Salt-rejecting membranes

The second major membrane type is a composite. Starting with a loose asymmetric membrane, usually a UF membrane, a coating is applied which is polymerized in situ to become the salt rejecting membrane. This process is used for most high-performance flat-sheet RO membranes, as well as for many commercial nanofiltration membranes. The chemistry of the leading RO membranes is known, but... 

Nanofiltrarion membranes were received from Fluid Systems in San Diego, USA (now Koch Membrane Systems). Thin film composite membranes were chosen due to their low fouling characteristics compared to poly sulphone membranes used in other studies. The CA-UF membrane is, as the name suggests, classed as a UF membrane and the material is cellulose acetate. However, it is treated as a NF membrane here as it is often used for similar applications according to the manufacturer, and also because it exhibits some salt rejection. Membrane characteristics as given from... 

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. 

E.M.V. Hock, M. Elimelech, Cake-enhanced concentration polarization A new fouling mechanism for salt-rejecting membranes. Environ Sci Technol, 37 (2003) 5581-5588. 

In other areas, POD has been used to improve the wear resistance of a mbber latex binder by incorporation of 25% of Oksalon fibers. Heat-resistant laminate films, made by coating a polyester film with POD, have been used as electrical insulators and show good resistance to abrasion and are capable of 126% elongation. In some instances, thin sheets of PODs have been used as mold release agents. For this appHcation a resin is placed between the two sheets of POD, which is then pressed in a mold, and the sheets simply peel off from the object and mold after the resin has cured. POD-based membranes exhibit salt rejection properties and hence find potential as reverse osmosis membranes in the purification of seawater. PODs have also been used in the manufacturing of electrophotographic plates as binders between the toner and plate. These improved binders produce sharper images than were possible before. 

Interfdci l Composite Membra.nes, A method of making asymmetric membranes involving interfacial polymerization was developed in the 1960s. This technique was used to produce reverse osmosis membranes with dramatically improved salt rejections and water fluxes compared to those prepared by the Loeb-Sourirajan process (28). In the interfacial polymerization method, an aqueous solution of a reactive prepolymer, such as polyamine, is first deposited in the pores of a microporous support membrane, typically a polysulfone ultrafUtration membrane. The amine-loaded support is then immersed in a water-immiscible solvent solution containing a reactant, for example, a diacid chloride in hexane. The amine and acid chloride then react at the interface of the two solutions to form a densely cross-linked, extremely thin membrane layer. This preparation method is shown schematically in Figure 15. The first membrane made was based on polyethylenimine cross-linked with toluene-2,4-diisocyanate (28). The process was later refined at FilmTec Corporation (29,30) and at UOP (31) in the United States, and at Nitto (32) in Japan. 

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. 

It foUows from these two equations that the water flux is proportional to the appHed pressure, but the salt flux is iadependent of pressure. This means the membrane becomes more selective as the pressure increases. Selectivity can be measured ia a number of ways, but conventionally, it is measured as the salt rejection coefficient, R, defined ia equation 6. 

Some data iEustrating the effect of pressure on the water and salt fluxes and the salt rejection of a good quaUty reverse osmosis membrane are shown ia Figure 34 (76). 

The pressure to be used for reverse osmosis depends on the salinity of the feedwater, the type of membrane, and the desired product purity. It ranges from about 1.5 MPa for low feed concentrations or high flux membranes, through 2.5—4 MPa for brackish waters, and to 6—8.4 MPa for seawater desalination. In desalination of brackish or sea water, typical product water fluxes through spiral-wound membranes are about 600—800 kg/m /d at a recovery ratio RR of 15% and an average salt rejection of 99.5%, where... 

The fluxes in hoUow-fiber membranes used in seawater desalination are 20—30-fold smaller, but the overall RO system size does not increase because the hoUow-fiber membranes have a much larger surface area per module unit volume. In use with seawater, their RR is about 12—17.5% and the salt rejection ratio is up to 99.5%. 

