Preparation Of PVDF/TPU Blend Membrane And Its Application To Dyeing Wastewater Treatment

In order to provide practical and theoretical solutions to the problems faced to Polyvinylidene fluoride (PVDF) hollow fiber membranes, preparing a novel PVDF/ Thermoplastic polyurethane (TPU) hollow fiber membrane, based on the design concept that the membrane structure and performance could be improved by adjusting the compatibility between PVDF and TPU blends, some aspects were studied, which included the studies on the compatibility of polymer blends, preparation and characterization of PVDF/TPU hollow fiber membranes, the effects on casting dope, membrane process, and influence factor in spinning and precipitation process; secondly, PVDF/TPU blend membrane applied in Hydrolysisacidrogenesis-sub membrane bioreactor process was used to treat reactive blue KN-R wastewater with addition of PAFS. A study on submerged membrane bioreactor (SMBR) with PVDF/TPU blend hollow fiber membranes for dyeing wastewater was carried out.For the first time the compatibility of PVDF and TPU was predicted and analyzed by means of calculation ofΔHm, Flory-huggins interaction parameters, Differential Scanning Calorimetry method and Dilute Solution Viscosity method respectively. It was demonstrated that PVDF and TPU possessed partial compatibility which was depended on and influenced by blend ratio, concentration of polymers in solution, temperature, etc. The compatibility of the systems with the percentage of PVDF in blends more than 60% was superior to systems with the percentage less than 60%. The compatibility in PVDF-rich phase was greater than in TPU-rich phase.The influence of the polymer concentration and PVDF/TPU ratio on such items was studied, as the viscosity of the casting solution, the compatibility, the cloud point, the gelation value, while as far as the system of DMAc-PVDF/TPU-H₂O was concerned, the formation mechanisms of the membrane were studied and explained based on thermodynamics and mass transfer dynamics. It was found that with the increase of polymer concentration, the viscosity of casting solution increased, the velocity of mass transfer between solvent and non solvent slowed down, which happened delay phase separation easily. Fewer big pores observed in the membrane structure, flux and porosity decreased. Due to the enhanced mutual function, molecule chain moved slowly. The diffusion was inhibited by TPU in the casting dope and the porosity reduced with the increase of the viscosity. The decrease of polymer concentration also reduced the viscosity of casting dope. Diffusion mass transfer between solvent and nonsolvent fasted, inducing spontaneous demixing and developing larger pores, while low polymer concentration resulted in poor mechanical properties. Cloud points experiments showed that with the increase of blends ratios, a very little amount of non-solvent can induce phase separation. When blends ratio reduced, the viscosity of casting dope increased, whereas the compatibility of the system reduced. Adding TPU to the casting dope can improve the membrane's mechanical properties. High concentration of TPU in casting dope made the separation too serious, water flux and porosity increased, while retention decreased sharply.According to the function of additives, it could be classified into high molecular weight organic additives and inorganic additives. In the system of Additives-DMAC-PVDF/TPU-H₂O, PVP was a complicated improver, which can prepare membranes with different property due to its high molecular weight. The separation property, microstructure and crystalline phase of membranes were characterized by bovine serum albumin (BSA) retention experiments, mechanical property scanning electron microscopy (SEM), Fourier transform infrared spectroscopy- attenuated total reflection (FTIR-ATR)and differential scanning calorimetric (DSC), respectively. The results showed that additives can affect Chemical Potential of solvent, change PVDF/TPU macromolecule soluble state. Additives also can affect the diffusion velocity between solvent and nonsolvent, forming different membrane structure. High PVP concentration would suppress macrovoid formation. PVP can not come to terms with the demixing enhancement, it was not because the PVP fails to induce thermodynamic enhancement but because it was overwhelmed by kinetic hindrance. Due to the increase of the dope viscosity, affected the exchange between Water and DMAc during the phase inversion and phase separation time increased and demixing delayed. Thus firstly the water flux increased while Crystallinity decreased, then flux reduced while crystallinity increased. PVP K30 and PEG6000 can reduce crystallinity of membrane significantly. Membranes added PVP K30 and PEG6000 possessed good retention and mechanical properties because the pores in membranes connected well. The