Advanced oxidation of bleached eucalypt kraft pulp mill effluent

In this study a poorly biodegradable (BOD/COD = 0.3) industrial alkaline ECF bleaching filtrate was treated using different advanced oxidation processes to evaluate their use in combined chemical–biological treatment aimed at increasing recalcitrant COD removal and improving final effluent quality. Oxidative treatments included ozonation combined with hydrogen peroxide (2, 5, 10, 20 mmol L⁻¹ O₃/0.7, 2, 5, 10 mmol L⁻¹ H₂O₂) and photocatalysis with hydrogen peroxide (UV/2, 4 and 8 mmol L⁻¹ H₂O₂) and with TiO₂ (UV/TiO₂/0.7 and 4 mmol L⁻¹ H₂O₂). The O₃/H₂O₂ process increased effluent biodegradability by up to 68% as a result of increasing BOD and decreasing COD. Increasing the O₃ dose had a greater effect on biodegradability improvement and lignin and colour removal efficiencies than increasing the H₂O₂ dose. A combined oxidant dose of 5 mmol L⁻¹ O₃ and 2 mmol L⁻¹ H₂O₂ resulted in 75% lignin removal, 40% colour removal and 6% carbohydrate loss without mineralizing the organic carbon. The photocatalytic processes led to a decrease in effluent biodegradability through combined decrease in BOD and increase in COD and did not result in efficient lignin or colour removal. Photocatalytic oxidation was apparently inhibited by the high chloride and COD levels in the alkaline filtrate, and may be more efficient in recalcitrant COD removal if performed after biological.