Electrodialysis. Electro dialysis processes transfer ions of dissolved salts across membranes, leaving purified water behind. Ion movement is induced by direct current electrical fields. A negative electrode (cathode) attracts cations, and a positive electrode (anode) attracts anions. Systems are compartmentalized in stacks by alternating cation and anion transfer membranes. Alternating compartments carry concentrated brine and purified permeate. Typically, 40—60% of dissolved ions are removed or rejected. Further improvement in water quaUty is obtained by staging (operation of stacks in series). ED processes do not remove particulate contaminants or weakly ionized contaminants, such as siUca. 

Rejection Rejection is defined in Background and Definitions. The highest-rejection membranes are those designed for single-pass production of potable water from the sea. The generally accepted criterion is 99.4 percent rejection of NaCl. Some membranes, notably cellulose triacetate fibers are rated even higher. A whole range of membranes is available as rejection requirements ease, and membranes with excellent chlorine resistance and hydrolytic stability can be made with salt rejection over 90 percent. 

Major problems inherent in general applications of RO systems have to do with (1) the presence of particulate and colloidal matter in feed water, (2) precipitation of soluble salts, and (3) physical and chemical makeup of the feed water. All RO membranes can become clogged, some more readily than others. This problem is most severe for spiral-wound and hollow-fiber modules, especially when submicron and colloidal particles enter the unit (larger particulate matter can be easily removed by standard filtration methods). A similar problem is the occurrence of concentration-polarization, previously discussed for ED processes. Concentration-polarization is caused by an accumulation of solute on or near the membrane surface and results in lower flux and reduced salt rejection. 

As RO membranes become looser their salt rejection falls (see Section 31.8.1). Eventually a point is reached at which there is no rejection of salts, but the membrane still rejects particulates, colloids and very large molecules. The membrane pore size can be tailored to a nominal molecular weight cut-off. The resulting filtering process is called ultra-filtration. 

A commercially available cellulose acetate film which we would now describe as homogeneous or isotropic, gave the results shown in Row 2 of Table I. The volumetric permeation rate of water per unit membrane area, called the water permeation flux Jl mVm day, and the water permeation constant. A, m m day atm were both very low, but a salt rejection of 94 percent was obtained. We define ... 

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

Performance. Figure 2 shows a rejection-flux pattern (r-f pattern). Compaction, as it is well known, results in the flux decline with salt rejection Increase. Contrary to this, other types of membrane deterioration give the flux increase with salt rejection decline. In case of scratching, vibration, or microbiological deterioration, small cracks or pinholes develop over membrane surfaces. If the flux Increase is solely attributed to the crack or pin-holes, and these sites do not reject salt at all, the relation between salt rejection and flux can be calculated. 

Figure 3 shows the surface structure of a physically deteriorated membrane by scanning electronmlcroscopes. Hard crystals of an inorganic salt might have scratched the membrane surface and the rough surface was developed. The salt rejection decreased to 60 % and the water flux doubled comparing that of normal membranes. 

Of the existing flat-sheet RO membranes, cellulose acetate membranes of the Loeb-Sourirajan type give the best results because their open microporous substrate minimizes internal concentration polarization. Conventional interfacial composite membranes, despite their high water permeabilities and good salt rejections, are not suitable for PRO because of severe internal concentration polarization. 

Useful PRO membranes do not require the very high permselectivity necessary in reverse osmosis, and a trade-off between flux and salt rejection in conventional RO membranes is possible. If the salt rejection is too low, however. Internal concentration polarization due to excessive salt leakage can limit the water flux. 

Membrane. TA-Cindpoxt. Yasuda and Lamaze have shown that salt rejection R. of a water-swollen membrane can be expressed as follows ... 

