| The significantly increasing production of sewage sludge from city wastewater treatment plant has been paid more attention because it has caused serious environmental pollution. Resource utilization is a fundamental way to dispose of sewage sludge. And preparation of activated carbon using sewage sludge as raw material is one of the most important development directions for resource utilization of sewage sludge. Zn Cl2 activation method is currently the main method for preparing sludge-based activated carbon due to the relatively low preparation temperature, high yield of activated carbon and low-priced and recyclable activator, compared to other preparation methods.The preparation process of sludge-based activated carbon through Zn Cl2 activation method is essentially the pyrolysis process of the sludge with Zn Cl2 impregnation. The researches on the mass loss characteristics, pyrolysis products and reaction kinetics during the pyrolysis of the sludge with Zn Cl2 impregnation are not only beneficial to explore the pyrolysis reaction mechanism but also can provide a theoretical basis and guiding role for preparation of the sludge-based activated carbon. In addition, pyrolysis conditions controlling and main influence factors in the preparation process of sludge-based activated carbon are still controversial. And the application of prepared sludge-based activated carbon in wastewater treatment is still limited, just applying to the adsorption of organic pollutant such as dyes and phenolic compounds and inorganic heavy metal ions.Based on the above points, firstly, thermogravimetry coupled with Fourier transform infrared spectroscopy(TG-FTIR) and thermogravimetry coupled with mass spectrometry(TG-MS) were used to study the mass loss characteristics and gaseous products generated during the pyrolysis of the sludge with Zn Cl2 impregnation in this thesis. Based on the above analysis results, the reaction kinetics of the pyrolysis process of the sludge with Zn Cl2 impregnation was studied. Meanwhile, the effect of Zn Cl2 on the pyrolysis process of the sludge was illuminated by comparing the pyrolysis characteristics and reaction kinetics between the sludge with and without Zn Cl2 impregnation. The results showed that the pyrolysis process of the sludge with Zn Cl2 impregnation could be divided into four stages including the dehydration of the sludge and depolymerization or decomposition of a small portion of organic matters, the initial pyrolysis of organic volatile, the volatilization of Zn Cl2 and further pyrolysis of intermediates and undecomposed organic matters, and the degradation of inorganic minerals and combustion of fixed carbon. The gaseous products generated during the the pyrolysis of the sludge with Zn Cl2 impregnation mainly consisted of H2, H2 O, CO2, CO, CH4, light hydrocarbon compounds such as C2H6 and C3H6, small molecular alcohols such as C2H5 OH and some small molecular aldehydes, ketones and carboxylic acids. The first and third pyrolysis stages of the sludge with Zn Cl2 impregnation accorded with the first-order chemical reaction model. The second pyrolysis stages accorded with the second-order chemical reaction model. The fourth pyrolysis stages accorded with the third-order chemical reaction model. The effect of Zn Cl2 on the pyrolysis process of the sludge was reflected in promoting the dehydration of sludge, inhibiting the formation of tar, improving the aromatization of organic matters in the sludge and enhancing the further pyrolysis of intermediates.Secondly, the effects of pyrolysis temperature, pyrolysis time, activator concentration and impregnation ratio on the pore characteristics of the produced sludge-based activated carbon were analyzed. In addition, orthogonal experimental design was applied to analyze the influential extent of different factors on the pore characteristics of the sludge-based activated carbon and optimize the preparation conditions. Furthermore, physico-chemical properties of the prepared activated carbon were studied by a series of advanced analysis equipments and test methods such as specific surface area and porosity analyzer and Fourier transform infrared spectroscopy(FTIR). From the research results, it was found that during the preparation process of sludge-based activated carbon through Zn Cl2 activation method, pyrolysis temperature was the most influential factor on the pore characteristics of the prepared activated carbon, followed by pyrolysis time, activator concentration and impregnation ratio. The optimal preparation conditions were designed as the following: pyrolysis temperature of 550℃, pyrolysis time of 50 min, activator concentration of 40wt% and impregnation ratio of 2.5. Under the above preparation conditions, the specific surface area of sludge-based activated carbon was 397.90m2/g. The pore structure was mainly mesoporous and the functional group such as hydroxyl, amino, aldehyde and carboxyl were found on the surface of activated carbon. Additionally, the content and proportion of the heavy metals with unstable state in the sample both decreased while those of the heavy metals with stable state both increased after the sludge was transformed into activated carbon by Zn Cl2 activation method, therefore causing the stability of heavy metals in the sample increasing significantly.Finally, a new approach for the application of sludge-based activated carbon was proposed in this thesis. The sludge-based activated carbon was applied to mitigate the membrane fouling in the membrane bioreactor(MBR). The sludge-based activated carbon with a dosage of 3g/L was added into MBR. The effects of the addition of the sludge-based activated carbon on the performance of MBR, sludge mixed liquor characteristics, membrane surface characteristics and membrane fouling in the MBR were studied. The results showed that the improvement effect on the performance in the MBR was not obvious, while the sludge mixed liquor and membrane surface characteristics were improved significantly after the addition of the sludge-based activated carbon. The sludge concentration and sludge particle size increased by 6.90% and 8.89% respectively. The viscosity of the sludge mixed liquor, contents of EPS and SMP in the sludge mixed liquor, sludge specific resistance and content of EPS on the membrane surface decreased by 13.45%, 17.83%, 13.73%, 6.26% and 9.27%, respectively. Furthermore, p/c ratios in EPS in the sludge mixed liquor and on the membrane surface and contact angle of the membrane surfacethe all decreased and the structure of cake layer on the membrane surface became looser. The above points resulted in the mitigation of the formation and growth of the cake layer on the membrance surface and the alleviation of the clogging in the membrane pore. The variation of the sludge mixed liquor and membrane surface characteristics in the MBR after the addition of the sludge-based activated carbon caused that the membrane pore resistence(Rp) and cake layer resistence(Rc) decreased by 22.95% and 18.50% respectively and the growth rate of Rp and Rc decreased by 22.22% and 16.67% respectively. The operation cycle of MBR was prolonged from 37 d to 46 d. |
PDF Full Text Request