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Abstract
Fouling indices have been extensively investigated in membrane applications, for evaluating the feed-water fouling potential and providing guidelines for the pretreatment process. For the past few decades, the silt density index (SDI) has been adopted as the most mature fouling index in many seawater reverse osmosis desalination plants. Recently, a modified fouling index (MFI) has gained attention since it compensates for the defects of the SDI. Its publication by ASTM Standard promoted more settlements in practical applications. To gain insight into the use of a membrane-fouling index, this paper reviewed SDI and MFI applications in laboratory-, pilot-, and plant-scale verifications, under conditions likely to be encountered in seawater desalination. The main focus was on the historical development of the fouling indices from theoretical basis to full-scale applications and the identification of future opportunities to expand their reliability in real applications. In particular, the practical implications associated with plant operation were documented to support an in-depth understanding of the MFI values.
Abbreviations
| CA | = | Hydrophilic, mixed cellulose and acetate |
| CEOP | = | Cake enhanced osmotic pressure (bar) |
| CFS-MFI | = | Crossflow sampler-modified fouling index (s L−2) |
| CFS-MFIUF | = | Crossflow sampler-modified fouling index ultrafiltration (s L−2) |
| DAF | = | Dissolved air flotation |
| DMF | = | Dual media filtration |
| DP | = | Differential pressure |
| FDR | = | Flux decline rate |
| GMF | = | Granular media filtration |
| ERI-PX | = | Energy recovery incorporated – pressure exchanger |
| HAB | = | Harmful algal bloom |
| MF | = | Microfiltration |
| MFI | = | Modified fouling index (s L−2) |
| MFI0.45 | = | Modified fouling index using a 0.45 μm membrane filter (s L−2) |
| MFIUF | = | Modified fouling index ultrafiltration (s L−2) |
| MFIUF-flux | = | Modified fouling index ultrafiltration measured under constant flux condition (s L−2) |
| MFIUF-pressure | = | Modified fouling index ultrafiltration measured under constant pressure condition (s L−2) |
| MFINF-pressure | = | Modified fouling index nanofiltration Ultrafiltration measured under constant pressure condition (s L−2) |
| MFINF-CRS | = | Modified fouling index nanofiltration-cake resistance simulator (s L−2) |
| PA | = | Polyacrylonitrile |
| PC | = | Hydrophilic, polycarbonate |
| PES | = | Polyethersulfone |
| PS | = | Polysulfone |
| PVDF | = | Polyvinylidene fluoride |
| SDI | = | Silt density index (% min−1) |
| SDI+ | = | Normalized silt density index (% min−1) |
| SDIV | = | Volume-based silt density index (% min−1) |
| SDI15 | = | Silt density index measured in 15 minutes |
| SWRO | = | Seawater desalination reverse osmosis |
| TMP | = | Transmembrane pressure (Pa) |
| UF | = | Ultrafiltration |
Symbols
| T | = | Filtration time (s) |
| ti | = | Initial time required to collect 500 mL of sample (s) |
| tf | = | Time required to collect 500 mL of sample after 15 min (s) |
| T | = | Total elapsed flow time (min) |
| V | = | Permeate volume (mL) |
| A | = | Filtration area (m2) |
| A0 | = | Reference surface area of a 0.45-μm membrane filter (m2) |
| Cb | = | Concentration of particles in feedwater (kg m−3) |
| I | = | Resistivity (m−2) |
| I’ | = | Resistivity modified by crossflow sampler (m−2) |
| J | = | Permeate flux (L m−2 h−1) |
| J0 | = | Initial permeate flux (L m−2 h−1) |
| Rc | = | Fouling-layer resistance (m−1) |
| Rm | = | Membrane resistance (m−1) |
| T | = | Filtration time (t) |
| V | = | Filtrate volume (m3) |
| α | = | (average) Specific cake resistance (m kg−1) |
| P | = | Applied pressure (kPa) |
| ΔP0 | = | Standard reference pressure (207 kPa) |
| ε | = | Cake porosity |
| μ | = | Viscosity of fluid (Pa s) |
| η20°C | = | Dynamic viscosity of water at 20°C (N s m−2) |
| Ω | = | Deposition factor (–) |
Acknowledgments
This research was supported by a grant from the Technology Innovation Program (10052814, technology development of ultrapure water process for semiconductor-level industrial water and localization of four kinds of consumable material), funded by the Ministry of Trade, Industry and Energy, South Korea.