Limited testing on chlorine sensitivity of poly(ether/amidel and poly(ether/urea) thin film composite membranes have been reported by Fluid Systems Division of UOP [4]. Poly(ether/amide] membrane (PA-300] exposed to 1 ppm chlorine in feedwater for 24 hours showed a significant decline in salt rejection. Additional experiments at Fluid Systems were directed toward improvement of membrane resistance to chlorine. Different amide polymers and fabrication techniques were attempted but these variations had little effect on chlorine resistance [5]. Chlorine sensitivity of polyamide membranes was also demonstrated by Spatz and Fried-lander [3]. It is generally concluded that polyamide type membranes deteriorate rapidly when exposed to low chlorine concentrations in water solution. 

Feed solution used in all experiments contained sodium chloride at a concentration level of 5,000 ppm. Membrane salt rejection is evaluated from conductance measurements of product water and expressed as percent rejection, %R, or desalination ratio, D. . These units are defined by the following equations in which Cp and Cf are sodium chloride concentrations in feed and product respectively. Note that D. is very sensitive to concentration changes and expands rapidly as 100% rejection is approached. 

Figure 5. Change in flux and salt rejection of U-1 membrane on continued exposure to 3,0 ppm chlorine at various pH levels...

UCLA Code Membrane ID Operating Pressure (psi) Product Flux (GFD) Desal. Ratio (Dr) % Salt Rejection ... 

The performance of membrane X-2 is strongly pH dependent, showing greatest flux change at pH 8.6 and appearing to tighten up at pH 3.0. For some unknown reason, salt rejection remains constant and near baseline for the entire 88 hour exposure period shown in Figure 7. 

Membrane Properties. The reverse osmosis performance of the bentonite-doped membrane under brackish water conditions is compared to that of the reference membrane in Figure 5 (I, reference membrane II, with organophilic bentonite). At low salt rejection the bentonite membrane again shows a higher initial flux than the reference membrane, the performance of the two becoming identical at the high rejection limit. 

Membrane Properties. The performance range of ammonia-modified membranes in low pressure operation is indicated in Figure 6 along with the performance of the reference membrane (I, reference membrane IV, ammonia-modified membrane). The lower boundary of the performance range refers to a solvent-to-polymer ratio of 3, the upper boundary to a ratio of 4. While the salt rejection towards univalent ions of the ammonia-modified membrane is limited to below 80 %, the maximum low pressure flux is over 15 m /m d (approaching 400 gfd) at a sodium chloride rejection of the order of 10 %. This membrane thus exhibits the flux capability of an ultrafiltration membrane while retaining the features of reverse osmosis membranes, viz. asymmetry and pressure resistance. 

Hollosep High Rejection Type is characterized by Cellulose Tri Acetate (CTA) hollow fiber with dense membrane structure and high salt rejection, and also by the module configuration favorable for uniform flow of feed water through hollow fiber layers (5 ). These features suggest that Hollosep may be operated under the conditions of higher recovery ratio compared to conventional conditions. 

Measurements of RO Performance. RO performances were measured by the simple apparatus, as shown in Figure 2. The water flux and salt rejection of the hollow fiber membranes under operating pressure in the range of 50 to 120 Kg/cm2c were determined using the feed water of 3.5 NaCl, at 25 C and at product water recovery ratio of less than 1 , after an elapsed time of 2 hrs. 

Characteristics of the hollow fiber membrane is shown in Table 2. The outer diameter and wall thickness of this hollow fiber membrane is fairly thick compared with those of other hollow fibers for seawater desalination. Salt rejection of hollow fiber membrane is high enough to be applied to one pass seawater desalination. 

Resistance of the hollow fiber membrane against high pressure was evaluated by measuring water flux rate and salt rejection under operating pressure of up to 120 Kg/cm2c in 3.5 NaCl feed water. The data obtained were analyzed in terms of... 

In the literature, there are many transport theories describing both salt and water movement across a reverse osmosis membrane. Many theories require specific models but only a few deal with phenomenological equations. Here a brief summary of various theories will be presented showing the relationships between the salt rejection and the volume flux. 

The solution-diffusion model (1 ) assumes that water and salt diffuse independently across the membrane and allows no convective salt transport. The reciprocal salt rejection, 1/r, is linearly related to the reciprocal volume flux, 1/q ... 

---