Decolourization and COD reduction of textile wastewater by ozonation in combination with biological treatment

Abstract

In this study, the characteristics of colour and chemical oxygen demand (COD) removal of azo dye by ozonation and biological treatment were evaluated for applying in industrial azo dye effluent treatment. Reactive Red 120 has been selected from among azo dyes due to its high solubility in an aquatic environment. The experimentation was arranged in two stages: during the first, only ozonation was performed, while, during the second stage, ozonation was integrated with biological treatment of completely mixed activated sludge (CMAS). COD removal was contributed simultaneously by the ozonation and biological treatment mechanism at lower ozone doses. However, it was significantly the result of direct oxidation at higher ozone doses. Ozonation of dyes usually leads to small organic molecular fragments that lead to a residual COD. The biological treatment is more efficient at mineralizing the by-products of the ozonation, but less efficient at colour reduction than the ozonation process itself. Thus, COD resulting from these small newly formed molecules can be further degraded by biological treatment. Therefore, ozonation should be applied at a medium ozone dose as a pre-treatment for a combined ozonation– biological treatment for azo dye removal.