| Ultrafiltration membrane separation technology has been increasingly concerned and widely employed in wastewater treatment due to its high removal efficiency, precision, simplicity and energy efficiency, etc. Membrane Bio-Reactor(MBR), which is used not only for efficient sludge separation but also in improving the efficiency and effect of bioreactor using ultrafiltration membrane instead of secondary sedimentation tank, is a cost effective wastewater treatment technology. Polyvinylidene fluoride(PVDF) membrane has gained increasing attention due to its excellent mechanical property, chemical stability, acid and alkali resistant property, simple preparation and low cost. However, decreasing membrane flux, frequent membrane cleaning and increasing energy consumption restrict the application of PVDF membrane in MBR due to its high hydrophobicity and high possibility of membrane fouling. Therefore, membrane fouling prevention and control is of great importance for the normal operation of MBR. The modification of membrane materials is regarded as an important way to increase membrane flux and decrease membrane fouling.In this study, 5 kinds of low temperature plasma modified methods were employed for the PVDF hollow fiber ultrafiltration membrane. Membrane flux, surface morphology, chemical composition and mechanical properties of modified membrane were investigated. The test methods of contact angle, synchronous measurement of membrane surface and membrane pore Zeta potential of PVDF hollow fiber membrane were developed. The antifouling performances of both modified and original membrane in MBR were investigated and compared with each other. Fouling model and the composition of fouling resistance were also analyzed. On the basis of physical and chemical characterization results, the influence law of modified membrane properties on membrane fouling was elucidated and the mechanism of low temperature plasma modification for improving membrane antifouling performance was explored. Finally, the performance of the optimal modified membrane in practical application was verified.PVDF hollow fiber ultrafiltration membrane was modified by plasma(PT), acrylic acid vapor phase graft initiated by nitrogen plasma(PIGGP), acrylic acid liquid phase graft initiated by nitrogen plasma(PIGLP),plasma chemical polymerization deposition of acrylic acid(SPCVD),mixed plasma chemical polymerization deposition of nitrogen and acrylic acid(MPCVD), respectively. Membrane intrinsic resistance, surface morphology, chemical composition,membrane pore size and mechanical properties of modified membrane were studied accordingly. The results showed that both the membrane flux and the change rate of corresponding membrane intrinsic resistance of the 5 plasma modified PVDF membranes increased but showed different variation rules with time. The membrane flux and change rates of the corresponding membrane intrinsic resistances of membrane modified with PT and SPCVD increased first and then decreased while those of modified membrane with other modified methods increased slowly. All the modified membranes reached a stable state nearly on the 20 th day. Finally, the sequence of average intrinsic resistance change rates of different plasma modified membranes was MPCVD > PT > PIGLP > PIGGP > SPCVD. Energy dispersive spectrum of X-ray(EDS) and Fourier transform infrared spectroscopy-attenuation total reflection(FTIR-ATR) analysis showed that the chemical composition and structure of membrane surface changed after plasma modification. It was observed that carboxylic acid, amide and hydroxyl groups were produced on membrane surface. Compared with original membrane, the carbon-nitrogen ratio(C/N) and carbon-oxygen ratio(C/O) of modified membrane decreased, among which, the latter decreased significantly. Scanning electron microscopy(SEM) detection showed that the membrane surface roughness degree of modified membranes increased and the membrane pore was blocked by layered or granular gel material. Mechanics property experiment results showed that the mechanical properties of PVDF membrane were improved after modification. The elastic modulus of modified membranes was greatly improved. The tensile strength of the modified membrane increased slightly. Whereas the break elongation of modified membranes decreased slightly, among which, PT modified membrane decreased significantly.Parallel experiments have been conducted to study the antifouling performance and cleaning effect of original PVDF membrane and 5 plasma modified membranes in MBR. The results showed that the flux decay of different membranes in MBR complied with the exponential decay law and the sequence of flux decay rates was: PIGGP<MPCVD <PIGLP< original membrane < PT < SPCVD. In terms of