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Comparison of two thin-film composite membranes: low pressure FT-30 to very low pressure NF40HF
Desulination, 62 (1987) 183-191 Elsevier Science Publishers B.V.. Amsterdam -Printed
in The Netherlands
I83
Comparison of Two Thin-Film Composite Membranes: Low Pressure FT-30 to Very Low Pressure NF40HF* SCOTT D.N. FREEMAN
and THOMAS F. STOCKER
FiJmTec Corpomtion, 7200 Ohms Lane, Minneapolis, MN 55435 (U.S.A.) TeJ. 612-835-5475, TeJefax 612-835-4996, Telex 290899 FILMTEC EDNA
SUMMARY
There is a need in industry for specialized membranes to purify process chemicals and wastewaters. A new type of very low pressure membrane, designated NF40HF, has been developed which allows NaCl to pass more freely than MgSO,. NF40HF has separation properties between reverse osmosis and ultrafiltration. This paper compares NF40HF to a classic RO membrane, FT-30. Both are polyamide thin-film composite membranes. Both membranes exhibit high flux with FT-30 yielding about 1 m3/m2d (25 gfd) at 1.4 MN/m2 (200 psi) net driving pressure (NDP), 25”C, and 0.2% NaCl. NF4OHF yields the same flux at half the pressure. FT-30 has the high rejections expected of an RO membrane, typically 98% for NaCl and 9% for MgSO,. NF40HF rejections are more dependant on the anion involved, typically lO--4% for NaCl and 95% for MgSO,. The key to NF40HF application is the low rejection of NaCl and other salts with monovalent anions, coupled with a high rejection of divalent anionic salts and organics with a mol.wt. of more than 300--400 at very low pressures. There are many applications in the processing of foods, pharmaceuticals, chemicals, and industrial waste. INTRODUCTION
Since their development as practical unit operations in the late 1950’s and early 1960’s, reverse osmosis (RO) and ultrafiltration (UF) have been expanding the scope of their applications. Initially, RO was applied to the desalination of seawater, brackish water, and municipal tap water. Typically, *Presentedat the International Symposium on Synthetic Membrane Science and Technology, Dalian, China, April 13-18,1986.
OOll-9164/87/$03.50
o 1987 Eisevier Science Publishers B.V.
184
the useful product from the RO process has been purified water, but this situation is changing as the desalination membrane market matures and as new membrane technologies are developed. There are increasing demands on industry to conserve water, reduce energy consumption, control pollution, and reclaim useful materials from process and waste streams. These demands present membrane technology with new challenges and new markets. Nontraditional RO applications include municipal and industrial wastewater treatment, as well as chemical processing. Examples of municipal wastewater treatment using RO include Water Factory 21 in Orange County, California [l] and the Denver, Colorado 1.0 million gal/d (3.8 million l/d) demonstration plant [2]. At Water Factory 21 the purified wastewater is injected into the aquifer to replenish the drinking water supply, while at Denver the long range goal is to use the purified wastewater as feed to the drinking water treatment plant if testing proves that it is safe for human consumption. Concerning industrial wastewater applications, Slater et al. [3] have written a good review article with 97 references. They discuss a wide range of applications including the following industries: metal finishing, electronics, chemicals, food, paper, and textiles. A more recent article [4] discusses a comparison of two commercially available membranes in a coal-liquefaction wastewater pilot plant which would conserve water usage. There are few good literature references concerning chemical processing with RO because the processes are generally considered proprietary, but we know of many existing and future applications. For example, in some biotechnological and pharmaceutical processes, membranes are replacing distillation, because membranes do not cause thermal degradation of the valuable biochemicals. The potential for reducing costs and increasing yields is enormous. In the traditional desalination industry there are only a few commercially available membranes. Many membrane experts agree [ 5, 61 that the diversity of industrial applications creates niches for a variety of specialized membranes. To widen the range of membrane science, FilmTec has developed a new type of very low pressure membrane which allows NaCl to pass more freely than MgSO,. Sometimes this type of membrane is called ‘loose RO’, because the separation properties are between RO and UF. One of these new membranes is called NF40HF, where the NF stands for nanofiltration. The significance of the name is that the pore size is on the order of a nanometer. The purpose of this paper is to compare FT-30 to NF4OHF. GENERAL
