Study Of PVA Modified Non-woven Preparation And Anti-fouling Performance

In recent years, Membrane Bioreactors (MBRs) have been widely used in wastewater treatment to achieve higher effluent quality, which is often difficult to be effectively met by conventional activated sludge process. The advantages of MBR are a high mixed liquid suspended solids (MLSS) concentration, simple process flow, small space occupation, high efficient separation of solid-liquid, a higher automatization, a lower excess sludge production. As a new and high efficient wastewater treatment process, MBR is attracting attention of water treatment pursuers in the world. But the major obstacles for the application of MBRs are the high cost of membrane filtration module and the issue of membrane fouling. So it is necessary to investigate the methods of decrease of membrane filtration module cost and reduction of membrane fouling. Thus, it can broaden its application fields if the crucial problems to be resolved.In this experiment, a low cost filtration material- hydrophobic nonwoven fabric (NWF) was used to replace high molecular membrane materials. The NWF is a local product. In order to reduce membrane fouling, the hydrophobic NWF surface was modified by polyvinyl alcohol (PVA) dip coating to prepare hydrophilic PVA-NWF that was used for wastewater treatment. In this work, the influence of membrane preparation processes and conditions on performance of PVA-NWF was investigated, anti-fouling characteristics of PVA-NWF was identified. Chemical structures and morphological changes of the NWF surface were characterized in detail by modern analysis technics i.e. attenuated total reflectance Fourier Transform Infrared (FTIR/ATR) spectroscopy, X-ray Photoelectron Spectroscopy (XPS), Scanning Electron Micrograph (SEM). This work includes:(1) To investigate the changes of pure water flux and static water contact angles with non hydrophilic pretreatment and hydrophilic pretreatment by PEG, and with the changes of the PVA concentrations, crystal concentration, dip coating time, heat treatment temperature and time. The results show that PVA can be equably coated on the NWF surface after hydrophilic pretreatment. But pure water membrane flux of pre treatment surface is obviously less than the flux of non pretreatment. Static contact angles of them are not big different. Pure water flux is decreased with the increase of PVA concentration, crosslink concentration, coating time, heat treatment temperature and time. PVA modification makes the contact angles of NWF surface decreased from 86°±1°(before modification) to 40°±1°(pretreatment) and 43°±3°(non pretreatment). The hydrophilicityof NWF surface is improved greatly.(2) To prepare membrane without hydrophilic pretreatment to investigate the influence of PVA concentration, crosslink concentrations, nominal size of NWF on pure water flux, rejection, immobilization, tensile strength and tear resistance. The results show that increasing the concentration of PVA and crosslinking, as well as decreasing nominal size of NWF, pure water flux is enhanced, and rejection of PAM (molecular weight 8.6 million) is increased. PVA immobilization on the NWF surface increases with increasing PVA concentration. The influence of variety of crosslinking concentration on immobilization is not obviously. In addition, Mechanical performance of NWF is increased after PVA modification. Both tensile strength and tear resistance are increased after modification, and they are influenced by PVA and crosslinking concentrations in coating solution.(3) After characterization and analysis of surface morphological and chemical structures by ATR-FTIR,XPS and SEM, the results show based on physical and chemical modification methods, i.e. absorption and sedimentation etc, the surface character and molecular structure of NWF are changed. The bases -OH and -C-O-C- are introduced to the NWF surface, and -OHs are on the surface in various forms. Interaction between molecules of PVA film is increased by chemical crosslinking. Basic structure of NWF is kept. There are two atoms (C, O) on the modified NWF surface, and three forms of C atom consist of C-H, C-O, O-C-O. Since O base is introduced on the NWF surface, relative content of O atom is enhanced obviously, and relative content of C atom is decreased. PVA Modification makes the pore size rate of NWF smaller and the surface smoother. An obvious interphase layer can be found in cross section of SEM, it reveals that NWF surface is coated by PVA film to form composite membrane.(4) Anti-fouling performance of composite membrane is investigated through static absorption of BSA and dynamic treatment of man-made wastewater. The results show that the prepared NWF at 0.8wt% of PVA can absorb least quantity of BSA. Compared with original NWF, the absorption of BSA is decreased by 83.4%. It reveals that PVA modification can restrict BSA absorption. Anti-fouling performance is enhanced. Membrane Fouling Index (MFI) of original and modified NWF is 0.9519 and 0.2238, respectively. Modified NWF has better anti-fouling performance. Correlation between EPS, P/C in soluble EPS (ESPs) composite, RH in activated sludge and fouling resistance is investigated by statistic analysis. The results show that the influence of EPSs, P/C in EPSs and RH on fouling resistance is very obvious. Pearson Correlation coefficients of original and modified NWF are 0.868/0.856, 0.840/0.866, 0.890/0.841. The modified surface can effectively restrict the absorption of EPSs, decrease P/C in EPSs, reduce sedimentation of activated sludge, and decrease fouling resistance.(5) PVA-NWF model is immerged in SMBR to be used for man-made wastewater treatment. The results show that PVA-NWF can reduce membrane fouling, can decrease the resistance of membrane fouling, but membrane fouling is still occurring during filtration. Filtration resistance is mainly from membrane fouling resistance. Based on the concept of specific fouling resistance and Darcy law that describes the relationship between flux and resistance during filtration driven by pressure, a module is established to explain therelationship between specific fouling resistance and time during filtration controlled byake resistance in SMBR of periodic operation:R_stf(t)=a(1-e^-t/b).The decline trend offlux can be predicted by this model, the expression is below J(t)=TMP/μ[R_m+a(1-e^-t/b)].This model is used to predict the decline trend of flux during pharmaceutical wastewater treatment. Compared with experimental data, the relative error is less than 10% at t>b, the relative error is less than 5% at t>2b. It shows that the model can predict the decline trend of flux during stable operation of SMBR. But the relative error is bigger during unstable operation at initial stage (t