Osmosis: A natural phenomenon in which the water passes through a semi-permeable membrane from a less concentrated solution to a more concentrated solution.
Reverse osmosis is called to the reverse natural process of osmosis by an external force.
Reverse Osmosis: A process in which water is forced to pass through a semi-permeable membrane from a solution more concentrated in dissolved salts and other contaminants to a less concentrated solution through the application of pressure. The purpose of the reverse osmosis is obtained purified water starting from a flow with large amount of salts such as the sea water.
Reverse Osmosis Membranes
The most commonly reverse osmosis membranes are in spiral configuration.
The spiral configuration of two flat sheets of membrane separated by a permeate collector mesh to form a kind of envelope. This assembly is sealed on three sides leaving the fourth side open for the permeate exit. Plastic mesh is added between each envelope as a spacer to allow the circulation of feed flow / concentration. A number of these units or envelopes revolve around a central plastic tube permeate collector. This tube is perforated to collect the permeate the set of all the envelopes. The typical element spiral wound membrane for industrial applications is about 100 cm (40 inches) long and 10 to 20 cm (4 or 8) inches in diameter.
The flow of feed / concentrate flows through the element in a longitudinal or axial direction from the feed end to the opposite to the concentrate or, running parallel to the surface of the membrane. The spacer mesh feed / concentrate induces turbulence and reduces the concentration by polarization. Manufacturers specify the concentrate flow requirements to control the concentration by polarization conversion, limiting conversion between 10 to 20 percent.
Therefore, the recovery (or conversion) is a function of the length of the element. To operate in acceptable recoveries, membrane systems usually consists spiral three to six membranes in series in a pressure vessel. The concentrate of the first element becomes the feed of the next item and so on for each element within the pressure tube.
The concentrate of the last item goes out of the pressure tube to the discharge or as a feed to the other vessel. Each Permeate element is collects in the permeate central tube collector and out one end of the pressure box. An individual pressure box with four or five membrane elements in serial connection can be operated up to 50 percent of recovery under normal design conditions. The O-ring, on feed side of the element prevents that flow of feed / concentrate pass to the next item.
The spiral wound elements are generally made using sheets of plastic film composite membrane composed of thin (thin film composite). A thin film composite consist in a thin molded layer of a polymer over a thicker layer of different polymer.
Nanofiltration is a process of filtration in the range between reverse osmosis and ultrafiltration that uses semi-permeable membranes with a pore size of about 0.001micras, rejecting organic molecules with molecular weight greater than 200 Daltons.
Differentiating characteristic of the reverse osmosis process, the monovalent ions are rejected by 30% and 60%, and divalent between 90% and 98%.
This difference in rejection can operate at very low pressures (4 to 12 bars depending on the type of waters).
Nanofiltration is ideal for applications requiring the removal of bacteria and viruses, and the reduction of water hardness.
Ultrafiltration is a membrane filtration process under pressure for separation or removal of bacteria, viruses, organic molecules and colloidal particles.
Its range is located between microfiltration and nanofiltration, with a pore size between 0.1 and 0.01 microns but the most common way of expressing the size exclusion in ultrafiltration is referring to the cut molecular size expressed in Daltons. The generally accepted definition of molecular court refers to the molecular weight of macromolecules, which are held by 90% by the ultrafiltration membrane.
Tangential Microfiltration is a process of physical-mechanical separation.As the medium flows tangentially through the membranes, the filtrate or permeate moves at right angles. This filtration method is called “dynamic filtering”. To separate the compounds do not undergo any chemical modification, which means they can be recovered.
Choosing the appropriate pore size, the low molecular weight substances can cross the membrane while the macromolecules are retained.
The continuous electrodeionization (CEDI) is the latest generation ion exchange technology. The electrodeionization is used for final refinement of the quality of water to obtain ultrapure water for pharmaceutical, power generation, microelectronics and food industry.
The electrodeionization is the process of removing ionized or ionizable species from water using ion exchange membranes, an electrically active field (typically ion exchange resin), and a source of electrical potential CD.
The ion exchange process efficiently removes ionized species of water by exchanging H + and OH-. The water passes through one or more chambers filled with ion exchange resins sustained between selective membranes of cations or anions.
Ions are bonded to the ion exchange resins go to a separate chamber under the influence of an externally applied electric field. This also produces H + and OH-required to maintain the resins in their regenerated state.
The main benefit of the electrodeionization is that it is a continuous process in which there are no stoppages for regeneration, and it is free of chemicals.