How Reverse Osmosis Works
The phenomenon of osmosis occurs when pure water flows from a dilute saline solution through a membrane into a higher concentrated saline solution.
The phenomenon of osmosis is illustrated in Figure 1. A semi-permeable membrane is placed between two compartments. ‘Semi-permeable' means that the membrane is permeable to some species and not permeable to others. Assume that this membrane is permeable to water but not to salt. Then, place a salt solution in one compartment and pure water in the other compartment. The membrane will allow water to permeate through it to either side but salt cannot pass through the membrane.
As a fundamental rule of nature, this system will try to reach equilibrium. That is, it will try to reach the same concentration on both sides of the membrane. The only possible way to reach equilibrium is for water to pass from the pure water compartment to the salt containing compartment to dilute the salt solution.
Osmosis can cause a rise in the height of the salt solution. This height will increase until the pressure of the column of water (salt solution) is so high that the force of this water column stops the water flow. The equilibrium point of this water column height in terms of water pressure against the membrane is called osmotic pressure.
If a force is applied to this column of water, the direction of water flow through the membrane can be reversed. This is the basis of the term Reverse Osmosis. Note that this reversed flow produces pure water from the salt solution, since the membrane is not permeable to salt.
How to Use Reverse Osmosis in Practice
With a high-pressure pump, pressurised saline feed water is continuously pumped to the module system. Within the module, consisting of a pressure vessel (housing) and a membrane element, the feed water will be split in a low saline product called permeate and a high saline brine called concentrate or reject. A flow regulating valve called concentrate valve, controls the percentage of feed water that is going to the concentrate stream and the permeate which will be obtained from the feed.