Study On The Efficiency And Mechanism Of Dye Wastewater Treatment By Non-Thermal Plasma

Non-thermal plasma(NTP) technology, as one kind of new advanced oxidation process, combines the ozonation, shockwaves, ultraviolet irradiation and pyrolysis effect together. This technology can remove different kinds of pollutants efficiently and non-selectively, without the need of adding chemical reagents. Furthermore, NTP can be operated under normal temperature and pressure. All those advantages led to its widely attention and decades development. However, the low mass transfer efficiency, low utilization of energy and high energy consumption undermine its application. In this thesis, the operating condition of NTP system was optimized firstly. Then the catalyst strengthening and gas adjustment system were investigated, in order to improve the feasibility of NTP technology for the degradation of dye wastewater. The hybrid process with discharge followed by biological process for the degradation of practical dye wastewater was studid at the same time. Some useful conclusions are:(1) To investigate the feasibility of N TP used for the degradation of dye wastewater, the degradation of model pollutant-reactive blue(RB-19) treated by dielectric barrier discharge(DBD) process was studied. The effects of discharge voltage, power, initial RB-19 concentrations and initial p H values on degradation were examined. And the changes of discoloratio n, p H values, COD, TOC and UV-Vis were analyzed. It shows that: the degradation efficiency increased with the discharge voltage and power. 65 k V and 62 W were chosen as the optimum discharge condition which could not only ensure the operation safety but also could achieve high degradation efficiency. Under the same discharge conditions, the higher initial concentration is in accordance with the lower degradation efficiency, but the absolute pollutant removal amount had a rising trend. The NTP system could remove the dye best under acid conditions; the degradation dynamics met the first-order kinetics well. Discharge power was the most responsible factor according to the RSM analysis, while the interaction effection of the three factors is not significant. This technology had good decolorization effect, the solution decolorization could reach more than 90% after 10 minutes discharge; but the COD and TOC removal rates w ere relatively low, which were 45.06% and 21.86% respectively. Results from the UV-Vis spectrum scan shown that: although the peak at 594 nm completely disappeared, the peaks in the ultraviolet area still existed and remained at a high level. It suggests that: the dye molecules were not fully biodegraded with structure of anthraquinone remain unchanged, and the dye molecules were only degraded to colorless intermediates. In conclusion, the DBD technology is effective on the dye decolorization, but inefficie nt on the dye mineralization.(2) Because of the ineffectiveness of NTP on the dye mineralization, the catalytic discharge system was used to enhance the generation of active species, which to improve the degradation ability. Through literature review and theoretical analysis, the sol- gel method was used to synthetize the Fe-doping titanium dioxide with butyl titanate and ferric nitrate as raw material. The synthesis conditions(Fe-doping amount and calcination) on the influence of catalytic properties were optimized and samples were characterized by XRD, SEM, FTIR and UV-Vis. Results showed that the samples synthesis with 0.05% of Fe-doping amount and calcination temperature 500 ? perform the best catalytic effect. The catalyst dosage, initial solution p H and initial concentration were examined in the catalytic-discharge coupling system for the degradation of dye wastewater. The changes of dye degradation parameters(decolorization rate, TOC removal, H2O2, O3, anion production, UV-Vis, FTIR, OES and intermediate product) were investigated; results showed that the coupling system could improve not only the dye degradation rate but also the mineralization efficiency. The strengthing mechanism of the coupling system was presented based on the experimental results: the coupling system can enhance the active species produce then improve the dye molecular degradation.(3) NTP could generate various active species, among them ozone(with atom oxygen in supplying gas) is one of the most important active species generated. O zone could damage the chromophore selectively to gain significant decolorization performance; however, the contribution rate of ozone for the dye degradation is still unclear. Different gas atmosphere(argon, oxygen, air) were introduced into a needle-plate discharge reactor for the degradation of reactive blue. The peak-peak voltage and power were 14, 25, 20 k V, and 0.29, 1.72, 1.57 W in argon, air and oxygen atmosphere respectively. The best TOC removal rate was obtained in argon, while the best decolorization was obtained in oxygen. By monitoring the ozone generation during the discharge, there was no ozone generation in argon, the ozone concentration increased rapidly over time and eventually reached nearly 400 ppm in oxygen; in air there was little ozone generation which did not exceed 50 ppm. The decolorization order was O2 > Ar > Air with the same energy density, while the p H decrease order was Air > Ar > O2. The quite different active species generated in different atmospheres were the essential reason for this phenomenon. Followed by hydroxyl radicals and NOx, ozone was the most efficient in dye decolorization. High oncentration of ozone generated in oxygen was the reason for the significant decolorization. Although there was no ozone in argon, the production of nonselective species(such as hydroxyl radicals, argon active species) was higher than that in oxygen and air, those species resulted in the TOC decrease. N itrate and nitrite generated in air was the major cause for the p H decrease. A one-dimension model was established to examine the ozone contribution of dye degradation, the ozone-dye reactions were simulated in the needle-plate reactor as well. The model consists of two parts, the first part was to determine the diffusion coefficient Dc, and the second part studied the ozone-dye reaction. Simulation results turned out that : about two-thirds of the dye decolorization was due to the ozonation. This result verified the hypothesis that ozone was most responsible for the dye decolorization in NTP system(with atom oxygen in supplying gas).(4) In order to investigate the feasibility of the practical dye wastewater degraded by NTP technology, the practical dye wastewater from an industrial park in Jiangsu province was treated by the DBD system. It turned out that the dielectric barrier discharge could remove part of the pollutants but could not meet the requirement of effluent standard. After 10 min discharge, the decolorization rate and COD removal rate of the dye wastewater were 71.45% and 44.42% respectively. As the discharge process could improve the wastewater biodegradability, B/C from 0.19 to 0.37, it could be then combined with the biological unit for the practical dye wastewater. The activated sludge taken from a sewage plant was acclimatized for the biological processing, and the optimal conditions for the microbiome were temperature 35 ? and p H 7. Through 8-hour biological treatment under the optimal conditions, 25.94% of decolorization rate and 31.07% of COD removal rate were obtained respectively without the discharge pretreatment. The hybrid process with discharge followed by biological process could achieve good treatment efficiency for the practical dye wastewater. The decolorization rate and COD removal rate were 88.91% and 80.09%, which had increased 2.43 and 1.58 times than the biological process respectively, the effluent COD and chromaticity were 82 mg/L and 68 times, which both met the discharge standards.