AbstractHardwood KP bleaching effluents, before (influent) and after (effluent) biological treatment, were characterized and compared to identify the main bio-recalcitrant organic compounds in the effluent. The results indicate that biological treatment reduces CODcr and BOD5 by 67% and 88%, respectively. However, various bio-recalcitrant organic compounds were still retained in the biologically-treated effluent, which exhibits an increase in effluent color. Spectroscopic analysis suggests that colored substances are the most recalcitrant materials in the effluent. Di-n-octyl phthalate and 6,6’-di-tert-butyl-2,2'-methylenedi-p-cresol are the predominant residual materials in the biologically treated effluent. These two materials are the main recalcitrant organic compounds in the effluent and contribute to the major part of the residual CODcr in the biologically treated effluent.