Reduced AOX and Biological Effects by Modification of a Mill ClO2-Stage, 1996 Pulping Conference Proceedings

S. Ljunggren, F. de Sousa, E. B. Gidner

STFI

The first C1O2 -stage in a modern OD(EPO)DD sequence for bleaching of softwood kraft pulp at the Iggesund Paperboard Mill has been modified by lowering the C1O,-charge, raising the pH-level and introducing a more effective reinforced EPO-stage. The C1O2 -charge in the last D-stage was, however, slightly raised to maintain the same high pulp brightness of 90%. The two mixed D and EPO effluents obtained before and after this modification were compared. The chemical analyses included AOX, EOX, COD, TOC, BOD7 , chloroform, chlorinated phenolics and extractives etc. The bioassays consisted of acute toxicity (Microtox), growth inhibition (green algae (S. carpicornutum), survival and reproduction (C. dubia), embryo-larval toxicity with zebrafish (B. rerio) tests. Effluents were also tested in a mini-model ecosystem. The chemical characterization study clearly showed that the process modifications resulted in a reduction in the amount of chlorinated matter in the bleach plant effluent. An AOX-reduction of 20% was observed, while chlorinated acetic acids and chlorinated phenolic compounds were reduced by 55% and 35% respectively. For some of the test methods used in the bioassay study, an improvement in effluent quality was observed.