FILDROP

Reverse Osmosis: Imitating a natural phenomenon

Salts have the tendency to absorb water. In nature the feeding of organisms occurs via the transport of mineral salts and acids dissolved in water passing through the cell membrane thanks to osmotic pressure, until the cellular pressure and osmotic pressure equalize.
In Reverse Osmosis, water with salts dissolved at a higher pressure than osmotic pressure passes through a membrane permeable only to pure water, resulting in a current of desalinated water (permeate) which is collected, while the remaining solution with a high saline content (concentrate) is expelled.

How it works

To explain the principle of Reverse Osmosis, one must first understand the phenomenon of osmosis. Osmosis can be defined as "a process by which molecules of a solvent tend to pass through a semi-permeable membrane (such as those of living cells) from a less concentrated solution into a more concentrated one, thus equalizing the concentrations on each side of the membrane." (See Fig. 1)
The vessel on the left is filled with water, and the tube is half-submerged in the water.The level of water in the tube is the same as the level of water in the vessel. In the central diagram the lower end of the tube has been closed off with a semi-permeable membrane and the tube has been partially filled with a saline solution and immersed. Initially the levels of the saline solution and that of the water are equal, but after a while the level of liquid inside the tube rises.This increase is due to "osmotic pressure". A semi-permeable membrane is a membrane that allows certain atoms or molecules to pass while blocking others. In Fig. 2, the membrane allows the water molecules to pass but not the salt molecules.
A model to understand osmotic pressure could be one to think that the water molecules are to be found on both sides of the membrane.The molecules are in continuous movement (Brownian motion). On the side with the salt water, some of the openings are obstructed by the salt molecules while on the other side this does not happen.
In this situation an higher number of water molecules passes from the water side to the salt side because there are more openings free for the passage. The level on the salt side increases until one of the following situations occurs: the saline concentration becomes equal on both sides of the membrane (impossible if on one side there is pure water and saline solution on the other); the pressure of the solution increases, in proportion to the height increase of the salt solution column, until it is the same as osmotic pressure.
At this point the osmosis gives no further visible effects. In Reverse Osmosis, applying a pressure to the more concentrated solution, the natural flow direction between two saline solutions separated by a semi-permeable membrane is inverted. In practice, the semi-permeable membranes, which are made of synthetic materials and installed in distinct groups (see Fig. 3), are the means by which this process is rendered possible.The water to be treated enters the separation module and exerts sufficient pressure on the membranes.
The part of the water deprived of salts (permeate), passes through the membranes and is collected in the permeate storage area.The remaining, more concentrated, part of the water leaves the module having passed through it completely. A minimum continuous flow of concentrate is fundamental to guarantee the flushing of possible deposits and the absence of salt precipitation.
Salts have the tendency to absorb water. In nature the feeding of organisms occurs via the transport of mineral salts and acids dissolved in water passing through the cell membrane thanks to osmotic pressure, until the cellular pressure and osmotic pressure equalize. [See Fig. 3]

Function and equipment

Pre-treatment

Possible uses

Working Conditions

Working Conditions