A Feasibility Study - Aaltodoc - Aalto-yliopisto

early 1970s (Buros 2000). This also represents the ever increasing interest in
desalination, with strong and continuously expanding markets (El-Ghonemy 2012).
The operating principle of an RO system is based on the natural phenomenon called
osmotic pressure, or to be more precise, in overcoming it. Different solutions have
different concentrations of ions and thus also different chemical potentials. When two
solutions of different concentrations are placed in connection through a semipermeable
membrane, the solution with a lower concentration of ions starts to lose its water to the
other one, thus striving for a chemical equilibrium (Wetterau 2011). The left side of
Figure 17 demonstrates the effect osmotic pressure has on a system.
The semipermeable membrane does not allow salt or other types of ions to pass.
Therefore it would be convenient if the flow of water could be reversed. That would
mean that the solution with a higher concentration of ions would increase its
concentration even more, as the water would pass through the membrane to the other
solution. In reverse osmosis systems this is achieved by applying pressure to the
concentrated solution, which is greater than that of the osmotic pressure (Wetterau
2011). The right side of Figure 17 gives an example of reverse osmosis.
Figure 17. Osmotic pressure, dots representing salt ions. Adapted from (Wetterau 2011).
The osmotic pressure of a solution, with a low concentration of salts, e.g. seawater,
can be calculated from the equation
where is the molar concentration of soluble ions in mol/m³, i.e. the salt ions, R is the
gas constant of 8,314 J/molK and T is the temperature in Kelvin degrees (Seppälä,
23
(2)