REDUCED AOX AND BIOLOGICAL EFFECTS BY MODIFICATION OF A MILL CLO2 STAGE

The first ClO2 stage in a modern OD(EPO)DD sequence for bleaching soft wood kraft pulp at the Iggesund paperboard mill has been modified by l owering the ClO2 charge, raising the pH level, and introducing a more effective reinforced EPO stage. The ClO2 charge in the last D-stage wa s, however, raised slightly to maintain the same high pulp brightness of 90%. The mixed D and EPO effluents obtained before and after this m odification were compared. The chemical analyses included AOX, EOX, CO D, TOC, BOD7, chloroform, and chlorinated phenolics and extractives. T he bioassays consisted of acute toxicity (Microtox), growth inhibition [green algae (S. carpicornutum)], survival and reproduction (C. dubia ), and embryo-larval toxicity with zebrafish (B. rerio) tests. Effluen ts were also tested in a minimodel ecosystem. The chemical characteriz ation study clearly showed that the process modifications resulted in a reduction in the amount of chlorinated matter in the bleach plant ef fluent. An AOX reduction of 20% was observed, while chlorinated acetic acids and chlorinated phenolic compounds were reduced by 55% and 35%, respectively. With some of the test methods used in the bioassay stud y, an improvement in effluent quality was observed.