Reverse osmosis elements

Conlon, W., Pilot field test data for protot5fpe ultra low pressure reverse osmosis elements. Desalination, 56, 203, 1985. 

An industrial reverse osmosis plant usually will consist of three separate sections which are shown in Figure 4.2. The first section is the pretreatment section in which the feedwater is treated to meet the requirements of reverse osmosis element manufacturers and the dictates of good engineering practice. Following pretreatment, the feedwater is introduced into the reverse osmosis section where the feedwater is pressurized and routed to the reverse osmosis elements which are in pressure vessels. The feedwater flows across the membrane surface where product water permeates through the membrane and a predetermined amount remains behind as reject. The reject is discharged to waste while the product water is routed to the posttreatment section. The third or posttreatment section treats the product water to remove carbon dioxide and adds chemicals as required for industrial use of the product water. 

The penalty of a high concentration polarization is not as severe with a low TDS feedwater as it is with a high TDS feedwater. The recommendations as to minimum flows or maximum recoveries which are specified by the reverse osmosis element manufacturer should be followed at all times, especially when the application is the desalination of high TDS waters. 

The thin film composite membrane exhibited superior overall rejection performance in these tests, with ammonia and nitrate rejection showing an outstanding improvement. It has also been reported that silica rejection by the thin film composite membranes is superior to that of cellulose acetate. While the above data indicates a marginal improvement in the rejection of chemical oxygen demand (COD), which is an indication of organic content, other tests conducted by membrane manufacturers show that the polyurea and polyamide membrane barrier layers exhibit an organic rejection that is clearly superior to that of cellulose acetate. Reverse osmosis element manufacturers should be contacted for rejection data on specific organic compounds. ... 

Once the pretreatment study had been completed, it will be possible to decide on the type of elements to be used in the reverse osmosis unit. If the SDI of the pretreated feed is 3.0 or less, then either the spiral wound or hollow fine fiber elements can be used. The choice will depend on economics (element price) and desalination characteristics (flux and rejection). If the pretreated feed SDI is more than 3.0, then the spiral wound element should be used. When the decision as to element type is made, then it is appropriate to forward a copy of the pretreated feed water analysis to reverse osmosis element manufacturers to obtain a prediction of product water quality, recommended type of element, total number of elements required, possible problems with sparingly soluble compounds in the feedwater, allowable recovery, and price and delivery. 

The maximum bore pressure in both cases is the pressure in the fiber at the furthest distance from the open end of the fiber. In the case of BOE type the maximum bore pressure is moved from the end of a longer fiber of length L to the midpoint of fiber length L/2, and the average pressure is reduced. The net effect of this change increases the amount of water flow since less bore pressure drop occurs for the same applied pressure. The BOE reverse osmosis element stmcture and flow pattern is illustrated in Figure 2.19. 

Pervaporation is a relatively new process with elements in common with reverse osmosis and gas separation. In pervaporation, a liquid mixture contacts one side of a membrane, and the permeate is removed as a vapor from the other. Currendy, the only industrial application of pervaporation is the dehydration of organic solvents, in particular, the dehydration of 90—95% ethanol solutions, a difficult separation problem because an ethanol—water azeotrope forms at 95% ethanol. However, pervaporation processes are also being developed for the removal of dissolved organics from water and the separation of organic solvent mixtures. These applications are likely to become commercial after the year 2000. 

Fig. 13. A hoUow-fibet reverse osmosis membrane element. Courtesy of DuPont Permasep. In this twin design, the feedwater is fed under pressure into a central distributor tube where half the water is forced out tadiaUy through the first, ie, left-hand, fiber bundle and thus desalted. The remaining portion of the feedwater flows through the interconnector to an annular feed tube of the second, ie, right-hand, fiber bundle. As in the first bundle, the pressurized feedwater is forced out tadiaUy and desalted. The product water flows through the hoUow fibers, coUects at each end of the element, and exits there. The concentrated brine from both bundles flows through the concentric tube in the center of the second bundle and exits the element on the right.

Peivaporation is a relatively new process that has elements in common with reverse osmosis and gas separation. In peivaporation, a liquid mixture contacts one side of a membrane, and the driving force for the process is low vapour pressure on the permeate side of the membrane generated by cooling and condensing the permeate vapour. The attraction of peivaporation is that the separation obtained is proportional to the rate of permeation of the components of the liquid mixture through the selective membrane. Therefore, peivaporation offers the possibility of separating closely boiling mixtures or azeotropes that are difficult to separate by distillation... 

Spiral-wound reverse osmosis membrane element, 26 75 Spiramycin... 

For removing low levels of priority metal pollutants from wastewater, using ferric chloride has been shown to be an effective and economical method [41]. The ferric salt forms iron oxyhydroxide, an amorphous precipitate in the wastewater. Pollutants are adsorbed onto and trapped within this precipitate, which is then settled out, leaving a clear effluent. The equipment is identical to that for metal hydroxide precipitation. Trace elements such as arsenic, selenium, chromium, cadmium, and lead can be removed by this method at varying pH values. Alternative methods of metals removal include ion exchange, oxidation or reduction, reverse osmosis, and activated carbon. 

Reverse osmosis, pervaporation and polymeric gas separation membranes have a dense polymer layer with no visible pores, in which the separation occurs. These membranes show different transport rates for molecules as small as 2-5 A in diameter. The fluxes of permeants through these membranes are also much lower than through the microporous membranes. Transport is best described by the solution-diffusion model. The spaces between the polymer chains in these membranes are less than 5 A in diameter and so are within the normal range of thermal motion of the polymer chains that make up the membrane matrix. Molecules permeate the membrane through free volume elements between the polymer chains that are transient on the timescale of the diffusion processes occurring. 

R.E. Larson, J.E. Cadotte and R.J. Petersen, The FT-30 Seawater Reverse Osmosis Membrane-element Test Results, Desalination 38, 473 (1981). 

Electrodeionization systems were first suggested to remove small amounts of radioactive elements from contaminated waters [27], but the principal current application is the preparation of ultrapure water for the electronics and pharmaceutical industries [28], The process is sometimes used as a polishing step after the water has been pretreated with a reverse osmosis unit. 

FPG-type deep cartridge filtering elements of 20 pm and 5 pm filtering capacity are used in microfilters. Element ERU-100-1016 (filtering capacity 10 nm) at a pressure < 0.3 MPa is used in ultrafilters in reverse-osmosis filters element ERO-KM-100-1016 (selectivity for NaCl 95-99%) at a pressure <7.0 MPa is used. Ion-exchange filters are... 

Fig. 12. A spkal-wound reverse osmosis membrane element (a) schematic depiction (b) cross section of a spiral-wound thin-film composite RO Fikntec...

Marinas, B.J. and Urama, R.I., Modeling concentration-polarization in reverse osmosis spiral wound elements, J. Environ. Eng., 122(4), 292, 1996. 

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