The Pretreatment Performance And Mechanism For Ultrafiltration Membrane Fouling Control In Drinking Water Treatment

Ultrafiltration?UF?has become an advanced technology for drinking water treatment since the last decade.It provides an effective removal of turbidity,particles and microorganisms from the feed water,which guarantees the drinking water security.However,the membrane fouling that occurs during the treatment process greatly restrict the wilder application of UF.In this research,several treatment units in a drinking water plant were taken as pretreatments to measure their ability in reliving the membrane fouling.Among them,effluent from coagulation/sedimentation still lead to the membrane fouling as the remained contaminants and coagulation flocs could deposit on the membrane.Adding sand filtration and active carbon adsorption could relive the fouling stepwise,while the effects were not very obvious.Chemical enhanced backwashing by HCl or NaClO showed a better performance than water backwashing,and among all the testing conditions,pretreatment of coagulation/sedimentation-sand filtration-active carbon adsorption combined with NaClO?10ppm as Cl2?backwashing was proved to have the best control of membrane fouling.By analyzing the composition of the foulants deposited on the membrane,polysaccharide was found to be the dominant part.Compared with other two NOM model compounds?humic acid and fulvic acid?,sodium alginate?as the representative of polysaccharide?had a much higher fouling ability under the same concertation.The pretreatments performance for polysaccharide fouling control was measured afterwards: sand filtration could only reject the particles so it was not effective to remove the dissolved sodium alginate;coagulation was able to remove sodium alginate from the water so as to eliminate the fouling;pre-oxidation by NaClO and O3 could change the structure of alginate and it was also proved to be effective.To better understand the mechanism of pre-oxidation in reliving the polysaccharide fouling,it was found that NaClO and O3 could both reduce the particle size of alginate-Ca complex,so that it could pass through the pores without being retained on the membrane,thus causing less fouling.By measuring the functions groups change in alginate molecules before and after oxidation,the particle size difference was found due to the structure transformation of carboxyl groups,which caused a lower ability to bound with Ca²⁺.Among the two oxidants,O3 was more effective than NaClO.