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Reverse osmosis unit

  • Added: 25.04.2019
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2.2.1 Purification unit Waste water, water from the pond-sump and fresh river water in the amount of 1000 m3/h after preliminary purification from coarse particles by pumps is supplied to two-layer filters 1. Two-layer filters are designed to remove suspended particulate matter from the source water. Before the two-layer filters, chlorine is introduced into the water stream in the form of a sodium hypochlorite solution, which contributes to the oxidation of bioorganic impurities contained in the water. Chlorine also contributes to the oxidation of ferrous iron into trivalent iron, which precipitates. Manganese dioxide contained in manganese sand significantly accelerates the iron oxidation process. To precipitate colloidal dispersed particles, a coagulant solution is added to the starting water. The coagulant facilitates the coarsening of colloidal particles and their precipitation. The supply of chlorine and coagulant helps to remove most of the suspended solids larger than 10 microns in the bilayer filters. The double-layer filter 1 is a vertical cylindrical apparatus filled with a filter nozzle of anthracite and manganese sand. Source water at a pressure of up to 0.6 MPa is supplied to the upper part of the filter through an inlet distributor, passes from top to bottom a layer of anthracite, where coarse filtration of water occurs, and then a layer of manganese wet sand intended for finer filtration. Mechanical admixtures of water are retained by filtering composition, and clarified water exits through drain collector system located in lower part of apparatus. The drainage system is covered with a layer of gravel with different grain size, which contributes to a more even distribution of the flow and prevents the removal of filter media from the apparatus. As the filter material is contaminated, the hydraulic resistance of the filter increases and its productivity decreases. In order to restore filter capacity of filters once a day, they are washed. To this end, the filters are first loosened with compressed air, as a result of which the filter material becomes suspended, and then washed with the source water supplied from the bottom up. Backwash is performed for 15 minutes. The flushing water is discharged to the sewage system. Upon completion of flashing backwash, filters are flushed with working water flow for 5 minutes. The filtrate is also discharged to the sewage system. After two-layer filters (1) water is supplied to pre-treatment filters (2). Prior to these filters, an anti-scale neutralizing agent is introduced into the water stream to form a scale and a sodium metabolisulfite to bind the chlorine present in the water because chlorine and scale-forming agents adversely affect the operation of reverse osmosis membranes. To prevent carbonate deposits, sulfuric acid is dosed into the water stream in an amount necessary to maintain the pH 3.0-7.5. The filters 2 act as a mixing chamber and ensure uniform distribution of reagents in the water stream, designed for the final purification of water from suspended particles larger than 5 microns. Cartridges of porous polypropylene with pore size of 5 microns are used as filtering material. Each node has 2 filters. Contamination of porous elements leads to increased hydraulic resistance of the filter. In case of pressure drop on the filter 0.1-0.12 MPa, cartridge filters should be replaced. Replacement is carried out at least 1 times per 10 days. 2.2.2 Reverse osmosis unit Water cleaned from suspensions by pumps (3) with pressure up to 2.5 MPa is supplied to reverse osmosis units (4). The reverse osmosis unit is designed to purify water from ions of salts dissolved in it, as well as organic substances and suspensions with a size of less than 5 microns. The reverse osmotic desalination process takes place in a roll type apparatus.It is a pipe into which roll filter elements are inserted. These elements are made by twisting around a central drainage tube with slits of semipermeable membranes separated by drainage devices and separation turbulators. During winding, materials forming membrane packs are impregnated with adhesive composition so that pressure and drain cavities separated from each other are obtained. The initial water is supplied to the outer surface of the roll filter element (REF), moves along the turbulizer-separator in a spiral to the center of the element. Purified water is collected here and a concentrated salt solution is removed through the end of the RPE. Thus, water is separated by membranes into two streams: purified water (permeate) and concentrated salt solution (concentrate). Permeate from each membrane element is collected in permeate collector and with residual pressure up to 0.4 MPa is supplied to decarbonizers 6. Check valve 5 is installed on permeate pipeline to prevent reverse flow. The concentrate is collected in the concentrate collector and then sent through the inter-mill pipeline to the industrial storm sewage well, from where it flows by gravity to the pond 7. The amount of concentrate is controlled by the restriction diaphragm installed on the pipeline, adjusted for a certain flow rate at the plant manufacturer. If it is necessary to change the flow rate, the concentrate flow is removed from the unit via bypass pipeline equipped with shutoff valves, and the restricting diaphragm is replaced by another one. The degree of water purification at the reverse