The main advantage of membrane water treatment technology is ecological cleanness of the process: the purification of water by reverse osmosis does not use chemicals and reagents. You can also select a low energy consumption process, and as a consequence, an acceptable cost of purified water. Furthermore, none of the available mass and water purification technologies can not reliably guarantee the same level and degree of purity. In reverse osmosis technology is simply no alternative.
The membrane is semi-permeable filter: it transmits water, but does not pass (or near misses) impurities - colloidal, disperse dissolved. Unlike traditional filters, in which impurities are deposited inside the filter in the membrane separation of water from the "no water" occurs only on the surface. It penetrates through the membrane only water and impurities remain on it. To avoid a rapid contamination of the filter surface, the membrane elements arranged two water flow via "clean" permeate side a permeate (cleaned water), and on the outer side of the membrane is constantly receiving new portions of the source water, which is washed from the surface do not pass through the membrane impurities (concentrate ).
In modern systems of reverse osmosis are used roll (spiral wound) flat (thin film) made of polyamide (PA, Nylon) membrane. The element is a so-called multi-layer envelope (or membrane sheet, if you will), the open part of the "glued" to the rod with holes. In order to compact envelope is wound on this same web. During operation, the membrane raw water supplied from the end of the membrane; permeate collected within the membrane envelope and extends helically in the shaft and into the drinking portion of the system. Concentrate while continuously washed into the drain, leaving the other end of the membrane.
Despite the prevalence and size of membrane technologies for water treatment by reverse osmosis, a single generally accepted theory of the process there. Here are several models of treatment.
1. Presentation of the membrane as a sieve with very, very small holes, comparable to the size of a water molecule. The simplest explanation for the consumer, but not the most honest. For example, it does not answer the question: why monovalent ions (monohydric, trihydric less H2O) does not pass through the membrane.
2. Model of diffusive transport. According to this theory, the water molecules through hydrogen bonds to form a film on the water surface; safely and water diffuses from the outside into the inner membrane, drinking. Also, a water film does not permeate through the membrane pores salts. Salts and ions also diffuse through the membrane, but the speed of their transport is much lower than that of water.
3. Capillary theory. According to this view, the water permeates through the membrane through both capillaries. The water associated with the membrane material by means of hydrogen bonds and structured. Bound water completely fills the pores of the membrane, and differs significantly from ordinary water, forming a barrier to other molecules and dissolved substances.
4. The model of selective permeability. The hydrophilic membranes whose pore size is not more than double the size of the water molecules inside the pores by adsorption on the walls, a layer of pure water, and dissolved substances can not pass through the pores.
To understand the characteristics of membrane separation processes of water should pay attention to the following laws.
The osmotic pressure (osmotic resistance). If it is very rude and "on the fingers" - the solution always wants to "draw" a clean water. And we in the reverse osmosis system is just one solution under pressure through a membrane trying to "squeeze out" pure water. Therefore, the operating pressure must exceed this membrane osmotic resistance (and much due still other nuances of the design of the system). The more initial water soluble salts, the higher is its osmotic resistance. For example, in Kharkiv total salinity of tap water is about 400-500 mg / liter; mainly calcium and magnesium salts. It is estimated that every 1,000 mg / liter of dissolved salts increase the osmotic resistance of 0.6 - 0.8 atmospheres (exact figures for different salts may vary).
The higher the concentration of impurities in the feed water, the lower the performance.
The water temperature also affects the performance of membrane water purification technology. As the temperature increases osmotic resistance increases, but decreases the viscosity of the liquid; the overall capacity of the membrane is increased by 3% for each degree Celsius (explains the drop in system performance in winter).
The higher the pressure applied to the membrane, the better the cleaning. It is believed that the high pressure seals the upper spacer layer, thereby narrowing the pores of the membrane; pure water flux through the membrane increases while the passage of foreign matter remains at the same level.
The first article was published on our website Osmosis Kharkiv 07.29.2014
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