Silt Density Index (SDI) employing 0.45 μm membrane has been widely used to evaluate the fouling potential of feed water in RO applications. Although the SDI value provides criteria of RO pretreatment quality, RO membranes still suffer from fouling which is often caused by colloids and organics. The...
Silt Density Index (SDI) employing 0.45 μm membrane has been widely used to evaluate the fouling potential of feed water in RO applications. Although the SDI value provides criteria of RO pretreatment quality, RO membranes still suffer from fouling which is often caused by colloids and organics. Therefore, modified fouling index (MFI) was continuously developed to substitute SDI. As the MFI promoted as another standard fouling index, further discussion on MFI is still required to integrate recent proposals, including measurement and application criteria.
This research firstly provides a comprehensive review on the development of MFI, from its theoretical basis to plant scale application. The historical transitions on MFI determination method have been thoroughly revisited. Moreover, the applicability of MFI in actual desalination processes was highlighted through two distinctive approaches. First, the MFI was introduced to the pretreatment process to verify its usefulness and to explore the implications of the MFI values during the seawater desalination RO (SWRO) operation. Second, the increment of transmembrane pressure (TMP) during the RO process was predicted through MFI.
To verify the usefulness of such MFI, the feasibilities of multiple MFIs (i.e., MFI0.45, MFI-UF100 kDa and MFI-UF10 kDa) were studied (Chapter 3) in a full-scale UF/RO seawater desalination plant located in the Middle East. For a year, MFI measurement was conducted with real RO feed water and correlated with increments in differential pressure (DP) among RO processes. The analyzed MFIs showed higher correlations with RO performance, while water parameters and SDI showed relatively low correlations. In addition, the regression study showed that MFI-UF100 kDa, series with a 0.45 μm membrane, is most sensitive to RO DP variation. We could find the significance of small pore-size membrane employment for the prediction of fouling potential, as confirmed through special events such as red tide and ship movement. This research demonstrated the feasibility of multiple MFIs approach, conducted by simultaneous separation and evaluation, for overcoming the limitations of the existing fouling index, SDI. Guidelines for fouling estimation and corresponding pre-treatment designs, based on MMAS techniques, were delineated.
On the other hand, the current fouling indices such as MFI0.45, MFI-UF and multiple MFIs have suffered greatly from their inability to predict actual fouling potential primarily due to the erroneous interpretation of fouling mechanisms. Therefore, the new concept of cake fouling index (CFI) was developed to accurately evaluate true fouling cake layer resistance (Chapter 4). Our findings clearly demonstrated that the effect of pore blocking should be excluded during fouling index measurements to simulate real RO applications. Specifically, the CFI was determined through consecutive filtration tests by subtracting the flux decline of the secondary filtration from that of the first one to eliminate the effect of pore blocking. The results proved that CFI better predicts the degree of fouling rate in RO experiments than MFI. It was also revealed that it could be utilized as a useful tool for identifying and evaluating the fouling mechanisms. Through a comparison of MFI and CFI, it was shown that pore blocking was enhanced as much as cake formation when pH decreased, while divalent cations (Ca2+) increased only cake formation on the membrane surface.
Finally, the MFI was further applied into the ultrapure water (UPW) production system to investigate the feasibility of UV as a pretreatment of RO, focusing on inorganic pollutants under UV exposure (Chapter 5). In this section, an appropriate fouling index using UF membrane, measured at constant flux mode, was developed and employed, which can effectively evaluate the fouling potential of UPW pretreatment process. This MFI-UFflux showed that UV radiation effectively degraded low molecular organic matter (e.g., IPA, a representative organic residual of spent UPW) and thus reduced organic fouling potential in RO. However, in the case of inorganic pollutants (e.g., silica), MFI-UFflux was increased after UV exposure. This phenomenon was intensified with medium pressure UV lamp compared to low pressure one, indicating that higher energy radiation enhanced silica fouling. This observation was verified by zeta analysis which proved that UV radiation derived the agglomeration of silica particles and thus increased the resistance of its fouling layer. The finding from fouling index measurements was also manifested in RO fouling tests, showing severe flux decline after UV radiation.
This research concluded with suggestions for future research to achieve MFI standardization through analysis of the results from the latest researches and plant verifications, thereby providing more substantial guidelines for improving the feasibility of RO membrane process. The improvement and application of MFI studies are expected to provide more precise information about RO membrane fouling, especially for the design of effective pretreatment processes.
