Biological treatment studies with bleached kraft mill effluents (BKME) assessed the removal of hepatic MFO induction potency towards fish. Bench-scale activated sludge, facultative stabilization basin, and aerated stabilization basin processes were compared under conservative conditions of elevated sludge age and hydraulic retention times as well as moderate temperatures for the treatment of bleachery effluents from a modernized bleached kraft mill using oxygen delignification and 60% chlorine dioxide substitution. Optimized treatment for the removal of hard-to-degrade organics also consistently removed more than 85–90% of effluent-induced MFO activity of fish, based on laboratory bioassays. Follow-up mill-site pilot scale testing with an activated sludge treatment process in extended-aeration mode also demonstrated >90% reductions in the MFO induction potential of whole mill effluent. Characterization of various in-plant process waters sampled at the mill indicated that the softwood-line bleach plant was a major contributor (>70%) to the MFO induction potential of untreated and biologically-treated BKME.