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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The operational stability of peroxidases was considerably enhanced by generating hydrogen peroxide in situ from glucose and oxygen. For example, the total turnover number of microperoxidase-11 in the oxidation of thioanisole was increased sevenfold compared with that obtained with continuous addition of H(2)O(2). Coimmobilization of peroxidases with glucose oxidase into polyurethane foams afforded heterogeneous biocatalysts in which the hydrogen peroxide is formed inside the polymeric matrix from glucose and oxygen. The total turnover number of chloroperoxidase in the oxidation of thioanisole and cis-2-heptene was increased to new maxima of 250. 10(3) and 10. 10(3), respectively, upon coimmobilization with glucose oxidase. Soybean peroxidase, which normally shows only classical peroxidase activity, was transformed into an oxygen-transfer catalyst when coimmobilized with glucose oxidase. The combination catalyst mediated the enantioselective oxidation of thioanisole [50% ee (S)] with 210 catalyst turnovers.

Two sequencing batch reactors (SBRs) with sequenced anaerobic/aerobic phases were used to study biological colour removal from a simulated cotton textile effluent containing an azo reactive dye. One of the reactors was daily fed with Remazol Brilliant Violet 5R dye and the other was used as control. When operating with a sludge retention time (SRT) of 15 days the total COD removal was around 80%, with 30% being removed anaerobically. After 40–50 days of acclimatization the colour removal efficiency reached a maximum, stable value of 90% from a feed dye concentration of 90 mg/l, almost all being removed during the anaerobic phase. This colour removal was attributed to microbial degradation rather than adsorption and colour removal capacity was not lost even after a seven-day absence of dye in the fed substrate. The dye-fed reactor experienced a reduction in the ORP values attained during the non-aerated phase, after acclimatization, an effect not observed in the dye-free control. Under denitrifying conditions it was observed that the decolouration levels achieved in the anaerobic phase decreased from 90% to 70% after only two cycles with a feed containing 45–60 mg NO3/l. Reduction of the SRT value from 15 to 10 days reduced the biomass concentration from 2.0 to 1.2 g VSS/l and lowered colour removal levels from 90% to 30–50%. When the SRT value was increased back to 15 days the colour removal capacity of the system was completely recovered, suggesting that with a SRT of 10 days the adequate microbial population could not be installed in the reactors.

Immobilisation of both enzymes and whole‐cell systems is of major importance in the improvement of the stability, activity and reusability of these biocatalysts. This review describes the use of the naturally occurring polysaccharide carrageenan as a support for the immobilisation of biocatalysts. Carrageenan is a food‐grade and biocompatible support material extracted from red seaweeds. Before focusing on the use of carrageenan as an immobilisation support, an overview is given of the present uses of biocatalysts in industrial processes. The basic concepts of enzyme and whole‐cell immobilisation are discussed, as well as the background of carrageenan as a biopolymer. Several examples of enzymes and whole‐cell systems immobilised in carrageenan are discussed. A list of the most relevant patents in this field is presented as well as a list of enzymes and cell systems immobilised in carrageenan as described in the literature.

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A great number of the reported examples of azo dye biodegradation comprise two main steps, the reductive cleavage of the azo bond under anaerobic conditions and the subsequent aerobic mineralization of the produced aromatic amines. Based on this possible metabolism a Sequencing Batch Reactor was chosen to study biologicalcolor removal from simulated cotton textile effluents containing a reactive azo dye. In previous studies high color removal levels of the azo dye Remazol Brilliant Violet 5R were achieved (up to 90% with an initial dye concentration of 100 mg l(-1)) during the anaerobic phase of Sequencing Batch Reactor operation. However, HPLC analyses revealed that the aromatic amines formed in the anaerobic phase were not mineralized during the subsequent aerobic phase. In an attempt to promote the aerobic biodegradation of these aromatic amines three different approaches were tested, the increase of the relative duration of the aerobic phase, the increase of the hydraulic retention time through the decrease of the daily fill flow and finally the increase of the dye/carbon source concentration ratio through the decrease of the fed volumetric organic load. The two aromatic amines directly resulting from azo bond reduction were detected by HPLC analysis. However, a third metabolite with significant peak area was also detected with a time profile suggesting an equilibrium with one of the aromatic amines In spite of the conversions occurring between metabolites during the cycles of the tested approaches, no effective biodegradation of these metabolites was observed during the experimental period of over 810 days.