Effect of H2O2 on characteristics and biological treatment of TCF bleached pulp mill effluent

Totally chlorine-free (TCF) bleaching technologies can result in high concentrations of residual hydrogen peroxide in combined mill effluent. This study was undertaken to determine the effect of hydrogen peroxide on effluent characteristics (biochemical oxygen demand (BOD), toxicity), biological sludge characteristics (oxygen uptake rate (OUR), catalase activity) and the efficiency with which laboratory-scale activated sludge reactors treat TCF bleached kraft mill (BKM) effluents. The presence of over 20 mg/L hydrogen peroxide in TCF-BKM effluent was found to decrease the BOD by up to 25%. The complete decomposition of hydrogen peroxide concentrations of up to 1000 mg/L was not found to have an effect upon the effluent toxicity, as measured by MicrotoxR. Increasing concentrations (5–1000 mg/L influent) of peroxide were continually added to a reactor treating combined TCF-bleached kraft mill effluent. Treatment efficiency, as measured by removal of BOD, chemical oxygen demand (COD) and toxicity, was found to be unaffected by hydrogen peroxide concentrations of up to 1000 mg/L influent. Several methods of gauging reactor acclimatization to hydrogen peroxide were performed throughout the study. Activated sludge mixed liquor samples from the continuous reactor were shocked with varying concentrations of hydrogen peroxide. Oxygen uptake rate of activated sludge samples, especially when unacclimated, was found to decrease dramatically when subjected to shock loads of increasing concentrations of peroxide. Activated sludge which had been acclimated to hydrogen peroxide in the reactor feed was more resistant to H2O2 shock loading. Catalase-equivalent induction in activated sludge was also monitored as a means of gauging reactor acclimatization to the presence of hydrogen peroxide. Catalase-equivalent activity was found to increase with increasing H2O2 feed concentration, and appeared to reach a maximum level of 1.3–1.5 (min g biomass)−1. It was concluded that activated sludge is able to treat TCF bleached kraft mill effluent which contained H2O2, and that the microorganisms present in the sludge were able to adapt to receiving such an effluent.