To answer this question it is first necessary to understand what
Osmosis is.
Osmosis is a natural process that
occurs in all living cells. Water permeates through a membrane
that excludes suspended solids, dissolved salts and larger organic
molecules. These semipermeable membranes have pores of approximately
0.0005 microns in size.
Water molecules have a stronger tendency to escape from pure
water than from a salt solution. Water flows through the semipermeable
membrane from the pure solution to the salt solution in an effort to equalise
the osmotic pressure of the two solutions.
The Osmosis process may be reversed by applying
pressure to the salt solution. In Reverse Osmosis,
water from the salt solution is forced back through the
semipermeable membrane to the pure solution. The
process stops when the osmotic pressure of the increasingly
salty solution equals the applied
pressure.
In practice the salt
solution must be continuously replaced before the osmotic pressure rises
significantly. This is achieved using a cross flow
mechanism where the surface of the semipermeable membrane is
continually flushed. Therefore, commercial membranes have an inlet
stream and two outlet streams. The inlet is known as the
Feedwater and the outlets are the Permeate
(pure water) and the Concentrate (reject water).
