Pulp mill modifications implemented by kraft mills for the purposes of controlling air emissions, such as the installation of indirect secondary condensers installed in blow heat handling systems, may result in altered characteristics of wastewater treatment plant influents. This study examines the impacts on biological treatment processes associated with the presence of blow heat condensates in the influent stream. In addition, the effectiveness of pretreatment of the condensate stream with hydrogen peroxide, in the presence of the ferric ion, is examined under controlled conditions through the use of bench-scale bioreactors. The study results indicate that the presence of blow heat condensates may adversely impact the mixed liquor settleability properties in the activated sludge process, although subsequent trials conducted to re-create this condition were not successful. Effluent soluble BOD5 was not affected by the presence of the condensates. Ferric/peroxide pretreatment was found not to be effective at destroying either the methanol or the turpentine present in the condensate stream. However, a significant improvement in Microtox biological response was observed, presumably due to a reaction between the ferric ion and reduced sulfur compounds. Furthermore, no adverse impacts on settleability were observed in a bio-reactor receiving condensates which were pretreated with ferric/ieroxide.