1.Concentration polarization
In the reverse osmosis desalination system, the selective permeability of the membrane allows water molecules to continuously permeate the membrane from the high-pressure side, while the solute molecules still remain in the original solution, resulting in a gap between the material liquid on the membrane surface and the inlet material liquid. The concentration difference will produce a very high concentration gradient when it is severe. This phenomenon is called concentration polarization. Concentration polarization increases the osmotic pressure of the material liquid, reduces the effective driving force, and causes the water permeability and desalination rate to decrease.
2. Inorganic salt scaling
Salts with small solubility products such as CaCO3, CaSO4, BaSO4, SrSO4, CaF2, and SiO2 may precipitate out due to concentration exceeding their solubility product during the reverse osmosis process, resulting in sediments staying on the membrane surface or entering water Scale is formed in the channel.
3. Adsorbent pollution
Some well water sources that belong to the range of brackish water generally contain low-valent iron and manganese ions, which have a certain degree of reducibility. The main cause of membrane fouling caused by such water sources is that iron, aluminum, manganese, etc. produce colloidal particle pollution on the membrane surface. Blocking. When O2 enters Fe2 plus -containing influent water, high-alkalinity water sources form iron carbonate and iron silicate. Reducing bacteria are blending in, iron scale formation becomes faster and faster, and colloidal iron caused by the conversion of iron flocculants. . . The characteristic performance after metal pollution is a decrease in water production and an increase in pressure difference.
4. Formation of biological sludge
For water sources from surface water and sewage, most of the problems encountered are biological sludge. When the surface of the membrane is covered with vigorous microbial sludge, the salt removed by the membrane will be trapped in the sticky layer and will not be easily washed away by water, providing abundant nutrients for the reproduction of microorganisms. At the same time, reverse osmosis is pre-treated before entering the water. The scale inhibitors, water softeners, etc. added at the time can promote the growth of microorganisms.
5. Colloid contamination
Groundwater and surface water all contain iron, aluminum, silicon, organic matter and other substances, which form colloids with coagulants, coagulants, scale inhibitors, etc. added during pretreatment and deposit on the surface of the membrane, causing colloidal pollution. Colloid pollution is difficult to deal with because it has the same kind of electric charge, is relatively stable, and is not easy to settle, but it is trapped on the surface of the membrane when the RO membrane filters water, forming hydrates, and easily fouling the membrane, resulting in a decrease in water flux. Generally, this trend is evaluated by the pollution index (SDI). Usually when SDI<3, no="" such="" pollution="" occurs="" on="" the="" membrane="" surface;="" when="" sdi="">3, fouling will occur. Note that this is the most difficult type of pollution! 3, fouling will occur. Note that this is the most difficult type of pollution!"
6. Water hammer
For the reverse osmosis system, due to the unreasonable design, there is a large amount of air in the membrane shell filled with the membrane at the beginning of the commissioning stage. When the air reaches a certain pressure in the membrane shell, it will suddenly burst and release, causing the reverse osmosis membranes to collide, squeeze and move in the membrane shell, resulting in a "water hammer" phenomenon. In the reverse osmosis system, the harm of water hammer lies in causing irreversible damage to the reverse osmosis membrane elements. Of course, this phenomenon can also be avoided through perfect design.
7. Suspended particulate matter
The security filter may cause leakage of filter media, corrosion fragments and foreign objects (such as small corduroy) due to "short circuit" or defects, or when the reverse osmosis is not thoroughly flushed for the first time, the membrane elements may be contaminated and the water inlet channel may be blocked. And an amorphous precipitate is formed on the film surface. But this situation is rarely encountered.
8. Pollution caused by other factors
Hydrocarbon and silicone-based oils and lipids cover the surface of the membrane, causing the membrane to be contaminated; the hydrolysis of the membrane, the erosion of organic solvents and oxidizing substances, etc. will also cause the nature of the membrane material to change.
