| Ultrafiltration membrane (UF) has received more and more application in surface watertreatment, it’s gotten consensus that UF could ensure the quality of effluent water. At present,in the process of water purification, membrane has been polluted inevitably. On the one hand,along with the increased membrane pressure, the water’s production cost has been becominghigher; on the other hand, irreversible membrane fouling would be serious, that may lead tochange of membrane module. How to reduce membrane fouling under the premise ofensuring water quality, it’s just the emphasis and difficulty in the field of water treatment.Pretreatment could effectively prevent the membrane fouling, the study selected therepresentative and operational ones-coagulation pretreatment, powder activated carbon (PAC)adsorption and coagulation-PAC adsorption, and investigated the effect those pretreatmentsusing different methods and different dosing quantity, in respectively combination with theUF treatment process, focused on the characteristics of the preprocessing method to preventmembrane fouling and its mechanism. The conclusions are attained as follows:(1)While raw water’s turbidity was within particular scope, raw waters with differentturbidity was filtered by UF membrane, this study discovered the removal of organic matterwas mainly focused on the non soluble organic matter, the less removal on soluble organicmatter. Compared with cross-flow filtration, we studied the effect that UF membrane directlyfiltered raw water with different turbidity with the dead end filtration, the membrane fluxcould be improved obviously. In the eighth Filtering cycle, the membrane flux of cross-flowfiltration could be recovered to68.2%, but the flux could be up to91.5%. Meanwhilecompared the results of filtering raw water with higher turbidity with the low turbidity one,the results showed that in the initial filtering stage of higher turbidity one, the flux could getcompletely recovery through the back flush; for the latter one, the initial flux could recoverafter the back flush, but as you progress through the filter, the flux’s falling trend was obvious.The possible reason might be that high turbidity could form a thick and loose filter cake layerfiltering with dead end flow mode, it could adsorb and intercept some potential pollution,colloid and fine suspended solids in particular. Directly filtering this raw water (havingfiltered by0.45μm microfiltration membrane) found that lower than1k Da organic mattercould be easily adsorbed by film hole. Although directly filtering high turbidity raw water,could effectively form loose filter cake layer on membrane surface, after a long run, thecolloid and solid organic matter may be deposited on the membrane surface, at the sametime, the lower than1k Da organic matter could go into and long-term accumulate in the filmhole, it was difficult to be removed and became the main material of membrane fouling. (2)While using PAC pretreatment process, the test found that the removal rate ofdissolving organic matter improved with increasing PAC quantity. DOC adsorbed by PACmainly focused on the lower than1k Da organic matter, but absorbed UV254was primarily inthe range of30kDa~1kDa, and most in the range of10kDa~1kDa. UF of raw water incombination with PAC treatment process, while raw water with low turbidity, although PACpretreatment nearly could not improve initial flux, the membrane flux has improvedsignificantly. And on the other hand, filtering raw water with high turbidity, the initial fluxcould be effectively improved, but the membrane flux was not. In the case of same PACquantity, hydrophilic modification of UF membrane was more obvious filtering the raw watercontaining less solid substances. Further analysis showed that the small molecule organicmatter and colloid is the main material of membrane fouling.(3)While using coagulation pretreatment process, the study found that removed DOCwas mainly concentrated in the range of higher than30kDa and lower than1kDa. Whileadding PAC and coagulant (PAC was first adding), removed DOC mainly focused on thethree ranges, higher than30kDa,10kDa~1kDa and lower than1kDa. Removed UV254mainly focused on the three ranges,30kDa~10kDa,10kDa~1kDa and lower than1kDa.Coagulation combined with membrane, which could effectively slow down the falling trendof membrane flux, and properly increased the dosing of coagulant, the membrane flux woulddecline more slowly. At the same time, the influence of coagulation pretreatment tomembrane flux existed a certain correlation between raw water turbidity, for the raw waterwith low turbidity, coagulation pretreatment could more effectively slow the attenuation ofmembrane flux. Than coagulating fluid improved membrane flux more significantly than thatupper fluid. Under the condition of separately adding Fecl3coagulant, coagulationpretreatment could improve the recovery level of membrane flux, but the falling trend ofmembrane flux could not be obviously improved. While adding PAC and coagulant in thesame time, dosing sequence had effect on membrane flux, membrane flux declined moreslowly under the condition of firstly adding PAC.(4)Compared every experiment, we found that removed the interference of solid matterfirstly, the best recovery extent of membrane flux was UF directly filtered raw water, the nextones were coagulation-0.45μm-UF, PAC-0.45μm-UF. The solid organic matter was morelikely to lead to membrane fouling than the small molecular organic compounds.Generally speaking, for Lanzhou section of upper Yellow River raw water, the processconsisted of ‘pre-precipitation, PAC and Fecl3coagulation’ could be more effective to slowmembrane fouling. |
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