5
Introduction
The principle of reverse osmosis (RO)
In natural sciences and technology, osmosis is perceived as the directed migration of
molecules through a semi permeable membrane. The chemical and physical structure of the
membrane determines which molecules are able to pass and which are not. For this reason,
it is called semi-permeable, which means as much as halfway or partial permeability.
RO Membrane
If one mixes different liquids, like in this case water with different salinities, they aim to
equalize their concentration. That way, the seawater would be thinned out and the drinking
water would be enriched with salts. The effect would be a less concentrated, homogeneous
dissolution. Pouring sea- and fresh water in equal amounts into a container where both
liquids are separated by an adequate semi-permeable membrane, there would be one side
with seawater that is highly loaded with salts, on the other side more or less “clean” water
without or with little dissolved components. The natural tendency of both liquids to equalize
their salinity leads to the migration of water molecules from the fresh water side towards the
seawater side. As a result, the volume of water on the fresh water side decreases while it
increases on the seawater side. This process of osmosis takes place until the pressure on
the seawater side is in accordance with the osmotic pressure. Then it stops. In this case, the
osmotic pressure is around 30 bar. The described process however is reversible by exposing
the liquid on the seawater side to mechanical pressure. At a pressure of 30 bar, the osmotic
process cannot take place or would rather be reversed. When pressure is increased beyond
30 bar, for instance 60 bar, water molecules from the seawater side migrate to the fresh
water side. All other components of the seawater dissolution are not able to pass the
membrane. As a result, the dissolution on the seawater side remains highly-concentrated
while there is a gain of fresh water on the other side of the membrane. This process is
referred to as reverse osmosis (R.O.)
The seawater is delivered to the pressure pipe’s entry side of a RO-system by a high-
pressure pump. The osmotic membrane is located in the pressure pipe and is merely
permeable for the vehicle “water“ (solvent) and detaining the soluted substances.
When the pressure difference is more than leveling out the osmotic head, the water
molecules are able to pass the membrane that works like a filter, while the
“unpurified” molecules are detained. In opposite to a classic membrane-filter,
osmosis membranes do not have continuous pores. In fact, the ions and molecules
are migrating through the membrane by diffusing through the membrane material.
Inside of the membrane, the permeate-tube is located which transports the fresh
water through one of the two end caps of the pressure pipe towards the fresh water
tank. The excess seawater, now referred to as concentrate (brine), is drained off
overboard through the outlet of the pressure pipe by a pressure control valve.