| With the improvement of water quality standards and the development of membrane technology,the application value of ultrafiltration(UF)technology in drinking water treatment has been widely recognized.However,ultrafiltration membranes will inevitably be exposed to membrane fouling during purifying water.In addition,the ultrafiltration alone is not good for the removal of organic pollutants with small molecules,and the effluent quality cannot be further improved.Therefore,it is a research focus and hotspot in the field of ultrafiltration membrane technology to study how to improve the effluent quality and reduce membrane fouling through pretreatment.In this paper,the effects of coagulation,adsorption,oxidation pretreatment and their combination processes on the treatment efficiency and ultrafiltration membrane fouling during the operation of flat membrane and dynamic hollow fiber membrane were studied.The formation mechanism of membrane fouling under different pretreatment conditions and the treatment efficiency of different pollutants were discussed.The mechanism of fouling formation of ultrafiltration membrane was discussed.The main research results are as follows:1.pre-coagulation could significantly improve the removal efficiency of UV254 and DOC in the effluent of UF system,and reduce the content of disinfection by-products precursors in effluent.Resistance of cake layer is the main source of resistance on UF membrane.Coagulation pretreatment can remove macromolecular hydrophobic substances from raw water,significantly reduce resistance of cake layer,thereby reducing membrane fouling.Sedimentation tank can reduce the accumulation of resistance of cake layer,which is helpful to alleviate membrane fouling in pre-coagulation-ultrafiltration(C-UF)system2.Appropriate amount of polymer organic coagulant promoter can increase zeta potential of the system and enhance the role of electricity,thus significantly improving the removal rate of organic pollutants in the pretreatment process and the whole ultrafiltration system.At the same time,the organic polymer coagulant can increase the floc size,make the cake layer become loose,and effectively alleviate the membrane fouling in the process of ultrafiltration.3.Adsorption pretreatment can improve the removal efficiency of UV254 and DOC in UF system.Its main advantage lies in the removal efficiency of hydrophilic small molecular organic matter which can not be intercepted by UF membrane.In addition,powder activated carbon(PAC)adsorption pretreatment can effectively reduce the production of disinfection by-products.4.In the pre-adsorption-ultrafiltration(A-UF)system,when the dosage of PAC was lower(<100 mg/L),the effect of PAC on membrane fouling was not obvious,but when the dosage was higher(>200 mg/L),the removal rate of organic matter from the adsorption process increased,and the membrane fouling could be effectively alleviated.5.Ozone oxidation can destroy unsaturated structures such as aromatic rings and double bonds in organic matter,thus removing part of organic matter from water.The removal rate of UV254 increased with the increase of ozone dosage.The removal rate of dissolved organic carbon(DOC)reached the maximum(17.5%)at the dosage of 2.0 mgO3/L.Oxidation-coagulation(O-UF)system can also control the production of disinfection by-products after effluent disinfection.6.Ozone oxidation has little effect on the transmembrane pressure difference in ultrafiltration system.In the initial stage,the increase of the transmembrane pressure difference in the system with or without pretreatment is not much different.After 30 hours of operation,the increase of the transmembrane pressure difference in the O-UF system tends to slow down compared with that in the non-oxidized ultrafiltration system.7.Using coagulation and adsorption as the pretreatment of ultrafiltration process can enhance the removal efficiency of DOC and UV254.The removal efficiency of coagulation followed by adsorption-ultrafiltration(C-A-UF)system is higher than that of simultaneous coagulation-adsorption-ultrafiltration(C+A-UF)system.Both C-A-UF system and C+A-UF system have better removal efficiency for chlorine disinfection by-product precursors.8.Compared with all pretreatment methods,C-A-UF system and C-UF system have the lowest transmembrane pressure difference and the lightest membrane fouling.PAC can reduce the chemical adsorption between pollutants and functional groups on the membrane surface,thereby reducing membrane fouling.Polysaccharides can significantly increase membrane fouling during ultrafiltration.For raw water rich in polysaccharides,the transmembrane pressure difference of C-A-UF process is the smallest and the effect of polysaccharides is the smallest when compared with other kinds of raw water.9.Compared with the single pretreatment,the combined pretreatment of coagulation and oxidation can improve the removal rate of organic matter slightly.The removal rate of organic matter is the highest in the system of coagulation followed by oxidation-ultrafiltration(C-O-UF);the removal rate of organic matter in the system of oxidation followed by coagulation-ultrafiltration(O-C-UF)is similar to that in the C-UF system,slightly lower than that in the C-O-UF system;and the removal rate of organic matter in the system of O-UF is slightly lower than that in the C-O-UF system.The removal rate is the lowest.10.The removal rate of organic matter with molecular weight>100 kDa and 10-100 kDa is the highest by oxidation followed by coagulation pretreatment.Therefore,the growth rate of transmembrane pressure difference in C-O-UF system is slower than that in O-C-UF system,and the membrane fouling is the lightest.Polysaccharides can significantly aggravate membrane fouling in C-UF,O-UF and O-C-UF systems.C-O-UF system can inhibit the formation of gel layer before polysaccharide and reduce the membrane fouling caused by polysaccharide. |
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