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This calculator is under construction This calculator determines the
osmotic pressure of a solution containing dissolved solids. The osmotic
pressure is based on theoretical calculations and gives you an indication
about the operational pressure of a reverse osmosis system. The osmotic
pressure can be calculated with the following formula:
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(1) | where:
 |
number of ions produced during dissociation of solute |  |
osmotic coefficient, unit less |  |
concentration of all solutes, moles/L |  |
universal gas constant, 0.083145 L·bar/moles·K |  |
absolute temperature, K | The factor
i is also called the dissociation factor or the van't Hoff factor.
When you have a water analysis you often have the TDS in mg/L. The TDS does
not give an indication about how the different solids are partitioned, it
means that you cannot know the molar concentrations of the different solids
dissolved in water. However, when you are making calculations for seawater
the calculator spreads the TDS over Sodium (Na+) and Chloride (Cl-).
This is possible because the concentration of these two elements is much
higher than the concentrations for other elements in seawater. The
theoretical formula (1) used above is adapted to operational conditions by
membrane manufacturers. Formula (2) is used by Dow / Filmtec to calculate
the osmotic pressure:
 |
(2) | where:
 |
:sum of molality concentration of all constituents in a solution (moles
of solute/kg of solvent) | The molality is
calculated with the following formula:
The obtained osmotic pressure with formula (2) is in psi (pounds per square
inch). This calculator converts automatically the pressure to bar with the
following conversion factor: 1 psi = 6.8948·10-2 bar. If
you do not have a water analysis you can use the values given in the right
column in the input table. The values an indication about the standard
seawater composition and brackish water. Click on the SW or BW button to put
the values in the input fields of the table.
The indication about the operational pressure is given by multiplying the
theoretical osmotic pressure by 2. As you can see in the formula the osmotic
pressure depends on the temperature. So if you are in an environment with
changing water temperatures calculate the osmotic pressure for different
temperatures. Warning: This calculator is accurate
for the following parameters:
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20000 mg/L < | TDS for
SW | < 43000 mg/L | |
100 mg/L < | TDS
for BW | < 15000 mg/L | |
10° C < | Temperature |
< 35° C | The calculations are made
using 1 membrane with a production of 0,2 m3/h or (200 L/h).
The membrane used for seawater is the SW 30-4040 from Dow/Filmtec.
The
membrane used for brackish water is the BW 30-4040 for Dow/Filmtec. | 
