Treatment of Textile Wastewater by Combining Biological Processes and Advanced Oxidation

Abstract: Popular Abstract in English Colors have always been part of human life and culture. Today natural pigments have been substituted by synthetic dyes that usually end up threatening nature itself by polluting the environment. To what extent can we use chemistry and biology to minimize the environmental impact of the textile industry? During the dyeing process a certain amount of dye, 2 to 50 percent, does not bind to the fabric and is washed away. Companies are working on improving this process. Meanwhile something needs to be done to avoid the dye effluents ending up untreated in the natural environment. Discharge of untreated textile effluents in the natural environment is a widespread problem where the production of textiles is concentrated, for example in the south of Asia. This is caused by lack of care and awareness, and defective regulations. Much research has been performed to reduce the environmental impact of the dyeing process units and appropriate technologies are now available. However, most units are very small, sometimes just the size of a family company, and they do not have money for treating their effluent. Why are colors threatening the environment? Aquatic plants and algae are indispensable for aquatic ecosystems. They use carbon dioxide and light to produce oxygen and food, which are consumed by higher organisms like fish. The presence of colors in water tends to stop the penetration of sunlight, hence the gradual decrease in living organisms in rivers. Dyes can be harmful for humans, acting as irritants for eyes and skin. Moreover, the problem does not disappear with the color! In fact, in time the dye molecules will eventually break down and become colorless. The resulting compounds are even more harmful than the original dyes and in some cases carcinogenic. Researchers all around the world are looking for an optimal treatment for textile effluents. Common solutions for wastewater treatment include aerobic biological processes, adsorption and coagulation. Can these be used for textile effluents as well? Microorganisms are the main actors in aerobic biological processes because they remove nutrients like organic carbon, nitrogen and phosphorous from water without the addition of chemicals. The downside is that a lot of oxygen is needed for this process and excess sludge that may contain pollutants is generated and needs to be handled in an appropriate way. In addition, most dyes used in textile industry are not fully degradable and the treated water could contain dangerous amines. Adsorption and coagulation remove dyes and other pollutants by transferring them from a liquid phase, the wastewater, to a solid phase, the adsorbent or coagulant. This is however only a temporary solution. In fact, the resulting sludge produced contains dyes and needs to be properly disposed of, with additional costs. Our research group is dealing with textile wastewater treatment. The project started some 10 years ago after visiting several textile dyeing units in the south of India. The research goal was to devise an ideal treatment which was cheap, easy to operate and efficient in terms of producing a good-quality effluent. The strategy is simple. First, the wastewater is treated with anaerobic microorganisms, which can reduce the total organic content of the effluent by metabolizing the degradable compounds without the need for oxygen. Secondly, an advanced oxidation process based on the activity of a powerful oxidant is applied to facilitate the removal of harmful amines. We have evaluated a few options for this last step. One is ozonation, energy intensive but “clean”. This means that it does not produce any sludge or secondary waste. In fact, ozone, apart from being a well-known pollutant, can oxidize aromatic compounds releasing oxygen. A very successful process is photo-Fenton oxidation. The effluent is treated with iron and hydrogen peroxide in the presence of light and what remains at the end is a mixture of salts rich in iron. To verify the effectiveness of the treatment, we used a series of toxicity tests. The costs and environmental impact assessment of the treatments suggested in this work indicate that the technology is ready to be implemented. Money cannot buy happiness, but it can certainly help to preserve our beautiful world. The technology for industrial wastewater treatment is mature enough to offer solutions to avoid pollutants from escaping into the environment. Dyestuff and other chemicals found in textile effluents are no exception. However, industries are often not ready to put money into waste handling, something that will not make them richer. Therefore, the future of wastewater should be centered on the concept of water reuse, which is profitable for industries as well as the environment.