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).
