Physicochemical pretreatment of seawater: fouling reduction and membrane characterization

The pretreatment of raw seawater is necessary to minimize the organic fouling of seawater reverse osmosis (SWRO) membranes. To predict the membrane fouling of the pretreated seawater, the modified fouling index (MFI) with ultrafiltration (UF) was investigated in terms of molecular weight distribution (MWD) and membrane characterization. The study was conducted with seawater drawn from Collioure, France. The concentration of total dissolved solids was 32,760 mg/L. The molecular weight (MW) of the initial seawater organic matter (SWOM) ranged from about 14160 Da to 280 Da. FeCl3 flocculation removed the majority of SWOM, while PAC adsorption could not remove the lowest MW fraction of organic matter (1110, 750 and 280 Da). The UF membranes with 30 kDa and 100 kDa MW cut-off removed the majority of of organic matter corresponding to the peaks 14,160 Da and 6560 Da in MWD. The MFI values obtained when using UF membranes of 30 kDa and 100 kDa with MF pretreatment were 19,700 s/L2 and 31,000 s/L2, respectively. The MFI values after pretreatments of FeCl3 flocculation and PAC adsorption significantly decreased to 6900 s/L2 and 6700 for 30 kDa UF and to 2300 s/L2 and 2500 s/L2 for 100 kDa UF, respectively. Some relation does exist for both membranes between the MFI–UF values and Spb values obtained during the first peiod of filtration (pore blocking). The pore blocking zone significantly decreased after flocculation and adsorption pretreatment. This suggests that the pore blocking can be used as an indicator to predict membrane propensity. The detailed membrane characterization on the clean and fouled membrane surface after MFI–UF experiments was made in terms of contact angle, zeta potential, functional group and microscopy.

The effect of different physicochemical pretreatment methods on MFI–UF was investigated. After each MFI experiment with pretreated seawater effluent the membrane was also characterized. The results led to the following conclusions: 1. The MW of the initial SWOM in seawater used ranged from about 14160 Da to 280 Da. 2. FeCl3 flocculation removed the majority of SWOM except 280 Da, while PAC adsorption could not remove 1110, 750 and 280 Da. Removal of different MWs by both UF membranes (30 and 100 kDa) showed a similar pattern. The UF membranes removed the majority of 14,160 Da and 6560 Da. 3. The MFI values of UF of 30 kDa and 100 kDa after MF pre-treatment were 19,700 s/L2 and 31,000 s/L2, respectively. This result, to be confirmed, shows that a small change in the membrane pore size (about 30%) is able to modify the characteristics of the cake being formed at the membrane surface. The MFI values after pretreatments of FeCl3 flocculation and PAC adsorption significantly decreased to 6900 s/L2 and 6700 for 30 kDa UF and to 2300 s/L2 and 2500 s/L2 for 100 kDa UF, respectively. The MFI–UF values look related to the Spb values for both membranes. The pore blocking zone significantly decrased after flocculation and adsorption pretreatment. Further experiments are performed with membranes of different (smaller) molecular weight cutoff to obtain precise uniormity concering the difference of the low MW fraction of the fouling. 4. The contact angle of UF of clean 30 kDa and 100 kDa was 94.2º and 92.4 º, respectively. The contact angles of fouled membranes were found to be more than 70º, suggesting that the short-term MFI–UF experiments did not change the hydrophobicity of the membranes. 5. After UF filtration, the intensity of the functional groups significantly decreased due to fouling. The fouled membrane without any pretreatment after both UF–MFI filtrations indicated the lowest intensity, while the intensity after the pretreatments of MF, flocculation and adsorption slightly decreased. 6. The zeta potential of the membrane decreased during the filtration of the pretreated effluents, while the zeta potential after flocculation pretreatment increased. 7. Both UF membrane surfaces were severely fouled with seawater without pretreatment, while the UF surface was preserved when a pretreat- ment of adsorption was adopted. A thick cake layer on the UF surfaces after flocculation was identified. The iron (Fe) content on the 30 kDa and 100 kDa UF increased by up to 1.39% and 4.84%, respectively.