Studies On Biodegradability Of Direct Dyestuffs

Direct dyes have been produced commercially for over 100 years. As an important and widely used type of dyes, its wastewater is one of the major sources of pollution. The ingredients of the wastewater are also complicated, which involves various organic dyes and their intermediates. What's more, the wastewater usually has those characters: deep color, high concentrations, long resistance to biodegradation, strong toxicity, wide fluctuation of pH, great variation of ingredients, large waste streams and so on. Therefore, dye wastewater treatment has increasingly become one of the heated topics in the field of environmental engineering.In this research, fourteen typical direct dyestuffs were selected to study their biodegradability. It was determined by testing both BOD₅/COD_Cr ratio and oxygen uptake rate under aerobic condition. While under facultative condition, the intermittence reaction was applied. In addition, the relation between the biodegradability of the dyes and their chemical structures was also analyzed. Then the simulated dye wastewater containing Direct Blue 15 was treated using a facultative-aerobic system. The conclusions obtained from the comprehensive analyses are as followings:1. According to the BOD₅/COD_Cr ratios, there were 3 dyes with poor degradability and all the other dyes were non-biodegradable. By the oxygen uptake rate method, the results of degradability yielded were much better: 9 direct dyes could be degraded to some degree, and only 5 dyes were difficult to be degraded. Of these 5 dyes there were 3 dyes which were toxic and inhibitory to the microbes badly.2. The degradability under aerobic condition shows the chemical structure of the dye molecule significantly affects its degradability. Dyeswith the benzidine were easiest to be degraded. In constrast, dyes with more than 3 R-N=N^R' azo bonds were the most difficult to be degraded. Meanwhile, dyes with heavier molecular weight got the relatively poorer degradability. Besides, the kinds and quantities of substituted groups on the aromatic ring of the dye were also the influential factors. Generally, the promotional groups will enhance the biodegradability, while the inhibitory groups will make the dye more stable.3. Under facultative condition without glucose, both CODCr and color reductions reached good results by the optimum retention time of 48h at 35°C. In contrast, when at 20 °C, the optimum retention time was as long as 72h. And the results were nearly the same as the former. When the glucose was added in, much better results were obtained compared to the condition without glucose. That demonstrated the co-metabolism contributed greatly to the degradability of the dyes when the glucose put in as the carbon source.4. The facultative-aerobic process run for two months turned out to be effective on treating the Direct Blue 15 containing wastewater. The system was rather stable after the first month's seeding and domestication. For the total system, CODCr and the color reductions of 87.7% and 91.0% were achieved respectively. While the facultative phase contributed greatly to the color removal and the biodegradability improvement in this system, the aerobic phase played an important role in reducing the CODoAll the above achievements reached in this research will provide a theoretical guide for developing the new environmental-friendly dyestuffs and the biological treatment of their wastewater.Shen Ting (Environmental Engineering) Supervised By Prof. Jiang Peihua...