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:

Calculate
the Net Pressure at which the membrane is rated (P_{r})
P_{r} = (Rate Pressure)  (Osmotic pressure of test
solution)

Calculate the net pressure under Operating
Conditions: (P_{op})
(P_{op}) = (Average applied pressure)  (average
osmotic pressure of the feed water)

Expected permeate flow at operating conditions =
(rated permeate flow) × (P_{op}) ÷
(P_{r})
EXAMPLE
QUESTION
For a thinfilm 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
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. 