© Color. Technol. , 117 (2001) 111 Web ref: 20010209 Reuse of decolourised wastewater of azo dyes containing dichlorotriazinyl reactive groups using an advanced oxidation method Ayse Uygur Textile Dept, Faculty of Fine Arts, University of Marmara, Istanbul 81018, Turkey Email: uygurayse@hotmail.com The possibility of reusing textile wastewater in dyeing processes is investigated in this study. Wastewater was first decolourised by an advanced oxidation method and was then reused in the dyeing of cotton fabric. It was found that decomposition products of the azo dyes used in this study released hydrogen peroxide and sodium sulphate, which were found to be present in the initial wastewater following decolourisation. Removal of this hydrogen peroxide from the wastewater prior to subsequent dyeings produced a dyeing colour strength of 98.4% as compared to that of the standard dyeing. A second reuse of this wastewater resulted in a slightly lower colour strength of 94.4% as compared to standard dyeing. Introduction The natural water resources of the world are becoming increasingly polluted and are unable to meet increasing industrial demands. Therefore the reuse or recycling of textile wastewater, as well as industrial wastewater, are currently important topics of investigation. Since textile wastewater is generally coloured, decolourisation processes must be carried out before the wastewater can be reused in subsequent dyeing procedures. Various decolourisation methods are utilised such as coagulation/flocculation, membrane, activated charcoal, biological adsorbers, electrochemical, reductive and oxidative methods [1]. All these methods have advantages and disadvantages in terms of cost, technological, environ- mental and practical considerations. Although some of these methods may be applied for the successful reuse of textile wastewater, one method is far superior to all of these and that is the advanced oxidation method. This method is preferable in terms of colour removal rate, effectiveness, removal of toxic chemicals, application to large amount of wastewater, etc. [2]. The previous paper in this series showed that hydrogen peroxide and ultraviolet irradiation effectively decolourised wastewater of azo dyes containing dichlorotriazinyl reactive groups, and demonstrated that total organic carbon (TOC) and chemical oxygen demand (COD) values decreased while biological oxygen demand (BOD) increased [3]. The decrease of TOC following this advanced oxidation method indicates that organic dyes are decomposed into carbon dioxide and water, but there are still some other decomposition products in decolourised wastewater which create difficulties when it is reused. There is also the risk of the presence of some metal ions, such as cobalt, nickel and copper, following the advanced oxidation decolourisation process when it is used with metal complex and formazan complex dyes [4]. However, in this research, the azo dyes selected did not release metal ions which might entail subreactions. Glauber salt is also present in decolourised wastewater which can be reused when the ‘all-in-new’ method is applied. In this method the Glauber salt is added to the dyebath at the start of the dyeing process. It is well known that hydrogen peroxide is an oxidising agent therefore it oxidises dyes easily in the wastewater or in the dyebath during reuse of treated wastewater. Therefore released hydrogen peroxide in treated wastewater must be removed before reuse. The probable applications of treated wastewater in dyeing treatments will be investigated in this paper. The first part of this research which dealt with ecological parameter determinations has been published previously [3]. Experimental Materials The fabric used was 153 g/m 2 knitted cotton. The dyes used were as follows: Procion Yellow MX-8G (BASF; CI Reactive Yellow 86), Procion Red MX-8B (BASF; CI Reactive Red 11) and Procion Blue MX-2R (BASF). Dyeing procedure Dyeing was carried out at a concentration of 2% owf on 5 g fabric samples. In addition, 35 g/l sodium sulphate (anhydrous) was included in the dye liquor. Dyeing was carried out at 30 °C and this temperature was held constant for 30 min before adding 4 g/l soda ash and the process continued for a further 45 min at liquor ratio 20:1. The fabric was then subjected to the following treatments: (a) rinse, 10 min at 20 °C, liquor ratio, 30:1; (b) rinse, 10–20 min at 40– 50 °C, liquor ratio 30:1; (c) boil with Kieralon (BASF) detergent for 30 min, liquor ratio 30:1; (d) rinse, 10 min at 80 °C, liquor ratio 30:1; and (e) rinse, 10 min at 20 °C, liquor ratio 30:1. Decolourisation procedure The wastewater produced was collected and diluted to 1 l with deionised water. Following the addition of hydrogen peroxide (1 ml/l), the coloured wastewater was placed in quartz cuvettes (3.3 ml) which were then placed 10 cm away from an ultraviolet (UV) lamp (Philips HPM 17, wavelength Coloration Technology Society of Dyers and Colourists