Improvement of eucalyptus bleached kraft pulp effluent treatment through combined ozone-biological treatment

Abstract

The aim of this work is to evaluate the combination of ozonation and biological treatment for reducing recalcitrant organic matter and enhancing the biological treatment efficiency of kraft pulp elemental chlorine-free (ECF) bleaching and similar effluents. Alkaline bleach filtrate was treated with ozone at doses of 100 and 250 mg/L, temperature of 70ºC, and pH 10 (mill conditions). The ozone-treated and untreated effluents were mixed with acid bleaching filtrate to prepare the combined effluents, which were then treated in a bench-scale activated sludge system. The constant biological treatment conditions used were: temperature = 35±2ºC, hydraulic retention time = 12 hours, and sludge retention time = 10 days. Effluents were also fractionated by molecular mass using a membrane with a 500-Da molecular-weight cutoff to characterize the high- and low-molecular-mass organic matter. Ozonation combined with biological treatment resulted in statistically significant increases in removal of COD, BOD5, TOC, lignin, and AOX compared to the effluent without ozone treatment. The high-molecular-mass fractions represented the majority of all components analyzed, and their contribution to total effluent load increased after biological treatment because of the greater removal efficiency of low-molecular-mass organic matter.