a m i a d w a t e r s y s t e m s

W A T E R S Y S T E M S

R E V R S E O S M O S I S ( R O )

Reverse Osmosis (RO) is the finest level of filtration available. Reverse Osmosis is a membrane process that

acts as a molecular filter to remove 95 to 99% of dissolved salts and inorganic molecules, as well as organic

molecules with a molecular weight greater than 100. The Reverse Osmosis membrane also removes more

than 98% of residual biological and colloidal matter from the feed water, resulting in a highly purified product

stream.

Osmosis is the natural process which occurs when water spontaneously flows from a purer solution, through

a semi-permeable membrane into a more concentrated solution.

The phenomenon of osmosis is illustrated in Figure 1 below. A semi-permeable membrane is placed between

two compartments. Salt water is placed in one compartment and fresh water in the other (see diagram (A)).

The semi-permeable membrane will allow water to pass through it but not salt.

In order for the system to reach equilibrium, that is for the concentration on both sides of the membrane to

be the same, water will flow from the fresh water compartment into the salt water compartment. This water

flow will dilute the salt water and concentrate the fresh water. As Diagram (B) illustrates this will lead to an

increase in the water height on the salt water side and a decrease on the fresh water side. The height will

increase until the pressure of the column of salt water is so high that it causes the water flow to stop. The

difference in the water levels on the two sides is the “Osmotic Pressure”

a m i a d w a t e r s y s t e m s

W A T E R S Y S T E M S

R E V E R S E O S M O S I S ( R O )

In Reverse Osmosis the natural osmosis forces are overcome by applying an external pressure to the

concentrated solution. Thus the flow of water is reversed and pure water is removed from the feed solution,

leaving a more concentrated salt solution. The process water flow in reverse osmosis desalination systems

can be divided into three basic streams.

1. FEED - the filtered raw water supplied to the machine.

2. PERMEATE - the purified water produced by forcing the feed through the semi-permeable membranes.

3. REJECT - the discarded waste water containing the concentrated salts removed from the feed water.

A flow schematic of the reverse osmosis process is as follows:

The applications for Reverse Osmosis include desalination of seawater or brackish water for potable water,

industrial water and wastewater/sewage recycling.