AbstractOzonation was used for tertiary treatment of hardwood KP bleaching effluent. The objectives were to investigate the performance of ozonation on reduction of recalcitrant COD and color in the biologically treated effluent and to investigate the change in biodegradability of the effluent during the ozonation process. The results indicate that COD removal was enhanced by increasing solution pH and temperature, but that the recalcitrant COD was not completely removed from the effluent due to ozone-resistant substances that remain in the effluent after ozonation treatment. Ozonation of the effluent was more effective for decoloration than for COD removal. The highest decoloration efficiency and COD reduction were 99% and 55% at pH 11.7 and 25 °C. It was observed that the biological oxygen demand (BOD5) and BOD/COD ratio were 93 mg/L and 0.40, after 30 min of ozonation, which was an increase of up to 102% and 185%, indicating enhanced biodegradability of the effluent after ozonation treatment. This obvious increase in biodegradability implies that the ozone-resistant substances can be decomposed by ozonation to produce compounds that are more biodegradable.