osmosis plant is 75%, that is, of the total amount of source water, 75% is permeate, and 25% is discharged as a concentrate. Cleaning and restoration of membrane retention capacity is achieved by circulation of detergent solutions through membranes. Orthophosphoric acid from inorganic deposits and caustic soda from organic deposits are used as detergent solutions. Cleaning is performed 1 times every 30 days. At the same time permeate and concentrate of the unit supplied for cleaning in the amount of 156 m3/h are supplied to the collector of the membrane cleaning unit 8 filled with the calculated amount of detergents. Circulation continues until the temperature of the detergent solution reaches 40 0C. Then the membranes are held in a detergent solution for 30 minutes. After that, the reverse osmosis unit is washed with clean water from detergent solutions. If necessary, the membrane cleaning operation is repeated. Water after flushing is discharged to the sewage system. 2.2.3 Decarbonization unit After reverse osmosis units permeate is supplied to decarbonizers (6). Decarbonizers are designed to remove carbon dioxide from permeate, which is formed when water is acidified with sulfuric acid before reverse osmosis plants. Decarbonization of permeate is necessary to increase the productivity of anionite, since absorption of free carbon dioxide by anionite leads to its rapid saturation. Dissolved carbon dioxide is removed from permeate by aeration. The decarbonizer is a cylindrical tank, inside of which the nozzle is located. Permeate is supplied to the upper part of the decarbonizer through a spraying device, air is supplied from below with counter current. The treated water washes the nozzle elements with a thin layer, and air flows towards it, supplied to the decarbonizer by the blower 9 through the branch pipe. Filters for high-efficiency air cleaning are installed on the suction of blowers. The carbon dioxide removed from the water passes into the air and with it is discharged from the decarbonizer to the atmosphere. Part of permeate is periodically withdrawn from the collector for its own needs. To increase the pH, a solution of sodium hydroxide is dosed into the permeate stream. 2.2.4 Water demineralization unit Decarbonized permeate (partially demineralized water) is supplied by pumps 3 with pressure of up to 0.6 MPa to ion exchange treatment in mixed-action filters 10, which are intended for final demineralization of permeate. Structurally, the mixed-action filter (MPS) is a vertical cylindrical apparatus loaded with cationite and anionite with four drainage and distribution devices: an inlet distributor, an alkali distributor, a middle-level collector, a drain collector. The mid-level collector plays an important role in the filter process. It is located at such a height from the lower drain collector that along its axis there is a line of separation of layers of cationite and anionite when they are separated. After mixing these ionites with each other, the interface disappears and a generally uniform filter bed is formed in the filter, through which the treated water passes from top to bottom, leaves the filter through the lower drain collector, is collected in the common collector of demineralized water. Permeate comes to the top part of the filter, is evenly distributed by means of the internal distributing device and from top to down passes through a layer of the mixed H+ grains - cation exchanger and IT - anion exchanger. On grains of cation exchanger processes of exchange of the cations which are contained in water for H+ ion proceed, on anion exchanger grains - processes of exchange of anions for an ion IT is. The H + and OH ions transferred to the solution bind to water molecules. Mixed-action filters are periodically output from the diagram for regeneration. The regeneration process of each FDS consists of three main stages: loosening and separation of ionites, their separate regeneration and washing from regeneration products. Loosening and separation of ionites is carried out with partially demineralized water (PMV), into the flow of which sulfuric acid and alkali are dosed. NDV is taken from the filter inlet header and supplied to the lower part of FPS. Passing the filter layer from the bottom upwards, the swelling water separates the anionite from the cationite due to the difference in their densities and through the upper distributor is withdrawn to the drain tray. Regeneration of ionites in the FDS, to reduce the total regeneration time, is carried out simultaneously. The operation cycle of the FPS consists of the following operations: - permeate softening; - backwashing (loosening) in the bottom-up direction to separate the cationite (bottom layer) from lighter anionite (upper layer); - settling of resins; - setting of flow dilution rate; - supply of regeneration solution of alkali; - supply of regeneration solutions of acid and alkali; - preliminary washing of ionites in the direction of passing of regeneration solutions; - lowering of water (draining); - mixing of ionite charge with compressed air in the bottom-up direction; - refilling; - final washing of ionites in the top-down direction. Demineralized (deionized) water is fed to the post-treatment filters 11 where the water is finally purified from suspended ion exchange resin particles. After fine filters, deep demineralized water is sent to consumers. The demineralized water production circuit is fully automated and controlled by a programmable logic device.

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