Brackish water membranes modules

For 8-inch diameter, brackish water membrane modules Microfiltration pore size < 0.5 microns. 

Membrane compaction involves the compressing of the membrane itself that, in essence, makes the membrane denser or thicker which reduces the flow and salt passage through it. Compaction can occur under higher feed pressure, high temperature, and water hammer. (Water hammer occurs when the RO high-pressure feed pump is started and there is air trapped in the membrane modules—see Chapter 6.2.) Most brackish water membranes, when operated properly, experience a minimal degree of compaction. However, seawater membranes and cellulose acetate membranes... 

The first reverse osmosis modules made from cellulose diacetate had a salt rejection of approximately 97—98%. This was enough to produce potable water (ie, water containing less than 500 ppm salt) from brackish water sources, but was not enough to desalinate seawater efficiently. In the 1970s, interfacial composite membranes with salt rejections greater than 99.5% were developed, making seawater desalination possible (29,30) a number of large plants are in operation worldwide. 

In 1968 we started investigations of RO applications for desalting brackish water. In the course of the investigations, we have found the spirally wound module of asymmetric cellulose acetate RO membrane shows excellent durabilities against fouling materials and free chlorine. 

Table V displays data recorded at the test facility in Roswell, New Mexico, maintained by the Office of Water Research and Technology, U. S. Department of Interior. This facility delivers a feed of brackish water pretreated to control bacterial growth and to deliver a feed free of chlorine. Modules 148 and 152 were nominally identical samples. They are constructed of fiber bundles approximately 2 inches in diameter, 10 inches in length and containing approximately 25 square feet of membrane surface area. The increase in productivity over time can be explained by an increase in feed temperature over the course of the test. The decline in rejection of module 148 is not fully understood. However, it is probable that the decline is similarly the responsibility of a temperature increase. Recent data indicates a stabilization at a rejection level of 94%.

Research effort at Albany International Research Co. has developed unit processes necessary for pilot scale production of several species of reverse osmosis hollow fiber composite membranes. These processes include spin-dope preparation, a proprietary apparatus for dry-jet wet-spinning of microporous polysul-fone hollow fibers, coating of these fibers with a variety of permselective materials, bundle winding using multifilament yarns and module assembly. Modules of the membrane identified as Quantro II are in field trial against brackish and seawater feeds. Brackish water rejections of 94+% at a flux of 5-7 gfd at 400 psi have been measured. Seawater rejections of 99+% at 1-2 gfd at 1000 psi have been measured. Membrane use requires sealing of some portion of the fiber bundle for installation in a pressure shell. Much effort has been devoted to identification of potting materials which exhibit satisfactory adhesion to the fiber while... 

A simplified flow scheme for a brackish water reverse osmosis plant is shown in Figure 5.24. In this example, it is assumed that the brackish water is heavily contaminated with suspended solids, so flocculation followed by a sand filter and a cartridge filter is used to remove particulates. The pH of the feed solution might be adjusted, followed by chlorination to sterilize the water to prevent bacterial growth on the membranes and addition of an anti-sealant to inhibit precipitation of multivalent salts on the membrane. Finally, if chlorine-sensitive interfacial composite membranes are used, sodium sulfite is added to remove excess chlorine before the water contacts the membrane. Generally, more pretreatment is required in plants using hollow fiber modules than in plants using spiral-wound modules. This is one reason why hollow fiber modules have been displaced by spiral-wound systems for most brackish water installations. 

Figure 5.24 Flow schematic of a typical brackish water reverse osmosis plant. The plant contains seven pressure vessels each containing six membrane modules. The pressure vessels are in a Christmas tree array to maintain a high feed velocity through the modules...

As discussed in Chapter 4.2.2, DuPont introduced linear aromatic polyamide membranes in hollow fine fiber form as the B-9 (brackish water) and B-10 (seawater) Permeators. These Permeators were available in 4-, 8- and 10-inch diameter models. The 4-, 8-, and 10-inch B-9 Permeators were capable of producing 4,200, 16,000, and 25,000 gallon per day of permeate, respectively, at 75% recovery (standard test conditions 1,500 ppm NaCl at 400 psig and 25°C).28 Permeators ranged from about 47 inches to 53 inches in length. DuPont discontinued these modules in 2001. 

Membrane research and development started in Du Pont in 1962 and culminated in the introduction of the first B-9 Permasep permeator for desalination of brackish water by reverse osmosis (RO) in 1969. The membrane in this B-9 Permasep module consisted of aramid hollow fibers. In 1969, proponents of RO technology had ambitious dreams and hopes. Today, RO is a major desalination process used worldwide to provide potable water from brackish and seawater feeds. Du Font s membrane modules for RO are sold under the trademark Permasep permeators. The RO business is a virtually autonomous profit center that resides in the Polymer Products Department. The growth and success of the Permasep products business is a direct result of Du Font s sustained research and development commitment to polyamides, a commitment that dates back to the 1930 s and the classic polymer researches of Wallace H. Carothers. Since 1969, improved and new Permasep permeators have been introduced six times, as shown in Table I. 

New membranes and modules PA spiral wound 24,000 mVday Brackish water Hypofiietical (pretreatment) ULP (100-150 psi) (membrane) [61]... 

The art of membrane manufacture has been perfected by a number of companies. Performance characteristics (water permeability as a function of salt rejection) of the standard 8 inch modules offered by several competitors are compared in Figures 15 and 16 for brackish water and seawater membranes, respectively. 

Figure 15 clearly illustrates the similar characteristics of the brackish water modules. For 99.5% salt rejection, membrane productivity varies slightly from 26 to 27 gallons/day/fl. At 99.7%, the spread increases to -24-27. Somewhat larger differences are evident in the seawater offerings - rejections of 99.75-99.8% are accompanied by productivities ranging fi om -17-22 gallons/day/ft. Manufacturers have had to match performance enhancements of competitors to the economic benefit of the end user. 

Commercial membrane separation processes include reverse osmosis, gas permeation, dialysis, electrodialysis, pervaporation, ultrafiltration, and microfiltration. Membranes are mainly synthetic or natural polymers in the form of sheets that are spiral wound or hollow fibers that are bundled together. Reverse osmosis, operating at a feed pressure of 1,000 psia, produces water of 99.95% purity from seawater (3.5 wt% dissolved salts) at a 45% recovery, or with a feed pressure of 250 psia from brackish water (less than 0.5 wt% dissolved salts). Bare-module costs of reverse osmosis plants based on purified water rate in gallons per day are included in Table 16.32. Other membrane separation costs in Table 16.32 are f.o.b. purchase costs. 

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