Removal of organic foulants from feed waters by dynamic membranes

DESALINATION ELSEVIER

Desalination 125 (1999) 65-75 www.elsevier.com/locate/desal

Removal of organic foulants from feed waters by dynamic membranes Marc Altman, Raphael Semiat*, David Hasson WRI Rabin Desalination Laboratory, Department of Chemical Engineering, Technion - Israel Institute of Technology, Haifa 32000, Israel Tel. + 972 (4) 829-2009; Fax +972 (4) 823-0476; email: cesemiat@techunix, technion, ac. il

Abstract

Seeking economic solutions for the pre-treatment of polluted waters is at the forefront of current reverse osmosis (RO) desalination research. The modem trend is to remove the polluting species by various UF/MF membrane pretreatment schemes. This paper describes progress in an ongoing project aiming to improve membrane pre-treatment processes by placing dynamic membranes in front of the UF/MF unit so as to relieve the load on the downstream processing. Dynamic membranes (DM), a class of membranes that has not yet found general industrial application, are formed in situ by colloid deposition on a porous support. They are an attractive method to relieve the load on the downstream pre-treatment membranes because, once fouled, they can be removed and reformed in place. This research has shown that DM formation conditions for organic foulant retention are not necessarily those suggested in the literature for other applications. The membranes tested were formed by dead end filtration of a hydrous zirconium oxide colloidal suspension on inexpensive non-woven fiat sheet supports and post-treated by a poly(acrylic acid) solution. The quality o f a DM membrane was characterized by measurements of permeate flux and retention of ovalbumin, used as a model contaminant. The properties of dynamic membranes are known to depend on a large number of variables. Results are described showing the effect of parameters found to exert a significant influence on membrane properties. They include the zirconium oxide colloid concentration, the tightness of the support fabric, the pH level of the colloidal suspension and the poly(acrylic acid) post-treatment. The best dynamic membrane achieved exhibited 85% ovalbumin retention at a concentration of 1000 ppm and over 95% retention at a concentration of 50 ppm. Permeation rates were in the range of UF fluxes (10-50 l/m2.h.bar). The work carried out to date has yielded encouraging results that are being currently extended. Keywords:

Dynamic membranes; Formed-in-place membrane; Membrane pre-treatment; Hydrous zirconium oxide; Membrane fouling

*Corresponding author. Presented at the Conference on Desalination and the Environment, Las Palmas, Gran Canaria, November 9-12, 1999. European Desalination Society and the International Water Services Association. 0011-9164/99/$- See front matter © 1999 Elsevier Science B.V. All rights reserved PII: S 0 0 1 1 - 9 1 6 4 ( 9 9 ) 0 0 1 2 4 -

1

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M. Altman et al. / Desalination 125 (1999) 65-75

1. Introduction Membranes exhibiting separation properties similar to those of conventional membranes can be dynamically created in-situ through deposition of colloidal particles of certain hydrous oxides on a porous substrate [1]. The potential benefits of dynamic membranes lie in the possibility to form them in situ by simple processes and that, once a membrane is too severely fouled, the layer can be removed and a new layer deposited on the same porous support. However, large-scale application of dynamic membranes has thus far been very limited due to their variability and many parameters affecting their performance. This paper describes progress in an ongoing project investigating the possible benefit of integrating dynamic membranes in RO pretreatment processes using UF/MF membranes for removing organic foulants of polluted surface waters. The aim is to test the feasibility of placing a dynamic membrane unit in front of the more expensive UF/MF units so as to relieve their fouling load. The variability in dynamic membrane performance would be of secondary importance in such an application.

