| Characteristic and structure of exopolysaccharide from biofouling layer of MBR were evaluated in this study. Relation of extracellular polymeric substances versus sludge properties in MBR and mechanism of membrane fouling were analysised. The theory of membrane fouling can be realized, the results were shown as follows:Separation and purification of extracellular polysaccharide from biofouling layer of MBR was studied in this paper. The amount of exopolysaccharide extracted by thermal treatment (80℃, 30 min) was 86%. Most of the protein was removed by enzyme- sevag method and the polysaccharide was purified by cellulose column chromatography. Analysis of polysaccharide by UV spectrum showed that there were no protein and nucleic acid.Exopolysaccharide was analyzed by HPLC, GC, FT-IR and SEM etc. The results showed that molecular weight (Mn) of exopolysaccharide was 2.6×106. FT-IR spectrum showed adsorption peaks of this polysaccharide. It was a complicated polysaccharide containing glucose, rhamnose, xylose and mannose with a multi-branched structure, mainly consisting of (1-3) and (1-6) glucosidic linkages. A method has been developed to give topography of polysaccharide by Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). It was found that, exopolysaccharide had a multi-branched structure. This phenomenon may be related to hydrogen bond of polysaccharide chains.Filtration characteristics and mechanism of membrane fouling were investigated.The fouling characteristics on PVDF membrane of exopolysaccharide were evaluated in this study. The relative flux was in the tendency to the exponential decay along with filtration time, and the filtration resistance increased with polysaccharide concentration. The relative flux decreased more quickly at higher pressure, and cake resistance increased at high pressure. The variation of filtration resistance was fit to the standard blocking filtration model in initial process, and then fit to cake filtration model in later process. Quantitative calculations showed that cake resistance made up over 90% of total resistance, cake resistance-limited dominated the most duration of microfiltration process. Membrane fouling can be mainly attributed to the polysaccharide deposition on the surface of membrane.The analysis of membrane fouling and cleaning of membrane polluted in submerged membrane bioreactor (SMBR). MLSS rose to 13g/L during the operation time, and extracellular polymeric substances has accumulated both in the mixed liquor and on the membrane which might have caused an increase in the viscosity of the mixed liquor and infitration resistance of membranes. The concentration of extracellular polymeric substances was 90mg/gMLSS in activated sludge mixed liquor. The linear correlations between extracellular polymeric substances content and viscosity of sludge was fairly good (r=0.9903), filtration resistance increased with concentration of extracellular polymeric substances and viscosity of sludge. Extracellular polymeric substances can cause membrane fouling severely worse, and it influenced characteristics of membrane filtration seriously.
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