| Membrane fouling is one of the most important obstacles hindering the wide application of nanofiltration membranes,and to reveal the in-situ formation of membrane fouling and to improve the anti-fouling performance of nanofiltration membranes are important ways to improve the application efficiency of nanofiltration membrane in water treatment.However,most of the studies focus on the static characteristics of the fouling layer,and there are few studies studying on the dynamic behavior of membrane fouling with time,and there are few reports on the relationship between membrane surface morphology and fouling behavior.Therefore,in-situ monitor techniques were used to non-intrusively reveal thefouling behavior and the influence of the cross-flow velocities on membrane fouling.Based on this,effect of the surface morphology on fouling was studied and a kind of patterned nanofiltration membrane was introduced to achieve high anti-fouling performance.Then,the anti-fouling mechanism of the optimized membrane was analyzed based on the hydraulic analysis.Firstly,the initial stage of the fouling and the long-term development process was visualized by the in-situ Micro Laser-Induced Fluorescence(Micro-LIF)technique and Optical Coherence Tomography(OCT)technique,respectively.As a result,the solute concentration distribution on the membrane surface was successfully determined from the LIF images,while the 3D view of the fouling layer development and top view of the pollutant deposition on the membrane surface was monitored using the OCT technique.Furthermore,a validated Computational Fluid Dynamics(CFD)model was established based on the visualization characterization results,to simulate the initial dynamic concentration polarization and study the role of surface morphology on the fouling reduction.It is found that compared with the flat membrane,the aggregation concentration within the CP layer was reduced by 13.0%,the polarization factor was reduced by 52.0%,and the mass transfer coefficient was increased by 3.3 times on the surface of morphological nanofiltration membranes,which indicates that the optimization of membrane surface morphology plays an important role in controlling fouling.Secondly,a type of enhanced nanostructure was created on a nanofiltration membrane surface by addition of the sodium bicarbonate during interfacial polymerization.The permeation water flux of the NF membrane with the enhanced nanostructure(E-NF)was nearly triple that of the control NF membrane(C-NF)membrane,whilst the rejection for salts was also improved.More importantly,the total resistance of the fouling layer formed on the surface of E-NF membrane decreased by30.8% and 57.5% respectively,while the flux reduction rate of E-NF membrane decreased by 2.0% and 15.6% respectively compared with C-NF membrane and S-NF membrane.The results of three-dimensional OCT showed that the surface fouling coverage of E-NF membrane was 3.0% and 29.0% lower than that of C-NF membrane and S-NF membrane,respectively.The results show that the anti-fouling performance of the E-NF membrane was improved to some extent.Thirdly,to further improve the anti-fouling performance of NF membranes,a kind of NF membrane possessing micro-eggette shaped patterns(M-NF)was fabricated on the patterned porous substrate obtained by micro-molding phase inversion.Microscopic characterization and crosslinking analysis confirmed the formation of uniform microsized(8.5 μm in width and 4 μm in height)patterns and thin-film polyamide layer.Filtration performance of the M-NF membrane against mono/di-valent salts and colloid particles was compared with the E-NF membrane and the in-house fabricated NF membrane(C-NF)which served as the control.The M-NF membrane outperformed the C-NF and E-NF membranes,demonstrating the highest rejection and anti-fouling ability.Compared with E-NF membrane,the flux of M-NF membrane was recovered by 19.4%,the fouling resistance decreased by 45.8%,and the irreversible fouling resistance decreased by 81.6%.Finally,the anti-fouling mechanism of the micro-patterned membrane(M-NF)was studied by investigating the micro-flow filed and stress analysis of feed particles.With the assistant of the CFD model,the dynamic flow field,velocity profiles,and particle deposition trajectory were studied under various hydraulic conditions between the control flat membrane and M-NF membrane.Analysis of membrane surface flow field showed that the flow field near the micro ridge-valley morphology of M-MF membrane surface was disturbed and the shear force was enhanced,which effectively prevented the deposition of trapped pollutants on the membrane and reduced membrane fouling.It is found that the hydrulic forces exerted on the particles plays an important role in preventing particle deposition.Compared with the flat membrane surface,the hydraulic lift of the fouling particles on the micro-morphology of M-NF membrane surface was increased by 2.3 ～ 2.6 times,while the drag force was significantly reduced.This study shows that the micro morphology plays an important role in improving the pollutant removal rate and anti-fouling performance of the nanofiltration membrane,and provides a potential and efficient method to improve the performance of composite nanofiltration membranes. |
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