2. Scope Dynamic membranes were first reported in 1965 by workers at the Oak Ridge Laboratories engaged in desalination research [1 ]. Interest in dynamic membranes initially centered on desalination applications. Since the early 1980s, interest in dynamic membranes has shifted to the research of UF applications [2-6], mostly for treatment of effluents from the textile industry [3] and recovery and disposal of dyes [2,4]. Some success was achieved in the treatment of textile wastewater streams. Studies have shown that a variety of colloidal species can be used to form dynamic membranes, but the most successful material appears to be hydrous zirconium oxide. Studies have also

shown that the rejection properties of hydrous zirconium oxide can be considerably improved by post-treatment with a polymer. Most investigations have adopted poly(acrylic acid) (PAA) for the post-treatment. A few studies [7-9] have attempted to model some aspects of the formation and performance of dynamic membranes. This work confirms that the simple models proposed by Freilich and Tanny [7], based on conventional expressions for filtration, adequately describe dynamic membrane formation. The properties of dynamic membranes are known to depend on a large number of variables. Guided by an extensive literature review and exploratory work, the influence of some of the important parameters was studied in systematic tests in which the following conditions were held constant: • the dynamic membrane was formed from a colloidal suspension of hydrous zirconium oxide • the exploratory work led to the adoption of the dead-end filtration technique for the creation of a dynamic membrane by colloids deposition on a non-woven fabric • cake formation was carried out at a filtration pressure of 4 to 5 bar • the contaminant selected for testing DM retention is the protein ovalbumin (MW45,000) • contaminant feed solutions were prepared in a saline medium of 2500ppm NaCI and contained normally 0.1% crude ovalbumin with 0.05% of the biocide sodium azide. The effect of the following parameters was tested by measuring membrane permeability, rate of fouling and contaminant rejection: • tightness of the substrate, by using two calendared non-woven fabrics of different permeability • pH of the hydrous zirconium oxide solution in the range of 2 to 6

67

M. Altman et al. / Desalination 125 (1999) 65-75

post-treatment of the hydrous zirconium oxide layer by PAA concentration of contaminant in the feed solution at several compositions in the range of 1000 to 10 ppm ovalbumin.

3. Experimental 3.1. Membrane system

Fig. 1 illustrates the experimental system. A rectangular passage stainless steel cell [6,11] was used to produce and test dynamic membranes. Flow within the cell was in a 4 mm high passage, 284mm long and 66mm wide. Grids at the entrance and exit of the cell served to provide a uniform flow distribution. A 321 glass feed tank held the zirconium hydroxide suspension during the membrane formation stage and the fouling solution during the dynamic membrane testing stage. The membrane cell was fed by a low-pressure centrifugal pump which provides pressures of up to 8 bar. The feed tank temperature, membrane exit pressure and permeate flow rate were measured and recorded continuously throughout all experiments.

3.2. Materials 3.2. I. Membrane substrate

The membrane supports selected were nonwoven calendared fabrics composed of random thermal-bound polypropylene and polyethylene fibres and were provided by Freudenberg Faservliessoff KG (Weinheim, Germany). The Viledon® FO-series fabrics are expressly marketed as membrane supports [12]. These relatively inexpensive supports were chosen based on preliminary experiments using a number of non-woven fabric supports. Experiments were conducted on two different fabrics (Table 1). The permeability of the looser fabric, FO2431, was about 2.5 times higher than that of the tighter fabric, FO2430. 3.2.2. Hydrous zirconium oxide

The hydrous zirconium oxide particles were formed by raising the pH of a solution containing Zr 4÷ions. The source of the ions was anhydrous ZrCI4 purchased from Fluka. The chemistry of hydrous zirconium oxide is complex, and the structure of the compound depends on such factors as the pH, ionic strength, Z r 4+ concentration and the presence of other

Membrane

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Fig. 1. Experimentalsystem.

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68

M. Altman et al. / Desalination 125 (1999) 65-75

Table 1 Characteristics of support fabrics Parameter

FO2430

FO2431

Specific weight, g/m2 Thickness, mm Max. cross-direction tensile strength, N/5 cm Max. machine-direction tensile strength, N/5 cm Air permeability at 2 mbar, 1/m2.s

