Salt passage

Salt flux across a membrane is due to effects coupled to water transport, usually negligible, and diffusion across the membrane. Eq. (22-60) describes the basic diffusion equation for solute passage. It is independent of pressure, so as AP — AH 0, rejection 0. This important factor is due to the kinetic nature of the separation. Salt passage through the membrane is concentration dependent. Water passage is dependent on P — H. Therefore, when the membrane is operating near the osmotic pressure of the feed, the salt passage is not diluted by much permeate water. 

Figure 2b. Plant performance history—salt passage...

Biological fouling was monitored using the customary indicators of RO performance, including flux (productivity), salt passage, and bundle pressure drops (AP). In addition, the total bacteria count (TBC) of the RO feed water and the reject brine stream were also obtained using the membrane filtration method (Ref. 6). The increase in TBC between the feed and reject portions of the system had been found in the preliminary stages of... 

Year Cost normalized (1980 US ) Productivity normalized (to 1980) Reciprocal salt passage normalized (to 1980) Figure of merit ... 

Figure of merit = (productivity) x (reciprocal salt passage/cost). Source Dave Furukawa. 

In addition to the progress shown in Table 1.1, some membranes now exhibit up to 99.75% rejection (a drop of 17% in salt passage over membranes exhibiting 99.7% rejection). Other advancements... 

Rejection is a term used to describe what percentage of an influent species a membrane retains. For example, 98% rejection of silica means that the membrane will retain 98% of the influent silica. It also means that 2% of influent silica will pass through the membrane into the permeate (known as "salt passage"). 

MPa), 77°F (25°C), pH = 8, and 15% recovery). The salt passage at this rejection is only 0.3%, while the salt passage for high-rejection CA membranes at 98% rejection is 2%. So, the salt passage is nearly 7-fold lower for polyamide membranes than for CA membranes. See Chapter 3, Table 3.2 for complete rejection capabilities for polyamide membranes. 

Hydrogen sulfide is typically found in well water that is devoid of oxygen. This compound easily oxidizes and releases elemental sulfur, which is very sticky and results in irreversible fouling of RO membranes. Metal sulfides can also form, which can precipitate. Deposits can be sooty-black or a pasty-gray.8 Fouling with elemental sulfur or metallic sulfides will cause a decrease if flux and an increase in salt passage. 

Initially, polyamide composite membranes that have been degraded due to chlorine attack will exhibit a loss in flux.4 This drop in flux is followed by an increase in flux and salt passage. 

Beta affects both the flux through an RO membrane and the salt rejection. The increase in Beta due to concentration polarization at the membrane surface results in increased osmotic pressure and decrease is water flux, as shown in Equation 9.1 (modified Equation 4.1). Salt passage also increases, as shown in Equation 9.2 (modified Equation 4.2). 

Figures 9.8 and 9.9 shows how Beta affects flux and salt passage (rejection), respectively, for two different brackish water concentrations (assumes membrane will deliver 20 gfd at 400 psi with a rejection of 99% at Beta equal to one (no concentration polarization)).8 From the Figures, it is shown that at Beta values greater than about 1.1, the water flux and salt passage (rejection) are significantly affected by Beta. Also shown is that the effect of Beta on performance is more pronounced at higher TDS feed water than with lower TDS feed water.

Figure 9.9 Beta s effect on salt passage for two different brackish water concentrations.

Other data that should be entered into the screen shown in Figure 10.8 includes the "Water Type." Clicking on this button will open up a box that allows the designer to select the feed water source (see Chapter 9.1 for details about the importance of the source of the feed water to the RO system). This box also lists the recommended percent salt passage increase per year and fouling factor that corresponds to the type of feed water selected. (These guidelines should be entered... 

Membrane age. For general projections, 3 years should be selected which assumes a 3-year membrane life. This input works closely with the flux decline and salt passage increase inputs discussed below. Selecting performance at end-of-life for the membranes will yield the operating parameters necessary after years of fouling and scaling of the membrane. 

Salt passage increase is located in the upper left of the screen, just below the flux decline input. Recommended percentage salt passage increase per year as a function of feed water quality is listed in Table 10.3. Note that the salt passage increases as the feed water quality goes down. 

Table 10.3 Hydranautics recommended salt passage increase percentages er year as a function of feed water quality.

Salt Passage Increase per year (%) RO Permeate Brackish Well Water Brackish Surface Water Tertiary Wastewater... 

SP = percent salt passage CFC = concentration of the feed-concentrate ... 

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