Study On Ultrafiltration Membrane Fouling Alleviated By Combined Pre-ozonation And PAC Adsorption Pre-treatment

Ultrafiltration(UF)is the most promising “green” technoloogy in water treatreament field.UF can efficiently remove particulate matter and microorganism,which ensures the biological security of drinking water.However,two inevitable impediments limiting for UF are low removal of soluble organic matter and the development of membrane fouling in the process progress.To this end,pre-treatment before UF is often applied to improve the performance of organic matter abatement and control membrane fouling.The effects of individual or combined pre-treatment of powdered activated carbon(PAC)adsorption and pre-ozonation on the UF process were systematically studied in this dissertation.Effect of PAC size on membrane fouling was investigated using surface water as feed water of UF.PAC of smaller size adsorbed more organic matter(such as humic acid,HA),resulting in better control efficiency of membrane fouling.When PAC particles after adsorption were not removed from the feed water of UF,a combined PAC and organic matter of cake layer developed,leading to the increase of fitration resistance.Therefore,the elimination of organic-adsorbed PAC particles before UF benefited the alleivation of membrane fouling.During UF of surface water,the increase of calcium ion enhanced the attachment of natural organic matter(NOM)on the membrane surface,resulting in the deterioration of hydraulically irreversible fouling.By comparing the short term and long run of PAC adsoption UF process,the fouling mechanism of UF initial period is pore block caused by HA,moreover,the fouling mechanisms of long term run are the concentration effect fouling resulting from protein-like organic matter concentration and the formation of proteindominating cake layer.Coconut-based PAC could adsorb protein-like and HA organic matter in the pre-coagulated water,therefore,the fouling of concentration effect and hydralically irreversible fouling were ameliorated by coconut-based PAC.Two types of UF membranes,hydrophilic regenerated cellulose(RC)and hydrophobic polyethersulfone(PES)were used to evaluate the effect of pre-ozonation on the membrane fouling in the UF of secondary effluent.The physicochemical property changes of effluent organic matter(Ef OM)matrix including biopolymer fractions by pre-ozonation were systemically investigated.The increase of ozone dosage led to greater alleviation of flux decline for these two types of membranes.In terms of fouling resistance analysis,the reversible fouling resistances of RC and PES membranes were improved by pre-ozonation.The irreversible fouling resistances of RC membranes were enhanced by pre-ozonation,however,preozonation exhibited no significant impact on those of PES membranes.Additionally,the classical pore blocking model and the extended Derjaguin-Landau-Verwey-Overbeek(XDLVO)theory were employed to scrutinize the fouling alleviation mechanism caused by pre-ozonation.As a result,the overarching mechanisms of fouling reduction were attributed to the following key reasons:(1)Ozone degraded biopolymers of Ef OM including proteins and polysaccharides into smaller fractions,thereby increasing free energy of cohesion of Ef OM and rendering them more hydrophilic and more stable;(2)pre-ozonation augmented the interfacial free energy of adhesion between foulants and the RC/PES membranes,leading to the increase of repulsions and/or the decrease of attractions;and(3)pre-ozonation prolonged the transition from pore blocking to cake filtration that was a dominant fouling mechanism,thereby reducing fouling.The effects of individual or combined pre-ozonation and PAC adsorption pretreatment on the purification performance and membrane fouling,using secondary effluent and Suwannee River natural organic matter(SRNOM).In the combined pretreatment,PAC adsorption followed by pre-ozonation and pre-ozonation followed by PAC adsorption were compared.The physicochemical characteristic changes of organic matter caused by combined pretreatment were detailly investigated.The macromolecular of organic matter(such as polysaccharide)and tryptophan-like matters were better removed by the combined pretreatment,especially by preozonation followed by PAC adsorption(abbreviation: O3-PAC)pretreatment.The model analysis showed that the transitive volume,that altered the fouling mechanism from pore blocking to cake layer filtration,was increased by the combined pretreatment.This resulted in the postponement of cake layer formation.The interfacial free energy results indicated that the cohesion interactions of foulant-foulant was altered from attractive interaction to repulsive interaction,and attractive interaction between foulant and membrane was decreased by combined pretreatment especially by O3-PAC pretreatment.These results implied that foulant had lower potential to aggregate and attach on the membrane surface.