Aquatec DC 500, Руководство пользователя, Страница: 11 / 25

11
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.
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 permeat-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.