100 + 10 0.22 + 0.03 170

60 + 5 0.14+ 0.02 110

260

170

180 ± 50

500 ± 70

membrane was constructed by depositing a layer of hydrous zirconium oxide on the porous support. This was followed by the post-treatment of the membrane by the addition of PAA. The membrane formation technique was based on elements of the methods described by Thomas [17] and Freilich and Tanny [7]. The most notable differences between the majority of works in the literature and this work are the deadend mode and choice of porous support. The procedure is described elsewhere in greater detail [6]. 3.3.1. Zirconium deposition

anions and cations. Commonly proposed structures include complexes such as [Zr(OH)4]n. Zr2(OH)7 +, [Zra(OH)s] s+, [Zr3(OH)4] s+ and ZrO(OH)2"H20 [ 13-15]. 3.2.3. Poly(acrylic acid) Sodium polyacrylate with an average molecular weight of 60,000 purchased from Fluka was used to modify the hydrous zirconium oxide dynamic membranes. 3.2. 4. Egg white powder The model foulant selected to simulate an organic foulant in membrane pre-treatment was crude dried egg white. The egg white is mainly a mixture of a variety of proteins, principally ovalbumin. Proteins constitute 88.2% of the weight of dried egg whites. The rest of the egg white consists of lipids (0.3%), carbohydrates (7.1%) and minerals (4.4%). The main proteins of typical egg white are ovalbumin (54%), conalbumin (13%) and ovomucoid (11%) with molecular weights of 45,000, 80,000 and 28,000, respectively [16]. 3.3. Procedure Each experiment was divided into three distinct consecutive stages. First, the dynamic

An aqueous solution containing 50 gNaCl and 3 g ZrCI4 was added to 201 of distilled water in the feed tank. Hydrous zirconium oxide colloids were formed by raising the pH by adding NaOH. Following about 1 h of mixing, the membrane cell entrance valve and bypass valve were open in the so-called dead-end configuration. The feed was circulated in the system and the experiment run at a pressure of 4-5 bar and a flow rate of 10-121/min for 4h. At regular intervals during the course of the experiment, the pH and turbidity of both permeate and feed samples were recorded. Turbidity reduction in the permeate and feed indicated the development of the deposited layer. 3.3.2. Modification with polyacrylic acid The PAA post-treatment is presumed to involve the insertion of the coiled polymeric molecules at low pH that are then entrapped as the molecules expand at a high pH. The following procedure was adopted. An aqueous solution of 1 g of PAA and 50g NaCI was mixed with 201 of distilled water in the feed tank. The feed solution was acidified to pH 2 with HCI. Dead-end flow of the PAA solution was carried for 45 min at this pH. This was then followed by dead-end operation at pH levels of 3, 4.5, 6 and 7, for 30 min each.

M. Altman et al. / Desalination 125 (1999) 65-75

3.3.3. Separation of egg albumin The model foulant solution was formed by dissolving the desired quantity of egg white powder, together with 50 g NaC1 and 10 g NaN3 in distilled water at pH 7. The NaN 3was added to ensure that the protein feed solution and samples collected could be used for several hours without any undesired parasitic growth. The powdered protein was dissolved in the pH adjusted saline solution to maximize its solubility and to avoid denaturation. The final feed consisted of a mixture of dissolved protein, protein agglomerations and suspended matter. Separation experiments were conducted in the crossflow configuration. As in the previous steps, the pH, turbidity and permeate flow rate were recorded periodically. In addition, each sample was saved for analysis of protein concentrations by either UV adsorption at 280nm or the Bradford test.

20

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4. Results and discussion

4.1. Membrane formation and character&ation

Fig. 2 shows a typical plot of the change in turbidity of the feed and the permeate with time during the deposition of the hydrous zirconium oxide. As the colloids are deposited on the support, the turbidity of the feed begins to decline. Concurrently, as the membrane begins to form the colloids begin to be rejected and the turbidity of the permeate declines to a final value of about 1NTU. The drop in feed turbidity is due to the deposition of particles on the support. The drop in permeate turbidity is due to the reduced concentration of the feed and to retention by the membrane being formed. The experiments confirmed that hydrous zirconium oxide deposition is adequately described by the mechanism proposed by Freilich and Tanny [7]. Initially, as the support pores are blocked, the flow through the membrane followed the "intermediate mechanism" of

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Hermans and Bred6e [ 18], given by Eq. 1" k't -

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70

M. Altman et al. / Desalination 125 (1999) 65-75 40

Once the pores are blocked, the "transition time" is said to have elapsed and the formation of a cake filtration layer begins. The cake formation law is given by Eq. 2 [7]:

