How to get rid of colour in textile wastewater
The textile industry is characterized by the fact that its activity requires a high consumption of water, energy and auxiliary chemical products. This is translated into the generation of a large amount of wastewater, with high concentrations of dyes, biodegradable and refractory organic pollutants, suspended matter, surfactants, salts and chlorinated compounds. In addition, because in the great majority of cases production is intermittent, there is a significant variability in the amount of the wastewater generated and the nature of its contamination. These characteristics make it an industrial effluent that is difficult to treat.
The regulatory requirements and the need to save energy and reuse water in industry make it necessary to develop new processes that will enable us to eliminate the water pollution and make it possible to reincorporate the effluent in the production process.
One of the parameters that requires the greatest effort for its elimination –with reasonable costs– is color. The Dyes are not usually toxic, but they are not very biodegradable. In an urban wastewater treatment plant it is estimated that only 20-30% of the color in the effluent will be eliminated. In addition, the dyes are evident in the water in very small concentrations, so the elimination yield must be very high.
The treatment of dyed effluent is an environmental problem that has not yet been resolved satisfactorily
Traditionally diverse technologies based on physical-chemical treatments have been applied for the elimination of the color from textile effluent. Nevertheless, there are other possibilities that are making progress depending on the type of dye to eliminate. The techniques that, according to each specific case, may be used to treat the color in the wastewater are listed below, indicating their advantages and disadvantages:
- Coagulation-flocculation: based on the addition of polyelectrolyte or inorganic flocculants (iron or aluminum salts), that form flocs with the dye molecules facilitating their elimination through settling. The elimination efficacies are high, but in the process slurry is generated that must be treated. The best performance is achieved when an excess of coagulant is applied, although this can increase the concentration of the pollutant in the effluent.
- Fenton Process: the dye is oxidized with a combination of hydrogen peroxide and ferrous sulfate (Fenton reagent), in acidic conditions. The agent responsible for the oxidation is the hydroxyl radical, which is very reactive; it is formed by the catalytic decomposition of the hydrogen peroxide in an acid medium. The hydroxyl radicals oxidize the dye, and the compound formed precipitates with the ferrous ion and organic compounds. This alternative offers various advantages: it achieves high decoloration speeds if the concentrations of the reagents involved are high, it does not form chlorinated compounds like other oxidizing techniques and there are no mass transfer limitations as it is a homogeneous system. Nevertheless, its main disadvantages are the costs associated with the treatment of slurry (a large amount of thin slurry that is difficult to settle) and the costs of the reagents (the continuous and stoichiometric addition of Fe(II) and H2O2).
- Ozonization: the dye molecules are destroyed based on the high oxidation ability of ozone. The oxidation reaction is quick, high flows can be treated, waste and slurry are not generated and a colorless effluent is obtained with a low COD. Nevertheless the toxicity of the effluent must be verified, as in some cases the compounds generated are more toxic than the initial dyes. Another great disadvantage of the ozonization is the short half life of ozone, around 20 minutes, which has a significant effect on the cost of the process. It has been observed that when the production of ozone is complemented with the addition of hydrogen peroxide, it achieves a significant increase in both the speed and elimination yield.
- Membrane technology: this enables an effective separation of the dye molecules and other compounds larger than the pore size of the membrane selected. Reverse osmosis and nanofiltration membranes are most often used. With this procedure it is possible to treat large volumes of effluent continually and with a high level of separation. The effluent is of excellent quality and in most cases can be reused. The main disadvantages of these techniques are the generation of waste with a high concentration of pollutants and the difficulty and cost of replacing the membrane.
- Adsorption: based on the physical retention of the dye molecules in the surface of the adsorbent that is used. The efficacy of the adsorption process is influenced by a wide variety of parameters, including the interaction between the dye and the adsorbent, its specific surface, the size of the dye molecule, the temperature, the pH and the contact time. Thus, the type of adsorbent selected is essential. An adsorbent that is widely used is activated carbon, although other inorganic adsorbents are also used. The adsorption processes generate high quality effluent, although there are a number of disadvantages that mean they are not competitive for treating effluents with dyes: they are slow processes; they are not selective, so there is competition between the dye molecules and other compounds present in the effluent; they are not destructive, generating a waste that must be eliminated; desorption is a difficult and costly process and, finally, the adsorbents are usually expensive.
- Electrochemical techniques: based on the hydrolysis of the dye through secondary agents generated electrolytically by applying a potential. The processes are clean, operate at a low temperature and in many cases do not require chemical products to be added to the wastewater. Nevertheless, its high energy consumption and the generation of secondary compounds due to parallel reactions decrease the potential of the procedure.
- Biotechnological processes: the application of microorganisms to the deterioration of water containing synthetic dyes is an interesting option due to the advantages derived from the biological treatment, as they are relatively economical processes and can enable the full or partial deterioration of the initial components. However, using the conventional aerobic process of activated slurry, the dye is not degraded and the low elimination yield is attributed to the adsorption on the slurry. Using anaerobic processes, a high elimination yield is achieved for a wide variety of dyes, although the kinetics of the process are slow. Moreover, systems are being implemented in which the dye is degraded by the action of enzymes produced by ligninolytic fungi in in vivo and in vitro cultures. They are very selective processes in which very high yields are reached. Nevertheless, they are not economical processes and they are being developed for ongoing application, recovering the enzymes used.
The treatment of dyed effluent is an environmental problem that has not yet been resolved satisfactorily to obtain, as a general rule, a high yield with a stable, sustainable and economical process. The choice of the most appropriate technology depends on numerous factors, such as the dye used, the amount and variety of pollutants in the water, the flow discharged, the production system, etc. In any event, to guarantee success in technology selection and treatment design, it is absolutely fundamental to perform a complete characterization of the effluent.