Effect of Residual Bleaching Oxidants in Kraft Mill Effluents on the Biomass of an Activated Sludge Treatment System, 2002 Environmental Conference Proceedings

L. C. Dalvi, C. M. Silva, S. M. Santos and R. Leroy

Hydrogen peroxide and chlorine dioxide have been largely used in kraft pulp bleaching plants. During process upsets or transient conditions, higher charges of these chemicals are required to maintain pulp brightness. The increase of ClO2 and H2O2 charges can lead to an increase in concentration of these oxidants in the effluent. The objective of this work was to evaluate their effect on the biomass of an activated sludge process. The effect of individual oxidants at constant and at transient concentrations was studied. Constant residual concentration of H2O2 and ClO2 as low as 1.7 mg/L were found to be deleterious to the biomass causing significant changes in the sludge floc structure, turning it disperse and light. Soluble COD removal was not substantially affected by the presence ClO2 in the effluent. However, in the presence of H2O2, a decrease in efficiency was observed. Transient concentrations of H2O2 and ClO2 in the effluent showed to be extremely harmful to the biomass. Sludge flocs became disperse and light, COD removal efficiency decreased and microorganism activity reduced. After stopping the oxidant application, the system was able to recover completely. H2O showed to be more deleterious to the activated sludge than ClO.