Reverse Osmosis RO Membrane Particle Fouling
Reverse Osmosis RO Membrane Particle Fouling is caused by organic and inorganic particles contained in the source seawater such as fine debris, plankton, detritus and silt, which cannot pass through the SWRO membranes. All suspended solids which naturally occur in insoluble form, if not removed by pretreatment, would be retained on the feed side of the Seawater Reverse Osmosis RO membrane. Depending on the hydrodynamic conditions on the membrane surface, as well as the size and charge of these particles, they would either migrate along the RO membrane leafs and ultimately exit with the concentrate, or would be trapped on the membrane surface and would begin to accumulate there causing loss of RO membrane productivity over time. This type of foulant can be effectively removed by pre-filtering of the source seawater prior to Seawater Reverse Osmosis RO membrane separation.
Reverse Osmosis RO Membrane Particle Fouling in raw source seawater vary in size. However, most of them, including pico-phytoplankton, are larger than 0.1 μm. Usually, over 90 % of the particulate foulants are larger than 1 μm. A well designed and operating pretreatment system will produce permeate that does not contain particles larger than 20 μm.
Reverse Osmosis RO Membrane Colloidal Fouling
Reverse Osmosis RO Membrane Colloidal fouling is caused by inorganic and organic compounds that naturally exist in suspension and may be concentrated by the SWRO desalination plant separation process, and precipitate on the Reverse Osmosis RO membrane surface thereby causing membrane flux decline over time. Colloidal solids have particle size of 0.001 to 1 μm. For prevention of Reverse Osmosis RO Membrane Colloidal Fouling, SWRO membrane manufacturers usually require RO membrane feed turbidity of less than 0.1 NTU, zeta potential higher than – 30 microvolts (mV) and SDI15 less than 3. The two most common culprits of this type of fouling are colloidal silica and iron.
Typically, seawater collected via open ocean intake does not contain significant amounts iron and silica in colloidal form and this type of fouling usually does not present a challenge in seawater desalination systems with open ocean intakes. Stability of colloids is reduced with the increase in source water salinity, and typical seawater with TDS concentration in a range of 30000 to 45000 mg/L, would contain silica and iron in dissolved and precipitated form rather than in colloidal form. However, if the source seawater is collected via subsurface well intake which is under the influence of brackish coastal aquifer with high content of colloidal silica and/or iron, or it is collected near an area where silt laden river enters into the ocean, than colloidal fouling may become a challenge. Reverse Osmosis RO Membrane Colloidal Fouling could be removed by coagulation, flocculation and filtration, which also applies to Reverse Osmosis RO Membrane particulate fouling.
Reverse Osmosis RO Membrane Scaling and Fouling: Causes
Reverse Osmosis RO Membrane Scaling and Fouling have many reasons which will be discussed in this article. First, we need to learn the mechanism of desalination plants. In desalination plants, Seawater is collected from the ocean using either subsurface intakes (wells, intake galleries, etc.) or open ocean intakes. Subsurface intakes naturally pre-screen and pre-filter the collected seawater and thereby they remove coarse debris, and most of the sand and particulates from the seawater. Open ocean intakes which collect ambient seawater directly from the ocean typically have equipment (bar racks, fine traveling screens, micro-screens, and/or strainers) to pre-screen large debris, floating materials, large aquatic organisms, coarse sand, and stringy materials from the source water. As a result, after preliminary screening by the intake facilities seawater typically contains the following five key groups of compounds that could cause Seawater Reverse Osmosis RO membrane scaling and fouling and therefore, would need to be removed by the pretreatment system:
- Particulate foulants (Mainly suspended solids and silt).
- Colloidal foulants: Compounds of relatively small size (0.2 μm to 1.0 μm) that are not in fully dissolved form. When concentrated during the membrane separation process they may coalesce and precipitate on the membrane surface (mainly clay-like substances).
- Mineral scaling foulants: Inorganic compounds (i.e., Ca, Mg, Ba, Sr salts) which during the salt separation process may precipitate and form a scale on the membrane surface (such as Calcium Carbonate CaCO3 and Calcium Sulfate CaSO4, and magnesium hydroxide) or may block the membrane diffusion layer (such as Iron and Manganese).
- Natural organic foulants: Natural organic matter that can attach to and foul the membranes.
- Microbial foulants: Marine organisms and soluble organic compounds that can serve as food to the microorganisms which inhabit in the source water and can form fouling biofilm reducing membrane transport.
Please visit the Hydrodex BLOG to learn more about each key group of compounds and how to prevent your RO membrane from scaling and fouling.