Causes of fouling in reverse osmosis membranes

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　　According to the Chemical Engineering 707 News Network: Membrane fouling refers to the physical and chemical interactions between particles, colloidal particles, or large solute molecules in the fluid in contact with the membrane, as well as concentration polarization that causes the concentration of certain solutes on the membrane surface to exceed their solubility limits. Mechanical forces can also lead to adsorption and deposition on the membrane surface or within its pores, resulting in a reduction in pore size or blockage. These factors cause irreversible changes in the membrane’s permeation rate and separation capabilities. 　　The decline in flux and the reduction in separation efficiency caused by the adsorption of pollutants, especially macromolecules such as proteins, on the membrane surface and within the membrane pores, are the main reasons for the decrease in membrane flux. However, the flux decay caused by membrane fouling often mixes with the reversible flux reduction resulting from concentration polarization, further reducing the membrane separation efficiency. 　　Causes of reverse osmosis membrane fouling During the operation of a reverse osmosis system, metal ions, microorganisms, insoluble precipitates, organic pollutants, biological sludge, colloids, oils, and other substances present in the wastewater come into prolonged contact with the membrane. This leads to membrane fouling, resulting in a significant decrease in the membrane’s flux and separation efficiency, as well as an increase in pressure drop. The main reasons for this include the following: 1) Concentration polarization. In reverse osmosis desalination systems, the selective permeability of the membrane allows water molecules to continuously pass through the membrane from the high-pressure side, while solute molecules remain in the original solution. This results in a concentration difference between the fluid on the membrane surface and the inlet fluid; in severe cases, a very high concentration gradient is formed. This phenomenon is known as concentration polarization. Concentration polarization increases the osmotic pressure of the feed solution, reducing the effective driving force and thus leading to a decrease in water permeation rate and desalination efficiency. 　　2) Ionic scaling: Salts with low solubility products such as CaCO3, CaSO4, BaSO4, SrSO4, CaF2, and SiO2 may precipitate during the reverse osmosis process due to concentration exceeding their solubility products; these deposits remain on the membrane surface and in the water inlet channels, forming scale. J.H. Bruus et al. found that extracting Ca2+ from sludge led to an increase in the number of small particles and filtration resistance. 　　3) Metal oxide deposition: Certain well water sources in the brackish water range, which generally contain low-valent iron and manganese ions, have a certain degree of reducibility. The main cause of membrane fouling in such sources is the formation of colloidal particles composed of iron, aluminum, manganese, etc. on the membrane surface. The lower pH required for iron oxidation leads to frequent iron fouling in reverse osmosis systems. Possible sources of contaminants that cause the deposition of soluble ferrous and ferric iron on membrane surfaces include: oxygen entering water containing ferrous iron; high alkalinity in the water source leading to the formation of FeCo3; reactions between iron and silicon resulting in insoluble iron silicates; the oxidation effect of iron-reducing bacteria, which exacerbates the growth of biofilms and the deposition of iron scale; colloidal iron formed as a result of changes in iron-containing coagulants; and metal contamination caused by iron, aluminum, manganese, etc., which is manifested by a decrease in water production volume and an increase in pressure difference. 　　4) Formation of biological sludge When the membrane surface is covered with microorganism sludge that is highly active, the salts removed by the membrane become trapped in this viscous layer and are not easily washed away by water. This provides abundant nutrients for the growth of microorganisms. Meanwhile, scale inhibitors (such as polymaleic acid, aminotrimethylphosphate, etc.) and water softeners added during the pre-treatment of water before reverse osmosis also contribute to the growth of these microorganisms. Organic and inorganic soluble substances as well as particulates can be removed through effective pretreatment. However, reproductive microbial particles can still reproduce using biodegradable substances in the water, even if only 0.01% of them remain after pretreatment; this is one of the main reasons why biological sludge causes pollution in any system. 　　5) Colloid contamination Both groundwater and surface water contain substances such as iron, aluminum, silicon, and organic matter. These, along with coagulants, flocculants, and scale inhibitors added during pretreatment, form colloidal deposits on the membrane surface, resulting in colloid contamination. Silica colloids undergo hydrolysis in water to produce Si(OH)4; under certain conditions, they undergo polymerization reactions: mSi(OH)4-(SiO2)m+2mH2O, which results in the formation of SiO2 colloid nuclei. These nuclei undergo stepwise ionization, releasing H+ ions and thus creating colloids with a negative charge. 　　Colloidal contamination is difficult to treat because particles with the same charge are relatively stable, do not settle easily, and can contaminate membranes, resulting in a decrease in water flux. Generally, this trend is evaluated using the Pollution Index (SDI). Usually, clogging occurs when SDI is at 3. 　　6) \"Water hammer\" phenomenon: In reverse osmosis systems, due to improper design and the presence of large amounts of air within the membrane housings during the initial commissioning phase, when the liquid to be treated enters these housings suddenly, the compressibility of air means that it cannot be completely expelled instantly. When the pressure of air in the housings reaches a certain level, it bursts out suddenly, causing the membranes inside to collide with each other, be compressed, and move around, thereby resulting in the \"water hammer\" phenomenon. In reverse osmosis systems, the hazard of water hammer lies in causing irreversible damage to the reverse osmosis membrane elements. 　　7) Pollution by suspended particulates 　When there is a \"short circuit\" or defect in the strainer, which leads to the leakage of filtering media, corrosion debris, and foreign objects (such as small fibers), or when the reverse osmosis system is not thoroughly flushed during its initial operation, this can contaminate the membrane elements, cause blockages in the water inlet channels, and result in the formation of amorphous deposits on the membrane surface. This situation is less common. 　　8) Pollution caused by other factors Hydrocarbons and silicon-based oils and fats can cover the surface of the membrane, leading to its contamination; hydrolysis of the membrane, as well as erosion by organic solvents and oxidizing substances, can also cause changes in the properties of the membrane material