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This paper aims to review the problem of colour in textile ef¯uents, the different classes of dyes available and their contribution to the problem. Through new regulations, pressure is being placed on water companies all over the world to reduce the amount of colour in sewage ef¯uent. Dyes exhibit low toxicity to mammals and aquatic organisms and therefore colour consents are normally applied for aesthetic and industrial reasons rather than for prevention of toxicity. The absorbance, ADMI values and concentrations of dyes in ef¯uent are examined here with particular reference to reactive azo dyes used in cotton processing. Colour consents, the problem of colour in textile wastewaters and the importance for research in this area are also discussed. Dye concentrations of 0.01 g dm À3 up to 0.25 g dm À3 have been cited as being present in dyehouse ef¯uent, depending on the dyes and processes used. ADMI values ranged from 50 to 3890 units for the dyeing of cotton. It was concluded that 1500 ADMI units was a reasonable value to aim for when simulating coloured ef¯uents. Simulated textile ef¯uents may be used for research purposes. These should resemble real wastes as closely as possible, but it is often dif®cult to replicate the ADMI values, absorbance and spectra of real ef¯uents. The concentrations of dye used in simulated ef¯uents examined in literature varied from 0.01 g dm À3 to 7 g dm À3.As absorbance and ADMI values change with the types of dye used, it is dif®cult to relate these values to dye concentrations. A concentration of 0.18 g dm À3 of a Red or Yellow dye or 0.43 g dm À3 of a blue dye would provide an ADMI of approximately 1500 units and ®ts within the range of dye concentrations presented in literature. A dye mixture simulating colour in a real textile ef¯uent is suggested and some limitations of simulating actual wastewaters discussed.

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Decolorisation of azo dyes during biological effluent treatment can involve both adsorption to cell biomass and degradation by azo-bond reduction during anaerobic digestion. Degradation is expected to form aromatic amines, which may be toxic and recalcitrant to anaerobic treatment but degradable aerobically. Methods for the quantitative detection of substituted aromatic amines arising from azo-dye cleavage are complex. A simple qualitative method is suggested as a way in which to investigate whether decolorisation is actually due to degradation, and whether the amines generated are successfully removed by aerobic treatment. Samples from a combined anaerobic-aerobic system used for treating a simulated textile wastewater containing the reactive azo dye Procion Red H-E7B were analysed by high-performance liquid chromatography/ultraviolet (HPLC-UV) methods. Anaerobic treatment gave significant decolorisation, and respiration-inhibition tests showed that the anaerobic effluent had an increased toxicity, suggesting azo-dye degradation. The HPLC method showed that more polar, UV-absorbing compounds had been generated. Aerobically, these compounds were removed or converted to highly polar compounds, as shown by HPLC analysis. Since the total organic nitrogen (TON) decreased aerobically as organic N-containing compounds were mineralised, aromatic amine degradation is suggested. Although only a simple qualitative HPLC method was used, colour removal, toxicity and TON removal all support its usefulness in analysing biotreatment of azo dyes.