Overview of textile wastewaters and textile dyes Characteristics of textile wastewaters Textile dyeing industries are facing problems to meet the green practices standards for safe discharge of wastewater due to its complex nature. TWWs are a complex mixture of salts, acids, heavy metals, organ-chlorine-based pesticides, pigments and dyes. 5,10 TWWs generated from the different wet processes are characterized by high pH, temperature, BOD, COD, detergents, surfactants, suspended and dissolved solids, dispersants, leveling agents, toxic organics, chlorinated compounds, sulphide and formaldehyde, may be added to improve dye adsorption onto the fibers 9 and more details are mentioned in the Figure 1. Such effluents are also characterized by the presence of heavy metals, such as Cr, Zn, Cu and Al due to metal-based complexes dyes. 10,14 The most common textile operates are desizing, bleaching, mercerizing, dyeing and finishing. 15 Characteristics and the amount of TWWs depend to the process, dyeing is the most one which requires large volumes of water not only in the step of adding color to the fibers, in dye bath, but also during the rinsing step. Mercerizing and finishing are also significant generators of TWWs. In addition, equipment, machines and chemicals, 5 such as detergents and stabilizers, alter significantly the nature of TWWs. Another important factor which contributes to the ecotoxicity and the volume of TWWs is that dyeing and finishing processes, especially, require the imput of a wide range of dyestuffs. The Variety of dyes depends to the fiber used. 16 For example, cellulose fiber requires the application of direct, reactive, vat, azo or sulfide dyes. Acid dyes are used essentially for wool and silk. Azo and disperse dyes are applied to the polyester fiber. A large quantity of these dyes is released in the TWWs due to their degree of fixation to fibers.

Textile wastewaters (TWWs) are characterized by high salinity and alkaline pH. Bioremediation using fungi were proved in many cases as inefficient tools to treat such effluent, giving the relay to haloalkaliphilic bacteria. Here, three extremophilic strains namely Halomonas desertis G11, Kocuria rosea BU22S and Microbaterium trichothecenolyticum TL13 have been selected to conduct textile dye decolorization experiments. The effect of different combinations of these strains was studied by a mixture design (MD) to assess Tubantin Brown GGL (TB GGL) color removal during species growth under optimzed conditions of dye concentration (100 mg/L), pH (9), salinity (5%), inoculum size (5%) and time (10 days). A remarkable decolorization was observed using mono and mixed cultures. Using the NemrodW software, the optimisation calculations were performed to find an optimum mixture proportions for maximum azo dye decolorization. High regression coefficients R2, between the variables and the response indicated excellent evaluation of experimental data by the polynomial regression model. The highest color removal (about 92%) was obtained with binary mixture composed by H. desertis G11and M. trichothecenolyticum TL13 and it was in close agreement with the estimated response value (93%). This finding shows a biotechnological potential of haloalkaliphilic bacteria in TWWs treatment.

Application of extremophilic plant growth promoting bacteria (PGPB) and their enzymes in bioremediation have been received increasing interest due to their eco-friendly nature and effectiveness for bio treatment of diverse industrial micro pollutants. In this work, the azo-dye decolorization potential of halophilic PGPB Halomonas desertis G11 was evaluated and optimized using central composite experimental design and response surface methodology. Interestingly, the increase of pH and NaCl concentration accelerated the dye decolorization. The model predicted a maximum removal of BEZACTIV blue S-2G dye (80%) at optimal operating conditions (dye concentration of 50 mg/L, inoculum size of 1.0%, pH of 8.2, NaCl of 5.0% and incubation time of 10 days). The experimental design model predictions are in good agreement with the experimental data, thereby providing the soundness of the developed model. The biodecolorization under pressures of high salinity and alkalinity seems to be correlated to azoreductase activity. The gene encoding FMN-dependent NADH azo-reductase from halophilic bacterium H. desertis G11 was identified and the structure and catalytic mechanism of dye decolorizing enzyme were elucidated. Results of this study provide evidence for the potential application of this azoreductase producing extremophilic bacterium as a novel candidate in the biological treatment of sediments and wastewaters contaminated by azo-dyes.