What is Reverse Osmosis? (RO)

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.

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.

Typical Osmotic Pressure at 25 deg C (77 deg F)

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).