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.

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