Pressure Correction

Pressure Correction

As net pressure goes down, so does the permeate flow rate. A fouled membrane's surface creates a higher TDS condition next to the membrane, resulting in higher permeate TDS levels.

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. Calculate the expected permeate flow at operating conditions =
    (rated permeate flow) × (Pop) ÷ (Pr)

Example

QUESTION

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?

ANSWER

  • 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