Treatment of textile wastewaters using combinations of biological and physico-chemical methods
Abstract: Considerable amounts of water and chemicals are used in the textile industry, and the presence of dyes in the wastewater is a problem of environmental concern. Synthetic dyes are designed to resist light, water and chemicals to improve the quality of the textiles; however, these properties also contribute to their persistence in the environment. The existence of wastewater treatment plants in connection to the textile industries varies from region to region, and while some treatment plants are quite efficient, others are not even in use. The pollution of dye-containing wastewater has clear visible effects and some dyes are also suspected to be biologically modified into carcinogenic and toxic compounds. Common techniques used for the treatment of dye wastewater are physical or chemical methods, such as precipitation, membrane filtration or adsorption in combination with aerobic biological treatment. Even though these techniques can be efficient, they can also be quite expensive and are based on transfer of the pollutant from one phase to another, thereby creating secondary waste. Moreover, the majority of synthetic dyes are recalcitrant to conventional aerobic biodegradation. With the purpose of finding a better alternative for the treatment of textile wastewaters, a number of methods were evaluated using azo dyes collected from the Indian and Bolivian textile industries. The studied treatment alternatives were both biological and physico-chemical methods selected on the basis to provide complete dye degradation and not only phase removal. Since textile effluents can be very diverse depending on the concentration of the dyes and additional chemicals, the focus was also to evaluate the robustness of the methods. To confirm the recalcitrance of the dyes under conventional aerobic conditions, studies using activated sludge collected from a municipal wastewater treatment plant in Lund, Sweden, were conducted. The results showed that the dyes were not degradable under aerobic conditions. The first alternative that was evaluated was degradation by white rot fungi. These fungi grow on trees and degrade lignin through the use of extracellular enzymes. Due to the unspecificity of these enzymes, it has been shown that they also can be used to degrade a wide range of recalcitrant pollutants. The process was evaluated using various fungal species as well as extracted enzymes. Even though high decolourisation efficiencies were obtained, the dyes were not fully degraded. Moreover, it was difficult to run a stable process when using the fungi in non-sterile reactors. Secondly, we evaluated a combined anaerobic-aerobic process using sludge from a municipal wastewater treatment plant, from a textile industry and from a chemical industry. Even though this process was both robust and efficient in decolourising the dyes, all intermediates were not degraded. Thereafter two physico-chemical methods, photocatalysis and photo-Fenton, were evaluated. Both these processes are based on the production of highly reactive hydroxyl radicals, which degrade the pollutant. In the photocatalysis titanium dioxide was used as a catalyst, while hydrogen peroxide and ferrous iron were added in the photo-Fenton treatment. Almost complete dye decolourisation and degradation could be achieved using both methods. The drawbacks with these methods, however, are the high energy use and the cost of the catalyst/reagents. Combining a physico-chemical method with a biological method could take advantage of both the cost efficiency of the biological process and the efficiency of the physico-chemical process. Using combined photocatalytic-biological treatment and combined photo-Fenton-biological treatment, it was shown that complete degradation could be achieved while reducing the chemical requirements of the photo-Fenton process as well as the treatment time of the photocatalysis. Comparing the two physico-chemical methods, the photo-Fenton process appeared to be a faster and more robust alternative, requiring lower catalyst concentrations. The combined biological-photo-Fenton process can thereby be considered as a promising alternative for treatment of textile wastewaters and should be further studied on real textile wastewater using sunlight as radiation source.
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