| Water pollution and shortage have become a critical and serious challenge to society progress and economy development in the world, especially in the coastal and economically developed areas. Nowadays, seawater desalination attracts extensive attention and is used as an important strategy to deal with the shortage of freshwater resources, because the product water supplied from seawater desalination is stable, high-quality and less affected by the climate. The high performance of seawater desalination system is guaranteed by the appropriately designed pretreatment which is to face the varying quality of raw seawater. It is great theoretic and practical importance to focus on Ultrafiltration (UF) as an effective alternative to conventional pretreatment system prior to seawater reverse osmosis (SWRO). To evaluate the performance of UF as the pretreatment method, membrane fouling in the filtration process should be considered. In this paper, the effects of inner diameter and molecular weight cut-off (MWCO) of the hollow fiber UF membranes on the UF performance for seawater in Qingdao, were systematically investigated, and experiments of seawater fraction using UF membranes were also carried out. To evaluate the reality of UF system, the critical operational mode of dead-end filtration and without backwash was applied. The main experimental contents and conclusions are as follows:1. In the operational mode of dead-end filtration and without backwash, hollow fiber UF membranes with the same material and MWCO yet different inner diameter (1.2 mm and 1.0 mm)were applied to treat seawater in Qingdao. Based on the results of the permeation flux and permeate quality in the same operating conditions, it indicated that UF II with a smaller inner fiber diameter had a higher performance for seawater, exhibiting a higher permeation, a lower potential of membrane fouling, even though a slightly high UV254 in permeate compared to UFI.2. Different cleaning strategies including acid washing, caustic washing, sodium hypochlorite washing, alkali-EDTA washing and ultrasonic cleaning, were used to restore the performance of the fouled UF membranes by seawater. Two parameters such as reversible fouling removal (R%) and pure water flux recovery (η) were applied to evaluate the efficiency of different cleaning strategies. The results showed that cleaning efficiency of Ultrasound and alkali-EDTA is more obvious, and the smaller inner diameter of the UFII cleaning efficiency is higher than UFI. In addition, characterization of cleaning efficiency of continuous filtration-cleaning operations, R% is more effective thanη; for the characterization of single filtration-cleaning experiments, R% andηwere similar to the symptoms.3. Modern characteristic means such as scanning electron microscope (SEM), infrared Spectroscopy (IR) and energy dispersive spectroscopy (EDS) were used to analyze the membrane fouling. The SEM pictures of the membrane surface showed that UF II fouling is more serious, and ultrasonic cleaning efficiency is more obvious. Although in this case, ultrasonic cleaning still can not remove the membrane fouling completely, which suggested that the irreversible pollution was occurred. The IR of foulants on the membrane surface indicated that the cleaning efficiency of ultrasonic and alkaline-EDTA washing were higher than other cleaning strategies, and lipid, polysaccharide, humus, lignin and other substances were the main ingredients of the foulants. The EDS of the membrane surface indicated that some metal elements mixed with the foulants, such as Na, K, Mg, Fe, etc.4. In the operational mode of dead-end filtration and without backwash, hollow fiber UF membranes with the same material and inner diameter yet different MWCO (67,50,30,6 kDa) were applied to treat seawater directly in Qingdao. The results showed that UF membrane with lower MWCO yielded a lower decrease of permeation flux, which indicated that the membrane with smaller MWCO has a lower potential of membrane fouling. 5. In the operational mode of dead-end filtration and without backwash, experiments of seawater fraction using UF membranes with different MWCO (67,50, 30,6 kDa) were also carried out. Compared to the experiments of seawater direct-ultrafiltration, the increases in flux of different UF membranes with MWCO of 50,30,6 kDa are7%,3%,2% in the seawater fraction process respectively. It indicates that larger than 67 kDa organic matter in seawater has a more obvious influence on the flux of UF membrane with MWCO of 50 kDa, and larger than 30 kDa organic matter in seawater has a minor influence on the flux of UF membrane with MWCO of 6 kDa.6. Compared to the experiments of seawater direct-ultrafiltration using the same UF membrane, both UV254 and median particle size in permeate are lower in the seawater fraction process. And, the difference of median particle size in permeate between different operations including seawater direct-ultrafiltration and seawater fraction increases with the decrease in MWCO of UF membrane. Moreover, there is a slight effect of different operations on the removal of organic matter in seawater, while for the removal of particle in seawater, the operation of seawater fraction is superior.
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