The effects of bleaching modifications on the biotreatability of the resulting Adsorbable Organic Halides (AOX) were studied using laboratory-scale bioreactors and mill-scale bleach plant filtrates. Under conventional pulping/bleaching conditions, biological removal of AOX averaged 40 percent. After modification to oxygen delignification and complete ClO2 substitution, the removal efficiency increased to 47 percent. Under the treatment conditions employed, the eight percent increase was found to be statistically significant at a 95 percent confidence level. These results indicate that AOX biotreatability efficiencies following bleaching sequence modifications to oxygen delignification and complete ClO2 substitution remain at least as high as those observed prior to such modifications.

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