COMPARISON
Both FT-30 and NF4OHF are thin-film composite membranes with the familiar construction of a support web coated with a microporous poly-
185
sulfone layer and then coated with an ultrathin barrier layer. FT-30 is a classic RO membrane with a long history of commercial application. Since its introduction in 1980 [7], over 50 million gal/d capacity have been installed around the world. These installations include the traditional RO applications of seawater, brackish water, and tap water desalinations, as well as interesting wastewater and chemical process separations. FT-30 has been ideal in many of these exotic applications because of its long life and good performance at extreme operating conditions. As discussed below, FT-30 spiral elements can tolerate wide ranges of pH (2-11 units, continuous operation; l-12 units, cleaning) and temperature (up to 45”C), as well as a variety of chemicals including some oxidizing agents that rapidly degrade other commercially available membranes. Dissolved oxygen causes no degradation, and short term exposure to low concentrations of free chlorine frequently causes no problem with system performance. One example is an accidental exposure of 0.5 mg/l free chlorine for 3 d. In addition, FT-30 spiral elements have shown stable flux and rejection over 5 y operating periods. NF40HF is a new type of membrane with separation properties between those of RO and UF. It became commercially available in 1985 after field testing of prototypes in 1984. Flux as a function
of pressure
The effects of pressure on permeate flux for spiral elements are shown in Figs. 1 and 2. Both membranes exhibit high flux with FT-30 yielding about 1 m3/mZd (25 gfd) at 1.4 MN/m* (200 psi) net driving pressure (NDP), 25°C and 0.2% NaCl feed concentration. NF40HF yields the same flux at half the pressure, 0.7 MN/m2 (100 psi) NDP. We intentionally cite the different pressures to yield 1 m3/m2d (25 gfd) flux, because we consider this a reasonably NoCl 00 ,
I
Pressure
(psi)
Fig. 1. Flux of FT-30 and NF40HF as a function of pressure based on a feed condition of 0.2% NaCl and 25°C with a spiral element.
186 MsW, 800
70-
E 0600 R,o;; x 403 IL 3020loo,
I 0
l,,,,i 40
60
,,,,,,, 120 160 200 Pressure (psi)
240
280
Fig. 2. Flux of FT-30 and NFIOHF as a function of pressure based on a feed condition of 0.2% MgSO, and 25°C with a spiral element.
typical flux for most field applications. Some references in the literature discuss fluxes in the range of 2-3 m3/mzd (50-75 gfd), which are only achievable in a laboratory setting. We consider these fluxes to be unrealistically high because it is our observation that fouling results with any membrane in actual field operation; therefore, we have limited our discussion to fluxes that can be achieved in actual field installations. The NF40HF flux is always greater than the FT-30 flux at identical conditions. Since FT-30 is well known as a membrane with high flux at low pres sure, then NH40HF is a membrane with high flux at very low pressure. The FT-30 flux is almost the same in both figures, while the NF40HF flux is lower in the MgSO, case (Fig. 2) than in the NaCl case (Fig. 1). This is caused by the difference in solute rejection. FT-30 rejects both solutes at roughly the same level, so the differential osmotic pressure is roughly the same for both solutes. NF40HF rejects NaCl at a much lower level than MgSO,; therefore, there is little differential osmotic pressure in the NaCl case. Solute rejection Table I compares typical rejections for a variety of solutes. The effects of pressure on solute rejection for spiral elements are shown in Fig. 3 (with NaCI) and 4 (with MgS04). FT-30 has the high rejections expected of an RO membrane, typically 98% for NaCl and 99% for MgSO,. NF40HF rejections are lower and are more dependant on the type of anion involved, typically lo-40% for NaCl and 95% for MgSO,. NF40HF exhibits low rejections of monovalent anions, but fairly high rejections of multivalent anions. The cationic concentration adjusts to maintain electrochemical balance.