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Fig. 3 shows the appropriate fit of the cake formation mechanism for the same experiment illustrated in Fig. 2. The cake formation equation describes the latter stage of the dynamic membrane formation accurately, as indicated in Fig. 3. For the particular experiment shown, the transition time was about 45 min. Depending on the experimental conditions, transition times observed ranged from about 45 to 120 min. The retention performance of a feed of 1000ppm model foulant in a saline solution is shown in Fig. 4. The membrane retained about 85% of the protein feed over the 4h of the experiments. Throughout the run a permeate turbidity of about 1 NTU was achieved. For this particular membrane permeability was about 50 l/m2"h'bar at the beginning of the separation experiment; this was reduced to about 8101/m2"h'bar after 4 h due to fouling.

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4.2. Parameters investigated The parameters investigated were primarily related to the formation of the dynamic membrane. These include the pH of the solution at formation and the nature of the porous support. In addition, a membrane was formed without the addition of PAA to verify that this is a vital part of the dynamic membrane formation. During the separation experiments, the concentration of egg protein in the feed solution was varied, as was the ionic strength.

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4. 3. pH of the membrane forming solution Fig. 5 shows the cake filtration plots for three membranes formed at different pH levels. Using

71

M. Altman et al. / Desalination 125 (1999) 65-75

the onset of cake formation to determine the transition time, there is a trend of increasing transition time with increasing pH. The transition time decreases from 120 min at pH 3.5 to only 45 min at pH 6. This seems to confirm suggestions that the particles forming the membrane are smallest at the lowest pH [7]. Ignoring any electrostatic effects of pH, smaller particles would be expected to take longer to clog the support than larger particles, according to either the standard filtration law or the intermediate mechanism. Fig. 6 presents permeabilities measured with feed concentrations of 1000 ppm with the same three membranes. The expected trend is observed: the permeability is lowest at pH 3.5 and highest at pH 6. It would be reasonable to assume that this is related to the size of the passages between the particles. Smaller particles can be more tightly packed and therefore offer more resistance than membranes formed of larger particles. Fig. 7 shows the protein retention and permeate turbidity of the three membranes of Fig. 5. No distinct trend can be observed in the relationship between the retention capability of a dynamic membrane and the pH at which was formed. The membranes formed at pH 6 and pH 3 are superior to those formed at pH 4. It is not clear if this is a physical effect connected to the electrostatic properties of the hydrous zirconium oxide at pH 4 or to an experimental defect.

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Fig. 8 compares the cake formation stage of dynamic membrane formation on the FO2430 fabric with one formed on the thinner, more porous FO2431. All other parameters were identical. With the less porous fabric, the permeate flow rate falls off much more quickly. After 4 h, the rejecting layer on the more porous fabric is about three times more permeable. As a

240

Fig. 6. Permeabilities of membranes formed at various pH levels.

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result, the "transition time" of 45 min on the FO2430 fabric is about half the "transition time" of 85 min on the FO2431 fabric.

M. Altman et al. / Desalination 125 (1999) 65-75

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Fig. 9. Foulant retention of membranesformed on fabric supports of different porosity.

While these two membranes have similar initial permeabilites, their performance with respect to protein rejection differ significantly. Both membranes were tested with 1000 ppm of egg white powder; the results are compared in Fig. 9. The graph shows that the membrane formed and tested under otherwise identical conditions on the FO2430 is far superior in terms of both protein and particulate retention. These results indicate that the choice of substrate has a profound impact on the flow and separation characteristics of the dynamic membrane. The tighter fabric resulted in a diminished permeate flow rate, indicating a tighter dynamic membrane, and as expected, provided better separation.

of ovalbumin. While PAA post-treatment has been used for RO dynamic membranes [17], it usually was not applied in UF dynamic membranes [19]. It is not entirely clear by what mechanism the PAA modifies the hydrous zirconium oxide layer. Tanny and Johnson [20] rule out the formation of a gel layer of PAA. EDS investigations in the present work did not show evidence of a distinct PAA layer on top of the colloid layer. It has been proposed [20] that the PAA molecules block the large residual pores in the metal oxide layer. In order to verify whether PAA post-treatment is essential, a membrane was formed and tested as described above, but without PAA posttreatment. Fig. 10 shows that only the membrane that underwent treatment with PAA demonstrated the integrity of an ultrafiltration membrane. An otherwise identical membrane with PAA posttreatment succeeded in retaining about 60% of the model feed foulant and produced a permeate with a turbidity of about 4-5 NTU over the 4h of the experiment. By contrast, the membrane

