The Improving Effect Of Fouling Cake Layer For PVDF Ultrafiltration Membranes Modified By Plasma Treatment In MBR

The secondary sedimentation tank was replaced by a membrane separation device in the membrane bioreactor(MBR)to realize the efficient interception of the sludge flocs and solute molecules,it is a green and stable technology of the efficient biochemical wastewater treatment.The PVDF membranes was paid highly attention by many scholars around the world due to the advantages of good mechanical,chemical,biological stability and higher separation accuracy.The hydrophobic proteins and colloidal substances was adsorbed easily by the PVDF membrane due to its high hydrophobiccity in the process of filtering,which resulted in aggravating cake deposition pollution and shortening the working life and cleaning frequency of the membrane.Studying the appropriate surface modification method to improve the hydrophilic modification of the PVDF membrane is regarded as an effective way to strengthen the antifouling performance of the cake layer.In this paper,PVDF-HFM was modified by plasma treatment(PT),plasma induced grafted in gaseous phase(PIGGP),mixture plasma chemical vapor deposition(MPCVD),plasma induced grafted in liquid phase(PIGLP),respectively.While,the variety of functional groups,surface morphology,? potential and deionized water flux of the membrane before and after its modification were investigated.In the research of the operation effect of the modified membrane in MBR,the antifouling performance was compared of the different modified membranes,the composition of membrane resistance and the cleaning effect were analyzed,the fouling types of ultrafiltration membrane were defined.The influence of modified membrane properties on cake layer fouling was researched with the cake layer deposition-compaction model.Finally,the effect laws and mechanism of cake layer were explored at the various gas-water ratio under the constant pressure and the various transmembrane pressure under the constant gas-water ratio.Specific experiment conclusions are as follows:FT-IR analysis showed that the composition of chemical elements changed on the PIGGP,MPCVD and PIGLP modified membrane surface,which is due to the hydroxyl and carboxyl functional groups introduced successfully.The SEM image showed that the membrane surface roughness increased with N2 plasma modification and the membrane pores were sheltered by graft polymers with tiled or superimposed on the modified membrane surface.The deionized water flux indicating that the average intrinsic resistance Rm of modified membrane were decreased in varying degrees,which was in perfect accordance with the change of the membrane surface ? potential,and the sequence of modified membranes was MPCVD>PIGLP>PT>PIGGP.Therefore,the ionization and hydration degree of the membrane can be improved by grafting acrylic acid.The parallel experiment on the operation effect of the original and modified membrane in MBR was conducted,the result showed that: the specific flux attenuation characteristicsand the evolution of cake layer structure were consistent with critical flux.For the ability of initial antifouling performance,the sequence of membrane is PIGGP > PIGLP >MPCVD >original membrane > PT.The static adsorption resistance degree of membrane followed the sequence: PIGLP > original membrane > PIGGP > PT > MPCVD.Quantitative calculations and cleaning effect showed that membrane resistance increasing is mainly caused by the cake layer reversible deposition,the cake layer resistance and residual contamination of PIGGP and MPCVD modified membrane have a significant improvement.The result of analyzing the ? potential of pollutant and clean membranes indicated that ?potential of PIGGP and MPCVD modified membrane had little influence on before and after pollution.Furthermore,the ? potential before experiment is bigger than the value after cleaning by a narrow margin.So PIGGP and MPCVD modified membrane,according to specific flux attenuation,resistance distribution and ? potential,showed the best separation efficiency and the more stable physical and chemical characteristics.The effect of modification on the cake layer was explored with cake layer compaction model,which showed that the compaction value followed the sequence:PIGLP > PT >original membrane > PIGGP > MPCVD.According to the various gas-water ratio,it showed that the most economical aeration intensity which reduce the cake layer pollution effectively existed between 17:1 and 24:1.It was found that the cake layer resistance increasing is mainly caused by the polysaccharide materials by the analysis of EPS components at the various gas-water ratio.Under the various pressure and the constant gas-water ratio,the higher or lower transmembrane pressure will exacerbate the cake layer pollution or degrade the sludge filtration characteristics.Furthermore,PIGGP and MPCVD modified membrane could acquire less special cake layer resistance in the experiments.Together,the PIGGP and MPCVD modification methods not only induced adsorption and interception pollution by the chemical thermodynamic behavior but also impacted the formation and evolution of the cake layer structure by dynamics on the membrane surface significantly.Which would prohibit sludge draw close to near and deposit on the membrane surface,form thinner and looser structure of cake deposition layer,and strengthen the mass transfer velocity and permeability coefficient.