187 TABLE I COMPARISON OF TYPICAL SOLUTE REJECTION (%)
Operating condition Net driving pressure (psi) Temperature (“C) Flux (gfd) Sodium chloride 0.2% 1.0% 2.0%
NF40HF
200 26 25
100 25 25
98 97 97
40 20 10
99 98 98
95 95 95
98 98
20 10
98
90
99
98
(NaCl)
Magnesium 0.2% 1.0% 2.0%
sulfate (MgSO J
Magnesium 0.2% 1.0%
chloride
Glucose (mol.wt. 0.2%
FT-30
(MgCl,)
180)
Sucrose (mol. wt. 342) 0.2%
Table II shows NF40HF flux and rejection as a function of concentration for a variety of sugars at 160 psi feed pressure with a spiral element. For example, with a 0.2-2.0% corn sugar mixture (which was 14% dextrose, NaCl
100
FT-30
s90& 2 60$ - 70 .: “:: 60 ;
20IOo,,.,,., 0
40
80
I I I I 120 160 200 Pressure (psi)
I
I 240
I
‘ 260
Fig. 3. Rejection by FT-30 and NF40HF a a function of pressure baaed on a feed condition of 0.2% NaCl and 26” C with a spiral element.
188
20IOo-7 0
1
1 40
I
I 80
I
, , , 120 160 Pressure
I (psi
, 200
1
,
, 240
I
, 200
Fig. 4. Rejection by FT-30 and NF40HF as a function of pressure based on a feed condition of 0.2% MgSO, and 25°C with a spiral element. TABLE II NF40HF FLUX AND REJECTION AS A FUNCTION SUGARS AT 150 psi FEED PRESSURE Sugar
OF CONCENTRATION
FOR
Concentration (%) 0.2
1.0
2.0
Fructose Flux (gfd) Rejection (%)
32 90
28 95
24 81
Sucrose Flux (gfd) Rejection (%)
31 98
32 99
38 99
Raffinose Flux (gfd) Rejection (%)
31 99
29 99
27 98
Corn sugar mixture Flux (gfd) Rejection (%)
31 97
28 97
27 97
12% maltose, 10% maltotriose, and 64% higher saccharides) an overall TOC rejection of 97% was observed. As an additional point of comparison, the nominal molecular weight cutoffs (which we define as about 98% rejection) are 100 for FT-30 and 300 for NF40HF. All data presented here in the tables and figures are based on single solute solutions, but it is interesting to consider mixed solute solutions. When
189
there is a mixture of solutes, there is preferential rejection of the multivalent anions. Therefore, in a mixture of NaCl and MgSO,, the NaCl rejection may drop to the 5% range. As discussed in the Applications section below, this is frequently advantageous because the goal may be to remove NaCl from compound X while simultaneously concentrating compound X. We do not fully understand the mechanism of this preferential rejection of multivalent anions, but since the membrane is negatively charged, we assume that the preferential rejection is a function of the ratio of monovalent anions to multivalent anions and also a function of the anionic charge densities. Effect of pH Table III compares the ranges of pH tolerance. Beyond these ranges there may be problems with long term membrane life. Salt rejection is not a strong function of pH. Operating an NF40HF element over the range of 3.9 to 8.7 pH units in the feed, the MgS04 rejection was essentially constant at about 95% (0.2% MgS04 feed, 150 psi, 25°C). Operating an FT-30 element over the range of 2.1 to 10.1 pH units in the feed, the NaCl rejection was essentially constant at 98% (0.2% NaCl feed, 250 psi, 25°C). TABLE III RANGE OF pH TOLERANCE
Continuous operation Periodic cleaning
FT-30
NFIOHF
2-11 l-12
5-8 4-9
Chlorine resistance FT-30 membrane has limited resistance to short term free chlorine exposure and is more resistant than other commercially available thin-film composite RO membranes. This ability to tolerate short term exposures has shown itself to be useful in field installations when a system upset has allowed free chlorine to breakthrough to the membrane. As a rule of thumb, short term exposure results in FT-30 degradation after about 200-1000 h at a concentration of 1 mg/l. The rate of degradation is dependent on many factors including the presence of heavy metals which catalyze the reaction; therefore, the allowable exposure time is shorter if there is iron (Fe) in the water. Qualitatively, NF40HF has more chlorine resistance than FT-30, but we have not finished enough tests to quantify this resistance. In one test, NF40HF failed after 2500 h exposure to 0.75 mg/l free chlorine; unfor-
190
tunately this test was tainted because of heavy ferric hydroxide (Fe(OH),) fouling which we assume catalyzed the chlorine attack.
