Blog

If a tank is set up divided into two compartments by a vertical barrier that is permeable to water only, and one compartment is partly filled with pure water and the other  is filled to the same level with a salt solution, then the chemistry of the system is such that water will flow through the barrier, from the pure water side into the solution, in an attempt to make the salt concentrations the same on both sides of the barrier (which it obviously cannot do, as that would require an infinitely great amount of water).

This flow of diluting water is termed osmosis. As osmosis continues, the amount of water on the pure water side decreases, and the volume of the solution increases by the same amount, the result being a steady increase in the separation of the
liquid levels on the two sides of the barrier. The increasing level of the solution over the pure water level creates a hydrostatic pressure difference across the barrier, which slows down the rate at which the water moves through to the solution side.
Eventually a physical equilibrium is reached, in which the hydrostatic head equals the force exerted by osmosis, and water flow ceases. The head at which this occurs is called the osmotic pressure of the solution, and this varies with the concentration
of salt in the solution: the higher the concentration, the greater the osmotic pressure, and also with the solution temperature.

If the solution side of the tank is now enclosed and pressurized, water is forced back through the barrier and out of solution, with the speed of reverse flow increasing as the applied pressure rises. This situation is called reverse osmosis, and is the
basis for the desalination of water – by the application of pressures in excess of the osmotic pressure to a solution restrained by a semi-permeable membrane.

It is, of course, true that no membrane can be 100% in rejecting the passage of salt through it, and therefore that the permeate from a reverse osmosis process will always have a slight salt content. The exact purity of this permeate depends on the concentration of the brine and on the salt permeation constant of the membrane.

This is a high pressure process. Sea water osmotic pressures can be in the region of 34 to 42 bar, and the net operating pressure for a reverse osmosis system, which is that required to provide an economic product water flow rate, ranges typically from 17 to 28 bar. As a result, the actual applied operating pressures are in the range of 50 to 70 bar. The applied pressures for brackish waters range from 14 to 48 bar, depending on the feed water salinity levels.

The rate of permeation of pure water through the membrane is proportional to the difference between the applied and the osmotic pressures, i.e. to the net driving force. As this is increased, the water flow rate increases while the salt flow remains
constant, so that increasing the pressure, and therefore the flow rate, gives decreasing salt concentration in the permeate or product water.

Look for more information, please click here.