Silt Density Index (SDI) employing 0.45 μm membrane has been widely used to evaluate the fouling potential of feed water in RO applications. Although the SDI value provides criteria of RO pretreatment quality, RO membranes still suffer from fouling which is often caused by colloids and organics. Therefore, modified fouling index (MFI) was continuously developed to substitute SDI. As the MFI promoted as another standard fouling index, further discussion on MFI is still required to integrate recent proposals, including measurement and application criteria.
This research firstly provides a comprehensive review on the development of MFI, from its theoretical basis to plant scale application. The historical transitions on MFI determination method have been thoroughly revisited. Moreover, the applicability of MFI in actual desalination processes was highlighted through two distinctive approaches. First, the MFI was introduced to the pretreatment process to verify its usefulness and to explore the implications of the MFI values during the seawater desalination RO (SWRO) operation. Second, the increment of transmembrane pressure (TMP) during the RO process was predicted through MFI.
To verify the usefulness of such MFI, the feasibilities of multiple MFIs (i.e., MFI0.45, MFI-UF100 kDa and MFI-UF10 kDa) were studied (Chapter 3) in a full-scale UF/RO seawater desalination plant located in the Middle East. For a year, MFI measurement was conducted with real RO feed water and correlated with increments in differential pressure (DP) among RO processes. The analyzed MFIs showed higher correlations with RO performance, while water parameters and SDI showed relatively low correlations. In addition, the regression study showed that MFI-UF100 kDa, series with a 0.45 μm membrane, is most sensitive to RO DP variation. We could find the significance of small pore-size membrane employment for the prediction of fouling potential, as confirmed through special events such as red tide and ship movement. This research demonstrated the feasibility of multiple MFIs approach, conducted by simultaneous separation and evaluation, for overcoming the limitations of the existing fouling index, SDI. Guidelines for fouling estimation and corresponding pre-treatment designs, based on MMAS techniques, were delineated.
On the other hand, the current fouling indices such as MFI0.45, MFI-UF and multiple MFIs have suffered greatly from their inability to predict actual fouling potential primarily due to the erroneous interpretation of fouling mechanisms. Therefore, the new concept of cake fouling index (CFI) was developed to accurately evaluate true fouling cake layer resistance (Chapter 4). Our findings clearly demonstrated that the effect of pore blocking should be excluded during fouling index measurements to simulate real RO applications. Specifically, the CFI was determined through consecutive filtration tests by subtracting the flux decline of the secondary filtration from that of the first one to eliminate the effect of pore blocking. The results proved that CFI better predicts the degree of fouling rate in RO experiments than MFI. It was also revealed that it could be utilized as a useful tool for identifying and evaluating the fouling mechanisms. Through a comparison of MFI and CFI, it was shown that pore blocking was enhanced as much as cake formation when pH decreased, while divalent cations (Ca2+) increased only cake formation on the membrane surface.
Finally, the MFI was further applied into the ultrapure water (UPW) production system to investigate the feasibility of UV as a pretreatment of RO, focusing on inorganic pollutants under UV exposure (Chapter 5). In this section, an appropriate fouling index using UF membrane, measured at constant flux mode, was developed and employed, which can effectively evaluate the fouling potential of UPW pretreatment process. This MFI-UFflux showed that UV radiation effectively degraded low molecular organic matter (e.g., IPA, a representative organic residual of spent UPW) and thus reduced organic fouling potential in RO. However, in the case of inorganic pollutants (e.g., silica), MFI-UFflux was increased after UV exposure. This phenomenon was intensified with medium pressure UV lamp compared to low pressure one, indicating that higher energy radiation enhanced silica fouling. This observation was verified by zeta analysis which proved that UV radiation derived the agglomeration of silica particles and thus increased the resistance of its fouling layer. The finding from fouling index measurements was also manifested in RO fouling tests, showing severe flux decline after UV radiation.
This research concluded with suggestions for future research to achieve MFI standardization through analysis of the results from the latest researches and plant verifications, thereby providing more substantial guidelines for improving the feasibility of RO membrane process. The improvement and application of MFI studies are expected to provide more precise information about RO membrane fouling, especially for the design of effective pretreatment processes.
주제어
#Fouling index RO membrane fouling Modified fouling index (MFI) Silt density index (SDI) Cake fouling index (CFI) Fouling mechanism Seawater RO desalination Multiple MFI measurements Pretreatment process
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