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Net Pressure Correction for Reverse Osmosis Membranes

Change In Net Pressure - Due To Fouling

Feed Reject Average Osmotic Net
100 90 95 10 85
100 80 90 10 80
100 70 85 10 75
  • As net pressure goes down, so does the permeate flow rate.

  • Fouled membranes surface creates a higher TDS condition next to the membrane, resulting in higher permeate TDS.

Performance of the reverse osmosis membrane element is affected by two key factors; temperature of the feed water and the net driving pressure across the element.  These two factors must be taken into account before comparing or evaluating the performance of a membrane element or a reverse osmosis system. 

The following information is provided to assist you in roughly calculating the affects of the pressure factor.  For information on temperature correction, please click here.

The higher the net pressure on a membrane element, the higher the permeate rate.  The osmotic pressure of water can be calculated roughly by the following rule of thumb:

Osmotic pressure (PSI) = Total dissolved solids (TDS) 100

 To estimate the effect of net pressure, follow these steps:

  1. Calculate the Net Pressure at which the membrane is rated (Pr)
    Pr = (Rate Pressure) - (Osmotic pressure of test solution)

  2. Calculate the net pressure under Operating Conditions: (Pop)
    (Pop) = (Average applied pressure) - (average osmotic pressure of the feed water)

  3. Expected permeate flow at operating conditions =
                    (rated permeate flow)   (Pop) (Pr)


For a thin-film membrane, 4 x 40 membrane element, using a 2000 pp, sodium chloride solution at 225 psi and 77 degrees Fahrenheit, the permeate rate is 1800 gallons/day.  What is the permeate rate at 150 psi, feed water with 1000 TDS and temperature of 59 degrees Fahrenheit?


  • From the above example for the temperature correction, the permeate rate at 59 degrees Fahrenheit is 1224 gallons/day.
  • Using the steps above, the effect of net pressure is:

1224 x (150 10) / (225- 20) = 1224 x (140) / (205) = 835.9 gallons/day

Please Note:  For systems, more detailed calculations are necessary to take into account the effect of pressure drop and variation in total dissolved solids (TDS) throughout the system.   For more information, please contact us.

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All Content on this site is  intended for informational purposes for experienced water treatment professionals only.  Applied Membranes, Inc. does not assume any liability for any damages caused by the misapplication or misinterpretation of any of the information contained on this website.
Applied Membranes, Inc. 2007