4.5. PAA post-treatment Most of the membranes constructed as part of this project were modified with the addition of 50 ppm (8.33" 10-4 mM) PAA in order to reduce the effective pore size and increase the retention

M. Altman et al. / Desalination 125 (1999) 65-75 40 35 30 Z . ~ 25

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Fig. 11. Foulant retention at various foulant concentrations.

without the PAA retained an insignificant 10% of the protein feed and produced a permeate that was about three times as turbid. The vast difference in effective pore size is also evident from the comparison of the permeabilities of the two membranes. The permeability of the membrane without PAA post-treatment was about 100 l/m2"h.bar, an order of magnitude greater of typical dynamic membranes tested under the same conditions. Such a high permeability would be expected of an ultrafiltration membrane with clean water feed. It is higher than would be expected from a membrane with a MWCO in the range of albumin proteins. It is, therefore, concluded that post-treatment with PAA of hydrous zirconium oxide membranes formed under these conditions is vital in order to obtain a membrane with good separation characteristics.

Retention and permeability were measured at an increasing foulant concentration, from a level of 50 ppm to a level of 1000 ppm. The test for each foulant concentration level lasted 4 h. The membrane was then rinsed with a 2500 ppm NaCI solution for 15 rain, after which a run with a higher foulant level was initiated. Fig. 11 shows the ovalbumin retention from feed solutions containing 50 ppm to 1000 ppm foulant, while Fig. 12 exhibits the permeate flows of the same feeds. It is seen that the retention of the foulant decreased with increasing feed concentration, as would be anticipated from a UF membrane. From retention levels of about 95% for low feed concentrations, the retention declined to about 60% at a concentration of 1000ppm. The permeate flow rate results (Fig. 12) exhibit the behaviour expected of a UF membrane. As the feed concentration increases, the flow rate declines. For all feed concentrations, the flow rate declines most dramatically in the first hour due to membrane fouling. It is important to recall that for this particular experiment the same membrane was used for all

4. 6. Foulant concentration

The effect of foulant concentration was tested with a single dynamic membrane constructed on the FO2430 fabric at pH 6, as described above.

74

M. Altman et aL / Desalination 125 (1999) 65-75

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Time [min] Fig. 12. Permeate flow rate from solutions containing various foulant concentrations.

feed concentrations, with the concentration being increased each day. Consequently, on any given day, the membrane was already partially fouled from the experiment conducted on the previous day. Nevertheless, the permeate flow for 1000 ppm feed is about the same as obtained previously on a similar new membrane at the same feed concentration. As shown in Fig. 1 I, the separation capabilities of dynamic membranes are diminished as the foulant concentration is increased. It should be noted, however, that the foulant concentration levels explored in this study, reaching as much as 1000ppm, are far beyond concentration levels likely to be encountered in practice. Therefore, the higher retention values measured in this study are more relevant.

5. Conclusions

Dynamic membranes, with reasonably good separation characteristics and permeabilities similar to those of commercial UF membranes,

can be constructed. Flat sheet membranes were formed in the dead-end configuration and operated at pressures of about 5 bar. The rate of hydrous zirconium oxide deposition obeyed conventional filtration laws. The colloid layer was modified with PAA. The modification was vital for producing a viable membrane; without PAA post-treatment almost no retention was observed. The porosity of the support fabric played an important role in membrane formation. The poreclogging stage of a tighter membrane was much shorter than for a looser fabric, leading to quicker membrane formation. More importantly, the membrane formed on the tighter fabric exhibited superior separation properties, affording protein rejection 40% higher than on the looser fabric, albeit at lower permeabilities. The membranes formed from particles in contact with an aqueous solution at pH 6 had higher retention rates those formed from more acidic solutions. Furthermore, the membranes formed at the higher pH also yielded higher permeabilities. The effect of increasing the model foulant concentration from 50 ppm to 1000 ppm was consistent with ultrafiltration membrane performance. At the higher protein concentrations the retention and permeability decreased. The permeate fluxes achieved were in the range ofultrafiltration fluxes (over 401/m2"h'bar) after 4h of continuous operation with a contaminant solution of 50ppm protein. Ovalbumin retention in the various tests ranged from less than 40% for mediocre membranes and over 95% for the best membranes. Experiments that produced membranes that lacked integrity showed much higher nominal fluxes with almost no retention. Of the combinations tested, the best dynamic membranes in terms of retention were formed on the thicker Viledon® FO2430 support with a concentration of 0.252mM ZrC14 at pH 6. For a feed of 1000 ppm ovalbumin, retention of about 85% was achieved over a period of 4h at a

