| Nanofiltration and ultrafiltration technology is used to replace traditional drinking water treatment process to meet the requirements of the strict water quality in some areas. Ultrafiltration was considered as an effective means to remove suspended partiCes, colloidal substances（0.1μm）, inorganic partiCes and microorganisms. However, serious ultra membrane fouling and limited removal efficiency of organic matter restricted the wide application of ultrafiltration technology. Thus coagulation pretreatment process can not only alleviate membrane fouling in some extent, but also can improve the removal efficiency of organic matter. Humic acid is one kind of polymer mixture with wide-range of molecular weight. The substances in various ranges of molecular weight have different influence extent on membrane fouling. However, coagulation pretreatment process has obvious distinction on removal efficiency for individual fractions of humic acid. The species of Al salt coagulant have some influence on the ultra membrane fouling and the removal efficiency of humic acid and other organic matter.Firstly, experiments regarding different fractions separated from humic acid and the impact of different humic acid fractions on membrane fouling were performed. Results showed the fractions with molecular weight of more than 50 kDa and less than 10 kDa were major part of humic acid, whereas the fractions with molecular weight of 1030kDa and 3050kDa were small portion of humic acid. The sequence of membrane fouling followed: the fraction with molecular weight of 3050kDa> the fraction with molecular weight of 1030kDa> the fraction with molecular weight of less than 10 kDa and more than 50 kDa.Then coagulation process was used to remove organic matter. For PAC, Al2（SO4）3 and Alb, coagulant dosage, raw water p H and floc breakage were applied to investigate the removal efficiency of each humic acid frantion and total removal efficiency of humic acid. The results showed that Alb achieved highest removal efficiency of humic acid in three coagulants. The dosage of 15 mg/L and the pH of 6 were the optimal conditions to remove humic acid for Alb. While Al2（SO4）3 achieved lowest removal efficiency of humic acid in three coagulants. The dosage of 20 mg/L and the pH of 6 were the optimal conditions to remove humic acid for Al2（SO4）3. The optimal coagulation condition for PAC was same with that for Al2（SO4）3.In this paper, the electronic camera was used to observe floc growth process in situ, and investigated floc morphology（floc average size, fractal dimension and strength） on ultrafiltration membrane pollution. This research focused on how to reduce membrane fouling by controlling floc morphology during coagulation. The results showed that flocs formed by Alb were small and dense. In three coagulants, the flocs formed by PAC have maximum size, whereas the flocs formed by Al2（SO4）3 have the loosest structure. With optimum coagulant dosage, the average size of flocs was largest and the removal efficiency of humic acid was best. It therefore could reduce membrane fouling effectively. Under neutral and alkaline conditions, the flocs formed during coagulation has larger size and looser structure, leading to loose, porosity and good permeability cake layer on membrane surface. Thus, membrane fouling was reduced effectively. For Alb, the flocs have maximum strength factor at 15mg/L dosage and thus, coagulation effluent has minimal impact on membrane fouling. It could infer that membrane fouling was positively correlated with floc size and strength factor, and was negatively correlated with fractal dimension of flocs. After breakage, additional PAC could increase the average size of flocs and reduce fractal dimension of flocs, generating large and loose flocs. It indicated the proportion of small flocs decreased and the proportion of large flocs increased, which can reduce the membrane fouling. |
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