Applied Membranes | Purchase Online |  Q & A | Technical Articles | Design Information | Search

What Is Reverse Osmosis?

By: Gil K. Dhawan Ph.D., P.E., Applied Membranes, Inc.

Before discussing membrane properties and performance, it is appropriate to define and discuss reverse osmosis briefly.

Osmosis can be defined as the spontaneous passage of a liquid from a dilute to a more concentrated solution across an ideal semipermeable membrane which allows the passage of the solvent (water) but not the dissolved solids (solutes).  (See Fig. 1.)  The transfer of the water from one side of the membrane to the other continues until the head or pressure (P) is large enough to prevent any net transfer of the solvent (water) to the more concentrated solution.  At equilibrium, the quantity of water passing in either direction is equal, and the pressure (P) is then defined as the osmotic pressure of the solution having that particular concentration of dissolved solids.

If a piston is placed on the more-concentrated solution side of a semipermeable membrane (see Fig. 2) and a pressure, P, is applied to the solution, the following conditions can be realized:  (1) P is less than the osmotic pressure of the solution and the solvent still flows spontaneously toward the more concentrated solution;  (2) P equals the osmotic pressure of the solution and solvent flows at the same rate in both directions, i.e., no net change in water levels; (3) P is greater than the osmotic pressure of the solution and solvent flows from the more concentrated solution to the "pure" solvent side of the membrane.  Condition (3) shown in Fig. II-2, represents the phenomenon of reverse osmosis.

Fig. 1. 
Normal Flow from Low to High Concentration

Fig. 2.
Reverse Osmosis
Flow Reversed by application of pressure to high concentration solution

The osmotic pressure of a solution increases with the concentration of a solution.  A rule of thumb, which is based on sodium chloride, is that the osmotic pressure increases by approximately 0.01 psi for each milligram/liter.  This approximation works well for most natural waters.  However, high-molecular-weight organics produce a much lower osmotic pressure.  For example, sucrose gives approximately 0.001 psi for each milligram/liter.

Several methods are available for measuring the osmotic pressure.  It can be calculated from the depression of the vapor pressure of a solution, by depression of the freezing point, and by the equivalent of the ideal gas law equation.  Some calculated values for common components are listed in Table 1.   Several devices are commercially available for direct measurement of the osmotic pressure.  These measure the pressure necessary to stop the flow of water through a membrane.

The procedure that we use to measure the osmotic pressure of a solution is to measure the water flux through a module under operating conditions at several pressures.  If a plot of water flux versus pressure is extrapolated to zero water flux, the intercept is the osmotic pressure.  This gives the effective osmotic pressure, including any concentration polarization.  Care must be taken to either maintain constant recovery or correct for the variation in concentration.

Attempting to measure the osmotic pressure of a solution directly by operating at a pressure just sufficient to obtain zero flow is impractical because the membranes are not perfect semipermeable membranes.  This technique would measure the difference in osmotic pressure between the feed and product water.  At low pressures the salt rejection is relatively  poor, so that a false osmotic pressure somewhat lower than the real value would be determined.

Table 1. 
Typical Osmotic Pressure at 25C (77F)

Compound Concentration
Osmotic Pressure
NaCl 35,000  0.6 398
NaCl   1,000 0.0171 11.4
NaHCO3   1,000  0.0119 12.8
Na2SO4   1,000 0.00705  6
MgSO4   1,000  0.00831  3.6
MgCl2   1,000  0.0105  9.7
CaCl2   1,000  0.009  8.3
Sucrose   1,000 0.00292  1.05
Dextrose   1,000  0.00555 2.0

Note:  Based on the above data for commonly present ionic species, a useful rule of thumb for estimating osmotic pressure of a natural water supply requiring demineralization is 10 psi per 1,000 mg/l (ppm).


Home Up Next

 What is RO Why RO RO & Water Chemistry Suspended Solids Membrane Fouling Bio. Growth on Mem. Fouling Solutions Correcting Fouling Membrane Disinfec. Mistakes to Avoid Pretreatment Seawater Desalination Activated Carbon Filtration Factors Affecting Perf. Achieving Brine Efficiency Using Pyrolox Media Residential Systems Oxygen

Applied Membranes Product Selection Links:
[Reverse Osmosis Membranes] [Reverse Osmosis Systems][Home RO Systems][Seawater Desalination Reverse Osmosis Systems] [Media Filters, Carbon Filters, Water Softeners, Etc.] [RO Filters & Housings] [RO Membrane Housings/Pressure Vessels] [Residential RO Components] [Commercial RO Components] [RO Pumps] [Membrane Chemicals] [Ultravioloet (UV) Systems] [Ozone][Water Quality Testers  (SDI)]


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