The natural phenomenon of osmosis is a fundamental rule of nature and occurs when pure water flows spontaneously from a dilute saline solution through a semi-permeable membrane into a saline solution of a higher concentration.
Figure 1 illustrates a semi-permeable membrane dividing two compartments, one containing pure water, the other a salt solution. Assume that this membrane is permeable to water but not to salt therefore it is “semi-permeable”. The membrane will allow water to permeate through it but will not allow salt to pass. This system will try to reach the same concentration on both sides of the membrane in the only way possible; that is for water to flow from the pure water side to the salt water side, thereby diluting the salt solution.
Figure 1 also shows that osmosis has caused a rise in the height of the salt solution column. This height will increase until the force or pressure caused by this column of water reaches a point which stops the water flow through the membrane. This final height difference between the salt and pure water columns, in terms of water pressure against the membrane, is called osmotic pressure.
Figure 2 shows a force or pressure being applied to the column of salt water that exceeds the osmotic pressure thereby causing the direction of water flow through the membrane to be reversed. Thus the term, reverse osmosis. This reversed flow is pure water produced from the salt solution since the membrane is not permeable to salt.
REVERSE OSMOSIS IN PRACTICE
As illustrated, a high pressure pump pressurises and continuously pumps saline feed water into a membrane/pressure vessel module where the low salinity permeate, is removed from the feed flow thereby producing a high salinity brine or reject flow. The operating pressure is controlled by the pressure regulation valve which determines the relative percentages of feed water going to the brine/reject flow and the permeate/potable water stream.