Parker AQUA DUO - A455C, Technical Manual

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3.0 THEORY OF OPERATION AND GENERAL DESCRIPTION
3.1 REVERSE OSMOSIS THEORY
Reverse osmosis, like many other practical scientific methods, has been developed from processes
first observed in nature. Osmosis is a naturally occurring phenomenon in which a semi-permeable
membrane separates a pure and a concentrated solution (a semi-permeable membrane is defined as one
that preferentially passes a substance). Every fluid has an inherent potential that is directly related to the
type and number of solids in solution. This potential, referred to as osmotic pressure, increases in
proportion to relative concentration of a solution. A concentrated solution, therefore, has an osmotic
pressure that is higher than that of a pure solution.
In a desalination system, the less concentrated solution will equalize the concentrations of both
solutions by migrating across the membrane. When enough pure solution migrates across the
membrane such that the inherent potential difference between the solutions is no longer higher than
the osmotic pressure of the membrane, the purer solution will stop flowing. If the pressure on
the concentrated solution is increased to above the osmotic pressure, fluid flow will be reversed. This
condition, called Reverse Osmosis, can be established by artificially pressurizing the more concentrated
solution using a high-pressure pump. In this type of system, the concentrated solution (normally referred to
as feedwater) will become more concentrated as pure water flows out of the solution and across the
membrane to the permeate side. Discounting the effects of feedwater temperature and salinity, the
operating pressure normally required to produce significant amounts of pure water is at least twice the
osmotic pressure of the membrane being used.
Figure 3.1: Simple (Reverse) Osmotic System
Seawater contains many kinds of solids dissolved in solution. The most prevalent is common table
salt (sodium chloride). Other minerals that may be present in solution are substances that usually
contain various compounds of calcium and sulfate. The sum of all of the solids dissolved in a particular
sample of water is referred to as
Total Dissolved Solids or TDS. Seawater normally averages 32,000 to