critical pressure, PIGLP modified membrane was about 7.7kPa while other modified membranes was between 14.2kPa and 12.1kPa. The initial adsorption fouling degree of membrane followed the sequence: SPCVD>original membrane>PT>PIGGP>PIGLP>MPCVD. Membrane fouling resistance distribution showed that the membrane fouling resistance in MBR mainly resulted from reversible filter cake fouling. However, there was a big difference between the irreversible fouling resistance and residual resistance. The irreversible fouling and residual resistance of PT, PIGLP, SPCVD, MPCVD modified membranes were less than 1.0×1011m-1, while those of PIGGP modified membrane and original membrane were almost twice that of other membranes. Their proportion in total resistance was larger than that of other modified membranes and that of original membrane the largest. MPCVD modified membrane showed the best antifouling performance in terms of membrane flux, pollution rate and fouling resistance,.The pollution mechanism of both modified and original membrane was analyzed with Hermia model. The results showed that in the initial 40 min membrane fouling was pore plugging pollution and the pollution degree of each membrane was in an order of PIGGP<MPCVD<original film<SPCVD<PIGLP<PT. The pollution mechanism was transformed into cake layer fouling subsequently. Through a mathematical model,it was found that the cake layer permeation of PIGGP and MPCVD modified membrane was smaller than that of original membrane, indicating that plasma modification not only influenced the thermodynamic behavior of the pollution in the membrane filtration process by changing the surface physical and chemical properties of the membrane but also impacted on the formation and evolution of the cake layer on the membrane surface significantly.SPASS 18.0 was employed to analyze the correlation between the pollution index factors reflecting the property and extent of membrane fouling like kln J-t(the slope of plug pollution model), kt/v-t(the slope of cake layer pollution model), τ(the constant of membrane pollution time), Rz(the total fouling resistance), Rrf(reversible membrane fouling resistance), Rirf(irreversible membrane fouling resistance), Rres(film residue fouling resistance), Ra(membrane adsorption resistance) and the index factors reflecting the characteristics of membrane physical and chemical performance like C/N, C/O, θ(water contact angle), Pmax(initial bubble pressure), ξs(Zeta-potential of membrane surface), ξp(Zeta-potential of membrane pore). The results showed that the characteristics of membrane pollution in MBR were mainly reversible resistance and irreversible fouling and residual contamination resulted from pore blocking. Moreover, the initial adsorption of membrane fouling promoted the above-mentioned resistance. Among the factors of membrane physical and chemical properties, Zeta-potential of both membrane surface and membrane pore had the most important and significant influence on membrane fouling. A greater Zeta-potential increased the electrostatic repulsion between membrane and pollutant and thus reduced the possible occurrence of pore blocking and cake layer pollution. The correlation between C/O and Rrf, Rirf and Rres showed that the increase of oxygen content on the membrane surface could increase the irreversible fouling and permanent irreversible residual pollution, indicating that the plasma modification of membrane should comply with the following conditions :controlling the effect of plasma etcher on the increase of membrane pore size, monomer selection favoring the increase of Zeta- potential of membrane surface and preventing the promotion effect of monomer chemical composition on microbial attachment.A verification experiment in engineering scale was conducted using 1m2 curtain membrane module made of MPCVD modified membrane with the best comprehensive performance. The results showed that the critical pressure of modified membrane increased by 3kPa. Pure water flux of membrane increased from 12.04% to 16.12% and initial adsorption fouling resistance decreased by 2/3. Cake resistance decreased and water production capacity was improved under the conditions of 20:1, 10:1, 40:1 gas-water ratio, especially under the low gas-water ratio condition, suggesting that MPCVD modified method showed great potential to be used in preventing the pollutant of MBR from approaching membrane surface. The cake layer was thinner and more permeable. Moreover, the average water production capacity of modified membrane was 1.58 times than that of original film. verifying that MPCVD modified method could effectively improve the anti-fouling performance of PVDF ultrafiltration membrane in MBR. |
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