| Summary: | 碩士 === 國立臺灣大學 === 環境工程學研究所 === 97 === Membrane or pressure-driven processes are used to remove contaminants such as dissolved solids, nature organic matters, inorganic ions, and some other hazardous compounds from water. One problem with this practice is membrane fouling, which causes not only permeate flux decline but also product quality deterioration. This research studied NF membrane fouling, identified associated foulants, assessed fouling mechanisms by the modified Hermia model, and finally developed optimal operation conditions using the respond surface method (RSM). Filtration was conducted with a cross-flow module using membrane (NF270) in plate form. Kim-Men Tai Lake water (natural) and effluent from the rapid sand filter (SF) (treated) of Kin-Men Water Treatment Plant were used at various pH levels, transmembrane pressures and cross-flow velocities. The physico-chemical properties of the raw water and the SF effluent were determined using instruments such as dissolved organic carbon (DOC) analyzer, gel filtration chromatography (GFC), Fourier transform infrared spectroscopy (FTIR) and so on.
The results showed that the DOC concentration were 8.49 ± 0.22 and 6.19 ± 0.22 mg L-1 for the raw water and the SF effluent, respectively. The hydrophobic fraction (49.5 and 54.2% for the raw water and the SF effluent, respectively) was approximately the same as the hydrophilic fraction for both water samples. The HPOA fraction (30.2%) of the raw water was the highest, whereas the HPIA and the HPON fraction of the SF effluent (35 and 35.7%, respectively) were the predominant components. The NOMs of the raw water showed that 30.8% of its molecular weight was in the range of 5 to 10k and 31.2% was in the range of less than 1k Da. The major NOMs of the SF effluent had molecular weight 49.4% in the range of 1 to 5k Da.
Results showed that sudden flux decline occurred in 7 hours generally and pH had significant influence over flux change. As expected, the flux decline increased with time. At pH 5, the fouling was mainly caused by organic materials; while at pH 9.5, for both water samples, inorganic scaling (e.g., calcium sulfate or magnesium sulfate formation) may be the main cause of flux decline as seen from results of SEM-EDX analysis of the fouled membrane. In the case of SF effluent, there was much less flux decline (7.3, 14.6 and 12.9% for pH 5, 6.5, and 8, respectively) than that for raw water (over 20% for all levels of pH), except at pH 9.5 (e.g., 28.43%). The NOM rejection and flux decline of raw water samples at all pH values could be divided into two phases in time, i.e., 0-7 and 7-48 hours. At pH 5, in 7 hours, irreversible fouling was the main cause of permeate flux decline; while reversible fouling controlled the permeate flux decline at 7 – 48 hours. At pH 8, in 7 hours, reversible fouling was the main cause of permeate flux decline; while irreversible fouling controlled the permeate flux decline at 7-48 hours. However, this tendency was not observed with the SF effluent (i.e., whose flux decline curves did not intersect at the same point).
Standard blocking was not the fouling mechanism for both water samples at all pH, transmembrane pressure, and cross-flow velocity. This indicated that there was no adsorption of solute onto the inner walls of the membrane pores. However, for raw water, intermediate blocking may be the dominant fouling mechanism at all pH levels; whereas for SF effluent, gel layer formation may be the major fouling mechanism regardless of pH. Finally, identification of the best operation condition was attempted using RSM program. Transmembrane pressure of 556.50 kPa, cross-flow velocity of 0.44 m s-1, and pH at 7.76 was the optimal condition. Under this optimal condition, it could be predicted that flux decline would be 7.95% and DOC/UV254 removal would be 98.29% (Desirability 0.64).
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