Catalytic ozonation of textile wastewater as a polishing step after industrial scale electrocoagulation.

The main objective of this study was to develop the treatment system to change wastewater into a reliable source of recyclable water within the textile plant. Therefore, a highly polluted industrial wastewater originated in the dyeing of cotton was subjected to a multi-step treatment. The raw wastewater was characterized by the concentration of Reactive Black 5, the azo dye, as high as 842 mg/L, extreme alkalinity (pH 11.26) and salinity (NaCl concentration 52,290 mg/L). Correspondingly, the chemical oxygen demand (COD) was equal to 3440 mg/L and the total organic carbon (TOC) was 1790 mg/L in this wastewater. This salty, hardly degradable wastewater underwent the electrocoagulation (EC) on an industrial scale in the first step of the treatment. Although the industrial EC resulted in 84% of color removal in a very short time of 8 min, the wastewater was still characterized by an extremally high absorbance which corresponded to 100 mg/L of RB5. Moreover, EC resulted in the occurrence of burdensome by-products, of which one was identified in this study as an aniline derivative. The by-products contributed to high residual COD and TOC after EC (2120 mg/L and 1052 mg/L, respectively). Consequently, the catalytic ozonation was used by us as a second, the polishing, step of the treatment. The catalytic ozonation was found efficient in the removal of the residual color and colorless by-products. The wastewater after catalytic ozonation was colorless and the final COD and TOC decreased to 1283 and 695 mg/L, respectively. The average oxidation state (AOS), spectra analysis, and the toxicity assay showed catalytic ozonation efficient in the by-products oxidation. Consequently, the catalytic action of activated carbon (AC) was proved for the ozonation of textile wastewater. Ultimately, the recycling of purified wastewater into dyeing resulted in a very good color quality of textile samples (DECMC values below limiting value equal to 1.0).

Influence of ozonation and biodegradation on toxicity of industrial textile wastewater.

The textile industry demands huge volumes of high quality water which converts into wastewater contaminated by wide spectrum of chemicals. Estimation of textile wastewater influence on the aquatic systems is a very important issue. Therefore, closing of the water cycle within the factories is a promising method of decreasing its environmental impact as well as operational costs. Taking both reasons into account, the aim of this work was to establish the acute toxicity of the textile wastewater before and after separate chemical, biological as well as combined chemical-biological treatment. For the first time the effects of three different combinations of chemical and biological methods were investigated. The acute toxicity analysis were evaluated using the Microtox® toxicity test. Ozonation in two reactors of working volume 1 dm3 (stirred cell) and 20 dm3 (bubble column) were tested as chemical process, while biodegradation was conducted in two, different systems - Sequence Batch Reactors (SBR; working volume 1.5 dm3) and Horizontal Continuous Flow Bioreactor (HCFB; working volume 12 dm3). The untreated wastewater had the highest toxicity (EC50 value in range: 3-6%). Ozonation caused lower reduction of the toxicity than biodegradation. In the system with SBR the best results were obtained for the biodegradation followed by the ozonation and additional biodegradation - 96% of the toxicity removal. In the second system (with HCFB) two-stage treatment (biodegradation followed by the ozonation) led to the highest toxicity reduction (98%).