Advanced treatment of biotreated textile industry wastewater with ozone, virgin/ozonated granular activated carbon and their combination

Biotreated textile wastewater (COD_o = 248 mg L^-1; TOC_o = 58 mg L^-1; A₆₂₀ = 0.007cm^-1; A₅₂₅=0.181 cm^-1; A₄₃₆ = 0.198 cm^-1) was subjected to advanced treatment with ozonation, granular activated carbon (GAC) adsorption in serial and simultaneous applications. Experiments were conducted to investigate the effects of applied ozone dose, ozone absorption rate, specific ozone absorption efficiency, GAC dose, and reaction pH on the treatment performance of the selected tertiary treatment scheme. In separate experiments, the impact of virgin GAC ozonation on its adsorptive capacity for biotreated and biotreated + ozonated textile effluent was also investigated. Ozonation appeared to be more effective for decolorization (k_d = 0.15 min^-1 at pH = 3), whereas GAC adsorption yielded higher COD removal rates (54% at pH = 3). It was also found that GAC addition (4g/L) at pH = 7 and 9 enhanced the COD abatement rate of the ozonation process significantly and that the sequential application of ozonation (at pH = 3-11, 675 mg L ^-1 O₃) followed by GAC adsorption (at pH = 3-7, 10 g L^-1 GAC) resulted in the highest treatment performances both in terms of color and COD reduction. Simultaneous application of GAC and ozone at acidic and alkaline pH seriously inhibited COD abatement rates as a consequence of competitive adsorption and partial oxidation of textile components and GAC. It could also be established that ozone absorption efficiency decreased after color removal was complete. Ozonation of biotreated textile wastewater with 113 mg L^-1 ozone resulted in an appreciable enhancement of GAC adsorptive capacity in terms of residual color removal. Ozonation of GAC at relatively low doses (=10.8 mg/g GAC) did not improve its overall adsorption capacity.