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Application value of reverse osmosis membrane in water treatment industry
The main reason for the performance decline of reverse osmosis membrane is that the surface of membrane is polluted, such as scaling, blocking of membrane surface, or chemical change of membrane itself. The physical change is mainly caused by the compaction effect, the chemical change is mainly caused by the fluctuation of pH value, such as hydrolysis of cellulose acetate membrane, and the performance of aromatic polyamide membrane is deteriorated by free chlorine. The main reasons for fouling and clogging of reverse osmosis membrane are the deposition of membrane surface and the growth of microorganisms. Microorganisms not only block the membrane, but also erode cellulose acetate. Therefore, a certain amount of residual chlorine must be kept in the membrane, but too high residual chlorine will lead to the degradation of membrane performance. Therefore, the residual chlorine should be kept 0.1 ~ 0.5mg/l before the cellulose acetate membrane, and less than 0.1mg/l in front of the aromatic polyamide membrane.
The cleaning of reverse osmosis membrane is a meticulous and complicated work, and it is easy to be damaged after repeated cleaning. In order to reduce the cleaning work, we must do a good job in pre-treatment and strictly control the water quality.
1、 Desalination principle of RO reverse osmosis membrane
To understand the principle of reverse osmosis desalination, we must first understand the concept of "osmosis". Infiltration is a physical phenomenon. When two kinds of water with different concentrations of salts are separated by a semi permeable membrane, it will be found that the water on the side with less salt content will penetrate through the membrane into the water with high salt content, but the salt content will not penetrate. In this way, the salt concentration on both sides will be gradually blended to an even level. However, it takes a long time to complete this process, which is also called natural infiltration. However, if a pressure is applied to the water side with high salt content, the result can also stop the above-mentioned infiltration, and the pressure is called osmotic pressure. If the pressure increases again, the water will permeate in the opposite direction and the salt will remain. Therefore, the principle of reverse osmosis desalination is to apply a greater pressure than the natural osmotic pressure in salty water (such as raw water) to make the infiltration proceed in the opposite direction. The water molecules in the raw water are pressed to the other side of the membrane to become clean water, so as to achieve the purpose of removing salt in water. This is the principle of reverse osmosis desalination.
2、 Reverse osmosis
Reverse osmosis is a membrane separation technology with high filtration accuracy. The filtration accuracy of the filter is 0.0001um. All substances in tap water are filtered, and there are no minerals and trace elements. This kind of water can be directly drunk. The pure water and the solution containing solute are separated by a semi permeable membrane which can only pass through water. At this time, the water on the pure water side spontaneously passes through the semi permeable membrane and enters into the solution side. The water surface on the solution side rises. This phenomenon is called infiltration. When the liquid level rises to a certain height, the pressure on both sides of the membrane reaches equilibrium, and the liquid level on the solution side no longer rises. At this time, there is a pressure difference between the two sides of the membrane, which is called osmotic pressure. If a pressure greater than osmotic pressure is applied to the solution side, the water molecules in the solution will be squeezed to the pure water side. This process is just the opposite of osmosis, which is called reverse osmosis. We can see from the process of reverse osmosis that due to the effect of pressure, the water molecules in the solution enter pure water, the amount of pure water increases, and the solution itself is concentrated. The reverse osmosis membrane used in this equipment is a semi permeable spiral wound membrane. When raw water is sent to the reverse osmosis membrane at a certain pressure, the water will pass through the small pore size on the membrane, and the pure water will be obtained after collection. The impurities in the water, such as soluble solids, organic matters, colloidal substances and bacteria, are intercepted by the reverse osmosis membrane and concentrated and removed in the intercepting liquid. The first stage reverse osmosis can remove more than 97% of the dissolved solids in the raw water.
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