| In the Twelfth Five-Year Plan, the development of high performance membrane material is a very important strategy content in the energy conservation and environmental protection industry. Howerver, there were few studies in this field in domestic, and most of them were only limited in laboratory stage, which were less oriented to practical application and pushed forward to the engineering effectively. Poly(vinylidene fluoride) (PVDF) is one of the most mainsteam commercial membrane materials, it occupy a very larger market share. But PVDF is relatively more hydrophobic because of its very low surface energy. This hydrophobic nature makes PVDF membrane susceptible to be contaminated by oil, protein and some other organic impurities, which directly results in flux serious decline, high energy consumption and short service life. Thus, the key approach to solve this problem is hydrophilic modification for membrane material. Recently, blending polymeric membrane with nano-inorganic material has been becoming a research hotspot of membrane material. The new membranes developed by this way have the characteristics of both organic polymer and inorganic nanomaterial, it shows super combination properties, and its hydroplilicity is improved as well.According to the previous literatures, the nanometer TiO2, SiO2 and Al2O3 were often used to modify polymeric membranes, while nano ZrO2 was used very rarely. But ZrO2 ceramic membranes have exhibited its superior performance in oily emulsion treatment due to its special surface character, such as high surface density and strong polar. ZrO2 membranes are chemically more stable than TiO2 and Al2O3 membranes, and therefore are more suitable for liquid phase applications under harsh conditions. From this viewpoint, ZrO2 could clearly be a nanomaterial reinforced phase for polymeric membrane, and so it has more development potential. Hence, further study PVDF membrane modified with nano-ZrO2 is likely to develop new kinds of high performance PVDF modified membranes, and the modified membranes would considered to be developmental potential for practical application, it has real research significance.Based on the background and points. ZrO2 nanoparticle was choosed to modify PVDF membrane material in this thesis. The new type nano-ZrO2/PVDF modified membranes were prepared by NIPS method, the preparation process, membrane structure and performance were studied systematically, and the influences of membrane forming mechanism with addition of ZrO2 nanoparticle were discussed as well. For application-oriented, emulsion oily wastewater is regarded as a current difficult problem existing in industrial oily wastewater, and this was selected as the breakthrough point for application research. In the thesis, the fouling mechanism of nano-ZrO2/PVDF modified membrane in the treatment of emulsion oily wastewater was studied in detail, which could provide understanding and grasping for substantial causes for the membrane fouling in the process. In addition, it could also provide guiding strategies for control membrane fouling effectively. For assessment effectiveness of actual application, the nano-ZrO2/PVDF modified membrane was fabricated into a MBR laboratory equipment, which was applied to the treatment for steel cold-rolled emulsion oily wastewater. All the above studies would provide some certain theories foundations and technical support for engineering. The main contents and results of this thesis were sumrized as follows:1. Preparation of nano-ZrO2/PVDF modified membrane and its membrane forming mechanism researchFirstly the membrane preparation process with NIPS method was studied, meanwhile a comprehensive discussion was carried out. In this research, some important parameters including solvent-nonsolvent pair, PVDF concerntration, addition type and content, were selected to have comparision experiments for obtaining optimization scheme for preparing PVDF membrane. On the basis, three different crystalline types of ZrO2 nanoparticles (m-ZrO2, t-ZrO2 and c-ZrO2) were applied for the modification of PVDF membrane material, and the nano-ZrO2/PVDF modified membranes were prepared simultaneously. By X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Scanning electron microscope (SEM), Energy dispersive spectrometry (EDS), water contact angle, membrane flux, Bovine serum albumin (BSA) rejection, and membrane flux recovery, etc. the key performance characterstics of the modified membranes were analyzed systemtically. Research findings as:The prepared ZrO2/PVDF modified membranes have been distributed the single crystalline type nano-ZrO2 nanoparticles successfully. There were no new chemical bonds generated obviously in membrane matrix, which presented it could be simple physical blending between nano-ZrO2 nanoparticles and PVDF polymer. The surface hydrophilicity of the modified