APPLICATIONS
The key to NF40HF application is the low rejection of NaCl and other salts with monovalent anions, coupled with a high rejection of divalent anionic salts and organic material. In pure solutions the NaCl rejection is in the lO---40% range, while in mixed solutions the NaCl rejection can be even lower. There are two general types of applications where NF40HF is useful. One, when it is advantageous to separate NaCl and related salts from organic and/or divalent anionic salts, while simultaneously concentrating the organic and/or divalent anionic salts. Two, when it is important to reject or concentrate organic and/or divalent anionic salts, but it is not important what happens with NaCl or related salts. Examples of application type one include “salty” (NaCI) food processing, pharmaceutical processing, and wastewater streams. NF40HF has been successfully applied to desalting and simultaneously concentrating valuable materials by allowing NaCl, ethanol, and/or water to pass through the membrane to the permeate. Approval by the U.S. government’s Food and Drug Administration (FDA) regarding NF40HF’s use in processing food for human consumption has been applied for and is expected to be readily approved. FT-30 received FDA approval in December, 1984. Application type two takes advantage of the membrane’s separation properties in a different way. Typically in RO, most of the osmotic pressure is caused by the presence of NaCl. Since NaCl rejection is low, the differential osmotic pressure is much lower, and therefore the required operating pressure is much lower than with traditional RO. Considering an extreme example, over 80% of the osmotic pressure of seawater is caused by NaCl. If a low rejection of NaCl and related salts is acceptable, then NF40HF’s very low pressure requirements are economically attractive.
CONCLUSIONS
(1) There is a need in industry for specialized membranes for chemical processing and wastewater treatment. (2) NF40HF (a ‘loose RO’ membrane) yields the same flux as FT-30 (a classic low-pressure RO membrane) at about half the pressure. (3) NF40HF salt rejections are typically lO-4% for NaCl and 95% for MgSO, compared to FT-30’s 98% for NaCl and 9% for MgSO,.
191
(4) The key to NF40HF application is the low rejection of NaCl and other salts with monovalent anions. (5) In one type of application, a valuable material is desalted and simultaneously concentrated by allowing NaCl, ethanol, and/or water to pass through the membrane to the permeate. (6) In another type of application, the rejection of NaCl and related salts is not important and NF40HF’s very low pressure requirements are economically attractive. REFERENCES 1 D.G. Argo, Water reuse: where are we headed ?, Environ. Sci. Technol., 19 (1985) 208-214. 2 W.C. Lauer, S.E. Rogers and J.M. Ray, The current status of Denver’s potable water reuse project, J. AWWA, July (1985) 52-59. 3 C.S. Slater, R.C. Ahlert and C.G. Uchrin, Applications of reverse osmosis to complex industrial wastewater treatment, Desalination, 48 (1983) 171-187. 4 D. Bhattacharyya, M. Jevtitch, J.K. Ghosal and J. Kozminsky, Reverse osmosis membrane for treating coal-liquefaction wastewater, Environ. Progress, 3 (1984) 95-102. 5 J.E. Cadotte, R.J. Petersen, R.E. Larson and E.E. Erickson, A new thin-film composite seawater reverse osmosis membrane, Desalination, 32 (1980) 25-31. 6 R.E. Larson, Industrial applications for RO and ultrafiltration: a technology driven market, Tech. Proc. 12th Ann. Conf., Water Supply Improvement Ass., Orlando, FL, May 13-18, 1984, Vol. 1. 7 M. Kurihara, T. Uemura, Y. Nakagawa and T. Tonomura, The thin-film composite low pressure reverse osmosis membranes, Desalination, 54 (1985) 75-88.
