Normalized Permeate Flow

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. 

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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 ... 

Normalizaed Salt Passage Cleaning Required Normalized Permeate Flow... 

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

Membrane fouling and scaling can both lead to a loss in normalized permeate flow. Additionally, membrane compaction will result in decreased permeate flow as well. 

An increase in normalized 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 are typically not cleaned due to a drop in salt rejection. This is because in most instances, there is a mechanical explanation for the drop in salt rejection (see Chapter 12.2.2). However, in the case where membrane scaling is responsible for a drop in salt rejection, normalized permeate flow is generally the first indicator of this phenomenon (see Chapter 11.3.1.2)... 

Data, particularly normalized data, is evaluated to determine the nature of the loss in membrane performance (see Chapter 11.3 for a complete discussion on data normalization). Normalized permeate flow, salt rejection, and differential pressure should be evaluated to determine trends in performance. 

Case Normalized Permeate Flow Normalized Salt Passage Normalized Differential Pressure Location Probable Causes... 

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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