membranes with 5 wt% content of PVP, comparing membranes without additive, flux improved from 9.25 L/m²Â·h to 346.73 L/m²Â·h, retention was from 82.54% to 86.88%, which can prepare excellent blend membranes with high water flux and retention..The influences of various parameters on the spinning technology included coagulation bath agent, coagulation bath temperature, bore liquid agent, bore liquid velocity, dry spinning gap distance, etc. Structure and performance of membranes were affected by spinning condition and the casting dope content. The dynamic diffusion was discussed. DMAc solvent as a coagulation bath, resulting in concrete slowing, chemical potential of diffusion mass transfer between solvent and non-solvent changed, which induced delay demixing, membrane structure changed from finger-like structure to spongy structure. Diffusion velocity increased with water as a coagulation bath. It is easy to desolventize and induce spontaneous demixing. Water flux increased with the development of macrovoid. The temperature of coagulation also effected solvent and non-solvent diffusion. At 25℃water flux and retention reached to their maximum. EtOH as bore liquid, different internal and external concerted velocity resulted in spongy layer location. With the increase of ethanol concentration, spongy layer removed to membrane outboard, finally forming macrovoid from the underneath of membrane skin to the bottom of a sublayer with water flux increase quickly Considering for other factors, when dry spinning gap distance being 14cm, water as bore liquid and velocity 2.5-3 ml/min, it can prepare excellent blend membranes with high water flux and retention.PVDF/TPU blends hollow fiber membranes with NaClO solvent post-treatment was studied and the results indicated that post-treatment with NaClO can enlarge the micropore on membrane surface, increase water flux significantly. NaClO had no distinct influence on membrane aperture size, which resulted in retention remained stable between 82% and 89%. PVDF/TPU/PVP blends hollow fiber membrane weight loss was below 3%, after membrane dipped in both acid and alkaline solution. Acid and alkaline soak had little effect on mechanical property. The results showed that PVDF/TPU blend hollow fiber membranes owned good ability of resistance to acid and alkali in the broad pH range. The membranes flux declined as the filtration time increased, followed by 60 minutes with a steady value. Reactive sapphire KN-R dyeing fluxes can be improved through increasing driving force, but excessive driving force would affect the membranes natural life, In the experiment, we would take 0.1 MPa as the transmembrane pressure, which also can remain high flux. Fouled membranes cleaned by water, membrane added PVP flux recovery higher than membrane free of PVP, It showed that PVP can improve hydrophilicity of membranes. Membranes modified by PVP had a more favorable antifouling performance and can be applied in wastewater treatment.Based on parallel experiment including biomorphic variety, particle diameter distributing in mixing sludge solution, PAFS addition can reduce membrane fouling. When PAFS was added in suludge, different kinds microorganism such as protozoan, metazoan had exited in sludge. Particle diameter increased, microorganism with particle diameter less than 15μm increased obviously, which can reduce membrane fouling and improve sludge characteristic. While the PVDF/TPU hollow fiber membrane was used in submerged membrane bioreactor (SMBR) process, Hydrolysisacidrogenesis-sub membrane bioreactor process was used to treat reactive sapphire KN-R and Polyvinyl Alcohol (PVA) dyeing wastewater. It showed different function. The results showed that decolorization was accomplished hydrolysisacidroginesis process in the beginning. COD was removed by SMBR system mostly. With the increase of running period, more and more coloring matter was flocculate in the sludge, which strengthened decolor bacteria culture, decolor bacteria can exist in the ferriferous and aluminous sludge, which resulted in the increase of color reduction. Based on the calculation result of dyestuff degradation dynamics parameters, COD of non-degradable pollutants account was only 7.6 % of the total COD. Based on the calculation result of dyestuff degradation dynamics parameters and the shortest bio-solid residence time, it can draw the conclusion that hydrolysisacidrogenesis-PAFS-SMBR process superior than anaerobic process and anaerobic-aerobic process. The fouling of membrane was mainly formed by gel polarity and sludge sediment. The alkali cleaning can reduce activated sludge sediment on the surface of membranes, which can ensure that SMBR system run stably for long time. It showed that a combined Hydrolysisacidroginesis-membrane bioreactor process adding flocculant PAFS was considered as a high efficient combination.