in The Netherlands
I83
Comparison of Two Thin-Film Composite Membranes: Low Pressure FT-30 to Very Low Pressure NF40HF* SCOTT D.N. FREEMAN
and THOMAS F. STOCKER
FiJmTec Corpomtion, 7200 Ohms Lane, Minneapolis, MN 55435 (U.S.A.) TeJ. 612-835-5475, TeJefax 612-835-4996, Telex 290899 FILMTEC EDNA
SUMMARY
There is a need in industry for specialized membranes to purify process chemicals and wastewaters. A new type of very low pressure membrane, designated NF40HF, has been developed which allows NaCl to pass more freely than MgSO,. NF40HF has separation properties between reverse osmosis and ultrafiltration. This paper compares NF40HF to a classic RO membrane, FT-30. Both are polyamide thin-film composite membranes. Both membranes exhibit high flux with FT-30 yielding about 1 m3/m2d (25 gfd) at 1.4 MN/m2 (200 psi) net driving pressure (NDP), 25”C, and 0.2% NaCl. NF4OHF yields the same flux at half the pressure. FT-30 has the high rejections expected of an RO membrane, typically 98% for NaCl and 9% for MgSO,. NF40HF rejections are more dependant on the anion involved, typically lO--4% for NaCl and 95% for MgSO,. The key to NF40HF application is the low rejection of NaCl and other salts with monovalent anions, coupled with a high rejection of divalent anionic salts and organics with a mol.wt. of more than 300--400 at very low pressures. There are many applications in the processing of foods, pharmaceuticals, chemicals, and industrial waste. INTRODUCTION
Since their development as practical unit operations in the late 1950’s and early 1960’s, reverse osmosis (RO) and ultrafiltration (UF) have been expanding the scope of their applications. Initially, RO was applied to the desalination of seawater, brackish water, and municipal tap water. Typically, *Presentedat the International Symposium on Synthetic Membrane Science and Technology, Dalian, China, April 13-18,1986.
OOll-9164/87/$03.50
o 1987 Eisevier Science Publishers B.V.
184
the useful product from the RO process has been purified water, but this situation is changing as the desalination membrane market matures and as new membrane technologies are developed. There are increasing demands on industry to conserve water, reduce energy consumption, control pollution, and reclaim useful materials from process and waste streams. These demands present membrane technology with new challenges and new markets. Nontraditional RO applications include municipal and industrial wastewater treatment, as well as chemical processing. Examples of municipal wastewater treatment using RO include Water Factory 21 in Orange County, California [l] and the Denver, Colorado 1.0 million gal/d (3.8 million l/d) demonstration plant [2]. At Water Factory 21 the purified wastewater is injected into the aquifer to replenish the drinking water supply, while at Denver the long range goal is to use the purified wastewater as feed to the drinking water treatment plant if testing proves that it is safe for human consumption. Concerning industrial wastewater applications, Slater et al. [3] have written a good review article with 97 references. They discuss a wide range of applications including the following industries: metal finishing, electronics, chemicals, food, paper, and textiles. A more recent article [4] discusses a comparison of two commercially available membranes in a coal-liquefaction wastewater pilot plant which would conserve water usage. There are few good literature references concerning chemical processing with RO because the processes are generally considered proprietary, but we know of many existing and future applications. For example, in some biotechnological and pharmaceutical processes, membranes are replacing distillation, because membranes do not cause thermal degradation of the valuable biochemicals. The potential for reducing costs and increasing yields is enormous. In the traditional desalination industry there are only a few commercially available membranes. Many membrane experts agree [ 5, 61 that the diversity of industrial applications creates niches for a variety of specialized membranes. To widen the range of membrane science, FilmTec has developed a new type of very low pressure membrane which allows NaCl to pass more freely than MgSO,. Sometimes this type of membrane is called ‘loose RO’, because the separation properties are between RO and UF. One of these new membranes is called NF40HF, where the NF stands for nanofiltration. The significance of the name is that the pore size is on the order of a nanometer. The purpose of this paper is to compare FT-30 to NF4OHF. GENERAL
