Abstract
The textile industry is one of the most polluting sectors in terms of volume and complexity of effluent discharge. Textile wastewater is characterized by high chemical oxygen demand (COD) and total organic carbon (TOC), as well as, strong color. Conventional treatment of textile wastewater generally consists of coupling chemical coagulation with biological treatment. However, these methods cannot be employed where the wastewater is resistant to biological treatment. An alternative path could be the application of electrochemical technology, which benefits from advantages such as versatility, environmental compatibility and potential cost effectiveness. The present study evaluates the efficiency of electrochemical (EC) and photo-assisted electrochemical (PAEC) degradation of real textile effluent under galvanostatic conditions. The effluent was provided by the Brazilian textile company Tecelagem Sao Carlos SA. It was collected in two stages: (a) without any pretreatment and (b) after biological treatment. A single-compartment photo-electrochemical filter-press cell was used with a commercially available Ti/Ru0.3Ti0.7O2 - DSA? electrode (area = 14 cm2) with a Ti-mesh cathode (area = 14 cm2). The UV radiation was provided by a 250 W Hg lamp. A constant temperature of 28?C was used. All electrolyses were performed adding NaCl and Na2SO4 as the supporting electrolyte in different proportions, keeping constant ionic strength at 0.15 mol L-1. For the study of the effect of current density the following values were: 10, 20, 40, 60, 80 and 120 mA cm-2. When the untreated (no electrolyte addition) effluent is treated there is little or no color or TOC removal, even at the higher current densities. As the conductivity of the effluent is low (~2 mS cm-1), electrochemical treatment of the as-received effluent is extremely inefficient. When salts are added to the effluent, the efficiency of the process improves. As a result when the current density is increased there is a concurrent increase in the extent of colour removal and TOC. The effect of substituting stepwise the Na2SO4 with NaCl was studied and it was observed that the color, COD and TOC removal increases almost linearly with the chloride ion concentration. This is due to the in-situ formation of the Cl2 and free chlorine species in solution (e.g. OCl-). With the addition of the salts the conductivity of the effluent increases (17 mS cm-1) and as a result the operating cell potential is reduced and the energy consumption is lower. Overall the photo-assisted method is observed to be more efficient than the purely electrochemical system. When the biologically treated effluent is electrochemically treated, analogous results to those obtained for the untreated effluent are obtained - inefficient color removal in the absence of salts and increased efficiency with NaCl - however, the energy efficiency is much greater. These results indicate that the EC and PAEC methods are best employed as a final treatment process (polishing) after biological treatment. As Cl2 is formed in- situ the results will also be presented with a determination of the extent of formation of harmful chlorinated degradation by-products.