Increase in Salt Rejection

Increases in salt rejection are typically due to membrane compaction (see Chapter 12.1.1.4). As the membrane becomes denser due to compaction, the passage of salts through the membrane is reduced, leading to a loss in salt passage and in increase in salt rejection. 

Pressure drop measures the loss in pressure from the feed to the concentrate. In effect, it measures the loss in driving force for water across the membrane (see Chapter 12.3.1.3 and Equation 12.6). Factors that result in an increase or decrease in pressure drop are discussed below. 

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Normalized salt rejection is a function of the concentration driving force across the membrane, as shown in Equation 11.3. Factors that lead to loss or increase in salt rejection are discussed below. 

The aspect of hole filling by plasma deposition can be demonstrated by the transport characteristics of LCVD-prepared membranes. First, the porosity as porous media calculated from the gas permeability dependence on the applied pressure can be correlated to the salt rejection of the composite membrane as shown in Figure 34.13. The effective porosity s/, where s is the porosity and q is the tortuosity factor, is measured in dry state and may not directly correlate to the porosity of the membranes in wet state. The effective porosity of LCVD-prepared membranes was measured before the reverse osmosis experiment. The decrease of porosity (as porous media) is clearly reflected in the increase in salt rejection in reverse osmosis. 

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. 

The effects of the most important operating parameters on membrane water flux and salt rejection are shown schematically in Figure 5.2 [14]. The effect of feed pressure on membrane performance is shown in Figure 5.2(a). As predicted by Equation (5.1), at a pressure equal to the osmotic pressure of the feed (350 psi), the water flux is zero thereafter, it increases linearly as the pressure is increased. The salt rejection also extrapolates to zero at a feed pressure of 350 psi as predicted by Equation (5.6), but increases very rapidly with increased pressure to reach salt rejections of more than 99% at an applied pressure of 700 psi (twice the feed solution osmotic pressure). 

With increase in salt concentration the approximations involved in the Debye-Hiickel theory become less acceptable. Indeed it is noteworthy that before this theory was published a quasi-lattice theory of salt solutions had been proposed and rejected (Ghosh, 1918). However, as the concentration of salt increases so log7 ,7 being the mean ionic activity coefficient, appears as a linear function of c1/3 (the requirement of a quasi-lattice theory) rather than c1/2, the DHLL prediction (Robinson and Stokes, 1959). Consequently, a quasi-lattice theory of salt solutions has attracted continuing interest (Lietzke et al., 1968 Desnoyers and Conway, 1964 Frank and Thompson, 1959 Bahe, 1972 Bennetto, 1973) and has recently received some experimental support (Neilson et al., 1975). 

High productivity can also be achieved with brief, measured exposure to free chlorine (see Chapter 8.2.1). Membrane manufacturers will sometimes treat their membranes with a very short exposure to free chlorine. This results in membranes that exhibit higher flux with no change in salt rejection. Longer exposure to free chlorine will result in a permanent loss of salt rejection. Note that exposure to free chlorine by the end user is a violation of the membrane warranty and should not be attempted to increase flux. 

Membranes fouled with suspended solids will exhibit lower productivity and an increase in pressure drop. Sometimes there is also a decrease in salt rejection. 

The flux of 0.03 gfd for the homogeneous polyamide membrane was more than two orders of magnitude too low for commercial desalination. The flux was increased 175 fold with no decrease in salt rejection by casting the membrane with asynmetric morphology. Even higher fluxes, up to 3.5 times that observed for the asymmetric MPD-l/T (100-70/30) polyamide membrane, were obtained with asymmetric membranes cast from polyhydrazides and polyamide-hydrazides. Permeation properties for the three types of aromatic polyamides are shown in Table IX. The RO properties of this group of membranes illustrate the combined effects of Structure Levels I, II and III on membrane performance. 

From the beginning of its development, problems with the long term stability of the NS-200 membrane were observed. A gradual increase in water flux occurred, accompanied by a corresponding decrease in salt rejection. Three factors may have been involved ... 

Experiments were carried out at 0.5, 2.5, and 4 mM CaCH. Results are shown in Figure 5.9B, and the trend observed is to that of the SPO data. Calcium can destabilise colloids that were stabilised by-organics. This was observed to occur at a concentration between 2.5 and 4 mM CaCl 2, with a resultant increase in colloid rejection from 15 to 95% and a greater flux decline, as shown in Table 5.7 (No 1, 4, 5). Tills corresponds to the effect of calcium on stabilised colloids reported by Amirbahman and Olson (1995), which was described in more detail in Chapter 2. Deposition increased with calcium concentration, which indicates that the destabilisation is always present to some extent. The calcium was added after the colloids were stabilised with HA, which is a different scenario to NOM, where salt (which is in the NOM powder) is added simultaneously. In this case, the calcium provides a full destabilisation of the organic-coated colloids at 4 mM, leading to complete rejection and deposition. 

Great improvements in the TFC membranes were also experienced by Chen et al. [56] by incorporating water-soluble amine reactants—sulfonated cardo poly(arylene ether sulfone) (SPES-NH2)—into an aqueous solution containing MPD. Under optimum preparation conditions, the TFC membranes prepared from SPES-NH2 showed remarkable increase in water permeability (51.2 L/m h) with a slight decrease in salt rejection (97.5% at 2000 ppm NaCl, 2 MPa) compared to membranes prepared without SPES-NH2 (37.4 L/m h and 99%). The improved results are attributed to the incorporation of hydrophilic SPES-NH2 to PAs and/or a higher degree of cross-linking formed in the thin selective layer. In view of the importance of hydrophilicity on TFC membrane performance, a novel amine monomer—3,5-diamino-A-(4-aminophenyl) benzamide (DABA)—with three amino... 

In most cases, biofouling is assessed based on performance characteristics (a) flux decline, (b) decrease in salt rejection, or (c) increase in feed-brine pressure drop as given in Table 2.5. These indirect methods, however, are not absolute indicators of biofouling, and cannot be used for any correlation. Common techniques and limitations for controlling biofouling include ... 

Li et al. 2009 Zhang et al. 2006 Mohammad et al. 2005 Du and Zhao 2004 Jegal et al. 2002). Furthermore, chemical stability of the membrane is a major concern, as PA in the top active layer tends to undergo ring chlorination following chlorine exposure during filtration processes. The deterioration of the top active layer will result in a drastic decrease in salt rejection, which in turn increases the maintenance cost. 

Fig. 5 the water content in the membrane and the membrane water flux increased monotonically with lEC value. On the other hand, a maximum was found in salt rejection between lEC value of 2.0 and 2.3 meq/g. 

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%. 

In a related case, FT-30 membrane elements were placed on chlorinated seawater feed at OWRT s Wrightsville Beach Test Facility. Flux and salt rejection were stable for 2000 hours at 0.5 to 1.0 ppm chlorine exposure. Chlorine attack did become noticeable after 2000 hours, and salt rejection had dropped to 97 percent at 2500 hours while flux increased significantly. Long term laboratory trials at different chlorine levels led to the conclusion that the membrane will withstand 0.2 ppm chlorine in sodium chloride solutions at pH 7 for more than a year of continuous exposure. 

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