COMPARISON
Both FT-30 and NF4OHF are thin-film composite membranes with the familiar construction of a support web coated with a microporous poly-
185
sulfone layer and then coated with an ultrathin barrier layer. FT-30 is a classic RO membrane with a long history of commercial application. Since its introduction in 1980 [7], over 50 million gal/d capacity have been installed around the world. These installations include the traditional RO applications of seawater, brackish water, and tap water desalinations, as well as interesting wastewater and chemical process separations. FT-30 has been ideal in many of these exotic applications because of its long life and good performance at extreme operating conditions. As discussed below, FT-30 spiral elements can tolerate wide ranges of pH (2-11 units, continuous operation; l-12 units, cleaning) and temperature (up to 45”C), as well as a variety of chemicals including some oxidizing agents that rapidly degrade other commercially available membranes. Dissolved oxygen causes no degradation, and short term exposure to low concentrations of free chlorine frequently causes no problem with system performance. One example is an accidental exposure of 0.5 mg/l free chlorine for 3 d. In addition, FT-30 spiral elements have shown stable flux and rejection over 5 y operating periods. NF40HF is a new type of membrane with separation properties between those of RO and UF. It became commercially available in 1985 after field testing of prototypes in 1984. Flux as a function
of pressure
The effects of pressure on permeate flux for spiral elements are shown in Figs. 1 and 2. Both membranes exhibit high flux with FT-30 yielding about 1 m3/mZd (25 gfd) at 1.4 MN/m* (200 psi) net driving pressure (NDP), 25°C and 0.2% NaCl feed concentration. NF40HF yields the same flux at half the pressure, 0.7 MN/m2 (100 psi) NDP. We intentionally cite the different pressures to yield 1 m3/m2d (25 gfd) flux, because we consider this a reasonably NoCl 00 ,
I
Pressure
(psi)
Fig. 1. Flux of FT-30 and NF40HF as a function of pressure based on a feed condition of 0.2% NaCl and 25°C with a spiral element.
186 MsW, 800
70-
E 0600 R,o;; x 403 IL 3020loo,
I 0
l,,,,i 40
60
,,,,,,, 120 160 200 Pressure (psi)
240
280
Fig. 2. Flux of FT-30 and NFIOHF as a function of pressure based on a feed condition of 0.2% MgSO, and 25°C with a spiral element.
typical flux for most field applications. Some references in the literature discuss fluxes in the range of 2-3 m3/mzd (50-75 gfd), which are only achievable in a laboratory setting. We consider these fluxes to be unrealistically high because it is our observation that fouling results with any membrane in actual field operation; therefore, we have limited our discussion to fluxes that can be achieved in actual field installations. The NF40HF flux is always greater than the FT-30 flux at identical conditions. Since FT-30 is well known as a membrane with high flux at low pres sure, then NH40HF is a membrane with high flux at very low pressure. The FT-30 flux is almost the same in both figures, while the NF40HF flux is lower in the MgSO, case (Fig. 2) than in the NaCl case (Fig. 1). This is caused by the difference in solute rejection. FT-30 rejects both solutes at roughly the same level, so the differential osmotic pressure is roughly the same for both solutes. NF40HF rejects NaCl at a much lower level than MgSO,; therefore, there is little differential osmotic pressure in the NaCl case. Solute rejection Table I compares typical rejections for a variety of solutes. The effects of pressure on solute rejection for spiral elements are shown in Fig. 3 (with NaCI) and 4 (with MgS04). FT-30 has the high rejections expected of an RO membrane, typically 98% for NaCl and 99% for MgSO,. NF40HF rejections are lower and are more dependant on the type of anion involved, typically lo-40% for NaCl and 95% for MgSO,. NF40HF exhibits low rejections of monovalent anions, but fairly high rejections of multivalent anions. The cationic concentration adjusts to maintain electrochemical balance.