membranes was slightly improved with the increasing amount of nano-ZrO2. Among the three crystalline type nano-Zr02, m-ZrO2 nanoparticles was more remarkable to improve the surface hydrophilicity. When compared to PVDF net membrane, the decline of water contact angles of the modified membranes was not very significant. The prepared membranes have a very type asymmetric porous ultrafiltration membrane structure, and with the addition of nano-ZrO2, nanoparticles aggregation increased which were exhibited in both membrane surfaces and cross-sections, but the morphological structures of the membranes have not been changed significantly. The distribution of nano-ZrO2 in membrane was relatively uniform, but there could existed a loss of nano-ZrO2 in the processes of membrane forming and subsequent use.The filtration experiments indicated the maximum water flux of modified membranes was obtained at 0.75wt% concentration of nanoparticles for all crystalline types of nano-ZrO2. when compared to PVDF net membrane, the flux of modified membranes were almost increased 30%, and Jw(m-ZrO2/PVDF membrane)> Jw(t-ZrO2/PVDF membrane)> Jw(c-ZrO2/PVDF membrane). The BSA rejection of the modified membranes were at 70～82%. it increased slightly with the increasement of nano-ZrO2. Meanwhile, the BSA flux and flux recovery increased obviously. These results showed the addition of nano-ZrO2 could increased membrane flux, and the membrane’s anti-fouling ability was improved simultaneously. The porosity rate of the modified membranes was in the scope of 80.5 ±0.7% to 82.5±0.6%, and it did not appear obvious change rule with the addition of nano-ZrO2. But for the all modified membranes adding with different crystalline type nano-ZrO2, its average pore size was decreased slightly with the increasing amount of nano-ZrO2.The surface energy of nanoparticles is related to atomic number, shape and the interaction potential between atoms, and so it depends largely on the crystal phase structure. For m-ZrCO2、 t-ZrO2与c-ZrO2, these three different crystal type of ZrO2 nanoparticles, tt is considered that the different crystal phase structure results in the different surface energy, which led to the different effects of membrane modification. In m-ZrO2、t-ZrO2、and c-ZrO2, m-ZrO2 has more·OH in particle surface for the reason of its crystal phase structure, and then has a relatively better modification effect.By studying the phase behaviors and gelation kinetics of the PVDF/DMAc/PVP/LiC1 casting solution, the effects of adding nano-ZrO2 particles on membrane forming mechanism were discussed. The results showed that, with addition of nano-ZrO2 particles, the viscosity of the castiong solution was increased, the original phase equilibrium was shattered, and the system capicity of containing the nosolvent (deionized water) was decreased. Therefore, the castiong solution tended to be thermodynamic unstable, and then the phase separation process was accelerated. The expericments showed the membrane forming times of the casting solutions were about 600 seconds, the bi-directional mass transfer diffusion between the solvent and nosolvent was speedup with adding nano-ZrO2, and then reduced membrane forming time. In the thesis, two kinds of gelation dynamics theory were discussed, and through the comparsion of membrane forming dynamic factors, the theory of Prof Sun Benhui was regarded as more reference for relevant studies. It divided the membrane forming process into different stages, and each stage has a corresponding gelation contant.2. Study on the modified membrane’s fouling mechanism in the treatment of emulsion oily wastewaterThe emulsion oily wastewater was prepared in lab as simulation solution, the prepared ZrO2/PVDF modified membranes and PVDF membranes were immersed into the emulsion oily wastewater to take static adsorption experiments. The adsorption characteristics of the ZrO2/PVDF modified membrane adsorbed emulsion oil were demonstrated via the studies of adsorption capacity, isothermal adsorption, thermodynamic parameters and adsorption dynamics. In addition, the modified membranes were also used to filtrate emulsion oily wastewater, by ultrafiltration resistance model, the modified membrane resistance and its distribution were investigated in filtration experiments, and the effects of operation parameters and environmental factors on membrane fouling were discussed as well. The conclusions as follows:Adsorption experiments showed the emulsion oil adsorbed on the ZrO2/PVDF modified membranes were less than that of PVDF membranes. The study of isotherm models indicated the adsorption capacity of the ZrO2/PVDF modified membranes on emulsion oil of was also less than that of PVDF membranes. Furthermore, the comparsion study of isotherm models reflected Temkin model was more suitable to fit experimental data, and it might be explained