Characteristics And Mechanisms Of Combined Fouling Of Ultrafiltration Membrane By Polysaccharide And Inorganic Particles

Membrane fouling is the main obstacle to the application of ultrafiltration membrane.In order to better understand and control membrane fouling,this study mainly focused on the characteristics and mechanisms of combined fouling on the ultrafiltration membrane during drinking water treatement.First,the composition and the structure of the membrane fouling layers were analysised.Results showed that polysaccharide and inorganic particles were the major membrane foulants of two chosen drinking water plants.Then the combined fouling characteristics and mechanisms of polysaccharide and different inorganic particles were studied.Significant interaction between Fe₂O₃ and polysaccharide was found.Therefore filterability and structure of the fouling layers of polysaccharide coexistingwith ferric iron in ultrafiltration membranewere further studied.High content of inorganic matters containing Fe,Ca,Si and Al elements and organic matters including polysaccharide and humic substances were found in the membrane foulants of drinking water plant A.The composition of inorganic matters in drinking water plant B were similar to plant A.However,the organic foulant of plant B were polysaccharide and protein.Based on these results,calcium alginate was chosen to be the modelpolysaccharide and Fe₂O₃,kaolinite,Al₂O₃ and SiO₂ were the model inorganic particles.Filtration results indicated that Fe₂O₃ could improve the filterability of calcium alginate fouling layer while kaolinite,Al₂O₃ and SiO₂ had an opposite effect.The different effects of the particles on the filterability were connected with the particle surface energy and its interaction with calcium alginate.Fe₂O₃ had a relatively high surface energy and strong interaction with calcium alginate,thus calcium alginate could connect with Fe₂O₃ on the combined fouling layer.The distribution of calcium alginate was evenand the pores among the particle layer had not been blocked.As a result,the filterability of the Fe₂O₃ and calcium alginate combined fouling layer was better than the calcium alginate layer.The surface energy and interaction ofkaolinite,Al₂O₃ and SiO₂ were weaker.Thereforethe interaction between these particles and calcium alginate on the combined fouling layer wasweaker.Calcium alginate distributed unevenly on these three particles fouling layers,a portion of which blocked the pores of the particles and the rest formed a gel layer on the surface or the bottom of the fouling layer.The filterability of these combined fouling layers was worse.The effect of ferric iron on the filterability and structure of calcium alginate fouling layer was further studied.Resulted showed that ferric iron could improve the filterability of calcium alginate fouling layer and high content ferric iron had a more significiant influence.Ferric iron-1 and ferric iron-2were considered here and ferric iron-1 had a more significant effect in improving the filterability.The aging of ferric iron could both happen in the alginate dispersion and on the well formed fouling layer.The change of the fouling layer filterability was due to the change of the fouing layer structure.Cake layer rather than gel layer was formed when high content fresh ferric iron existed and the porosity of the cake layer was much higher.