Normalized salt rejection

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. 

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

Virtually all normalization programs will calculate the normalized permeate flow, normalized salt rejection and/or passage, and differential pressure (some programs normalize it, some do not). Some programs also include net driving pressure as an output as well as the following outputs ... 

Figure 11.7 Normalized permeate flow and normalized salt rejection as functions of time. Data courtesy of Madalyn Epple, Toray Membrane, USA, Inc.

Performance of an RO system, specifically the permeate flow rate, salt rejection and pressure drop, is a function of membrane fouling, scaling, and degradation, as previously discussed (See Chapter 11.3). This chapter covers the detailed effects of membrane fouling, scaling, and degradation have on normalized product flow, normalized salt rejection, and pressure drop. 

Membrane scaling and degradation can lead to a loss in normalized salt rejection as can breaches O-rings and permeate tube. 

If the normalized salt rejection is low or the normalized permeate flow is high, the integrity of the membrane may be in question. The vacuum decay test is a direct test for the integrity of a spiral wound RO membrane module. The test is best used to identify leaks within the membrane modules rather than leaks due to chemical attack. The test requires the isolation of an individual membrane module or the entire pressure vessel. A vacuum is then pulled on the membrane(s) and the rate of decay in pressure is observed. A decay of greater than 100 millibar per minute is indicative of a leaky membrane. Refer to ASTM Standards D39235 and D69086 for a more detailed review of the technique. 

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