that oil droplets adsorbed on membrane surface trend to spread out to generate oil film, which would form a face to face membrane fouling.The results of thermodynamic parameters revealed the membrane absorbing oil droplets was an exothermic process, which illustrated the increasing of temperature could be benefit to reduce the oil-adsorption-fouling on membrane in wastewater. The caculation results showed the enthalpy change (△rHθm) of PVDF membrane and ZrO2/PVDF modified membrane was-12.09 and-13.17 KJ/mol respectively, the absolute value of the two ArHθm were all less than 20KJ/mol. It could be explained the physical adsorption was primary, and the adsorption forces between membrane and oil droplets were main physical adsorption forces. Based on this understanding, Van der Waals force was adopted to explain the effects between the membrane and emulsified oil droplets.Four common kinetic models (pseudo-first order, pseudo-second order, Elovich, and dual constant model) were used to analyze the adsorption kinetic process. The results showed the adsorption equilibrium time was about 20h for the membrane-emulsion oil wastewater system, the kinetic process could be divided into rapid adsorption stage (0-20h) and slow adsorption stage (20-40h). Through the compasion of parameters of the four kinetic models, it was clearly depicted that the pseudo first order kinetic model could be considered to be more suitable to describe the adsorption kinetic process than other models. According to the meaning of pseudo-first order kinetic model, a conclusion could be drawn that, the emulsion oil adsorption rate on membrane was directly proportional to the difference value between equilibrium concentration and instantaneous concentration, and the adsorption amount was mainly effected by the diffusion of emulsion oil.The filtration experiments of the ZrO2/PVDF modified membrane showed that the membrane itself resistance Rm had occupied the largest proportion in the total membrane resistance Rt. Through the studies on membrane resistance and its distribution, a conclusion could be obtained that, in order to overcome the membrane fouling, the emphases should be list to reduce concentration polarization, cake layer and adsorption pollution in turn. This understanding would be benefit to optimize operation condition and cleaning methods. Moreover, the results also indicated the total membrane resistance of the modified ZrO2/PVDF membrane Rt was less than that of the PVDF membrane, and its every parts of membrane resistance (membrane itself resistance Rm, concentration polarization resistance Rc, filter cake resistance Rcake, adsorption resistance Ra) were all less than that of the PVDF membrane, these results revealed that the addition of nano-ZrO2 particles would reduce the membrane resistances in every parts.The effects of operation parameter on membrane resistances were investigated through filtration experiments. The results showed that the total resistance Rt increased with pressure increase. At the same increase conditions of operation pressure, Rm was basically unchanged, but the other parts of membrane resistances were all showed the growth trends, their increment were sorted as follows:△Rc> △ Rcake> △ Ra> △ Rm. These revealed that the pressure increase could seriously enlarge the concentration polarization and filter cake pollution, while it had a relatively small impact on adsorption fouling. Moreover, the experiments also indicated that the rising temperature could make the decline of Rt, but the effects of each parts of the membrane resistances were not consistent. Among them, the intrinsic membrane resistance Rm was not obvious changed, the membrane adsorption resistance Ra and the concentration polarization resistance Rc were reduced, but the filter cake resistance Rcake was significantly increased. After the overall analysis, the phenomenon was considered to be the competition result by various factors, such as the viscosity drop of water, the acceleration of the Brownian motion of emulsion oil droplets, the solubility increase and the accumulation speed of oil pollutant on membrane surface, etc.According to practical process, the membrane resistances effected by water environmental factors were analyzed from the angle of Zeta potential, such as pH, inorganic salt. It was found that the pH of water had a severe impact on the total membrane resistance Rt. Compared to alkaline conditions, the filtration of the modified membrane on emulsion oil was relatively good under the low pH acid conditions, at that time Rt was lower, flux was higher, and rejection of emulsion oil droplets was improved as well. In addition, the effects of inorganic salt anion and cation’s concentration and types on Zeta potential were also discussed, its essence was due to the changes of Zeta potential make the aggregative states of