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The relationship between the thickness of the reverse osmosis membrane water supply pipe network and the possibility of pollution


The relationship between the thickness of the reverse osmosis membrane water supply pipe network and the possibility of pollution


In the water treatment industry, discussions on the relationship between the thickness of the water supply network of reverse osmosis membrane components and the likelihood of membrane fouling, especially in the case of high concentrations of organics, colloids and suspended solids in the water supply, often lead to different opinions. There is an opinion that the use of thick water supply channels, i.e. 0.031 inch (0.78 mm) thick elements, results in less frequent membrane cleaning and therefore longer membrane life than common, 0.028 inch (0.71 mm) thick elements. long. But there is no actual data to support this view.


Before entering the discussion, we need to understand the relevant principles of membrane module fouling. Water contains insoluble salts, colloids, microorganisms, organic matter, metal oxides and other various impurity particles, which will cause pollution of membrane modules. Taking colloid as an example, most of the colloids in natural water are negatively charged. This colloid consists of a positively charged rubber core and a negatively charged outer layer. Due to the multi-layered structure and hydration of the colloid, the colloid can float in the water.


Since the negatively charged air mass used in the manufacturing process of the composite membrane is not fully reacted, the surface of the composite membrane generally has a negative charge. This negative charge is intentionally controlled during the manufacturing process to better remove the negatively charged air. charged substance. When water is supplied to the membrane module, most of the colloids will pass through the water separation net along with the water flow and be discharged from the membrane module. The two forces will affect the moving speed of these colloids in the membrane module. The bell force moves the colloidal particles in a direction parallel to the membrane surface. The second force has colloidal particles that move vertically to the membrane surface, displacing the space left by the passage of water. The speed at which colloidal particles reach the membrane surface is related to the water output. The higher the water flux (such as the smaller the membrane area of ??the system), the higher the colloid concentration on the membrane surface. Due to the boundary layer effect near the membrane surface, the water resistance is greater, so the horizontal When the flow rate is close to 0, some colloidal particles adhere and bond with each other, maintain a sufficient horizontal flow rate of the membrane surface in water, and wash and peel off MOIN colloid and other particles on the membrane surface in time, maintaining a certain boundary layer thickness, which has a positive impact on maintaining the membrane yield. . With an understanding of how membrane fouling machines are, the importance of maintaining an arithmetic flow rate for the lateral flow rate of the water supply to reduce fouling is clear. The high side flow velocity can increase the turbulence of the water, reduce the accumulation of particulate matter in the gaps of the separator, and reduce the precipitation on the membrane surface. The high lateral flow velocity can also improve the diffusion rate of high-concentration salts on the membrane surface, reduce the risk of concentration polarization, reduce the thickness of the boundary layer, and prevent the precipitation and fouling of insoluble salts on the membrane surface.


The transverse velocity of the membrane surface is determined by the water supply flow, the width and thickness of the membrane element water supply channel, etc. The water supply channel, i.e., the thicker the water supply partition, requires a higher lateral flow rate to achieve the same turbulence and boundary layer effects. The smaller the area is, which means that in order to achieve a more suitable transverse flow rate on the membrane surface, a membrane element with a smaller membrane area (thickness of the water supply separator) must be used.


According to the above analysis, in order to reduce pollution, maintain water volume, and prolong the washing cycle, it is necessary to use the appropriate thickness of the water supply network. Under certain conditions, choosing a thin water supply network thickness can effectively increase the transverse flow velocity and improve the comprehensive performance of the membrane components.


Some membrane manufacturers advocate that water separation nets encourage the use of thick-membrane elements (eg, nominal thickness of 0.031 inches, membrane area of ??330 square feet), which they believe is less prone to fouling and can reduce cleaning frequency and improve cleaning results. Hyde Energy also manufactures the modules, but mainly supplies modules with a water separation mesh thickness of 0.028 inches and a membrane area of ??365 square feet by 400 square feet. Through field application testing, it was found that there was no apparent benefit in the fouling resistance of membrane elements with a separator thickness of 0.031 inches and a membrane area of ??330 square feet.


The comparative test was also carried out at a semiconductor factory in the southwestern United States, where the water contained high concentrations of organics and colloidal particles. They also used Hyde Energy's 400-square-foot membrane section with a 0.028-inch diaphragm thickness and a 330-square-foot membrane section with a 0.031-inch diaphragm thickness from other companies. The results show that the fouling speed of the two membrane elements is basically the same as the recovery speed of the water flux after cleaning when working under the same conditions.


In fact, first, once the water flow is determined, a membrane element with a baffle thickness of 0.031 inches will have a lower lateral flow rate than a membrane element with a water supply baffle of 0.028 inches. In the reverse osmosis system with the same membrane area, if different process parameters are the same, the lateral flow rate of the system with thicker water separator is lower, thereby increasing the membrane fouling rate. 2. The water supply flow and water production flow are the same. For the reverse osmosis system such as the number of membrane elements used, when membrane elements with small membrane area (water separator thickness) are used, the water production flux of the system, that is, the unit membrane area The water production load is large, and the membrane fouling rate increases. For example, the membrane area of ??the 400-square-foot and 365-square-foot membrane elements is 10.6% and 21% larger than that of the 330-square-foot membrane element, respectively. Among the systems using the same number of membrane elements, the system using 330 square feet of membrane has a large water flux and high water supply pressure due to the small membrane area. The mountain water flux will greatly increase the pollution tendency, and the high water supply pressure will increase the cost and power consumption of the feed pump. Another option at this point is to increase the number of membrane modules (and pressure vessels), but this will increase equipment costs.