What is Reverse
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
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).