Study On The Pollution Mechanism Of N0M Ultrafiltration Membrane By Backwashing Components

The components of backwashing water have remarkable influence on the process of UF membrane fouling.To study the effect of different backwash water component on PVDF UF membrane fouling,experiments were performed under dead-end filtration mode,humic acid(HA),Na+ and Ca2+ were applied to simulate low-molecular-weight organics,monovalent and bivalent mental cations in backwash solution.Humic acid,sodium alginate(SA)and bovine serum albumin(BSA)was regarded as the main typical foulant ofnatural organic matter(NOM)in real micro-polluted surface water.By dead-end filtration,effect of the components in backwashing solution on the flux of PVDF ultra filtration membrane and its recovery rate after backwashing was studied.With the aid of mass balance,reversible fouling and irreversible fouling on membrane wall after backwash were quantitatively analyzed in a precise way.Through the experiment comparison and analysis,it can be concluded that the small molecular foulant and the lagger one produce an even more compact cake layer,which leads to a sharp decline of membrane flux.The interaction between metal cations and NOM foulants is the dominant factor affecting membrane flux.There is a certain law lying in irreversible fouling accumulated by NOM components on the membrane surface that the protein material,BSA,makes the biggest contribution the irreversible fouling,following by the polysaccharide,SA,and the micro mo leculer humic substances,HA,causes minimal irreversible fouling.Through analysis of foulant with the law of conservation of mass,it is found that,after hydraulic cleaning,the cake layer becomes bulky and the membrane flux is restored,howerver,the foulants are not effectively removed,it is recompacted to the membrane surface by continual filtration.The divalent metal cation Ca2+ both in the feed soulution and in backwashing water can produce bridging action and electrostatic shielding effect between foulants and membrane,strengthening the interaction between membrane surface and foulants,then producing more irreversible fouling.As Na+ has stronger discharge capacity than Ca2+,it can get more opportunities to combine with NOM,thendecreasing the fouling caused by Ca2+ bridging effect,reducing the irreversible fouling.Na+ in the backwashing water also occurs the ion exchange with Ca2+ in the cake layer,destructing part of the bridge,increasing the reversibility of menbrane fouling.The presence of small molecules,HA in the backwashing solution alonewill lead to a certain fouling on membrane suralce.However,in the presence of Ca2+,the effect of HA on membrane surface fouling is enhanced obviously.On the one hand,Ca2+ bridging strengthens the adsorption of small molecules to the membrane surface.On the other,HA molecules and Ca2+ form even larger organic aggregates,blocking the wash channel,coming into being invalid wash hole,resulting in even more irreversible foulingobviously.