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 Mineral Scaling
Reverse Osmosis RO Membrane Mineral Scaling Causes: All minerals contained in the source seawater are concentrated during the process of membrane salt separation. As their concentration increases during the desalination process, ions of Calcium, Magnesium, Barium, Strontium, Sulfate and Carbonate can form sparingly insoluble salts which could precipitate on the membrane surface. The mineral scales that typically form during seawater desalination are these of Calcium Carbonate CaCO3, Calcium and Magnesium Sulfate (CaSO4 and MgSO4), and Barium and Strontium Sulfate (BaSO4 and SrSO4). Formation of mineral scales on the membrane surface is balanced by the high salinity of the seawater which tends to increase the solubility of all salts. This means that the higher the salinity of the source seawater the less likely a mineral scale forms on the membrane surface at typical seawater pH of 7.6 to 8.3 and desalination system recovery of 40% to 50 %. In brackish seawater desalination systems which typically operate at much higher recoveries (75% to 85%), and ionic strength of the source water is relatively lower, mineral scaling is a frequent problem. In typical seawater desalination systems, Reverse Osmosis RO Membrane Mineral Scaling is usually not a challenge, unless source seawater pH would need to be increased to 8.8 or above to enhance Boron removal.
Calcium Carbonate and Magnesium Hydroxide are the most common causes for Reverse Osmosis RO Membrane membrane scaling, when source seawater pH is increased for enhanced Boron removal. Scale formation in this case can be prevented by addition of antiscalant / dispersant to the source seawater. Commonly used parameters which can be used to predict the seawater’s potential to form mineral scaling of calcium carbonate are the Langelier Saturation Index (LSI) and the Stiff and Davis Index (SDI). These indexes are function of the source seawater pH, Calcium concentration, alkalinity, temperature and TDS concentration/ionic strength. It is important to note that although source seawater temperature usually has limited influence on scale formation, when this temperature exceeds 35°C, Calcium Carbonate scale would form at accelerated rate. Metal oxide and hydroxide foulants most frequently encountered during seawater desalination are iron, manganese, copper, zinc and aluminum.
Typically, open ocean seawater contains very low levels of these metal foulants and therefore, if such fouling is encountered on the membrane elements, the usual sources are overdosing of coagulant (iron salt) or corrosion of pipes, fittings, tanks and other metal equipment located upstream of the SWRO system / desalination plant. Iron and manganese fouling may occur if source seawater is collected via subsurface intake from a coastal aquifer which is under the influence of fresh groundwater that contains high levels of these metals in reduced form (iron of more than 2 mg/L (as ferrous) and manganese of more than 0.5 mg/L). This type of scaling problem has been observed in desalination plants with beach well intakes collecting seawater from alluvial coastal aquifers, which are located near the entrance of a river or a creek into the ocean.
If iron and manganese are in reduced form and they are below 1.0 mg/L and 0.1 mg/L respectively, than they can be removed by the SWRO membranes without causing accelerated fouling. However, if iron and manganese are in oxidized form, their levels should be reduced below 0.05 mg/Land 0.02 mg/L, respectively to prevent mineral fouling. Another mineral fouling compound frequently encountered in fresh surface water and brackish aquifers is silica. Open ocean seawater contains silica of less than 20 mg/L and therefore, this compound in not a typical cause of Seawater Reverser Osmosis RO membrane mineral fouling. Silica content above this level usually indicates that the source seawater quality is influenced by a fresh water discharge or coastal aquifer of high silica content located near the desalination plant intake area.
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.
