Perceived negative environmental effects associated with chlorine bleaching have led to the development and implementation of pulp bleaching technologies which eliminate the use of elemental chlorine (ECF) or any chlorine containing compounds (TCF). Commercial implementation of these technologies has moved forward; however, the research necessary to fully understand the impact of effluents from these new bleaching technologies on the environment and on existing biological treatment processes has lagged behind, and is for many novel bleaching sequences, non-existent. This study examined the impact of hydrogen peroxide (H2O2) and chelating agents on the characteristics and treatment of TCF and ECF kraft effluents.
Effluent BOD was reduced approximately 25% by addition of H2O2 concentrations from 20-640 mg/L, however, effluent toxicity was not affected by hydrogen peroxide concentrations of up to 640 mg/L. Unacclimated activated sludge was inhibited by sudden exposure to shock doses of hydrogen peroxide. While the viability of the sludge (as measured by the rate of substrate metabolism) was profoundly affected, the effect was reversible, with full recovery of metabolic activity restored within approximately 10 hours of the shock. In continuous trials, as the activated sludge reactor became acclimated to H2O2, the kinetics of degradation of hydrogen peroxide increased.
Chelating agents, particularly diethylene triamine pentaacetic acid (DTPA), were also found to have a dramatic impact on sludge viability and reactor performance. Batch tests indicated a DTPA dose-dependent decrease in oxygen uptake rate. Introduction of DTPA at levels commonly found in TCF effluents to continuous reactors resulted in a disruption of floc structure and a 39% decrease in BOD removal efficiency. Removal of acute toxicity as measured by Microtox was maintained despite the poor BOD removal efficiency.