187 TABLE I COMPARISON OF TYPICAL SOLUTE REJECTION (%)
Operating condition Net driving pressure (psi) Temperature (“C) Flux (gfd) Sodium chloride 0.2% 1.0% 2.0%
NF40HF
200 26 25
100 25 25
98 97 97
40 20 10
99 98 98
95 95 95
98 98
20 10
98
90
99
98
(NaCl)
Magnesium 0.2% 1.0% 2.0%
sulfate (MgSO J
Magnesium 0.2% 1.0%
chloride
Glucose (mol.wt. 0.2%
FT-30
(MgCl,)
180)
Sucrose (mol. wt. 342) 0.2%
Table II shows NF40HF flux and rejection as a function of concentration for a variety of sugars at 160 psi feed pressure with a spiral element. For example, with a 0.2-2.0% corn sugar mixture (which was 14% dextrose, NaCl
100
FT-30
s90& 2 60$ - 70 .: “:: 60 ;
20IOo,,.,,., 0
40
80
I I I I 120 160 200 Pressure (psi)
I
I 240
I
‘ 260
Fig. 3. Rejection by FT-30 and NF40HF a a function of pressure baaed on a feed condition of 0.2% NaCl and 26” C with a spiral element.
188
20IOo-7 0
1
1 40
I
I 80
I
, , , 120 160 Pressure
I (psi
, 200
1
,
, 240
I
, 200
Fig. 4. Rejection by FT-30 and NF40HF as a function of pressure based on a feed condition of 0.2% MgSO, and 25°C with a spiral element. TABLE II NF40HF FLUX AND REJECTION AS A FUNCTION SUGARS AT 150 psi FEED PRESSURE Sugar
OF CONCENTRATION
FOR
Concentration (%) 0.2
1.0
2.0
Fructose Flux (gfd) Rejection (%)
32 90
28 95
24 81
Sucrose Flux (gfd) Rejection (%)
31 98
32 99
38 99
Raffinose Flux (gfd) Rejection (%)
31 99
29 99
27 98
Corn sugar mixture Flux (gfd) Rejection (%)
31 97
28 97
27 97
12% maltose, 10% maltotriose, and 64% higher saccharides) an overall TOC rejection of 97% was observed. As an additional point of comparison, the nominal molecular weight cutoffs (which we define as about 98% rejection) are 100 for FT-30 and 300 for NF40HF. All data presented here in the tables and figures are based on single solute solutions, but it is interesting to consider mixed solute solutions. When
189
there is a mixture of solutes, there is preferential rejection of the multivalent anions. Therefore, in a mixture of NaCl and MgSO,, the NaCl rejection may drop to the 5% range. As discussed in the Applications section below, this is frequently advantageous because the goal may be to remove NaCl from compound X while simultaneously concentrating compound X. We do not fully understand the mechanism of this preferential rejection of multivalent anions, but since the membrane is negatively charged, we assume that the preferential rejection is a function of the ratio of monovalent anions to multivalent anions and also a function of the anionic charge densities. Effect of pH Table III compares the ranges of pH tolerance. Beyond these ranges there may be problems with long term membrane life. Salt rejection is not a strong function of pH. Operating an NF40HF element over the range of 3.9 to 8.7 pH units in the feed, the MgS04 rejection was essentially constant at about 95% (0.2% MgS04 feed, 150 psi, 25°C). Operating an FT-30 element over the range of 2.1 to 10.1 pH units in the feed, the NaCl rejection was essentially constant at 98% (0.2% NaCl feed, 250 psi, 25°C). TABLE III RANGE OF pH TOLERANCE
Continuous operation Periodic cleaning
FT-30
NFIOHF
2-11 l-12
5-8 4-9
Chlorine resistance FT-30 membrane has limited resistance to short term free chlorine exposure and is more resistant than other commercially available thin-film composite RO membranes. This ability to tolerate short term exposures has shown itself to be useful in field installations when a system upset has allowed free chlorine to breakthrough to the membrane. As a rule of thumb, short term exposure results in FT-30 degradation after about 200-1000 h at a concentration of 1 mg/l. The rate of degradation is dependent on many factors including the presence of heavy metals which catalyze the reaction; therefore, the allowable exposure time is shorter if there is iron (Fe) in the water. Qualitatively, NF40HF has more chlorine resistance than FT-30, but we have not finished enough tests to quantify this resistance. In one test, NF40HF failed after 2500 h exposure to 0.75 mg/l free chlorine; unfor-
190
tunately this test was tainted because of heavy ferric hydroxide (Fe(OH),) fouling which we assume catalyzed the chlorine attack.
