## 1 Introduction  
The environmental impact of the textiles industries is associated with large quantities of water consumption and the variety and amount of chemicals used along all the steps of textile processing. The effluents from textile manufacturing, dyeing and finishing processes are highly variable in composition and contain a complex mixture of toxic substances such as dyes and pigments, salts, metals, biocides, surfactants and many other organic and inorganic components [1; 2; 3; 4]. Along the textile processing process, there are extreme variations in different parameters such as chemical oxygen demand (COD), biochemical oxygen demand (BOD5), pH, colour and total dissolved solids (TDS). The primary concern about the release of coloured effluents into the environment is their negative aesthetic impact and eutrophication in the receiving waters; however, many of the dyes and degradation products used in the textile industry have been found to be toxic and genotoxic [4; 5; 6; 7]. Another great concern regarding the textile industries from the environmental point of view is that in many regions, most of the water used during the textile processing is discharged in the aquatic ecosystems as waste and without adequate treatment, leading to contamination problems [8; 9; 10]. Due to the high complexity and variability of these effluents, there is no general procedure for the treatment of this waste and a simple treatment technology does not guarantee the satisfactory removal of all contaminants, especially in terms of toxicity [8]. The conventional methods used for the treatment of these effluents rely on biological and physico-chemical methods, including, in the last few years, the advanced oxidation processes [6; 11]. Due to the complexity of these effluents, the wastewater treatment is inefficient in most cases, leading to negative environmental impacts [12]. Unfortunately, most of the evaluations of these effluents before discharge into freshwater ecosystems are made based on chemical methods that do not provide sufficient information about the potential adverse effects of contaminant mixtures on organisms [3; 9; 13]. However European legislation, namely the Water Framework Directive (WFD, Directive 2000/60/EC of 23 October 2000) and daughter directives, extended the analysis of water quality not only to physico-chemical and microbiological analysis, but also to the characterisation of the biological communities inhabiting the receiving ecosystems. Also, measures and targets were proposed to reduce and control the pollution caused by priority substances and to reduce or eliminate discharges of priority hazardous substances [14]. The decision n\({}^{\circ}\) 2455/2001/EC of the European Parliament and of the Council of 20 November 2001 established a first list of 33 priority substances including priority hazardous substances, which, according to existing information, pose a significant risk to the aquatic environment [15]. In order to regulate the discharge of these substances, the Directive 2008/105/EC of 16 of December 2008, established environmental quality standards (EQS) for the priority substances and other included pollutants [16]. However, the majority of chemical substances present in the textile effluents are not included in the previous list for which EQS were defined, but they can be detected, at least indirectly, if they harm the ecological communities of receptor systems. Also, it is important to note that although individual compounds may be present at concentrations below the established limit, the combined exposure to the complexity of an industrial effluent can be underestimated [17]. It is therefore of utmost importance to include a whole effluent toxicity for an effective assessment of the risk, hence the importance of this study, as this is not enforced by European and national regulations. Therefore, the toxicity bioassays continue to be a useful tool for evaluating the effect of these complex effluents on living organisms.