Technical analysis of fluoride removal by reverse osmosis membrane

October 18, 2020

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Reverse osmosis technology is a treatment process that changes the flow direction of natural osmotic pressure by means of higher pressure than osmotic pressure, so as to press the solvent (water) in the concentrated solution to one side of semi permeable membrane. Inorganic ions of 1 nm or above are used as the main separation objects in reverse osmosis membrane. The applied pressure is just opposite to osmotic pressure and exceeds osmotic pressure, which leads to reverse osmosis of water in concentrated solution to one side of dilute solution. Because the effective treatment range of reverse osmosis membrane is more than 0.1nm, and the diameter of F-ion is 0.266nm, the fluoride ion in solution can be effectively removed by using reverse osmosis pressure. After 10 mg / L water passes through reverse osmosis membrane, the fluoride ion removal rate reaches more than 90%, while the allowable concentration of fluoride ion in European community drinking water standard is about 0.5 ~ 1.5 mg / L, even if it is lower in China, there is no problem. The only major problem is that the ability of the reverse osmosis membrane to retain univalent ions is much worse than that of bivalent ions. When the fluoride content reaches the requirements, the hardness is reduced to a level not allowed by the health standards.

Salt content is one of the important factors that affect the efficiency of fluoride removal by reverse osmosis. When the salt content is more than 5g / L, the removal rate will be significantly reduced. Reverse osmosis can be used to effectively remove fluoride in groundwater with medium salinity.

Reverse osmosis (RO) is a kind of membrane separation technology, which has been widely used in various fields of water treatment, such as electronics, power and desalination of brackish water.

1、 Basic principle of reverse osmosis technology

Many natural or man-made films have selectivity for the flow and permeation of substances. When two solutions of different concentrations are separated by a semi permeable membrane, only water can pass through, but solutes cannot. In the natural state, water seeps from the fresh water side to the concentrated water side. When the infiltration reaches the dynamic equilibrium, there is a height difference on the surface of two solutions with different concentrations. This pressure difference is called osmotic pressure, and this process is called osmosis. If the pressure is applied on the concentrated water side to exceed the osmotic pressure, the water in the concentrated water quality will penetrate into the fresh water side through the semi permeable membrane. This process is just opposite to the infiltration process in the natural state, so it is called reverse osmosis.

The process of water purification by reverse osmosis equipment is a physical process. Under a certain pressure, various substances including molecules and ions dissolved in the water are intercepted on one side of the reverse osmosis membrane, and water molecules penetrate into the other side of the reverse osmosis membrane, thus realizing the filtration and purification of water quality. Generally, the recovery rate of primary reverse osmosis is controlled at about 75%, and the desalination rate can reach about 98%. In the desalting water treatment system, the reverse osmosis equipment is usually used as the pre desalting device, and the ion exchange equipment is used as the fine desalting device.

2、 Application of reverse osmosis technology

At present, the main reason why most power plants in our country do not adopt reverse osmosis technology is because of the price restriction. However, in fact, the cost of preparing desalted water by reverse osmosis plus ion-exchange desalination is 2-3 times higher than that by ion exchange desalination A the cost of acid and alkali saved is equivalent to the investment of reverse osmosis and its pretreatment system. As long as the operation and maintenance are proper, the service life of reverse osmosis membrane elements can exceed 6 years, and the harmful pollution to the environment can be greatly reduced.

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Author:

Mr. Zheng

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zheng@rmiplast.com

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+86 15888074637