The increasingly stringent limits on the discharge of adsorbable organic halogen (AOX) from bleached kraft pulp mills have resulted in a need to optimize the performance of biological wastewater treatment systems with respect to the removal of these compounds. To investigate whether anoxic conditioning of sludge can enhance AOX removal in activated sludge systems, a series of batch experiments was conducted where reactors were seeded with various proportions of activated and anoxic-conditioned sludges. AOX removal was found to be significantly enhanced through the addition of anoxic-conditioned sludge to activated sludge, resulting in overall AOX removal of up to 75%. The extent of enhancement ranged from 3 to approximately 10% as the conditioning time was increased from 5 to 20 days. The benefit then declined as the conditioning times increased further beyond 20 days and up to 30 days. The effect of conditioning time on COD removal followed a similar trend as for AOX removal. An optimal AOX removal was achieved when the proportion of conditioned sludge within the biomass was 30%. A good correlation between the enhanced AOX removal and increasing levels of facultative bacteria developed in the conditioned sludge was observed. The addition of sulphate into the sludge during conditioning further increased the enhancement of AOX removal when compared to sludges conditioned under nitrogen.

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