emulsified oil droplet changed, thus influenced the membrane resistances.For further study the modified membrane’s plugging types in the treatment of emulsion oily wastewater, the classical Hernia theoretical model was adopted in the thesis, and some modern testing and analyzing techniques were also used. All of the models and techniques analysis were to set up the corresponding methods for study on membrane fouling. The main conclusions were:The analysis of Hernia theoretical model showed that, there was not either one single model could completely agree with the change rule of membrane flux. The fitting coefficients of single model were listed in turn:cake filtration> intermediate pore blocking>standard pore blocking> complete pore blocking, it could be seen that the coefficient of cake filtration model was highest among them, which reflected that the process was mainly cake filtration, but did not exclude the combination impact of other blocking types. It was also revealed that if we want to overcome the blockage pollution, we should give priority to choose appropriate measurements to eliminate the filter cake layer on membrane surface.SEM analysis showed that after the filtration of the modified membranes on emulsion oily wastewater, the nanoscale micro-pores spreading on membrane surface has been covered completely by a layer of micelle pollutants, but the cross-section of the membranes has not been obviously changed, which indicated that the membrane surface fouling was primary, the conclusion was consistent with that of membrane plugging model analysis. AFM analysis showed that the surface roughness of the modified membrane was less than that of the PVDF membrane, and when after filtration, the decline of the modified membrane surface roughness was also less than that of the PVDF membrane. These results reflected the surface of the modified membrane was more neat and orderly, and it filled less deposition pollutants. These further conformed that by the way of adding suitable amount of ZrO2 nanoparticles effectively improved the PVDF membrane surface roughness, and made the membrane surface smooth, and the anti-pollution ability of the modified membrane was better relatively. EDS analysis reflected the main inorganic elements of membrane fouling were Fe, S, Si, Al, Ca, Mg contents and etc., it could be inferred that the inorganic foulants of membrane surface was possibly composed of various salts, such as ferric salt, silicon oxide gel, sulfate, metal sulfide, aluminium salt, calcium salt and magnesium salt, etc. Moreover, when compared with PVDF membrane, the oxygen content on the modified membranes before and after filtration has not been obviously changed, which may be explained that the anti-oxygen containing pollution resistance of the modified membrane was better than that of PVDF membrane in some level.3. Experiment study on the modified membrane in the treatment of steel cold-rolled emulsion oily wastewater According to the current status and water quality characteristics of a steel cold-rolled emulsion oily wastewater, followed by the new standard for the iron and steel industrial water pollutant discharge (GB13456-2012), a suitable combination process with ceramic membrane and electric catalytic and MBR was presented in the thesis, which could not only satisfied the requirements of discharge, but also could created excellent water-quality for regeneration.Using the trial production test equipment of the ZrO2/PVDF modified MBR, the treatment effects on the steel cold-rolled emulsion oily wastewater were studied through experiments. Under different operation conditions of water HRT and DO, the removals of COD and oil were investigated. The results showed when HRT at 10h, and DO at 2～4mg/L, the treatemnt effect was best. The COD removal was about 88～91%, effluent COD was almost 50～60mg/L. Meanwhile, the oil removal reached 90～92%, and effluent oil content was about 4～5mg/L. the water qualities of effluent met the requirements of the national new standard, and created advantages for next subsequent deep treatment.Finally, techno-economic analysis of engineering investment and treatment cost for the cold rolling wastewater in this plant was conducted. The estimates indicated the investment of the MBR engineering cost almost 780,000 yuan, the invest cost per tonne of wastewater was about 2,600 yuan, the operation cost per tonne was around 1.464 yuan. Compared MBR to the current traditional physicochemical process, the conclusions could be draw that, although the investment and operation cost of MBR were relatively higher, but it had the advantages of process stability, water quality and less land use, and it would be benefit to wastewater regeneration. Therefore, it was considered to be more comprehensive advantages and had a better prospect. |
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