Increase in Normalized Permeate Flow

An increase in normahzed permeate flow is typically the result of a leak, either due to a breach in the membrane itself or because of problems with the membrane module hardware, or to exposure to oxidizers such as chlorine. 

Membranes can also be oxidized in the presence of iron, manganese, and other transition metals. These metals catalyze the oxidation of RO membranes. This type of oxidation tends to involve the entire RO skid rather than focus on the lead membranes. Again, when this type of degradation occurs, feed water passes into the permeate, resulting in an increase in permeate flow and a decrease in product quality. 

Exposure to high temperature at pH extremes can hydrolyze the membranes, leading to loss of membrane integrity. (See Chapter 4.2.1, Table 4.2, and Chapters 9.2, 9.8, and Table 13.1 for more detailed discussions on the effect of temperature and pH on polyamide composite membranes.) Hydrolysis also tends to involve the entire RO skid rather than focus on only the lead membranes. Just as with oxidation of the membrane, feed water will pass into the permeate resulting in an increase in permeate flow and decrease in the product quality. 

Membrane degradation can also be a physical phenomenon. Particles, such as sharp, granular, activated carbon fines, abrade the membrane surface and cause microscopic tears in the membrane through which feed water can breach the membrane, and increase the permeate flow (see 

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Normalized permeate flow (NPF) is a function of the average applied transmembrane pressure, the osmotic pressures of the feed and permeate, and temperature, as shown in Equation 11.1. Factors that cause an increase or decrease in the NPF are discussed below. 

The critical operational assumption that makes it possible to draw conclusions in a given comparison situation about the effect of plate and pore amount is that a constant volume and a constant absolute amount of solute was injected per column to normalize comparisons. If pore amount per column is constant, then increase in resolution with several columns of the same kind in series is due only to the increased amount of plates. Conversely, if plates of a column bank are the same, then differences in resolution are due to differences in the amount of pores of appropriate size. Also, all the other appropriate operating parameters are constant for each comparison. The following group of comparisons will illustrate different issues involving the interplay of pores, plates, and resolving power. The times on the figures are maximum values for total permeation volumes at a flow rate of 1 ml/min. 

For effective ultrafiltration, equipment must be optimized to promote the highest transmembrane flow and selectivity. A major problem which must be overcome is concentration polarization, the accumulation of a gradient of retained macrosolute above the membrane. The extent of polarization is determined by the macrosolute concentration and diffusivity, temperature effects on solution viscosity and system geometry. If left undisturbed, concentration polarization restricts solvent and solute transport through the membrane and can even alter membrane selectivity by forming a gel layer on the membrane surface—in effect, a secondary membrane — increasing rejection of normally permeating species. 

Figure 16. Permeate flow rate per unit membrane area (gallons/day/fl ) and NaCl rejection of seawater membranes offered by GE (K>), FilmTec/Dow (x), Koch (o), Toray (n), and Nitto Denko/Hydranautics (Us). All values taken from the manufacturers web sites. Test conditions for all membranes were 32,000ppm NaCl feed concentration, 800 psi feed pressure, and 77 °F feed temperature. Permeate recovery varied slightly in the tests from 7-10% andfeed pH variedfrom 6.5-8. Note that a 32,800 ppm NaCl feed was used to obtain the Koch values normalization to 32,000 ppm NaCl increases permeability by 2%.

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