Seawater membrane

Seawater membranes are used to treat high-salinity (35,000 to 50,000 ppm total dissolved solids (TDS)) feed waters. These membranes can operate at pressures up to 1,500 psi. Typical membrane test conditions are as follows  

Test conditions are important to take note of as these are the conditions under which rated performance is based. Operating under 

Manufacturer Location Brand Membrane Material Spiral Wound Configuration 

Koch Membrane Systems, Inc. Wilmington, MA, Koch PA Standard 

Dow Water Process Solutions Edina, MN FilmTec PA Standard 

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Fig. 1. Water flux and NaCl rejection of several membrane types (10), where (D) represents seawater membranes, which operate at 5.5 MPa and 25°C ...

Figure 4. Normalized salt rejection vs. operating time for seawater membranes tested at Eilat site...

As a result of prior field experience with the furan system, a qualifying test has been employed at Albany International Research Co. to measure the durability. Table III displays data of typical samples tested against synthetic seawater at 1000 psig maintained at a temperature of 50°C. Samples of cellulosic seawater membrane and polyamide membrane were found to fail in several hours of challenge by these conditions. 

Koyuncu et al. [56] presented pilot-scale studies on the treatment of pulp and paper mill effluents using two-stage membrane filtrations, ultrafiltration and reverse osmosis [56]. The combination of UF and RO resulted in very high removals of COD, color, and conductivity from the effluents. At the end of a single pass with seawater membrane, the initial COD, color and conductivity values were reduced to 10-20 mg/L, 0-100 PCCU (platinum cobalt color units) and 200-300 ps/cm, respectively. Nearly complete color removals were achieved in the RO experiments with seawater membranes. 

Fig. 1. Water flux and NaCl rejection of several membrane types (10), where (D) represents seawater membranes, which operate at 5.5 MPa and 25°C ( ), brackish water membranes, which operate at 1500 mg/L NaCl feed, 1.5 MPa, and 25°C and (SSI) nanofiltration membranes, which operate at 500 mg/L NaCl feed, 0.74 MPa, and 25°C. A represents cellulose acetate—cellulose triacetate B, linear aromatic polyamide C, cross-linked polyether D, cross-linked fully aromatic polyamide E, other thin-film composite membranes F, asymmetric membranes G, BW-30 (FilmTec) H, SU-700 (Toray) I, A-15 (Du Pont) J, NTR-739HF (Nitto-Denko) K, NTR-729HF (Nitto-Denko) L, NTR-7250 (Nitto-Denko) M, NF40 (FilmTec) N, NF40HF (FilmTec) O, UTC-40HF (Toray) P, NF70 (FilmTec) Q, UTC-60 (Toray) R, UTC-20HF (Toray) and S, NF50 (FilmTec). To convert MPa to psi,...

Parameter Seawater membrane (SW-30) Brackish water membrane (CA) Nanofiltration membrane (NTR-7250)... 

Seawater has a salt concentration of 3.2-4.0%, depending on the region of the world. Because of this high salinity, only membranes with salt rejections of 99.3 % or more can produce potable water in a single pass. Application to seawater desalination of the first-generation cellulose acetate membranes, with rejections of 97-99 %, was limited. With the development of the polyamide hollow fine fibers and interfacial composites, suitable seawater membranes became available, and many plants have been installed. In general, membranes are not... 

Table 4.5 Comparison of Koch Membrane Systems seawater membranes.28...

Koch Membrane Systems Seawater Membrane Permeate Flow gpd Rejection (%)... 

Within the classification of seawater membranes, there are subsets of membrane that are rated for different performance. For example, Koch Membrane Systems offers a standard high rejection seawater membrane module plus a high-flow seawater element seawater membrane module. Table 4.5 lists the productivity and rejection for two seawater membrane types. Other membrane suppliers offer similar variety in seawater membranes. 

As with seawater membranes, there is no one uniform test condition for all brackish water membranes of the same type. Thus, a direct comparison between manufacturers requires a close look at the test conditions. 

Seawater Membrane— The seawater test is similar to the brackish water test except that the feedwater is an aqueous sodium chloride solution with a concentration of 35,000 mg/ and the test pressure is 800 psig. 

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 16. Permeate flow rate per unit membrane area (gallons/day/fl ) and NaCl rejection of seawater membranes offered by GE (K>), FilmTec/Dow (x), Koch (o), Toray (n), and Nitto Denko/Hydranautics (Us). All values taken from the manufacturers web sites. Test conditions for all membranes were 32,000ppm NaCl feed concentration, 800 psi feed pressure, and 77 °F feed temperature. Permeate recovery varied slightly in the tests from 7-10% andfeed pH variedfrom 6.5-8. Note that a 32,800 ppm NaCl feed was used to obtain the Koch values normalization to 32,000 ppm NaCl increases permeability by 2%.

Test conditions are important to take note of as these are the conditions xmder which rated performance is based. Operating under different conditions will result in performance that differs from the rated performance. (Chapter 9 discusses the effect of varying operating conditions on the performance of RO membranes). Notice that there is not one uniform test condition to which all membrane manufacturers adhere. Therefore, because of the difference in pH and recovery xmder such membranes are tested, the rated performance of seawater membranes from different manufacturers cannot be directly compared. 

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