Brackish water membranes test conditions

Brackish water membranes are designed to treat lower-salinity feed waters, up to about 4,000 to 5,000 ppm (TDS). Maximum operating pressure for brackish water membranes is typically 600 psi. These membranes are typically tested at the following conditions ... 

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. 

Figure 15. Permeate flow rate per unit membrane area (gallons/day/ft ) and NaCl rejection of brackish water membranes offered by GE (<>), FilmTec/Dow (x), Koch (o), Toray (a), Trisep (+), andNitto Denko/Hydranautics (Is). All values taken from the manufacturers web sites. Test conditions for all membranes were 2000 ppm NaCl feed concentration, 225 psi feed pressure, 77 F feed temperature, and 15% permeate recovery. Feed pH for test conditions varied slightly in the tests from 6.5-8. Note that a 1500 ppm NaCl feed was used to obtain the Nitto Denko/Hydranautics values normalization to 2000 ppm NaCl reduces permeability by S%.

The objective of employing organophilic bentonite is flux stabilization. In terms of the membrane compaction slope the stabilizing effect is exemplified by the following figures (brackish water conditions) reference, -0.10 bentonite-doped, -0.06. In a field test over 1300 hours on well water of 5200 ppm TDS at a pressure of 60 bar, starting with an initial flux of 1780 1/m d and 95 % rejection, a compaction slope of -0.058 was found under the same conditions the reference membrane had a compaction slope of -0.094. 

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. 

Properties of FT-30. The properties of FT-30 membranes have been reviewed in several publications. Therefore, only the salient features that relate to the chemistry of the barrier layer will be considered here. Reverse osmosis performance of FT-30 under seawater and brackish water test conditions was described by Cadotte et al (48) and by Larson et al (51). In commercially produced spiral-wound elements the FT-30 membrane typically gives 99.0 to 99.2 percent salt rejection at 24 gfd (40 L/sq m/hr) flux in seawater reverse osmosis tests with 3.5 percent synthetic seawater at 800 psi (5516 kPascaJJand 25°C. 

The properties of FT-30 membranes have been reviewed in several publications, including reverse osmosis performance under seawater and brackish water test conditions.60"62 In commercially produced spiral-wound elements, the FT-30 membrane typically gives 99.1 to 99.3% salt rejection at 24 gfd flux in seawater desalination at 800 psi and 25°C. In brackish water applications, FT-30 spiral elements can be operated at system pressures of as low as 225 psi while producing water at 22 to 24 gfd. Similar flux levels are possible with the TFC-202 and LP-300 membranes, as mentioned earlier. But it is notable that those membranes achieve such high fluxes through use of extremely thin surface barrier layers about only one-tenth the thickness of the FT-30 barrier layer. 

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