M. Altman et al. / Desalination 125 (1999) 65-75

transmembrane pressure of 5 bar. The permeate turbidity throughout the experiment was about 1 NTU. Membrane fouling, however, continued throughout the experiment, reaching 101/m2"h'bar after 4h. At a lower feed concentration of 50 ppm, the same membrane afforded retention of about 95% with permeabilities of over 40 l/mZ.h.bar. The work carried out to date has yielded encouraging results that are being currently extended.

Acknowledgements The research forms part of the EC Joule Programme Project JOE3-CT97-0053 undertaken by Suez-Lyonnaise des Eaux, CPERI of the University of Thessaloniki, AGBAR of Barcelona, the Mekorot Water Company and the Water Research Institute (WRI) of the TechnionIsrael Institute of Technology. Permission to publish this paper is gratefully acknowledged. The authors with to thank WRI for supporting the exploratory phase of this research. Thanks are also due to Mrs. Anat Shauli for her involvement and useful suggestions. This work forms part of a thesis presented by M.A. to the Technion in partial fulfillment of the requirements of a Masters degree.

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75

[2] J.L. Gaddis, H.G. Spencer and S.C. Wilson, AIChE Symp. Series 75, 190 [Water 1978] (1979) 156. [3] G.R. Groves et al., Desalination,47 (1983) 305. [4] A.K. Turkson, J.A. Mikhlin and M.E. Weber, Sep. Sci. Tech., 24 (1989) 1261. [5] C.C Chen and B.H. Chiang, J. Membr. Sci., 143 (1998) 65. [6] M.S. Altman,Applications of Dynamic Membranes, Masters Thesis, Technion,Haifa, 1999. [7] D. Freilich and G.B. Tanny, J. Coll. Int. Sci., 77 (1980) 369. [8] D. Szaniawskaand H.G. Spencer, Desalination, 95 (1994) 121. [9] C. Diaper, V. Correia and S. Judd, J. Membr. Sci., 138 (1998) 135. [10] X. Xu and H.G. Spencer, Desalination, 113 (1997) 95. [11] R. Levenstein,D. Hasson and R. Semiat, J. Membr. Sci., 116 (1996) 77. [12] FreudenbergNonwovens,Nonwovensfor Membrane Support, technical data, 1997. [13] A. Veyland et al., Eur. J. Inorg. Chem., 1 (1998) 1765. [14] S.U. Aja, S.A. Wood and A.E. Williams-Jones,Appl. Geochem., 10 (1995) 603. [15] L.M. Zaitsev, Russ. J. Inorg. Chem., I 1 (1966) 900. [16] J.-C. Cheftel, J.-L. Cuq and D. Lorient, Prot6ines alimentaires,Lavoisier,Paris, 1985. [17] D.G. Thomas,in: S. Soudrajan,ed., ReverseOsmosis and Synthetic Membranes:Theory, Technology and Engineering,NRCC, Ottawa, 1977, pp. 295-312. [18] R.P. Hermans and H.L. Bred6e, J. Soc. Chem. Ind., 55 (1936) IT. [19] C.A. Brandon,J.L. Gaddisand H.G. Spencer,in: A.F. Turbak, ed., Synthetic Membranes, Vol. 2, ACS, Washington DC, 1981, pp. 435--453. [20] G.B. Tanny and J.S. Johnson Jr., J. Appl. Polymer Sci., 22 (1978) 289.