APPLICATIONS
The key to NF40HF application is the low rejection of NaCl and other salts with monovalent anions, coupled with a high rejection of divalent anionic salts and organic material. In pure solutions the NaCl rejection is in the lO---40% range, while in mixed solutions the NaCl rejection can be even lower. There are two general types of applications where NF40HF is useful. One, when it is advantageous to separate NaCl and related salts from organic and/or divalent anionic salts, while simultaneously concentrating the organic and/or divalent anionic salts. Two, when it is important to reject or concentrate organic and/or divalent anionic salts, but it is not important what happens with NaCl or related salts. Examples of application type one include “salty” (NaCI) food processing, pharmaceutical processing, and wastewater streams. NF40HF has been successfully applied to desalting and simultaneously concentrating valuable materials by allowing NaCl, ethanol, and/or water to pass through the membrane to the permeate. Approval by the U.S. government’s Food and Drug Administration (FDA) regarding NF40HF’s use in processing food for human consumption has been applied for and is expected to be readily approved. FT-30 received FDA approval in December, 1984. Application type two takes advantage of the membrane’s separation properties in a different way. Typically in RO, most of the osmotic pressure is caused by the presence of NaCl. Since NaCl rejection is low, the differential osmotic pressure is much lower, and therefore the required operating pressure is much lower than with traditional RO. Considering an extreme example, over 80% of the osmotic pressure of seawater is caused by NaCl. If a low rejection of NaCl and related salts is acceptable, then NF40HF’s very low pressure requirements are economically attractive.
CONCLUSIONS
(1) There is a need in industry for specialized membranes for chemical processing and wastewater treatment. (2) NF40HF (a ‘loose RO’ membrane) yields the same flux as FT-30 (a classic low-pressure RO membrane) at about half the pressure. (3) NF40HF salt rejections are typically lO-4% for NaCl and 95% for MgSO, compared to FT-30’s 98% for NaCl and 9% for MgSO,.
191
(4) The key to NF40HF application is the low rejection of NaCl and other salts with monovalent anions. (5) In one type of application, a valuable material is desalted and simultaneously concentrated by allowing NaCl, ethanol, and/or water to pass through the membrane to the permeate. (6) In another type of application, the rejection of NaCl and related salts is not important and NF40HF’s very low pressure requirements are economically attractive. REFERENCES 1 D.G. Argo, Water reuse: where are we headed ?, Environ. Sci. Technol., 19 (1985) 208-214. 2 W.C. Lauer, S.E. Rogers and J.M. Ray, The current status of Denver’s potable water reuse project, J. AWWA, July (1985) 52-59. 3 C.S. Slater, R.C. Ahlert and C.G. Uchrin, Applications of reverse osmosis to complex industrial wastewater treatment, Desalination, 48 (1983) 171-187. 4 D. Bhattacharyya, M. Jevtitch, J.K. Ghosal and J. Kozminsky, Reverse osmosis membrane for treating coal-liquefaction wastewater, Environ. Progress, 3 (1984) 95-102. 5 J.E. Cadotte, R.J. Petersen, R.E. Larson and E.E. Erickson, A new thin-film composite seawater reverse osmosis membrane, Desalination, 32 (1980) 25-31. 6 R.E. Larson, Industrial applications for RO and ultrafiltration: a technology driven market, Tech. Proc. 12th Ann. Conf., Water Supply Improvement Ass., Orlando, FL, May 13-18, 1984, Vol. 1. 7 M. Kurihara, T. Uemura, Y. Nakagawa and T. Tonomura, The thin-film composite low pressure reverse osmosis membranes, Desalination, 54 (1985) 75-88.