Reduced AOX and biological effects by modification of a mill ClO2 stage

The first ClO 2 stage in a modern OD(EPO)DD sequence for bleaching softwood kraft pulp at the Iggesund paperboard mill has been modified by lowering the ClO 2 charge, raising the pH level, and introducing a more effective reinforced EPO stage. The ClO 2 charge in the last D-stage was, however, raised slightly to maintain the same high pulp birghtness of 90%. The mixed D and EPO effluents obtained before and after this modification were compared. The chemical analyses included AOX, EOX, COD, TOC, BOD, chloroform, and chlorinated phenolics and extractives. The bioassays consisted of acute toxicity (Microtox), growth inhibition [green algae (S. carpicornutum)], survival and reproduction (C. dudia), and embryo-larval toxicity with zebrafish (B. rerio) tests. Effluents were also tested in a minimodel 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. With some of the test methods used in the bioassay study, an improvement in effluent quality was observed. Application : by using this process modification, it is possible for a mill to reduce organic chlorine material such as AOX. From the presented results of the chemical and biological analysis of the bleaching effluent, no apparent significant disturbance of an ecosystem will occur.