Study On The Characteristics Of Organic Matter In Raw Water And Its Membrane Fouling During The Ultrafiltration

The nature of organic matter in raw water is often different due to the existence of different regions,seasonal climate,and surrounding environment,and the fouling behavior of these organic matters to the ultrafiltration membrane is more complicated.Therefore,understanding the characteristics of natural organic matter（NOM）in raw water and conducting its research on membrane fouling will provide a reference for alleviating membrane fouling and promoting the application of membrane technology.In this study,the Ganjiang River and Poyang Lake in Jiangxi Province were taken as water sources several times.The NOM in the raw water were classified for hydrophilic and hydrophobic properties by Solid-Phase Extraction technology.The hydrophilic and hydrophobic properties of NOM in different raw waters were studied.Ultrafiltration experiments were performed using the classified water sample.Fouling characteristics of two kinds of polyvinylidene fluoride（PVDF）ultrafiltration membranes（cut-off molecular weights of 100kDa and 50kDa,respectively）were investigated by NOM with different hydrophilic and hydrophobic properties.Meanswhile,based on the molecular weight distribution,fluorescence characteristics and hydrophilic and hydrophobic properties of NOM in raw water,combined with Hermia model,the mechanism of membrane fouling caused by organic matter in different raw water was analyzed.The main findings are as follows:Studies on the characteristics of organic matter in different raw waters showed that in terms of conventional water quality indicators,the turbidity,UV₂₅₄,DOC,and zeta potential of the two raw waters during the water intake are:Ganjiang River:21.22 NTU,0.2067 cm^-1,4.824 mg/L,-20.19 mV;Poyang Lake:7.631 NTU,0.1410 cm^-1,4.860 mg/L,-23.01 mV.In terms of hydrophilic and hydrophobic properties,the proportions of hydrophobic fraction（HPO）,transphilic fraction（TPI）,neutral hydrophilic fraction（N-HPI）,and charged hydrophilic fraction（C-HPI）components in the raw water（based on DOC）were:Ganjiang River:40.06%,18.91%,29.69%,11.34%;Poyang Lake:34.98%,15.31%,29.92%,19.78%.The relative magnitudes of the SUVA values of the components in the raw water of the Ganjiang River were:C-HPI(3.79 L·mg^-1·m^-1)>HPO(2.34 L·mg^-1·m^-1)>TPI(1.73 L·mg^-1·m-1)>N-HPI(0.70 L·mg^-1·m^-1),and the SUVA values of the HPO and TPI components in the Poyang Lake(1.49 L·mg^-1·m^-1,1.51 L·mg^-1·m^-1),respectively)were slightly higher than the C-HPI and N-HPI(respectively 1.00 L·mg^-1·m^-1,0.70 L·mg^-1·m^-1).In terms of molecular weight distribution,the molecular weights of NOM in the raw water of the Ganjiang River and Poyang Lake showed a similar law,thosewere,they were mainly medium molecular weight（1～10kDa）organic matter,and also contained a small amount of small molecular weight（<1000Da）organic matter.However,there was no responce in the large molecular weight（>10kDa）region.From the fluorescence properties of NOM,it could be seen that the two raw waters were mainly humic-like hydrophobic organic matter such as fulvic acid and humic acid,but the protein-like fluorescence peaks in the raw water of Poyang Lake were more obvious than the raw water of Ganjiang River.The HPO and TPI components mostly contained fulvic and humic acids,but the positions of the fluorescence peaks in the two components were different.The N-HPI and C-HPI components were mainly hydrophilic protein substances and metabolites of soluble microorganisms（such as polysaccharides,proteins,etc.）,and also contained a small amount of humus substances.In the study of the fouling characteristics of ultrafiltration membranes,theraw water of Ganjiang River with different initial UV₂₅₄ passing through the same type of molecular weight ultrafiltration membrane continuously had a similar specific flux change trend.The effect of the same raw water and components on the specific flux changes of the two kinds of ultrafiltration membranes with molecular weight cutoff was different,and different raw waters were also different.When the Ganjiang River’s raw water and each component passed through the 100kDa ultrafiltration membrane,the order of the decline in the specific flux was:N-HPI>HPO>raw water>TPI>C-HPI,and the terminal specific flux caused by the N-HPI and C-HPI components were:0.71 and 0.82,respectively.When they past through a 50kDa ultrafiltration membrane,raw water would cause a more serious decrease in flux,and the end specific flux value caused by raw water is 0.63.As for Poyang Lake,the raw water and its components had similar effects on the decline of the specific flux of the two ultrafiltration membranes.The order of the decline of the specific flux is raw water>N-HPI>HPO>TPI>C-HPI.The terminal specific flux values of raw water to the two ultrafiltration membranes were 0.70 and 0.81,respectively.In terms of organic matter retention,as for UV₂₅₄,the Ganjiang River showed that the C-HPI components were more likely to be retained by two ultrafiltration membranes.After two cycles,the retention rates were 44.04%and 50.24%.However,the rejection rates of raw water and other components were lower,and the retention efficiency of TPI components in the two ultrafiltration membranes were the lowest,which were:8.14%and 13.57%.And Poyang Lake had similar retention laws.In terms of DOC,the two types of ultrafiltration membranes were more inclined to retain the hydrophobic organics,especially the HPO components,which in the raw water of Ganjiang River their highest removal rates are:27.34%and 38.54%.Additionally,the two ultrafiltration membranes also had higher rejection rates for hydrophilic organics in Poyang Lake,among which the rejection rates for the C-HPI components are 13.17%and18.39%,respectively.In the study of reversibility of membrane fouling,the HPO and N-HPI components in the Ganjiang River were more likely to cause reversible fouling to 100kDa ultrafiltration membranes,while TPI and C-HPI components were more likely to form irreversible pollution,while 50kDa ultrafiltration membranes were the opposite.For Poyang Lake,each component was more inclined to form irreversible fouling to 100kDa ultrafiltration membrane,and reversible foulingwas more formed on 50kDa ultrafiltration membrane.The test results of membrane fouling resistance showed that the ultrafiltration test of each group is at the stage dominated by the membrane’s inherent fouling resistance,and its value was between 4.5×10¹¹～7.8×10¹¹m^-1.The contribution of reversible fouling resistance and irreversible fouling resistance were small,with values ranging from 0 to 3.0×10¹¹m^-1.Studies on the identification and mechanism analysis of membrane fouling substances had shown that humic acid and fulvic acid organics and biological macromolecular organics,which were hydrophobic medium-and large-molecular-weight,were the major substances that cause membrane flux decline.Most of this organic matter could be washed by hydraulic recoil.Polysaccharides and protein organics would also contribute to membrane fouling.These organics were attached to the inner wall of membrane pores,and most of them were irreversible membrane fouling.The fitting results of the Hermia membrane pollution model showed that the membrane pollution of the two raw waters and each component were not caused by a single membrane pollution mechanism,but a combination of multiple mechanisms.Membrane fouling mechanism,such as complete blockage,middle blockage and cake filtration would also contribute to a certain degree of membrane fouling.